Fluid-Structure-Sound Interactions and Control

For the past 50 years or so, Kolmogorov’s (1962) correction (K62) to his 1941 hypotheses (K41) has been embraced by an overwhelming majority of turbulence researchers. Our recent work suggests that there are no valid reasons for abandoning K41. In particular, analytical considerations, based on the NS equations, which take into account the finite Reynolds number (FRN) effect, together with the available experimental laboratory data, seem to confirm a tendency towards the simple and elegant predictions of K41 as the Reynolds number increases. This is especially true when the focus is on the length scales which lie in the dissipative range. Incorrectly accounting for the FRN effect and the inclusion of the atmospheric surface layer (ASL) data, likely to have been affected by the proximity to the surface, appear to be the major factors which have contributed to a nearly unchallenged acceptance of K62.

Jets and waves, whose origin is in gravity force, are often observed in fluids. When the fluid has a vertical density stratification, both can be generated due to the buoyancy force. The jet appears when an obstacle descends vertically in a stratified fluid. The generation is supported by the molecular diffusion of the stratifying scalar such as salt. If there is no scalar/density diffusion, the density must be conserved along the fluid motion, and the originally horizontal isopycnal surfaces are simply deformed as they are dragged down by the obstacle, so that the dragged fluid will move downward indefinitely and will never go back to its original height. If the diffusion exists, the fluid can change its original density, and move away from the isopycnal surface to which the fluid originally belonged. In this study, we demonstrate the generation process of the jet, along with its interaction with the internal gravity waves. As another example of waves in fluids, water waves with capillary effects in a homogeneous fluid are numerically simulated, showing the excitation of short capillary waves by a long solitary wave. The phenomenon has been predicted by a weakly nonlinear theory, but has not yet been observed.

Feedback turbulence control is a rapidly evolving, interdisciplinary field of research. The range of current and future engineering applications of closed-loop turbulence control has truly epic proportions, including cars, trains, airplanes, noise, air conditioning, medical applications, wind turbines, combustors, and energy systems. A key feature, opportunity and technical challenge of closed-loop turbulence control is the inherent nonlinearity of the actuation response. For instance, excitation at a given frequency will affect also other frequencies. This frequency crosstalk is not accessible in any linear control framework. This paper will address these nonlinear actuation mechanisms in three parts. First, success stories of human learning in turbulence control are presented, i.e. cases in which the nonlinear actuation mechanism has been modelled and understood. A large class of literature studies can be categorized in terms of surprisingly few mechanisms. Second, we discuss model-free machine learning control (MLC) and selected applications. MLC detects and exploits the winning actuation mechanisms in the experiment in an unsupervised manner. In all studies MLC has reproduced or outperformed existing optimized control strategies. Finally, future directions of turbulence control are outlined. Methods of machine learning are a disruptive technology will contribute to rapidly accelerating progress in turbulence control—both for performance and for physical understanding.

The suitability of two universal wake numbers, namely Roshko’s universal Strouhal number and the Griffin number, was explored for surface-mounted finite-height bluff bodies. Mean drag force coefficient, Strouhal number, and mean base pressure coefficient data for finite cylinders and finite square prisms, for various Reynolds numbers and aspect ratios, were considered. Comparison was made to established values for the two-dimensional cylinder and square prism for Reynolds numbers in the range of 104 < Re < 105. The Griffin number was found to be the most suitable universal wake number, and was reasonably successful at collapsing data for finite cylinders and finite square prisms for a wide range of aspect ratios and incidence angles, particularly if the body’s aspect ratio was higher than the critical aspect ratio.

As passive flow-control devices disrupting flow separation, leading-edge protuberances can provide superior hydrodynamic performance to hydrofoils at high angles of attack. Most experimental and numerical investigations carried out for low Reynolds number conditions have related the relative improvements observed post-stall to “bi-periodic” flow structures, developing over tubercles pairs. In this study, a numerical approach is employed to show the emergence of higher-order patterns in the flow over a stalling NACA 0021 hydrofoil with sinusoidal leading edge. The effect of the number of sinusoidal tubercles defining the leading edge of the hydrofoil model on the prediction of “bi-periodic” or “tri-periodic” flow structures is particularly analyzed to interpret the uncertainty found on the resulting hydrodynamic performance.

