Advances in Turbulence

The flow around surface-mounted finite-height square prisms is complex due to the strong three-dimensionality of the flow, especially for prisms of small aspect ratio AR = H/D (where D is the prism width and H is its height). Different flow models have been proposed in the literature to describe the dynamic flow structures in the wake of surface-mounted prisms, although gaps and inconsistencies remain—for instance, in explaining the origins of regions of high streamwise vorticity in the upper part of the wake. This study aims to explore the dynamic flow structures in the wake of a surface-mounted finite-height square prism of AR = 3 at a Reynolds number of Re = 500 based on the prism width, with a thin boundary layer on the ground plane. Large-eddy simulation results were analyzed using a phase-average approach to extract the dominant periodic features of the wake. The phase-averaged flow fields show the occurrence of alternating structures similar to half-loops, although the regions of highest streamwise vorticity take place outside the near-wake formation region and are associated with the downwash and with a large-scale motion of the wake in the transverse direction. The dynamic behavior of the wake reveals the alternate entrainment of fluid close to the ground plane. In contrast, the upper region of the wake is more symmetric.

The slug flow is one of the most complex flow patterns due to the unstable behavior of phase distribution. This pattern occurs in a wide range of flow rates and therefore is observed in different industrial processes. The prediction and understanding of the hydrodynamic parameters of this flow regime have a significant engineering value. In this context, Computational Fluid Dynamics (CFD) has been shown to be an efficient tool for the prediction of this type of flow. However, to ensure the accuracy of the numerical solution, adequate modeling of interfacial properties transfer is necessary. One of the most important interface transfer phenomena is momentum transfer between phases. Therefore, it is necessary to use a robust approach to model the gas–liquid interface region. The aim of this study is to evaluate the effect of adding the damping of turbulent diffusion at the interface on flow modeling. For this, different cases of simulations were elaborated for a pipe with 2 m in length and 26 mm inner diameter. In all the cases, the multiphase approach used was the Volume of Fluid (VOF) with the Geo-Reconstruct scheme. The interface between the fluids was modeled with constant surface tension equal to 0.0728 N/m. The discontinuities present at the interface were treated in a “continuous surface stress” (CSS) manner. The turbulence was modeled using kω-SST with and without turbulence damping. The independence of the numerical solution in relation to the grid was evaluated by the Grid Convergence Index (GCI) method in which four levels of grid were used. Preliminary results showed that, in the cases with turbulence damping, a better representation of the flow pattern morphology was obtained. Regarding the quantitative parameters, the prominent frequency of the Power Spectral Density (PSD) of the pressure signal was under-predicted when the turbulence damping was not used.

Large Eddy Simulation (LES) is a useful tool in the study of smooth channel flows of high Reynolds number, but when the domain is large enough computational cost restricts the correct representation of the viscous sublayer. In this study, we test the use of a one-dimensional stochastic model (ODT) as an alternative to simulate the flow close to the wall within the LES. This approach comprises the use of one independent ODT (a vertical line) inside each LES grid close to the wall, driven by the LES at the top and providing the lower boundary condition to the LES (two-way coupling). Results of mean velocity and total stress for Re_τ = 590 and 5200 are similar to Direct Numerical Simulation, and they have the correct order of magnitude for velocity variances.

Theoretical and experimental interest in transport and deposition of sediments from rivers to oceans has increased rapidly over the last two decades. The marine ecosystem is strongly affected by mixing at river mouths, with, for instance, anthropogenic actions like pollutant spreading. Particle-laden flows entering a lighter ambient fluid (hyperpycnal flows) can plunge at a sufficient depth, and their deposits might preserve a remarkable record across a variety of climatic and tectonic settings. Numerical simulations play an essential role in this context since they provide information on all flow variables for any point of time and space. This work offers valuable spatio-temporal information generated by turbulence-resolving 3D simulations of poly-disperse hyperpycnal plumes over a tilted bed. The simulations are performed with the high-order flow solver Xcompact3d, which solves the incompressible Navier–Stokes equations on a Cartesian mesh using high-order finite-difference schemes. Five cases are presented, with different values for flow discharge and sediment concentration at the inlet. A detailed comparison with experimental data and analytical models is already available in the literature. The main objective of this work is to present a new dataset that shows the entire three-dimensional spatio-temporal evolution of the plunge phenomenon and all the relevant quantities of interest.

