Fluid Mechanics and Fluid Power, Volume 5

Coupled Level Set and Volume Of Fluid (CLSVOF) method is an Eulerian interface capturing technique in Computational multi-Fluid Dynamic (CmFD) problems. It is broadly classified into geometric and algebraic types of method. A geometric CLSVOF method requires construction of a sharp interface, which is avoided in an algebraic method by considering a numerically diffused interface. Geometric methods are generally more accurate than algebraic methods; however, numerical implementation of algebraic CLSVOF schemes is relatively straightforward. This paper presents a relative performance of the two types of CLSVOF methods; geometric Piecewise Linear Interface Calculation (PLIC) and algebraic Weighted Line Interface Calculation-Tangent of the Hyperbola for INterface Capturing (WLIC-THINC); for gravity dominated dam-break problem and surface tension dominated buoyant rise of a bubble. Incompressible Navier–Stokes equations are solved on a co-located grid using a balanced-force method to avoid decoupling of pressure and interfacial forces. Both the types of CLSVOF method perform satisfactorily when there is a smooth change in the interface topology, demonstrated for the dam-break problem. However, when severe interface deformation occurs for the bubble rise problem, results show that the geometric-based method offers modestly better accuracy than algebraic methods owing to their sharp treatment of the interface.

We extended the numerical evaporation model for pure liquid sessile droplets to include a binary droplet on hydrophilic and hydrophobic surfaces. The extended model comprises a diffusion-limited species transport equation to estimate the vapour diffusion from the liquid–vapour interface. The model employs an axis-symmetric cylindrical coordinate using the finite element method. The species’ volatility produces different saturation concentrations in the vicinity of liquid–vapour interface. The concentration at the interface depends on the composition and activity of species in binary droplets. The respective activity coefficient is estimated using the AIOMFAC model. The experimental study shows the nonlinear variation of volume with time for different ethanol concentrations. The nonlinearity is due to ethanol, the highly volatile species, controls the initial evaporation, and the latter stage is controlled by water, the less volatile species. The comparative results of the present model show a nonlinear variation of drying droplet with time, which is consistent with the published experimental findings of the evaporating binary droplet.

A novel pintle injector with a bi-swirl arrangement is designed and developed using additive manufacturing methods. Experimental investigations are conducted to capture the atomization processes at the injector exit using laser diagnostic techniques. The spray structure is captured using high-speed direct laser sheet imaging and time-resolved particle image velocimetry experiment is conducted to obtain the component velocity field. Different linear spray parameters such as cone angle and spray penetration are estimated. The pintle injector develops a uniform spray at the exit, with a periodic droplet/ligament shedding is observed at the injector exit. The velocity field shows the dominance of the liquid phase on the overall flow field.

Direct contact condensation phenomenon between steam and water takes place in various industrial applications. In this work, we experimentally investigate the bubble formation and collapsing processes for three different nozzles with a diameter of 1.7, 2.7, and 3.5 mm and with changing pool temperatures. The frequency of bubble creation, maximum bubble equivalent diameter at the bubble development stage, and bubble collapsing duration is investigated. The bubble equivalent diameter increases with rising pool water temperature for a particular nozzle size. Additionally, when the nozzle diameter increases at a given pool temperature, the equivalent bubble diameter also increases. Bubble formation frequency increases for lower pool water temperature and the opposite trend is found for increasing pool water temperature.

The present work investigates heat and mass transport phenomena inside a thermosyphon using computational fluid dynamics (CFD) simulations by considering water as working fluid. The CFD model was developed using the volume of fluid (VOF) approach to simulate the two-phase flow hydrodynamic during steady state operation of the thermosyphon. The predicted results were compared with the experimentally obtained temperature distribution data along the height of the thermosyphon. A good agreement was found between the predicted and measured temperature data, thus confirming the predictive capability of the computational model. The heat transfer inside the thermosyphon was analyzed using temporal variation of temperature inside the thermosyphon. The water boiling concept was used to demonstrate the phase change phenomena in the evaporator region, while film boiling was demonstrated at the condenser zone. The temperature distribution was used to estimate the thermal conductivity of the thermosyphon. The study will be helpful to address all aspects of two-phase flow hydrodynamic and heat transfer phenomena during the operation of a straight thermosyphon and suggest modifications to improve the thermal conductivity.

This study presents the numerical investigation on effects of inertial force in dispersed phase and channel hydrophobicity on the hydrodynamics of droplet generation under squeezing regime in a cylindrical T-junction micro geometry using 3D finite element and level set conservative methods. The results are elucidated in terms of the phase profiles, point pressure profiles and velocity magnitude and recirculation zones. The results depict that with increase in the contact angle (θ) from 120° to 135°, the time required by the droplet to stabilize increases. The pressure profiles, recirculation zone contours and velocity magnitude graphs are then presented to elucidate the droplet breakup phenomena and recirculation zones in both phases.

The coalescence of two disc-shaped droplets falling under the effect of gravity inside a quiescent liquid medium is studied using particle image velocimetry (PIV). A smaller droplet, followed by a larger droplet, falls from a rest position inside a confined channel. The larger droplet with the higher terminal velocity collides with the smaller droplet and merges to form a single droplet that continues to fall. Here, three stages of coalescence (film drainage between droplets, film rupture, and growth of connection) of the droplets are shown qualitatively. The glycerol-water mixture is used as a droplet and silicon oil as a continuous surrounding liquid. The droplet is seeded with a 40 μm PMMA particle, which is neutrally buoyant and faithfully follows the flow. Images are recorded and processed at different time intervals to capture the droplet coalescence. It is shown that larger droplet approaches smaller droplet due to higher terminal velocity resulting in movement or leakage of surrounding liquid away from the droplets. Distance between droplets keeps reducing to zero where film rupture occurs. Reshaping of a larger diameter disc-shaped droplet is observed at the end, which keeps falling with a new higher terminal velocity.

In this work, a case of axisymmetric perfect core-annular flow (PCAF) is considered, where both the fluids are assumed to be incompressible and immiscible having different flow-behaviour and consistency indices, respectively. A steady and fully developed flow is studied theoretically with a focus on role of power law characteristics on CAF velocity profiles. It is noted that a low viscosity shear thinning fluid in the core significantly affects the velocity profiles. However, if the core has very high viscosity, the CAF velocity profile is almost independent of its flow-behaviour index. Further, a low viscosity shear thinning fluid can be the most promising option to be used as annular fluid in the CAF arrangement while transporting high viscous fluid in the CAF arrangement.

The present work investigates the evaporation of droplets containing bidispersed colloidal particles (0.5 and 1 μm) on hydrophilic glass substrates under different ambient air temperatures (25 and 50 °C) and relative humidity (30, 50 and 70%). Various experimental techniques such as high-speed visualization, 3D profilometry, fluorescence microscopy and SEM microscopy are implemented to study the evaporating droplets. The findings show that by varying the ambient conditions, there exists a significant change in the final deposit patterns and the ring profiles. It is observed that at a given temperature as the Relative Humidity (RH) is increased, the deposit morphology changes from, a thin ring with non-uniform inner deposit to a thick ring with uniform inner deposit. Also, the ring width and height reduced with decreasing RH; moreover, the reduction is more pronounced at higher ambient temperature. These findings suggest possible alteration in the internal flow due to the variation of ambient air temperature and RH. Other aspects, such as sorting of particles near the vicinity of the CL is also investigated. This study is fundamental to understanding the effect of varying ambient conditions in governing the evaporation dynamics of the colloidal suspension droplets and, eventually, the morphology of the final deposition.

Two-dimensional numerical simulation has been performed to investigate the heat transfer and melting characteristics of the Phase Change Materials (PCM) filled inside the square domain equipped with the different fins configurations. Accordingly, five different cases have been considered in the current work where each square domain holds equal fin surface area which is 30 mm2. For all the cases, paraffin wax is taken as a PCM. Left wall of the domain is maintained at a constant temperature of 350 K. This study primarily estimates the melting process of the PCM, besides, melting rate, enhancement ratio and impact natural convection current are also discussed. Findings of this study reveals that inclusion of fins in the domain certainly increased the melting rate as compared to case 1 (without fin). Further, case 3 (with two identical rectangular fins) is found to be the most efficient among all the considered cases. Melting rate of the PCM in this domain is 60.44% higher than case 1, it is attributed to rapid heat propagation in the finned domains followed by the evolution of favourable natural convection current.