Several vortex identification methods along with a newly proposed $$\Omega $$ method are examined in the Burgers vortex and the Sullivan vortex. Of particular interest is the physical meaning of the parameter, especially the thresholds. While all the methods are capable of capturing precise vortex boundaries in these two analytical vortices, only the parameter $$\Omega $$ seems to have a clear physical meaning, i.e. to what extent the local fluid particles tend to rotate like a rigid-body. Therefore, the parameter $$\Omega $$ might be helpful and informative when utilized to investigate the interaction of vortical structures.

To study the effect of a hole on the wake structures of a low aspect ratio circular cylinder, an inclined hole going from the front face to the top end face is drilled inside cylinder having height H and diameter D of 70 mm (an aspect ratio H/D = 1). In order to compare the flow structures between the hole and no-hole cylinders, the PIV measurement of Reynolds number of 8,570 is carried out in a water tunnel. Furthermore, to evaluate the position effect of the hole, three kinds of the hole cylinders having different height of hole from the flat plate are used. It was found that the separation region of the hole cylinder on the top face is evident smaller than that of the standard cylinder. An area of a high negative u-component velocity was observed in a rear recirculation region by using the hole, indicated a strong separation flow. The Reynolds shear stresses were evidently suppressed by the blowing flow from the hole, but their distribution spread out.

The dynamic behavior of burst point (BP) around delta wing (DW) during oscillation has been a hot topic for its close relationship with the stability and control of an aircraft. Delayed detached eddy simulation (DDES) combined with rigid moving mesh techniques was implemented to investigate the dynamic response of the BP around an 80°/65° double delta wing (DDW) during sinusoidal pitching motion at critical angle of attack (AOA). The effects of reduced frequency (RF) on the performance of BP were discussed in detail. The movement of BP is locked in the frequency of pitching motion with a large phase lag. The time-averaged location, oscillation amplitude and phase lag are significantly determined by RF. The time-averaged location reaches its most downstream at RF of 0.2. When the RF is near 0.2, the Root-mean-Square (rms) of displacement of BP is much larger. The phase lag increases linearly with the growth of RF.

Active drag reduction of an Ahmed body with slant angle of 25° has been experimentally investigated at a Reynolds number Re of 1.67 × 105, based on the square root of the body cross-sectional area. A combination of steady blowing is deployed along the upper and two side edges of the rear window and the upper edge of the vertical base, resulting in a maximum reduction in drag by 25%, which is higher than those reported in the literature for the same body. Measurements are conducted using particle image velocimetry. A marked modification is identified on the separation region over the rear window, the upper and lower recirculation bubbles behind the vertical base, and the C-pillar vortices, which is connected to the high drag reduction.

Steady slot suction is applied near the leading edge of the free end of a cantilevered square cylinder to investigate its effects on the aerodynamic forces. The slot suction significantly changes the flow separation on the free end and also the aerodynamic forces on the entire cylinder span. The best control result appears at the suction coefficient Q = 1 (Q = U _s /U_∞, where U _s is the suction velocity at the slot, and U_∞ is the oncoming flow velocity), with the fluctuation drag and lift reduced by 17.8% and 45.5%, respectively. At Q = 1, the shear flow at the leading edge is weakened and reattaches on the cylinder free end, which results in stronger momentum transport between the free-end shear flow and the wake, thus suppressing the vortex shedding and aerodynamic forces efficiently.

In this study, a jet control using the coaxial type DBD plasma actuator (=DBD-PA). The coaxial type DBD-PA is an axisymmetric nozzle, and the jet is ejected from this nozzle. DBD-PA is driven by burst modulation control and the induced flow is intermittently generated. The coaxial type DBD-PA controls the jet by controlling vortex generation in the jet. We control the jet at Reynolds number Re = 10,000. As a result, the induced flow of the coaxial type DBD-PA synchronizes vortex generation of the jet in a specific burst modulation control frequency range. This phenomenon is the phenomenon of lock-in. Driving the coaxial type DBD-PA with burst modulation, it is possible to generate an axisymmetric vortex within frequency of burst modulation control in which the phenomenon of lock-in occurs. Consequently, the jet is controlled by the coaxial type DBD-PA within frequency of burst modulation control in which the phenomenon of lock-in occurs.

Unsteady flow structure behind an Ahmed body with a slant angle of 35°, corresponding to the low-drag regime, has been experimentally investigated at Re of 6.59 × 104. Measurements are conducted using single hotwire and flow visualization techniques. The major flow characteristics are examined. A total of five distinct Strouhal numbers (St) have been identified in the wake, as the case of the high-drag regime. Two detected over the rear window are linked to vortices generated along the roof and side surfaces of the model, respectively, and two found near the ground surface are ascribed to vortices produced by the struts (wheels) of the body. The fifth one, with St = 0.3, captured behind the base is distinct from its counterpart found in the high-drag regime and a physical explanation is proposed for this St.