The present work aims to investigate the flow-induced noise generation in a turbulent pipe flow by means of Computational Fluid Dynamics. The flow behavior and noise production at a high Reynolds number internal flow are analyzed in a heating ventilation air conditioning (HVAC) system. In order to develop the present analysis and to find the most accurate approach for this application, we evaluated different advective discretization schemes in the context of DES simulations. Comparisons are established with experimental data of benchmark cases, for which gauge pressure in specific regions, sound pressure level, and velocity fields are evaluated. Simulations are performed using an in-house code, i.e., UNSCYFL3D, which was developed at the Federal University of Uberlândia and has been validated in several internal and external flows. By means of this work, we were able to find the most accurate setup parameters for such a problem. Reynolds Averaged Navier–Stokes turbulence models did not yield satisfactory results, whereas the Detached Eddy Simulation (DES) was found to produce much more accurate results, obviously at a higher computational cost. It is concluded that DES, along with the central difference scheme for the advection, is a viable approach for computational aeroacoustics for HVAC ducts.

This study is dedicated to the implementation, verification, and measurement of execution speed, using the method of manufactured solutions, of a new temporal integration method of the implicit–explicit kind, the Stepwise-IMEX or SIMEX, in the Aeroacoustics, Transition and Turbulence Group’s (GATT) Direct Numerical Simulation (DNS) code. In simulations where \(\mathrm{Ma}\approx 0.1\), or less, SIMEX should be able to reduce the explicit fourth-order Runge–Kutta’s (ERK) execution time, which is currently used. A pilot program was created, in Python, with the Burgers’ 1D equation, using fourth-order compact finite differences, to test from second- to fifth-order SIMEX methods and the mentioned explicit method. Indeed, in these preliminary tests, SIMEX proved to be faster than ERK only with linear Burgers’ equation and only after a certain stiffness level. However, there are indications that some characteristics of the DNS, such as the \(\mathrm{y}\)-axis’ greater refinement, not found in the pilot are favorable to SIMEX. Both the decomposition of the ODE and the iterative method for implicit systems and the chosen tableaus pair have a huge impact on the SIMEX efficiency and will be targeted in next stage of DNS code implementation.

We performed direct numerical simulations (DNS) of bi-disperse particle-laden gravity currents on a lock-exchange configuration with different values of Schmidt number (\(\mathrm{Sc}\)) for each particle fraction, to investigate the impact of double mass diffusivity on flow dynamics and deposition. We used the high-order code Incompact3d to solve the incompressible Navier–Stokes equations and the scalar transport equation. We compared our results with previous physical and numerical experiments available in the bibliography, obtaining a good agreement. We simulated two cases: (i) \(\mathrm{Sc}=1\) for both particle fractions and (ii) \(\mathrm{Sc}=3\) and \(\mathrm{Sc}=1\), for coarse and fine fraction, respectively. Case (ii) shows higher reduction of the front velocity of current during the deceleration phase. For case (i), the current has higher amount of suspended fine particle during all the experiments, which could explain why the front velocity has lesser decreasing during the deceleration phase. The configuration of the final deposit profile shows that case (ii) has the highest deposit peak nearer to the lock-exchange gate than the case (i). We also calculated the temporal evolution of the energy budget of our simulations, and we find that the energy is conserved during all time of our simulations and the term related to turbulent dissipation is the principal responsible for energy loss.