In this work, we studied numerically the dynamic behaviour of droplet impact over two different wettability surfaces having equilibrium contact angles of 91° and 147° in hydrophilic and hydrophobic range, respectively. Numerical simulations are performed using an in-house code based on Dual Grid Level Set Method (DGLSM) (Gada and Sharma in Numerical Heat Transfer, Part B: Fundamentals 59:26–57, 2011) as an interface tracking method. We implemented Yokoi’s dynamic contact angle model in solver, in which the dynamic contact angle varies as the function of contact line velocity. We considered the computational domain as 2D cylindrical axisymmetric. An in-house code developed by (Gada, V. H. (2012). A novel level-set based CMFD methodology in 2D/3D Cartesian and cylindrical coordinates and its application for analysis of stratified flow and film boiling. IIT Bombay.) with modifications on contact line velocity is implemented, considering the dynamic contact angle as a boundary condition in the DGLSM (Yokoi et al. in Phys Fluids 21, 2009). Sudden jumps in thermo-physical properties across interface are smoothened by considering the interface to be diffused. It has been found that implementation of Second-Order Upwind (SOU) and First-Order Upwind (FOU) schemes to approximate contact line velocity show a good match of numerically obtained temporal variation of droplet diameter and droplet height, with published results for both the bouncing and non-bouncing droplets. But the Quadratic Upstream Interpolation for Convective Kinematics (QUICK) scheme gives a comparatively better result specifically for bouncing and errors for non-bouncing cases.

Droplet impact and wetting on non-wetting micro-textured surfaces have applications in manufacturing innovative heat-transfer devices. Inspired by examples in nature, we examine the interaction of sessile droplets with the micro-pillared surfaces of superhydrophobic nature (contact angle $$\sim$$ ∼ 150°), with square and hexagonal arrangements of pillars. These physically textured surfaces were fabricated using photo-lithography and are quantitatively characterized by surface height correlations. The solid-area fraction for the two surfaces is kept constant. Experiments show that the contact angles are almost similar on both types of arrangement in the ‘air-pocket’ (Cassie) state in a wide range of solid area fractions, however, a significant difference is observed for the lower values of solid area fraction: in this regime, the contact angles for squared-pillar arrangement exhibits a plummet. The droplet impact revealed that, at low solid-area fraction, hexagonal arrangement continues to show complete bouncing behaviour for higher Weber number while in square arrangement droplet transits to Wenzel-state at significantly lower Weber number.

In many engineering applications, such as in compressors, evaporators, and boilers, wavy film flow acts as a heat transfer barrier. Numerous researchers used long wave approximation and linear stability analysis to characterize the wavy flow. It is still necessary to study the film flow over the vertical plate in the pseudo-laminar region using numerical simulation and analytical models such that varying heat transfer coefficients with time and space can be obtained later. In the current study, we developed an analytical model that depicts the distribution of forces as a function of the film thickness and change in wave amplitude with time and space, variation of film velocity profile, and wall shear stress along the plate. We also performed a multiphase VOF simulation using ANSYS Fluent for two films free stream Reynolds number 2000 and 4000, several parameters including velocity profile, variation in film thickness along the plate length and time, variation in film Reynolds number and wall shear stress along the plate length, were obtained. Film thickness and Reynolds number data from the simulation show a 1.13% average error with the Kapitsa average film thickness equation.

In the present study, we numerically analyse the temporal evolution of internal advection pattern and corresponding thermal contours of an evaporating sessile droplet. For this purpose, a numerical evaporation model based on Arbitrary Lagrangian–Eulerian (ALE) framework is adopted. The governing differential equations for the transient heat and mass transfer phenomenon are solved in a fully coupled manner. To understand the role of wettability of the underlying substrate, the droplet volume was considered to be constant while the contact angle was varied over a wide range. Results show formation of multi-vortex pattern inside the droplet at initial stages for the hydrophilic surfaces. These findings may be attributed to the local internal thermal imbalance due to evaporative cooling effect that set off buoyancy driven advection and thermal Marangoni flow. On super-hydrophobic surfaces, buoyancy effects play dominant role at initial stages due to large contact angle. With progressing evaporation, the effect of Marangoni convection becomes significant gradually. Also, the overall internal circulation velocity in such cases is greatly enhanced due to large temperature gradient across liquid–vapour interface near the periphery. These findings may have strong implication towards understanding the mixing characteristics inside an evaporating droplet during transient stage.

An experimental study has been implemented to examine water pool sudden flash evaporation in a cuboidal chamber. Some key parameters have been studied such as initial vacuum tank pressure, superheat, and initial water temperature. The initial water volume is set to one liter only. Initial conditions of the liquid vary from 50 °C to 85 °C temperature and vacuum tank pressure 6.33 kPa and 20.63 kPa (abs.) which corresponds to 2 °C to 38 °C superheat. Moreover, the influence of these key parameters on the thermo- properties like water pool temperature, mass evaporated, nonequilibrium fraction (NEF), and heat transfer coefficient (h) are analyzed. The experimental results showed that lower vacuum tank pressure and greater superheat have larger impact on flashing phenomenon. Results suggested that a larger value of the NEF and h take place at the beginning of the flash. The value of heat transfer coefficient found to be time dependent function tends to decrease with flash time passes.

In this chapter, droplet impact and bounce off over different hydrophobic surfaces have been studied numerically. For that volume of fluid based solver has been used. The solver has been validated with the experimental observations of droplet impact on a flat surface. At first droplet, impact over a flat surface with and without pillars and stripes has been studied and directional spreading dynamics has been investigated. The study has been further extended for cylindrical surfaces and droplet split-off has been observed. Thereafter effect of different types of structured surfaces (pillared, axially striped, and circumferentially stripped) on the droplet split-off phenomena has been observed and the fluidic reason behind has been analyzed. Also, droplet angle of wrap and height over cylinder has been compared for different surface structures.

The cavitation machining process is non-traditional, gaining importance in the research community due to various advantages such as sustainability, versatility, and clean machining process. The present work examines the effect of various bubble dynamics parameters for cavitation machining. The parameter such as downstream pressure, the density of carrier fluid, initial bubble radius, and initial pressure inside the bubble on output response, i.e. implosion pressure, because the intensity of implosion pressure creates plastic deformation and behaves as a tool to remove material in cavitation machining. In bubble dynamics, the downstream pressure and the density of carrier fluid are controllable factors. The initial radius of the bubble and the initial pressure inside it are considered uncontrollable factors because it varies with time, and the process engineer has little control over these. Three different levels of control factors were planned, and a full factorial design has been considered to analyse the impact on implosion pressure. While simulating the experiments, three sets of uncontrollable factors are chosen to derive the effect of these factors. The analysis of variance (ANOVA) is used to determine the statistically significant controllable factors using the outcomes. The determined implosion pressure is capable of machining most of the engineering materials. The analysis indicates that the downstream pressure is statistically significant, while the fluid density has an insignificant effect on the implosion pressure.

The development of electronic component cooling technology has enormous potential for a special kind of wickless structure called a pulsating heat pipe (PHP). In general, performance of PHP is significantly influenced by three parameters—geometric parameter, working fluid physical qualities, and operational parameter. Surface tension, one of the thermo-physical fluid properties, has an impact on PHP's hydrodynamics and reduces heat transfer, whereas surfactant increases nucleation sites, which lowers the energy required to form more and smaller bubbles, and more bubbles overall, increasing the heat transfer coefficient in the surfactant solution. A substance called a surfactant is used to lower surface tension and produce more stable fluid. Closed loop PHP uses a 5 turns of copper tube, 2 mm of an inner diameter, and 50% of a constant filling ratio. In order to study how PHP can improve heat transmission while reducing thermal resistance, the experiment uses water-based binary fluids in a 1:1 ratio, such as distilled water, ethanol, and acetone. The surfactant employed is sodium dodecyl sulfate. Along with bottom mode heating, the heat input is adjusted throughout steps of 10 W. When comparing the experimental results for pure and binary fluids with increasing heat rate, the surfactant mixture solution produced better results. In comparison to pure fluid with surfactant solution, water–acetone mixture has the highest efficiency and the lowest thermal resistance of any of these fluids, which is about 17%.