The authors numerically investigate the influence of the forced oscillation upon the three-dimensional thermal convection in a cubic cavity heated from one wall and chilled from its opposite wall in the gravity and zero-gravitaty fields without/with a forced sinusoidal oscillation. The direction of the forced oscillation is parallel to the temperature gradient direction. In addition, the direction is parallel to the direction of the gravity, in the gravity field. The authors assume incompressible fluid with a Rayleigh number Ra = 8.0 × 104 in the gravity field without the forced oscillation or Ra = 1.0 × 104 in the gravity field and a Prandtl number Pr = 7.1 (water). The forced-oscillation parameters are a vibrational Rayleigh number Ra _η and a non-dimentional forced-oscillation frequency ω. In the gravity field, Ra _η  = 1.0 × 105 and ω = 5.0 × 100, and in the zero-gravity field, Ra _η  = 1.1 × 105 and ω = 5.0 × 100. As a result, supposes reports a new flow structure in laminar and steady thermal convection, which consists of a pair of trident currents, namely, three ascending streams and three matching descending streams in a cube heated from a bottom wall and chilled from its opposite top wall. This flow is rather robust. Then, it can be observed in a stationary cube under the gravity, and can be observed in an oscillating cube under the gravity or zero gravity besides.

High-speed water jet shows a peculiar processing property in cutting of submerged objects but its processing ability decreases quickly with the increase of standoff distance. Aiming at to improve the performance of submerged water jet an experiment investigation on the velocity distribution of cavitating submerged jet is carried out by PIV method where micro bubbles are used as flow tracers. Further, a sheathed orifice nozzle with ventilation hole is developed and the effect of air ventilation is investigated. The result reveals that the core velocity of no-ventilation cavitating jet is higher than ventilated ones near the sheath exit. However it becomes reverse at the downstream when x/d ≥ 40. The core velocity of ventilated jets decays much slowly compared to the case of no-ventilation jet. Air ventilation is demonstrated to be an effective way to enhance the perframnce of submerged water jets.

The control of a two-dimensional turbulent flow separation from a 25°—backward-facing-ramp is experimentally studied using an unsteady upstream blowing through a spanwise slit with a view to reducing the recirculation length. The Reynolds number examined is 9.38 × 104, based on the ramp height h. Three control parameters are examined, i.e. the blowing amplitude, frequency and duty cycle. Extensive measurements of the flow are carried out using PIV, pressure and hotwire techniques. The results are compared with those of the natural flow and under unsteady blowing. The time-averaged recirculation bubble is found to contract in length by 40% under the optimum control parameters, which correspond to a very small fraction of input energy. It is further found that the optimization control frequency is close to the shedding frequency of the uncontrolled shear layer captured at 1 h downstream of the separation point.

This work aims to understand the mechanism behind friction drag reduction in a dielectric barrier discharge (DBD)-plasma-controlled flat-plate turbulent boundary layer (TBL). Streamwise-oriented DBD plasma actuators are deployed to generate streamwise counter-rotating vortices in the TBL. The variation in the local friction drag is measured using a single hotwire, and the change in the flow structure is captured using a high-speed PIV. At a voltage V _a of only 4.25 kV, the drag reduction over an area (90 mm long and 200 mm wide) behind the plasma actuators reaches 14%. In fact, the drag reduction area stretches longitudinally to about 300 mm or 2000 wall units. The drag reduction is found to be linked to the decrease in the near-wall turbulent kinetic energy production, pointing to that the plasma-actuator-generated streamwise vortices interrupt effectively the turbulence generation cycle, thus stabilizing near-wall velocity streaks and resulting in friction drag reduction.

Water or air-water bubble flow passing through tube bundle can be seen in many industrial equipments such as heat exchanger and chemical equipment. In addition, tube bundle can be used as a flow-straightener, -mixer, -resistor, and -damping device. In this study, the effects of tube arrangement of equally or unequally spaced in-line and staggered tube bundle and of void fraction on the flow characteristics such as flow pattern and flow resistance of a tube or tube bundle are examined experimentally.