Simulation results based on the lattice Boltzmann method are shown for the turbulent lid-driven two-dimensional cavity flow at Reynold number 100,000. We use a boundary condition scheme that significantly improves stability for simulation of turbulent flows within the lattice Boltzmann method framework using a regularized form. Velocity profiles as well as explicit expressions for the turbulent kinetic energy budget are presented.

The transient flow regime related with the formation of laminar separation bubbles (LSB) is examined by time-resolved measurements of the velocity field. The investigated scenario aims at studying the flow in multiple stage turbines, where the wake of airfoils in previous compression stages induces periodic variation of turbulence level in the subsequent airfoils. This can lead to periodic removal and formation of LSB. This process is simulated here by exciting controlled disturbances with a vibrating ribbon. Experiments are carried out in a laminar water channel at PUC-Rio. Longitudinal PIV measurements are performed on a flat plate subjected to an adverse pressure gradient. The pressure gradient is set by false walls with adjustable geometry. Suction is applied on the false wall, in order to avoid the boundary layer separation at this surface. Bubble topology and disturbance growth along the streamwise direction are measured during the transient of bubble formation. Results show spatial amplification over a narrow frequency bandwidth. Dominant non-dimensional frequencies (St = fδ*_s/U) at late stages of bubble formation are in close agreement with those reported in literature. During the transient the intensity of reverse flow is significantly changed, pointing out for possible changes on the stability mechanisms involved on the bubble reattachment.

In this work, the simulation of an airfoil with NACA 63-618 cross section was performed to obtain the curves of the lift and drag coefficients as a function of the angle of attack. The interest in this profile is its application in operational current turbines, such as SeaGen, which can harness the kinetic energy of the currents of rivers and seas. A two-dimensional model was used to simulate the airfoil with a chord of 0.23 m. The Reynolds number was Re = 5.3 × 10⁵ and the angle of attack varied between −5° < α < 15°. The values obtained were compared with experimental tests and other simulations. Several conventional RANS turbulence models have been applied, such as k-ε, optimized k-ε, SST and RSM, however none of them presented results that were in accordance with the literature. The Transition SST model showed better agreement with the experiments. It was demonstrated in the present study that the Transition SST model is fundamental in this specific case. This happens because there is a laminar-turbulent transition on the airfoil. Therefore, computational fluid dynamics (CFD) models accurately simulate provided appropriate numerical techniques are employed.

The present work experimentally investigates the spatial evolution of controlled artificial disturbances inserted by a harmonic point source in a Blasius boundary layer over a flat plate. A small hole of 0.80 mm diameter is responsible to introduce a blowing and suction flow inside the boundary layer induced by an embedded loudspeaker. The model employed consists of a plexiglass plate with an 1800 mm chord, 1000 mm span, and 10 mm thickness, vertically assembled inside the test section of the Low Acoustic Noise and Turbulence (LANT) wind tunnel at EESC-USP. Aluminum leading edge, flap, and tab were attached to the model in order to promote a practically constant pressure distribution. Hot-Wire Anemometry was carried to measure base flow, turbulence level, and Tollmien-Schlichting profiles. Good agreement with Blasius was obtained in at least a 300 mm span range, on two different streamwise positions. Tollmien-Schlichting eigenfunction profiles were measured for different positions in a chordwise direction. The amplification region matches that predicted by Linear Stability Theory. Through spectral analysis, it was possible to identify the presence of fundamental and first harmonic oscillation, which led to the conclusion that the flow conditions corresponded to a weakly non-linear regime.