Miniature loop heat pipe (mLHP) can meet the demand of high heat flux dissipation, arising out of miniaturization of modern electronic devices. Capillary wick is an essential component of mLHP because it serves as a thermal and hydraulic locking in the device, and also provides capillary pumping that renders the device passive. In the current study, a bi-porous composite capillary wick made of copper and aluminium powder was fabricated for flat mLHP, with a diameter of 50 mm and a thickness of 4 mm. Capillary performance index of the wick such as capillary rise, wettability, porosity, permeability, and pore size were studied. The process of the capillary rise was observed using an infrared camera. The capillary rise height was determined to be 9.5 mm, utilizing the mass conservation method. Static contact angles (CA) on aluminium and Copper surfaces were observed for acetone and methanol fluids by sessile drop method. Acetone was found to be a better choice as it has low CA, hence higher wettability. The wick was found to have a porosity of 42% and permeability of 0.031 µm2. The average pore diameter of the aluminium and the copper wick is found 6.82 µm and 0.38 µm, respectively.

The present numerical analysis deals with cavitation with and without air injection in a convergent–divergent (CD) nozzle. Numerical analysis covers for three primary stages of cavitation, such as cavitation inception, sheet cavitation and cloud cavitation. These cavitation stages are further described by injecting air bubbles in the convergent side. Results, based on vapour fraction and turbulent kinetic energy, are explored at different stages of cavitation. It is found that cavitation phenomena increase significantly with air injection at upstream of CD nozzle. This method of air injection can be a potential method of manipulating the cavitating bubble cluster particularly increasing the cavity dynamics.

The experimental investigations of droplet impingement on surfaces are quite common. However, the studies on the impact of droplet on particle embedded on flat surfaces are quite rare and need special attention. The image of droplet impact on flat hydrophobic surface with and without particle has been thoroughly analysed with the help of high-speed imagimetry technique. At low Weber numbers, rebounding was observed for flat surface without particle, whereas high Weber numbers resulted in jetting and fragmentation. For particle embedded flat surfaces, at low Weber numbers, pinning of droplet is observed, and at higher Weber numbers, fragmentation takes place.

The time evolution of spreading and evaporation following droplet impact on a non-porous substrate is experimentally investigated. Ethanol is considered the working liquid in all cases. The droplet impact occurs on a glass substrate. The main objective is to understand the influence of impact velocity on the dynamics of the spreading and evaporation rate of the droplet. The impact velocity range is selected to avoid splashing after the impact. The experimental results show no considerable variation in the maximum spread diameter and total evaporation time for the different impact velocities. However, the above parameters are always higher for the sessile droplet, where the droplet is gently placed on the substrate.

Being a two-phase flow passive heat transfer device, pulsating heat pipes can solve some of the current thermal management issues. Their distinctive feature involves thermally induced oscillations of the working fluid within a capillary tube connecting hot and cold ends. The effectiveness of heat transport in pulsating heat pipes is intricately linked to the amplitude and frequency of these oscillations. A U-shaped tube having one liquid slug and two vapor plugs is considered for mathematical modeling of the oscillatory flow in the present study. The amplitude of oscillations, the variation of mass, temperatures, and the pressure of vapor plugs are analyzed from the conservation of mass, energy, and momentum. A modified mass transfer equation for vapor plugs is introduced to account for the evaporation and condensation processes. The fourth-order Runge–Kutta method is employed to solve the non-dimensional form of the governing equations through a MATLAB program. A dimensionless time step of 0.0001 was considered for the time marching solution. The improvements are found in the amplitude of oscillation compared to the existing model. The plots for displacement of slug, mass, temperature, and pressure of plugs show the smooth variation having a phase difference around π.

In this study, a numerical investigation of droplets’ formation and their hydrodynamics of the two-phase immiscible flow has been performed in a T-junction microchannel using the conservative level set method for a broad range of flow governing parameters. The flow dynamics is described by the time evolutions of the pressure profiles for various flow regimes like squeezing, transition, parallel and jetting. The findings are presented as phase contours and pressure profiles. The pressure value in the continuous phase is coming closer to the dispersed phase when the droplet breakup is happening in the droplet zone. On the other hand, the pressure profiles exhibit contrasting behavior in the non-droplet zone. Insights of the present study help design microfluidic devices precisely.

In a heat transfer system, the point of onset of nucleate boiling (ONB) is important as it distinguishes boiling regime from the natural convection regime. Its importance is similar to that of critical Reynolds number which demarcates laminar and turbulent flow in fluid dynamics (Frost and Dzakowic in Manual of boiling heat-transfer design correlations, 1969). This makes the study of ONB important in design and sizing of heat transfer surfaces to get the optimum performance. ONB prediction models available in the literature take into account the effects of subcooling, contact angle and wettability. However, the effect of heater size factors like the thickness and width of the heater on ONB is not considered, and such studies are scarce. The present study focuses on trying to unravel the dependence of ONB on the aforementioned parameters. Experimental investigation was undertaken to study the effect of different widths of stainless steel 316 strips on ONB during steady heating. It was found that the heat flux and wall superheat at ONB increase with decrease in width of the heater. A presence of sharp transition point was observed between 4 and 2 mm width which is under further investigation.

Morphology of the bubble plays a vital role in determining heat transfer behaviour in nucleate pool boiling. Surface wettability, thermal conditions and the gravity level have a remarkable influence on morphological evolution of the bubble. Present work deals with the study of dynamic and heat transfer behaviour of a growing bubble in nucleate pool boiling on a hydrophilic surface by considering water as the working fluid. Computations have been performed by using commercial software ANSYS Fluent 2021 R1 in which volume of fluid method (VOF) has been employed for interface capturing at the liquid–vapour interface. The contribution of evaporation in the micro region has been considered. The effect of super heat level at normal and microgravity conditions has been analysed for a particular contact angle (Ca) between the surface and the bubble. Results indicate that the influence of increase in wall superheat on morphological evaluation of bubble is more pronounced under reduced gravity conditions. The impact of Ca also has been studied. It is observed that an increase in Ca leads to increment in both the departure diameter (Dd) and growth time of the bubble.

An experimental investigation has been performed to analyze the frequency associated with pressure, acoustics, and bubble evolution characteristics during the vertical injection of steam into a subcooled water pool. The frequency analysis is crucial for the structural integrity of the pool. The continuous wavelet transform is used for time domain frequency analysis of the data acquired from the pressure transducer and hydrophone at different steam mass flow rates and submergence of pipe outlet within the pool. The range of mass flow rate for the present study is 10–30 kg/hr, and pipe submergence depth is 1–9 cm. The bubble shape continuously changes with time due to the unstable nature of the direct contact condensation (DCC) phenomenon. The analysis shows that the frequency of pressure and acoustic sound signal increases with the mass flow rate. In addition, the analysis has revealed that the pressure spikes appear relatively in shorter time with an increase of mass flow rate in magnitude scalogram. The amplitude of pressure and acoustic signals decreases with the increase of steam mass flow rate.