The longitudinal vortex wind turbine (LV-WT) is a novel propeller type, but the turbine blade is the circular cylinder. The blade is driven by lift force of the longitudinal vortex (LV). This work is to examine a simple single-cylinder blade to suppress the negative driving-force portion by cutting and keeping only the useful area. Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulation is used for investigation. For the original single blade, the positive driving force was generated around the dominance of the LV at the crossing section. Meanwhile, the negative driving force appears far from the ring regime. When the original blade is diminished to avoid the unpleasant parts, the negative regions are suppressed completely. The distribution of the aerodynamic forces along the blade length is symmetric for the length extended equally. Final, the equally extended blade with the length of three times of the ring width is built and tested in the wind tunnel. The tendency of the experimental data and the CFD results correlate very well.

This study investigates by LES the formation and evolution of counter-rotating vortex pair (CRVP) from a streamwise-inclined 35° turbulent jet in a laminar crossflow (JICF). A new method is developed by which the JICF consists exclusively of a jet marked by artificial XH₂O and crossflow by real H₂O. Predictions by LES agree well with the experimental results of (Dai et al., Int J Heat Fluid Flow 58:11–18, 2016) [3]. It is shown that the flow patterns, visualized by the three-dimensional flow topologies, vary with the velocity ratio (VR) considerably. The hairpin vortices are illustrated to be actually the instantaneous forms of the time-averaged CRVP. Analysing the jet mass fraction distributions around the jet exit finds that the CRVP mainly comes from the coherent structures within the nozzle for VR = 0.5 and from the shear vortex at the jet/crossflow interface for VR = 2.0.

Water jets issuing from a fan jet nozzle (Fan Jets: FJs) are widely used in cleaning, decontamination of radiological substances, and removal of plasma spray coating and asbestos. The material removal performance of abrasive suspension jets issuing from a fan jet nozzle (Abrasive Fan Jets: AFJs) are much higher than that of FJs. In the present study, flow structure and velocity distribution of AFJs are investigated by using PIV method to clarify the material removal characteristics of AFJs.

Micro-bubble is one of the most promising methods for reduction of skin friction drag. The injection of gas bubbles into a turbulent boundary layer may have multiple impacts on the turbulent flow structure. This work aims to understand the interaction between micro-bubbles and the turbulent boundary layer, especially the effect of the bubble layer thickness on the skin-friction drag reduction. Large eddy simulation was conducted for a turbulent boundary layer over a flat plate, injected with micro-bubbles, with a view to reduce skin-friction drag. The Reynolds number Re _θ examined was 1430 based on the momentum thickness θ and free-stream velocity $$ U_{\infty } $$. A three-dimensional perturbation method was deployed to generate a turbulent boundary layer within a short distance of inflow.

This paper reports numerical simulations of the film cooling performance and flow structure from two staggered rows of coolant jets. The RANS modeling is conducted for the cases that are validated by and matched to the measurements of Sinha et al. (J Turbomach 113:442, 1991) [8]. The cooling effectiveness of two rows of jets is calculated for blowing ratios of 0.5 and 1.0. It is found that the interaction of counter-rotating vortex pairs (CRVPs) generated by two rows of jets enhances the cooling effectiveness significantly. Moreover, the calculations quantify the contribution from each row of coolant jets to the overall cooling performance and find that the second row makes more contribution than does the first row.

The wake of a three-dimensional (3D) time-averaged flow field over a surface-mounted finite-height square prism of aspect ratio AR = 3 at a Reynolds number Re = 500 has been investigated using Large Eddy Simulation (LES). The topological characteristics and interactions between the dynamic structures in the prism wake were assessed using planar streamlines and the second invariant vortex identification criterion. The shear layer from the prism free end, which descends into a pair of counter-rotating tip vortices due to downwash, is seen in the streamwise planes in the wake. Other features identified in the simulation include the mean recirculation zone behind the prism and a complex set of discrete streamwise vortex tubes in the wake.

The present study investigated the jet diffusion mechanism using a coaxial dielectric barrier discharge plasma actuator. A coaxial plasma actuator was installed at a nozzle exit to promote mixing, and a hotwire sensor was used to measure the jet velocity fluctuations downstream of the nozzle exit. It was found that the jet velocity near the nozzle exit exhibited three different fluctuation modes depending on the duty cycle of the plasma actuation. Modifications to the flow structure and the corresponding mechanism under different actuation conditions are described in this paper.