Circulating Fluidized Bed (CFB) reactors have been extensively used for industrial applications such as mixing, drying, catalytic, and non-catalytic reactions. Due to the nonlinear and non-equilibrium gas-solid flow structures, the dynamic behaviors in a gas-solid multiphase flow remain far from being completely understood. In addition, considerable differences in the flow state occur under different operation conditions, resulting in different structures. These structures cause an impact on gas-solid momentum, mass, and heat transfer, affecting productivity. Pressure fluctuations are usually used to characterize dynamic behaviors of heterogeneous structures in fluidization. The signal of measured fluctuation contains the information about the multiscale flow characteristics and may also be associated with different phenomena. Identifying which scales of the flow are the most affected can help to reveal the dynamics of these different structures in gas-solid flow. The present work investigates pressure signals obtained through physical experiments at different experimental conditions in order to identify which scales were affected by the turbulent gas-solid flow. These signals were obtained in a CFB riser using glass beads particles, which were classified as group B of Geldart. Additionally, the gas velocity and mass flow were varied with the purpose of evaluating their influence over the turbulent scales. The obtained signals were investigated on the frequency and time-frequency domains. The power spectrum density (PSD) was applied to identify the dominant frequencies, as well the Wavelet transform was used as a mechanism to obtain the scales where its fluctuations were evaluated. The combination of such analyses resulted in the identification of the most affected scales, where it was observed that the mesoscales were attenuated with the addition of particles resulting in an increase in the fluctuation of the microscales.

Circulating fluidized beds (CFB) are used in several industrial applications, in which more homogeneous fluidization of solid particles is desired. In these devices, the core-annulus profile and the clusters of particles decrease the gas-solid contact. Two alternatives to improve solids dispersion are the use of aerodynamic ring-type baffles and ultrasonic waves. In this work, the influence of these devices on the solids dispersion in a lab-scale CFB riser was evaluated using Phase Doppler Anemometry (PDA) measurements. We used five airfoil-shaped ring baffles with 10 mm of thickness and 20 transducers with a frequency of 40 kHz and an input power of 10, respectively, above the riser inlet. The ring baffles increase the solid velocity near the wall and the ultrasound decreases it. With a velocity of 5.6 m/s, the solids distribution in the cross section improves with the rings and worsens with the influence of ultrasound, but with 8.3 m/s rings and ultrasound improve the solids distribution. The gas-solid dispersion improved by 18% with baffles at 8.3 m/s and 17% with ultrasound at 5.6 m/s. This study indicates that both the ring baffles and the ultrasonic field applied in the riser inlet region increase the gas-solid dispersion and contribute to reducing the concentration of particles near the riser walls and the development of clusters.

Stirred tanks agitated by impellers are used in a wide range of industries, e.g., chemical, food, pharmaceutical, and petroleum. The tank design, the impellers, and the number and type of baffles are often associated with their application. Thus, the experimental investigation of these parameters in the turbulent flow is crucial for the control and optimization of this equipment. Particle Image Velocimetry (PIV) is a non-intrusive and quantitative technique that allows determining the vector fields of the flow using tracers. The distribution obtained by this method can also assist in the validation of CFD simulations. The objective of this work is to estimate the energy dissipation rate (EDR) of a stirred tank from PIV 2C-2D measurements and its relation with the spatial resolution. The work was conducted in 0.38 m diameter tank (\(\mathrm{T}\)) with a pitch blade turbine (PBT) impeller of diameter D (\(\mathrm{D}/\mathrm{T}=1/3\)) in water. The angle-resolved PIV enables a number of turbulent features to be identified. Hence, measurements were taken for three angles, 0°, 45°, and 75°. The EDR was estimated using four methodologies: by the assumption of local axisymmetry (AS), by direct estimation (DE), by modified direct estimation (MDE), and by large eddy simulation (LES). For the optimization and reduction of possible errors, different processing strategies were used to decrease the noise level of the PIV measurements. This study showed that values of EDR were found to vary by two orders of magnitude from near the impeller to the circulation region of the tank. Herein, the effect of measuring angle on EDR was analyzed and provided an insight into the anisotropy of the turbulence in the stirred tank. However, EDR estimation is exceptionally challenging due to the lack of knowledge to distinguish its accuracy and the influence of spatial resolution.