The online water washing technology refers to the injection of water spray into the intake air in land-based gas turbine engines to clean the compressor blades fouled due to air-borne dirt deposition. In such a case, typically, a series of flat-fan nozzles are employed at the engine bell mouth. The present research work focuses on the multi-phase simulation of the spray injected from a single flat-fan atomizer into coflowing air within a wind tunnel configuration. The simulations are carried out using Ansys Fluent based on a coupled Eulerian–Lagrangian framework. The discrete phase model (DPM) is adopted to track droplets, whilst the primary breakup, secondary atomization, droplet drag, evaporation and collision are suitably modelled. The numerical results are validated against in-house experimental data. The simulations are carried out for a wide range of coflow air velocity (0–70 m/s) for the same injected liquid volume flow rate. The preheated liquid injection is also considered. The spray tends to be narrower and the cone angle decreases with an increase in coflow air velocity. The axial evolution of spray characteristics (including characteristic droplet size, velocity and temperature) downstream of the injector exit is studied for different operating flow conditions mentioned above.

We present experimental measurements of transient capillary rise in the corners of an interstice formed between an array of tubes/rods. The capillary rise shows an initial regime where the height of the corner meniscus shows a steeper dependence of $${t}^{4/5}$$ t 4 / 5 compared to the $${t}^{1/3}$$ t 1 / 3 dependence in a later regime. We develop a scaling law for the later regime by minimization of the sum of the free energy and viscous dissipation, using Onsager's principle. The obtained scaling law for the capillary height in the corner $${Z}_{m} \propto {\left(\frac{t(\sigma \mathrm{cos}\theta {)}^{2}}{\rho \mu g}\right)}^{1/3}$$ Z m ∝ t ( σ cos θ ) 2 ρ μ g 1 / 3 matches the measurements in the later regime.

Understanding the flow patterns emerging near the contact line is a primary concern of moving contact line dynamics. A contact line occurs at the intersection of a solid surface with an interface between two immiscible fluids. The moving contact line dynamics can be observed in several flow phenomena, including inkjet printing, spreading of drops across a surface, and tertiary oil recovery, among others. Displacement of the contact line over the solid surface generates flow patterns in both the fluid phases. Based on flow kinematics and boundary conditions (Huh and Scriven, JCIS, 1971; Cox, JFM, 1986), two distinct flow patterns, a rolling motion and a split-streamline motion, occur in one of the fluid phases and the problem is primarily governed by viscosity ratio, $$\lambda$$ λ , and dynamic contact angle, $${\theta }_{d}$$ θ d . Most of the existing experimental studies are for obtuse contact angles, $${\theta }_{d}$$ θ d > $${90}^{\circ }$$ 90 ∘ with very low viscosity ratio, $$\lambda \ll 1$$ λ ≪ 1 and at very low $$Re$$ Re (Chen et al., JFM, 1997). The present study aims to obtain flow patterns at low to moderate values of $$Re$$ Re and with a relatively higher viscosity ratio using a combination of experiments and simulations. We also present a comparison of our results to existing analytical models.

The interaction of an ethanol drop with a swirl flow is studied experimentally by shadowgraphy technique. In a swirling airflow, the drop has opposed, cross, and co-flow situations based on its detaching position, swirl strength, and aerodynamic force resulting in a different droplet morphology. As the droplet interacts with a differential flow field produced by the wake of the vane in swirling airflow, we observe a new breakup dynamics known as “retracting bag breakup.” For different dimensionless variables affecting the droplet morphology and its trajectories, a regime map is presented, outlining the multiple modes, such as no droplet breakup, vibrational only, retracting bag, and bag breakup modes. The breakup time for different locations and Weber number is demonstrated. The breakup time is significantly less when the drop experiences two flow configurations simultaneously.

Condensation over a tubular surface has a wide range of applications. A steady-state mathematical model for droplet condensation of saturated water vapor on the outer surface of a hydrophobic tube is proposed. The tube curvature effect on a single drop heat transfer as well as on the drop size distributions has been incorporated. A single droplet growth model utilizing heat conducted through the droplet population to the condensing surface is established. Thermal resistances that include curvature, interfacial, drop and the hydrophobic promoter layer have been considered for model development. The tube curvature effect on a single droplet heat transfer is considered based on condensation over a concave substrate (Zhang and Zhang in ACS Omega 5:22,560–22,567, 2020). The surface wetting phenomena and their characteristics are defined on the basis of an equilibrium contact angle and hysteresis. The developed model is built on the definition of an apparent contact angle. A general transcendental equation for the apparent contact angle via droplet and the tube radius to the intrinsic contact angle is utilized. Population balance theory is used for the droplet number density distribution. The droplet number densities for gravity-driven drop removal and drainage have been iteratively determined. The numerically obtained drop size distribution is integrated with the single droplet heat flux for obtaining the overall heat transfer rate during droplet condensation outside a vertical tube.

Analysis of accidental sodium leak resultant fire events is important in the safety evaluation of Sodium-cooled Fast Reactor (SFR). The sodium leaked out from the heat transport system can disintegrate into numerous droplets of various sizes and their burning in the surrounding air atmosphere constitutes the spray fire event. Accurate evaluation of droplet velocity is essential in the spray fire analysis, as the residence time and burning rate of spray droplets in the air depends on the falling velocity. In this work, a numerical model has been developed to evaluate the falling sodium droplet motion in the gas medium along with the possible droplet deformation effects. The model has been validated using the settling velocity data of free falling liquid droplet experiments in the literature and model predictions shown good agreement with the experimental results. The model has been used to evaluate the settling velocity of sodium droplets of different sizes and these results revealed the effect of droplet deformation on the falling velocity of sodium droplets. This model is incorporated into the sodium fire analysis code for the realistic simulation of postulated sodium spray fire events in the safety evaluation of SFR.

This paper presents an experimental study on drop formation of suspension prepared with moderately viscous liquid and non-colloidal spherical particles. We study the dynamics of successive droplet formation to understand the particle size effects for different particle volume fractions. We show that the droplet breakup length increases with increase in the particle volume fraction and if the particle size is comparable to the filament thickness, the axisymmetric breakup structure is modified to an asymmetric drop breakup. The retraction of the filament as well as the de-stabilization of the filament leading to quick breakup strongly depend on the particle size.

This study explores the behaviour of oppositely charged submicron particles attracted by a pendant DC-charged spherical collector in still air. Quantitative analyses of dynamic particle behaviour are conducted. There are inertial, viscous, gravitational, electrostatic, and dielectrophoretic forces involved in analysis. Electrostatic forces cause submicron particles with opposite charges to move towards the charged collector. Electrostatic attractions and distance between collector and particle are tightly related. Particles have different initial attractions based on their position on the plate. It is determined that dielectrophoretic and gravitational forces are the weakest. The particle closest to the collector has the shortest drift time and greatest force, whereas the particle farthest from the collector has the longest drift time and least force. This study outlines charged particle movement and capture using a charged spherical collector.

The electronic components, instruments, sensors, and other satellite payload subsystems generate significant heat during repeated transient duty cycles. The thermal management of such satellite payload subsystems becomes more challenging under the influence of the microgravity environment. The rapid temperature fluctuations caused due to stringent space environment may lead to the overheating/failure of electronic devices. The phase change materials (PCM) are the natural fit for the thermal control of such satellite subsystems where the heat dissipation is non-continuous. Moreover, during the melting and solidification processes, the PCMs have a tendency to either expand or contract. Designing the containment system for PCM must take both thermal and structural factors into account. Due to the harsh environmental conditions, designing the containment system to accommodate PCM volume change, particularly for space applications, provides extra challenges. Consequently, the present work deliberates two different mass accommodation methods (i.e., an open boundary and a free/movable surface), which takes into account the effect of volume expansion on the melting cycle of PCM. The computational domain consists of an enclosure filled with paraffin-based PCM, and a comprehensive comparative analysis of the PCM melting accommodating volume expansion effects is discussed in the present work.

This work employs computations based on the discrete element method to examine the gravity-driven flow of grains through apertures in a silo with two outlets positioned asymmetrically at the base. Analysis entails studying the effect of the relative positions of the apertures and their sizes. We demonstrate the existence of the self-similarity of velocity profiles in agreement with previously reported studies employing single outlets.