This research reports on an experimental investigation of the flow characteristics of multiple round jets issuing from 6 × 6 in-line nozzle arrangement at low-Reynolds number, Re ≈ 4 × 103, with five spacing ratios of l/d = 1.5, 2, 3, 4 and 5 (where d is a diameter of nozzle, and l is a spacing between the center of nozzles). The mean and fluctuating velocities of the jet flow were measured by constant-temperature type hot-wire anemometer. We found that the bending of the outside jet decreases with increasing the spacing ratio l/d. In the further downstream, the multiple jets merge into a single jet flow and the merging of multiple jets occur more downstream with increasing the spacing ratio l/d. In the case of the small spacing ratio, the maximum velocity of merged jet increases and the merged jet spreads from the near field of jet exit.

Acoustic excitation, moving tabs, a plasma actuator and so on have been presented as methods of active control for a jet. In this study, a round jet is locally excited by a local sound wave radiated from a loudspeaker inputting a sine wave. The excited frequencies were determined with reference to the preferred frequency in the column mode of the jet. The jet was visualized using planar laser Mie scattering. Mean velocity and turbulent intensity on the jet centerline were measured using a hot wire anemometer. As a result, it is found that the acoustic excitation produces anisotropy to the spatial development of the jet, and changes of the flow structure by the frequency of the sound wave. For exciting at the preferred mode, one side of the jet expands and the other side becomes narrow. For exciting at 1/2 frequency of the preferred mode, the asymmetric structure appears in the jet.

In this study, measurement of wall shear stress on an airfoil surface under the lifting condition by using oil film interferometry (OFI) is performed. Reynolds number of the airfoil flow is 8 × 104. Silicon oil and sodium lamp are used for Fizeau fringes formation. The 300–600 images with time interval snap shot in 1–2 s depends on velocity by digital camera are acquired for the calculation of thickness of oil film and moving velocity on surface of it. Also, airfoil surface is covered with PET film for more clearly Fizeau fringes. Time-averaged wall shear stress measured by OFI are compared to numerical simulation by the LES. Maximum difference in the distributions between by the OFI and by the LES has at x/C = 0.58, and it has C _f  = 0.00439 by the OFI and C _f  = 0.00455 by the LES.

A new method for the formation of free jets with long laminar regions using a small-size device is proposed. A free jet with a 0.12 m diameter at Reynolds numbers in the range of 2000–12560 is experimentally studied using thermoanemometer measurements of velocity and turbulent fluctuations profiles and laser visualisation of the flow. It is shown that the designed technology forms the free jet with a laminar region length of 5.5 jet diameters for an optimal regime with Re $$\sim 10000$$. Numerical simulation of the flow in the forming unit and an inviscid hydrodynamic instability analysis of the jet with calculated profiles have been conducted to explain the existence of the optimal regime.

Fluid-structure interactions of large amplitude vibrations are investigated for flows around an elastically mounted rigid circular cylinder, a long flexible circular cylinder, and the Tacoma Narrows Bridge. The governing equations of fluid flow and structure motion are solved implicitly for a stable solution. Weak coupling is introduced with predictors to simulate fluid-structure interactions without any iteration per time step. For an elastically mounted rigid circular cylinder and a long flexible circular cylinder, large amplitude vibrations are caused by starting vortices generated in the shear layers. On the other hand, for the Tacoma Narrows Bridge, large-amplitude torsional vibrations are maintained in two segments of the deck due to the matching of the dominant frequency of the rotational angles of the deck with that of vortex shedding.

This study analyses the fluid-elastic instability occurring in turbulent wakes around tandem cylinders as well as the morphing effect in order to control the instability amplification and to improve the aerodynamic performances particularly concerning the Airbus-A320 wing-flap configuration. These studies have been carried out by means of High-Fidelity numerical simulations by using the NSMB—Navier Stokes MultiBlock code (Grossi et al. AIAA J 52(10):2300–2312, 2006) [1], as well as by refined physical experiments, after having built advanced morphing wing prototypes and tested in wind tunnel by using Time-Resolved PIV (TRPIV) and aerodynamic balance measurements. It has been shown that the electroactive morphing concepts are able to provide significant improvements of the aerodynamic performances and to decrease the rms of the displacement and of the forces in the case of the tandem cylinders configuration undergoing Flow Induced Vibration.

A numerical method for massively parallel computing to solve fluid-structure interaction problems was developed and the method was employed for solving the multiscale problems in biomedical applications. As one of the examples, a platelet adhesion process to the vessel wall, which occurs at the initial stage of a thrombosis, was analyzed using the multiscale method of coupling continuum scale finite difference method with the molecular scale Monte Carlo method. The platelets adhesion to the injured vessel wall is caused by the protein-protein binding (GP1b-α on the platelet—VWF on the wall.). This protein-protein binding force is evaluated by Monte Carlo simulation, solving the stochastic process of each biding. Adhered platelets also feel the fluid mechanical force from blood flow and this force is affected by the presence of red blood cells, which causes the drastic change to the adhesion process. As another example of multiscale simulations, ultrasound therapy method using microbubbles are also explained.