The dynamic features of a water droplet impingement and consecutive penetration around the conical surface are investigated numerically. As a means of analyzing the complex hydrodynamic behaviours of the system, axisymmetric simulations based on the finite volume method (FVM) have been employed. This numerical work will be solved by using conservation equations related to mass, momentum, and volume fraction which has been derived from conservation equations. While approaching and penetrating around the conical substrate, droplets impinge upon the surface and undergo a continuous spreading process resulting in a variety of stages, most notably free fall, hitting, cap formation, uncovering, oscillation, and detachment. Apart from this, the most important parameter is the cone base-to-droplet diameter ratio (Dc/Do) that ranges from 0.5 to 1 and to be taken for the present numerical study, to observe several deformation characteristics of the droplet continuing to spread during the whole impinging process. The influence of contact angle, cone base-to-droplet diameter ratio, and Weber number (We) on droplet impinging is investigated further by integrating droplet profile and deformation during penetration. In the current investigation, the maximum deformation factor rises with higher Weber number and significantly reduces contact angle.

Although the impinging dynamics of droplets on solid surfaces has been extensively studied, a complete study on the impact of impinging droplets on direct current electrowetting on dielectric (DC-EWOD) substrates with different configurations of electrodes and voltages is found limited. In order to investigate the dielectric performance and dynamics of EWOD with different voltages, experiments are performed on DC-EWOD. Parametric studies involving droplet diameter, droplet height, applied voltage, electrode spacing, etc., are studied in this work. At a Weber number of 20.02, jetting is inhibited by the application of direct current to the EWOD chip and the droplet shift to one side, and the magnitude of voltages shows a dominant effect on this phenomenon.

Here we numerically investigated the deformation kinematics of a ferro-droplet in the joint dominance of uniform magnetic field and uniaxial extensional flow. Coupled the phase-field technique with the Cahn–Hilliard-Navier–Stokes equation into our two-dimensional model, we have captured important interfacial dynamic characteristics for an extensive range of governing parameters. It is seen that the droplet can monotonically evolve into a prolate or oblate shape based on the relative magnitude between magnetic stress and hydrodynamic stress. This study further shows that the magneto-physical properties of the dispersed phase play a significant role in controlling the morphology of the droplet. This study can help to understand the various operations related to various process engineering and biomedical applications such as modifying emulsion rheology, delivering drugs, and others.

Water collection from natural and industrial fogs has recently been viewed as a viable freshwater source. An interesting outgrowth of the relevant research is focused on arresting the drift losses (un-evaporated and re-condensed water droplets present in the exhaust plume from industrial cooling towers. Such exploits in fog collection have implemented metal and polyester meshes as fog water collectors (FWC). Fog droplets impinge and deposit on mesh fibers. They coalesce with previously deposited liquid to evolve as larger drops before detaching from the fibers under their own weight, an event largely dependent on the mesh fiber wettability, diameter and its arrangement relative to the fog flow. To better estimate drainage and hence collection from these fibers, the study focuses on droplet detachment from differently wetted horizontally positioned cylindrical fibers of various diameters, placed orthogonally in the path of an oncoming fog. Droplet detachment volume is found to increase with fiber diameter and fiber surface wettability. Interestingly, in a typical fogging condition, the detachment volume is also found to exhibit a time-dependent behavior, altering the droplet detachment criteria otherwise predicted from emulation. Our current study sheds light on this unexplored phenomenon.

The nonlinear evolution equations (NLEE) of fourth-order for the propagation of two surface gravity wavetrains (SGW) in infinite depth of water are derived. Based on these evolution equations, the properties of sideband instability of uniform gravity waves are investigated. Considerable deviations are observed from the results obtained from the cubic NLEE, which are the nonlinear Schrödinger equations (NLSE). An expression of the instability growth rate is obtained, and this expression shows a key result that the fourth-order terms in the evolution equations significantly modify the modulational instability properties and produce a decrease in the growth rate for the case of acute angle.

The simultaneous drop size and velocity distribution are often desirable to optimize a liquid fuel atomization system and understand the spray dynamics completely. In the present experimental work, the swirl airblast atomizer is employed to fragment the central liquid jet with the fast-moving annular swirl atomizing air at ambient conditions. The spray characteristics of the vegetable oil are contrasted with the baseline water. The drop size and velocity components are measured simultaneously using the Phase Doppler Particle Analyzer technique. The two sprays are contrasted based on the Weber number distributions, droplet size, and velocities joint probability distributions (JPDFs). The results show that the adverse physical properties of the liquid fluid deteriorate the atomization process by raising the input energy required to overcome the viscous effects and suppress the radial growth of the spray due to the dampening of the tangential momentum while preserving the breakup morphology.

The present study aims numerical investigation of oil–water two-phase flow through horizontal pipe with sudden contraction having diameter ratio (D2/D1) of 0.47 and D1 is 25.4 mm. The ratio of length to diameter (L/D) before and after contraction was 78.74 and 83.33, respectively. For the different combinations of superficial velocities, the velocity and pressure profiles have been plotted. Result shows singularity causes a huge amount of pressure drop with the increase of superficial velocities of oil and water. For a constant superficial velocity of oil, the static pressure profile gets steeper with the increase in superficial velocity of water after singularity. At higher superficial velocity of oil (Usk = 0.9 m/s), the fall in static pressure is drastically high as compared to the low superficial velocity of oil for same combination of water superficial velocities. Vena contracta phenomena are observed just after the sudden contraction region, which can be confirmed by pressure profile.

In a stratified gas–liquid (water–air in our case) horizontal pipe flow, the growing long wavelength waves may reach up to the top of a pipe and form a slug or evolve into the roll waves. At certain flow conditions, the slugs may grow to become extremely long, up to 500 times pipe diameter (Kadri et al. “On the development of waves into roll waves and slugs in gas/liquid horizontal pipe flow,” in ICMF 98, 2007). Existence of such a long slug may create problems like reduction in operational efficiency (Andritsos et al. in Int J Multiph Flow 15:877–892, 1989). It is important to know at what flow conditions the roll waves start to occur from a wavy stratified flow. It is also necessary to find out the certain critical parameters beyond which the flow starts to form a slug. This can be analysed by tracing the horizontal and vertical time required by the crest to reach at the top pipe wall with the help of the high speed camera (Kadri et al. in Int J Multiph Flow 35:1001–1010, 2009). In this paper, the wave transition time model proposed by (Kadri et al. “On the development of waves into roll waves and slugs in gas/liquid horizontal pipe flow,” in ICMF 98, 2007) is validated using the flow visualization technique considering the average height of liquid $${\mathrm{h}}_{\mathrm{avg}}$$ h avg as the input parameter. When the time required by the wave crest to overtake downstream end is less, the roll wave is formed, if not slug formation occurs. Experiments for the validation were performed on the gas–liquid horizontal pipe set-up with 0.05 m internal pipe diameter. The obtained results are in adequate concurrence with the time model (Kadri et al. “On the development of waves into roll waves and slugs in gas/liquid horizontal pipe flow,” in ICMF 98, 2007). The flow visualization analysis is also extended to obtain the wave crest tracking and its characteristics.

Knowledge of the basic mechanisms of the bed expansion ratio in gas fluidized beds is essential for an optimum design. The influence of bed height and bed particle sizes on the bed expansion ratio in a cylindrical fluidized bed was investigated. Four sizes of alumina Geldart Type-B particles of mean diameters 125, 177, 250 and 320 μm respectively were investigated at varying bed heights and superficial gas velocity under ambient conditions. Results showed that the expansion ratio of bed increased by increasing superficial gas velocity of all bed particle sizes employed. Smaller bed particles gave rise to a higher bed expansion when compared to larger-sized bed particles. The use of bed particles of relatively large size resulted in reduced bed expansion owing to decreased bubble formation. The expansion ratio was also determined to be inversely proportional to the bed height; this being a result of the larger resistance offered to the flow of fluidizing gas by the high static bed.