Vortex induced vibration of a circular cylinder with multiple small-diameter control rods at a relatively low Reynolds number Re = 200 is numerically investigated in this study. The numerical model is based on the Reynolds-Averaged Navier-Stokes equations. The Arbitrary Lagrangian-Eulerian (ALE) method is employed to consider the motion of the circular cylinders. The Petrove Galerkin Finite Element Method (PG-FEM) is used to discretize the governing equations. The numerical results show that the maximum oscillation amplitude for the small gap ratio of G/D = 0.1 is almost the same as that of an isolated circular cylinder. However, for larger gap ratios, the maximum oscillation amplitudes are much smaller. Therefore, the VIV response of the circular cylinder can be successfully suppressed by the six control rods in present study.

A cylinder is constructed to have its aspect ratio (AR = H/D) varied between 0.5 and 11 in increments of 0.5. A force balance is used to measure a normal force developed on the free end of the cylinder, along with the drag force and corresponding bending moment. The forces are seen to be influenced by two critical aspect ratios, occurring at AR = 2.5 and AR = 6. The drag coefficient increases rapidly for low AR, is stable between the critical AR, and rises linearly for high AR. The normal force coefficient reaches a plateau between the critical AR. The bending moment coefficient and the point of action for the drag force are stable for AR higher than 6, and increase rapidly at low AR. Below AR = 2.5, the boundary layer is dominant, and above AR = 6, the free end effects begin to reduce as infinite cylinder behaviour is approached.

In this paper, three-dimensional turbulent flow fields in a low specific speed Francis turbine have been obtained by CFD methods, and the performance prediction for the turbine have been made based on the simulation results. Two types of runner have been compared for the turbine: a conventional runner with uniform blades and another runner with splitter blades. The comparison of results shows that by adding splitter blades, the inlet vortex on the suction surface of the long runner blade is inhibited obviously, leading to an increase in turbine efficiency. Furthermore, the minimum pressure on the runner blade with splitter is higher than for the conventional runner, resulting in a better cavitation performance. In addition, the turbulence kinetic energy and vorticity for the runner with splitter are also smaller than that of the conventional runner.

This paper present a numerical study of vortex-induced vibration of four rigidly connected cylinders in a square arrangement in an oscillatory flow. The cylinders are elastically mounted and are only allowed to vibrate in the cross-flow direction. The gap between two neighbouring cylinders is 3D, where D is the cylinder diameter. The Reynolds number and Keulegan–Carpenter (KC) are kept at constants of 2 × 104 and 10, respectively. The reduced velocity is in the range of 1–15. The vibration amplitude and frequency are compared with those of a single cylinder. Two lock-in regimes of the reduced velocity are found to be the same as that of a single cylinder. Between the two lock-in regimes, the vibration amplitude is zero because of the symmetric vortex shedding flow pattern.

This paper presents the cross-flow induced vibration response of a both-end-spring-mounted circular cylinder (diameter D) placed in the wake of a rigid circular cylinder of a smaller diameter d. The cylinder diameter ratio d/D and the spacing ratio L/d are 0.4 and 2.0, respectively, where L is the distance between the center of the upstream cylinder to the forward stagnation point of the downstream cylinder. The focus is given to investigate how the initiation of the vibration occurs, gaining insight into physics in the transition period where the cylinder starts to vibrate. The transition period can be divided into pre-initial, initial and late transitions. The role of added mass, added damping and work done in the transition process is studied in detail.

In this paper, a twin cantilever system is spring-mounted on a bearing and investigated experimentally in a cross flow in a wind tunnel; two types of bearing are used, cylindrical with one degree of freedom and spherical with three. By comparing the two systems, it is found that the spherical bearing system is slightly more unstable and operates at a higher oscillation frequency, hence having more favourable conditions for energy harvesting. The physical manifestation of the destabilising effect of the spherical bearing is seen in the yawing figure-of-eight forward and backward motion of the cantilevers about the mount, which can be observed by looking directly down on the set-up. For the range of spring stiffnesses used in this investigation, the power producing capabilities of both systems continue to increase as system natural frequency reaches higher values.