There is chance of presence of argon bubbles in the secondary sodium circuit of sodium cooled fast reactor (SFR). The source of these argon bubbles can be from the locked up gas inside the secondary sodium loop during filling. When the gas bubbles reach the surge tank, these bubbles may escape in to the cover gas plenum depending on the buoyancy and inertia forces acting on these bubbles inside surge tank. The escape probability of gas bubble is also a function of surge tank geometry. Therefore, if the escape probability of gas bubble through surge tank is sufficiently high, then over time, the secondary sodium loop will be free from re-circulating gas bubbles. In order to quantify the escape probability of circulating gas volume in the secondary sodium circuit to the surge tank cover gas plenum in prototype fast breeder reactor (PFBR), an experimental study is carried out using a 5/8 scale size geometrically similar water model of surge tank in PFBR. The transportation of argon bubbles in sodium has been simulated by injecting air bubble to water through inlet lines of surge tank. The variation of bubble escape probability inside surge tank with (i) water flow rate, (ii) with air injection rate, and (iii) bubble size are estimated.

Mathematical Modelling is prepared for Cylindrical Heat Pipe in this paper. Heat transfer limitations are calculated for optimization of axial orientation and parameters of Cylindrical Heat Pipe. Cylindrical Heat pipes are considered for Mathematical Modelling on the basis of axial orientations and design parameters. Capillary limitation of Cylindrical Heat Pipe is the main heat transfer limitation for heat pipe design. Various heat transfer limitations are plotted for different parameters of Cylindrical Heat Pipe, thermo-physical properties of the working fluids and wick structure parameters. Copper and water are considered as container material and working fluid of Cylindrical Heat Pipe, respectively. Mathematical model has been prepared for axial orientations of 180º and 90º with width of wick structure as 0.28 mm and 0.48 mm and spherical diameter of copper powder as 50 and 56 μm, respectively. Cylindrical Heat Pipe with 180º axial orientation can transfer more heat than Cylindrical Heat pipe with 90º axial orientation at operating temperature range. Cylindrical Heat Pipe with 0.48 mm width of wick structure and 56 μm spherical diameter of copper powder has more heat transfer limitation within range of operating temperature. Cylindrical Heat Pipe is optimized for good thermal performance.

The present study investigates the motion of a rigid sphere in a quiescent liquid following an impact from the air. An extensive set of experiments are conducted by varying parameters such as sphere diameter (d), density ratio (ρ/ρl), and impact Weber number (We). High-speed imaging technology is used to study the underwater behaviour of the sphere. According to prior reports, high-density (ρ/ρl $$\ge $$ ≥ 7.926) spheres travel in a straight line. However, spheres with a lower density ratio (ρ/ρl $$\le $$ ≤ 2.771) are observed to deviate from their vertical impact position. It is observed that the resulting deviation in the radial direction is random. Interestingly, the penetration depth is repeatable across various experiments, irrespective of the deviation in the radial direction. This anomalous deviation in the trajectory of the spheres is attributed to the rotation of the sphere, leading to asymmetric vortices in the wake. In the present work, we also show that this anomalous motion of the sphere can be controlled by modifying the wettability of the surface. By making the surface superhydrophobic, we observe the formation of a cavity in the wake of the sphere due to air entrapment, resulting in a straight trajectory for the sphere.

The release and axial dispersion of fission gas-fragmented fuel particle mixture through the voided coolant channel following the fuel pin rupture in sodium fast reactor under severe accident condition is investigated. The motion of fuel particles released from the ruptured fuel pin is evaluated using Euler–Lagrangian approach. The fission gas velocity in the triangular voided coolant channel is described using an exponentially decaying pressure gradient. The decaying pressure gradient has a characteristic time typical of fuel pin rupture resultant fission gas release process. A parametric analysis for gas-particle flow in the range of 50 and 1000 μm diameter fuel particles is carried out. Results show that the fuel particle velocity and displacement for particles of diameter 1000 μm and above are negligible and not expected to participate in the fuel particle dispersion following the pin rupture. Also, dispersion behavior of fuel particles less than 100 μm is identical to that of fission gas. The result brings out the influence of pressure decay time constant and the fuel particle relaxation time on the behavior of fission gas-fuel particle mixture following the fuel pin rupture. It contributes to the evaluation of in-vessel radiological source term due to refractory material under severe accident condition in sodium fast reactor.

The phenomena of wetting and spreading on solid surfaces and in porous media are being studied for a wide range of applications. Two processes interact to cause drop motion over a porous layer: (a) the spreading of the drop over already saturated parts of the porous layer, which causes the drop base to expand; and (b) the imbibition of the liquid from the drop into the porous substrate, which causes the drop base to shrink and the wetted region inside the porous layer to expand. This study presented an experimental investigation of various size droplets spreading over a thin porous layer. Ink and silicone oil droplets are used to determine drop-spreading behaviour. A drop spreading on porous media increases its contact diameter initially and then decreases. Conversely, the size of the wetted region increases monotonically. Although overall behaviour for silicon oil and ink was comparable, there were qualitative and quantitative variances. Furthermore, it is demonstrated that the initial drop size affects how long it takes for a drop to imbibe into a porous medium.

Propagation mechanics of the liquid sheet formed by the impingement of a like-on-unlike impinging injector is investigated experimentally. The liquid sheet formation and breakup dynamics are captured using the backlight imaging technique. Characteristics of the instability waves on the sheet surface and the ligament shedding are quantified using proper orthogonal decomposition technique and frequency spectra analysis. Propagation of impact waves and Kelvin–Helmholtz instability waves is identified, and related frequencies are correlated with the ligament/droplet shedding frequencies.

Reactor safety is one of the major concerns in nuclear power plants. The occurrence of critical heat flux (CHF) results in the sudden raise in the wall temperature. During the post-CHF conditions, the temperature of the fuel rods in the reactors can reach more than 1850 K. To maintain the temperature of the fuel rods under the permissible limit is essential to safeguard the structural integrity of the reactor fuel core. Therefore, the study of post-CHF regime plays a vital role in nuclear safety. In the present numerical study, numerical simulations were performed using the mixture model. For solving turbulence, RNG $$k -\epsilon$$ k - ϵ model is employed. The model framework is validated against the experimental data and a good agreement was noticed. Further, the influence of critical quality and mass flux on the temperature, velocity and Nusselt number are also obtained. It is noticed that the fuel rod surface temperature is found to increase with increase in heat flux, decrease in mass flux, and increase in critical quality.

In the past, attention has been focused on symmetric superheated surfaces, such as spherical and flat geometries. However, more research needs to be done on how droplet dynamics affect asymmetric curvatures. The focus of the current study is on the impingement of water droplets on asymmetric surfaces, such as cylindrical concave heated surfaces with drop-sized curvatures. Concave surfaces main-trained in the extremely high-temperature range of 250–450 °C are impinged with water droplets. A high-speed imaging technique is utilized to follow a droplet, while the Weber number is varied between 8 and 50. The experiment showed that the droplet shape and spreading characteristics on the aforementioned concave surface are entirely different from those on flat surfaces due to non-uniform momentum distribution. Furthermore, it is demonstrated that impact Weber number and enhanced superheat have an effect on the maximum spread factor and resident time. Since gravity opposes the momentum spreading in concave geometry, it is obvious that around 56% less contact time occurs when compared to flat surfaces. In addition, conversion of enthalpy into vapour pressure contributes to the reduction in contact time.

The interaction of molten fuel and coolant occurring during a severe accident case in a nuclear reactor is studied in this paper. The molten jet of MOX interacts with the sodium in dripping flow regime. An in-house code DROP formulated using slenderjet approximation of Navier–stokes equations is used in the present analysis. Usually in the study of jet breakup, surface tension of the jet material with respect to air is considered and crust formation is neglected. Uncertainties related to interfacial tension in a liquid–liquid system and crust formation due to solidification affecting the jet breakup are discussed here. The variation of breakup length, breakup time and droplet diameter is recorded when the interfacial tension is accounted in the calculation. It is noted that a maximum deviation of 11% is observed in the breakup length, whereas a deviation of 17% is observed in breakup time for all possible ranges of interfacial tension. To account for the heat transfer to the coolant and crust formation, a module for heat transfer with solidification is incorporated into the existing code. Reduction in breakup time and breakup length is quantified. It is found that the combined effect of these two factors leads to a considerable change in breakup length and time which is brought out in this analysis.

Due to the low Re flow in a microchannel, turbulent mixing is absent. Hence, there is a need to explore novel unconventional mechanisms to improve mixing efficiency. In the present study, we theoretically analyze the interplay of an externally applied electromagnetic field and wall wettability in the mixing hydrodynamics. We observe that while the discharge rate through the microchannel depends on both the electric and the magnetic field, the interplay of wall wettability and the magnetic field governs the local vortex generation, which controls the mixing phenomenon. However, the discharge rate reduces with increased magnetic field strength.

In contrast to the fluidization of mono-disperse particles, where the minimum fluidization velocity is uniquely defined, more than one characteristic velocities are required to define the fluidization behaviour of bi-disperse particles. Literature suggests that there are two characteristic velocities; however, the conclusion is based on only the measurement of bed pressure drop. The aim of the present study is to investigate the fluidization behaviour of initially segregated bi-disperse particles through laboratory scale visualization experiments and numerical simulations. Particles used in the experiments are of the same density but of two different mean diameters. Bed pressure drop is measured and the physical appearance of the bed is recorded simultaneously. In addition, simulations are performed following the continuum description of the particle phase. Simulations are performed on the open-source software MFIX. Experimental observations show that there are at least three characteristic velocities in a fluidized bed with an initial segregated state. The preliminary continuum-based simulations show good agreement with the experimental observations.

In this study, we revisit the large bubble entrapment phenomenon during the impact of a drop in a deep liquid pool. By considering second and third modes of oscillation of Lamb’s theory, we have replicated the exact shape of a deformed drop. The third mode incorporates the asymmetric oscillation of the drop. The effect of asymmetric oscillation on the impact dynamics on a deep pool has been investigated using numerical simulations, emphasizing the large bubble entrapment process. It has been observed that asymmetric oscillation significantly affects the large bubble entrapment dynamics.

The aim of this numerical work is to understand the electrospray process of a conducting, viscoelastic liquid inside an ambient dielectric, highly viscous liquid. We use the Oldroyd-B model (which is solved with the help of Log conformation approach) to capture the dynamics of the viscoelastic liquid. The interface between the viscoelastic liquid and the surrounding highly viscous liquid is captured using the volume-of-fluid (VoF) method. The bulk charge conservation equation is solved for the movement of free charge in the viscoelastic liquid under an electric field. The numerical results depict high-volume charge density at the tip of the initial interfacial profile which leads to the formation of a thin viscoelastic jet. This jet finally undergoes Rayleigh plateau instability due to which small droplets detach under the dripping mode. The electric charge remains trapped inside the droplets as these detach from the jet. Higher magnitude of the $$\tau_{p,xx}$$ τ p , x x stress component appears on the jet interface near the orifice while such values for the $$\tau_{p,yy}$$ τ p , y y component appear in the thin, stretched jet. This technique can be used to generate an emulsion of uniformly sized, charged droplets of a viscoelastic fluid inside an ambient viscous liquid.

The rebounding dynamics of a falling droplet of varied surface tension (0.25 and 1 CMC) impacting a sessile water droplet sitting on a glass substrate is captured experimentally. The deformation of the impacting droplet as well as the sessile droplet is evaluated and the results are plotted against time. Also, the dynamic contact angle of both the impacting and the sessile drop is extracted and plotted to see the contact line dynamics. The deformations plot reveals that the lower surface tension droplet (1 CMC) deforms faster than the droplet of relatively higher surface tension (0.25 CMC). The contact angle plot reveals that it reaches a maximum value at the onset of the rebounding stage.

Natural convection is a density-driven flow within the fluid in the presence of gravity. In thermally stable systems, i.e., those with a colder temperature at the bottom, natural convection is primarily driven by dissolved solutes. The phenomenon of freezing mixtures offers an ideal case for studying such flows. The presence of small-scale solid-phase structures further enhances the complexity. The present work shows the evolution and strengthening of natural convection and its effects, such as the fragmentation of growing solids while freezing binary alloys. Furthermore, these fragmented solids remelt in bulk due to composition and temperature differences. The work highlights fundamental aspects of an interplay between the compositional and thermal distributions of freezing mixtures. In the experiment, we used an SCN-15 wt.% acetone mixture, where SCN (985 kg/m3) solidifies during bottom-cooled solidification. Acetone (784 kg/m3)-enriched liquid is rejected in bulk, which is lower density, leading to solutal convection. Convective velocity magnitude is estimated by adding neutrally buoyant particles in binary solutions. The study has contributed to understanding convective flow in solidification, which relates to fluid dynamics, experimental techniques, and metallurgical aspects.

The study of vapour bubble growth is very much essential to estimate the heat transfer rates in thermal and energy systems that involve coolant phase change. In the present study, a numerical model based on energy and force balance method is employed to predict the growth rate and departure diameter of vapour bubbles on a vibrating surface under subcooled flow boiling conditions. The energy balance considers the conduction heat transfer through the superheated layer only. The model is validated against the experimental data without and with vibrating heated surface and a good agreement is obtained. Further, a numerical simulation is performed by varying the vibration frequency of heated surface along the flow and the degree of subcooling. It was noticed that the vibration of heated surface along the coolant flow causes early departure of the vapour bubbles from the heated surface and results in enhanced heat transfer.

Droplet impact on the liquid surface with sufficient impact strength results in splashing; it is an undesirable parameter in many applications, especially in the case of fire extinguishing. To design the fire extinguishing equipment, one must know the droplet impacts characteristics before starting the fire extinguishing. Both the miscible and immiscible droplet impacts were considered for the current investigation. For the experiments, five different viscous liquids (water, petrol, kerosene, diesel and rice-bran oil) for the droplet impacts and four different liquids (petrol, kerosene, diesel and rice-bran oil) for liquid pool. Different velocities are considered as a function of three droplet impact heights to link impact strength and impact characteristics. The jet height and the secondary droplet diameters were measured using the image processing techniques and efforts to relate them with the non-dimensional numbers.

Linear stability of Newtonian fluids flowing in two layers through an infinitely long 2-D channel is carried out. Top and bottom plates of the channel are provided with constant concentrations of soluble surfactant. The current study extends the work of (Picardo et al. in J Fluid Mech 793:280–315, 2016) to investigate the effect of Peclet number and diffusivity ratio on the stability of the system. Varying the Peclet number shows non-monotonic growth in instability for two most unstable modes (M1 and M2). Neutral curves confirm that the system becomes unstable only for some range of Peclet number. The region of instability on the Peclet number space spreads with increasing Marangoni number. Upon increasing the diffusivity ratio, $$D_{r}$$ D r , the system shows a destabilizing effect upto a certain value, beyond which the system stabilizes with an increase in $$D_{r}$$ D r .

Granular flows are observed in natural processes such as landslides, snow avalanches and exhibit inhomogeneous stress distribution. Forces applied by snow avalanches on obstacles such as large electric poles, aerial lifts, cable cars, etc. need to be taken into account in the design of its construction. In granular media, drag is a function of model geometry (D), grain size (d), inclination angle (ϕ) and mass flux (m). The objective of the present study is to analyze the force variation and flow behavior past standard geometries. For lab scale experiments, a novel granular flow chute is fabricated to facilitate the force measurement and shock wave visualization for various channel inclinations. Experiments are carried out on cylinders with different diameters. Unlike previous studies reported in the literature, the chute is kept at different inclination angles. The storage hopper is placed on the top of the chute and feeds glass particles into the channel at a specific mass flux. Velocity profiles are estimated using the particle image velocimetry method. The force acting on the models is measured using the strain gauge force measurement system. Studies show that the force acting on the cylinder is independent of the mean flow velocity and that the force increases with cylinder diameter for the vertical channel.