We study the asymptotic stability of steady and pulsatile Poiseuille flow through a compliant channel to investigate the effect of wall structural stiffness on the different instability branches that may exist in the Fluid-Structure Interaction (FSI) system. We implement both vertical (V) and axial-vertical (AV) structural displacement models in which the dynamics are respectively controlled by the vertical stiffness and its ratio to the axial stiffness. It is shown that reducing the wall stiffness in the axial direction causes destabilisation of the inflectional instability (II) while reducing vertical stiffness causes Divergence (D) modes to yield the critical instability. Finally, it is shown that pulsatile Poiseuille flow at the low-frequency modulation used herein is more unstable regarding the inflectional instability but more stable regarding the Divergence instability than the steady Poiseuille flow for the range of wall stiffness and streamwise disturbance wavenumber studied in the present work.

Blood flow oscillations of 0.001–0.2 Hz are called vasomotion whose physiological mechanism has not been understood. This vasomotion can mirror human body conditions and initiate the pathogenesis sequence in some diseases. In the preliminary measurement of blood flow oscillations in radial artery at the wrist, a strong power spectral density (PSD) at ~0.1 Hz was found, indicating that low frequency flow oscillations play a dominate role in radial pulse pattern. To understand the interaction between vasomotion and cardiac rhythm in radial artery, numerical simulations were carried out. It is found that the natural frequency of the system decreases with the complexity of microvasculature system, and the inlet oscillating velocity interacts with the natural frequency to generate subharmonics. As the natural frequency in the constructed vessel system can be as low as 0.37 Hz, we speculate that the natural frequency of actual microcirculation is much lower, and the mechanism of vasomotion is actually due to the interaction of cardiac rhythm and microvasculature natural frequency.

In this work, we focus on the vortex evolutions in the wake behind three non-axisymmetric rigid cylinders under high-speed, low-damping galloping in a wind tunnel. Laser displace sensor is used to measure the displacement amplitude and two branches of amplitude curves are observed—rising and falling range of galloping range. Particle Image Velocimetry is then used to investigate coherent vortex structures in the wake. It has been found that a D-section cylinder has a galloping response at the lowest free stream velocity, which also has the smallest formation length and recirculation bubble length when at static condition. The wake widths show a similar growth rate, which is independent of the galloping amplitudes.

This paper presents a study on the control of two-dimensional vortex-induced vibration (VIV) of a single circular cylinder at a low Reynolds number of 100 using a synthetic jet (SJ) pair. To facilitate this study, a lattice Boltzmann method based numerical framework is adopted. While its strength is fixed, the SJ pair operates either in phase or anti-phase over a wide excitation frequency range. The effects of the SJ excitation phase difference and frequency are systematically investigated. Simulation results reveal that both the in-phase and anti-phase SJ pairs are able to mitigate the VIV at higher excitation frequencies, while either the cross-flow or streamwise resonance (associated with large-amplitude VIV) may be induced by the SJ pair at lower frequencies.

Spontaneous rhythmic oscillations in microvessel diameter are known as vasomotion. Vasomotion is the intrinsic property of small arteries and arterioles and is dependent of heartbeat, respiration, or neuronal input. The skin microcirculation is an anastomotic network of vessels with many crucial functions in which the blood flow must be finely regulated and tuned in order to fulfill all the demands of the organism. The laser Doppler Flowmetry (LDF) can be used to measure dynamic changes in skin blood flow over a small area. Traditional Chinese medicine (TCM) considers that there exist acupoints around body which are connected by meridian, and stimulating the acupoint is a typical therapeutic technique in TCM. It was found that stimulating one acupoint could enhance the vasomotion in another acupoint significantly. We argue that the vasomotions in these two acupoints could be correlated to each other. To verify our argument, we used the laser Doppler Flowmetry (LDF) to measure the skin blood flow at two acupoints, and carried out spectral and correlation analyses. It is found that the vasomotions related to myogenic activity are quite strong at acupoints and the correlation is discernable. The vasomotion related to myogenic activity at non-acupoint is much weaker and there is no significant correlation between the acupoint and non-acupoint.