Mass transfer during the gas liquid interactions in a monolith reactor has been an area of paramount importance due to its prevalence in process intensification. The present study experimentally investigates the mass transfer from slug bubble train in a glass tube by using the colorimetric method and a high-speed camera. An oxygen sensitive dye resazurin is used with the pure oxygen cap/slug bubble train for the current study. An in-house code has been developed for image thresholding, interface detection, bubble mask generation, and concentration measurement. It is investigated that the overall mass transfer coefficient increases with the gas superficial velocity. The role of the Taylor recirculation vortices in the mass transfer enhancement has been observed from the radial concentration profile.

This paper presents an experimental investigation of a new combination of alkali solution for generating superhydrophilic copper surfaces and its characterization based on spreading parameters. We employ the versatile alkali solution-based oxidation technique, where different aqueous solutions (KOH, K2S2O8, (NH4)2S2O8, and NaOH) are used to induce the growth of copper oxide (CuO) microstructure on the copper surface. CuO microstructure growth is controlled by varying parameters such as immersion time, temperature, and heat treatment in the traditional sol–gel method. The effect of three different chemical combinations on the spreading area is investigated. Also, the critical heat flux (CHF) calculated from the spreading parameters and droplet evaporation technique is discussed. Different techniques like X-ray diffraction (XRD), contact angle measurement (θ), and scanning electron microscopy (SEM) are utilized to study the crystal structure, surface chemical composition, wettability, and morphologies of microstructure on copper substrate. The chemical combinations used in this study changed the wettability of the pristine copper (90.3°) to superhydrophilicity (~0°), which significantly affects the spreading parameter and hence the critical heat flux (CHF). The CHF increases by ~200% compared to the bare copper surface. Our work provides a novel chemical combination to obtain the superhydrophilic (SHP) copper oxide surface with high CHF which can be produced on a mass scale in the industry.

In this paper, we have investigated the role of surface wettability during subcooled flow boiling in a microchannel heat sink. The Cahn–Hilliard phase field method is used to numerically simulate the bubble nucleation from a single artificial cavity inside a microchannel heat sink. The pressure and velocity distribution inside the microchannel is studied at different time levels. Further, simulations have been performed by varying the surface wettability of the heater surface to investigate the role of surface wettability in flow boiling. The role of bubble dynamics and heat transfer characteristics have been studied for different wettabilities of the heating surface. The bubble departure time increases with the decrease in the wettability of the heater surface.

The maneuvering capability of a fighter aircraft employing delta wings, at low speeds, can be significantly improved by enhancing the high angle of attack aerodynamic stability and control characteristics. Provision of additional control power through certain unconventional means would help in overcoming the loss of control power happening at high angles of attack due to certain flow features like vortex break down and fore-body induced asymmetrical flow features. This paper explores the possibility of using an asymmetric deflection of canard surface for generating yawing moments on a close coupled canard-delta wing fighter aircraft, at subsonic Mach numbers. RANS computations using CFD++ solver were carried out for the configuration with asymmetric canard deflection. The results from the computations were compared with the experimental wind tunnel test results for the same configuration. The yawing moments produced by the asymmetric canard deflection have been compared against that produced by the rudder and presented. The results from the CFD simulations were used to explain the interactions of vortices shed by the up and down deflected canard and the delta wing vortical flow, when compared to the undeflected canards.

This paper presents an in-house code-based physiological fluid–structure interaction (FSI) study for pulsatile blood flow in abdominal aortic aneurysms (AAA) with varying height and width. An in-house FSI solver developed using arbitrary Lagrangian–Eulerian (ALE) framework with finite-volume discretization is employed to perform axisymmetric blood flow simulations in artery with fusiform aneurysm. Considering a pulsatile inlet flow at Womersley number Wo = 16.5, a parametric study is presented with various height (H) to diameter (D) ratio H/D = 0.3, 0.5, 0.7, 1.0 and 1.2 and width (W) to diameter (D) ratio W/D = 0.5 and 1. For identification of severity of aneurysm, the present work attempts to analyze the variation of rupture risk (required for planning treatment strategy) and qualitative hemo-acoustic indicator. After analyzing the two parameters, a correlation which can be harnessed to predict rupture risk based on actual acoustic signals is presented. Integrated pressure force rate (IPFR), which correlates directly with acoustic fluctuation sensed by a digital stethoscope is calculated on the artery surface. Also, rupture potential index (RPI) is calculated based on the ratio of wall stress and ultimate tensile strength of the artery. It is observed that geometries with H/D ≥ 0.7 lead to RPI ≥ 0.3 which is considered critical. Based on the data of RPI and IPFR spectrum, a nonlinear relation is proposed to calculate rupture risk through frequency data. The current work establishes the efficacy of phonoangiography-based diagnosis to assist medical practitioners in planning treatment strategy.

The problem of pollution dispersion in urban areas is significant in the densely populated cities. The topography and barriers in the form of buildings impact the atmospheric fluid flow. The resulting phenomena known as pollution traps cause an artificial dispersion in the buildings’ proximity, affecting the health of ordinary road commuters. The primary source of pollution on the street canyons is exhaust gases from the vehicle movements. However, the concern is associated with the poor dispersion of pollutants under normal wind conditions. The primary reason behind the poor dispersion is the buildings that act as obstacles to the atmospheric wind flow. Thereby it is essential to comprehend the behaviour of pollutants under given shape constraints and flow conditions to improve urban air quality. The present study investigates the wind flow in the proximity of a six-storey building for a medium street canyon configuration under the logarithm inlet velocity profile that acts as atmospheric boundary layer (ABL). Effect of important parameters such as the building height, the wind direction (0, 30, 45, 60, and 90°), and building configurations (straight road, both side building, and only upwind side building with downwind side building) are investigated to gain valuable insights into pollutant dispersion. The analysis of turbulence and velocity profile in the domain at nose level (1.5 m above ground level) leeward sidewalk and windward sidewalk shows turbulent intensity decreases at the nose (breathing) level with building height; however, it increases when the approach angle is 450 suggesting the formation of dominant pockets of pollutants.

We experimentally and theoretically study the vibrational dynamics of a thin cantilevered flexible plate mounted to the lee side of a circular cylinder subjected to a free stream airflow. Plates of four different lengths having constant span and thickness are tested in a wind tunnel by varying wind velocity. Based on the dynamics of the plate’s motion, we find three regimes, namely pre-critical, transition, and post-critical regimes. In the post-critical regime, the plate reaches self-sustained limit cycle oscillation. A reduced-order wake oscillator model is developed to explain the experiment. The plate is modeled as an Euler–Bernoulli cantilever beam and the fluctuating lift force is modeled using van der Pol oscillator. This coupled wake oscillator model is able to describe the qualitative behavior of the experiment in a lock-in regime. We find that the coupled dynamics of the plate and flow exhibit similar characteristics as that of the vortex-induced vibration (VIV) of a cylinder.

The paper presents numerical investigation of flow-induced vibration of an elastically mounted circular cylinder in proximity of identical downstream stationary cylinder at Reynolds number Re = 100. The problems are solved using in-house code based on Level-Set function-based Immersed Interface method (LS-IIM). The elastically mounted cylinder has mass ratio $$m^{*} = 2.0$$ m ∗ = 2.0 and damping ratio $$\zeta = 0.005$$ ζ = 0.005 . The two cylinders are placed in tandem arrangement with varying gap ratios of $$G^{*} = 2.5 - 0.1$$ G ∗ = 2.5 - 0.1 . . Three distinct vibration responses named as vortex-induced vibration (VIV), proximity-induced galloping (PIG), and proximity pressure-induced staggered vibration (PPISV) are observed at larger ( $$G^{*} \ge 1.5$$ G ∗ ≥ 1.5 ), intermediate ( $$G^{*} = 0.5,0.3$$ G ∗ = 0.5 , 0.3 ), and smaller gaps ( $$G^{*} = 0.3, 0.1$$ G ∗ = 0.3 , 0.1 ), respectively. The novel vibration response PPISV is caused by enhanced influence of proximity pressure of downstream cylinder. Similar to galloping, large amplitude A*, low frequency in-phase oscillation occurs in the PIG vibration response. The three vibration responses have distinct flow dynamics occurring in gap, called as flow states and each flow state is discussed in this paper.