This work aims to investigate experimentally the fluid-structure interaction FSI for a flexible cylinder subjected to axial turbulent flows. Two configurations are considered; a solitary flexible cylinder with simple support ends and is free to vibrate in the transverse direction, another configuration is a non-flexible cylinder which installed parallel and adjacent to the flexible cylinder in various cylinder centre-to-centre pitch P* (=P/D, where D is the diameter of the flexible cylinder). Investigation on the effect of axial-flow turbulence intensity T _u at 0.7 and 2.9% on the FSI for the two configurations over a range of freestream velocity U_∞ (0.19–2.14 m/s) is performed by means of simultaneous measurement of flexible cylinder vibration and velocity field adjacent to the cylinder. We found that T _u has a strong effect on the flexible cylinder vibration in both configurations. At high _∞ and T _u , we observed substantial interaction of flow structures between cylinders which provokes the buckling of the flexible cylinder at a lower critical velocity compared with its counterpart of a solitary cylinder.

In this paper we model the fluid-structure interaction of non-linear flutter of a cantilever mounted upon a non-linear spring at the clamp in a uniform axial flow. This permits us to compare results with those from a hybrid non-linear system (a linear system mounted on a non-linear spring) and so to assess the change in fundamental physical phenomena owing to the introduction of full non-linear structural- and fluid-mechanics. We use numerical simulation for the non-linear system while our state-space solution of the corresponding linear system is used to guide the choice of parameters in the investigation. We show that above the flow speed of flutter-onset for small disturbances, amplitude growth leads to non-linear saturation so that the system settles into finite-amplitude oscillations. The frequencies of these oscillations evidence the dual-frequency characteristics of mount oscillation observed in physical experiments. When the natural frequency of the mount is low, we show that for a range of increases above the linear critical speed the linear hybrid and non-linear systems evidence the same frequency phenomena. However, the linear hybrid system evidences larger oscillation amplitudes than the non-linear system. Therefore, the stabilising effect of the non-linear structural terms outweighs the destabilising effect of the non-linear fluid terms.

A numerical study is conducted on the effect of inlet turbulent intensity on the axial-flow-induced vibration of an elastic cylinder subjected to axial tubular flow. The cylinder with fix-supported ends is free to vibrate in the lateral direction. While a large eddy simulation is used to calculate the turbulent flow field, the Ansys mechanical + Fluent two-way coupling has been deployed to capture the fluid-structure interaction. The calculation agrees qualitatively with experimental data. Various inlet turbulence intensities, T _u , i.e., 0, 0.3, 5.0 and 10.0%, are examined at two non-dimensional flow velocities, $$ \overline{U} $$, i.e., 3.30 and 7.62. The results show that T _u has a significant effect on the cylinder vibration. At $$ \overline{U} $$ = 3.30, the maximum displacement grows with T _u and the vibration is classified as the subcritical vibration; the instability of cylinder is not induced with increasing T _u . At $$ \overline{U} $$ = 7.62, the buckling occurs at T _u = 0%, while the flutter takes place at T _u = 0.3%; both are associated with an asymmetric pressure distribution around the cylinder.

Particle Image Velocimetry (PIV) and direct force measurements are used to investigate the effect of transverse perforations on the flow-induced loading on and the associated flow structure around flat plates that are aligned with the oncoming flow. Plates with different characteristic diameter of the perforations, as well as a reference configuration without perforations are compared in terms of the spectra of the flow-induced forces, frequencies of the trailing edge vortex shedding and boundary layer profiles at the trailing edge at different planes across the perforation patterns for a range of inflow velocities. At high inflow velocities, boundary layer thickness increased as the diameter of the perforations increased and the distance from the perforation to the trailing edge of the plate decreased.

The purpose of this research is to control the particles motion by the ultrasonic standing wave in air. In this research, simulation have been compared with experiment for the confirmation of validity and examined the possibility for control of particles motion by ultrasonic standing wave. As the result, simulation corresponded with experiments qualitatively. Also, maximum sound pressure can be increased by converging the acoustic wave with a reflector. Then slow speed particles are more easily affected by acoustic radiation force. But, the motion control of small particle was difficult because acoustic radiation force acting on particle was weak. Additionally, motion of small particle is susceptible to acoustic streaming. Therefore influence of acoustic streaming must be taken into consideration when we want to control the motion of small particle.

Acoustic impedance is one of the most important parameters for fluid media and is determined by effective mass and elastic modulus. For an acoustic absorber with a shallow depth, the system has high characteristic resonance frequency and its performance is controlled by the system stiffness which is inversely proportional to the depth. This study begins with the analysis of a common porous material as the benchmark and compares with the traditional micro-perforated panels (MPP) before introducing advanced designs. It is shown that MPP can indeed yield similar performance as the porous material when the aperture is very small hence high cost. However, the performance at very low frequencies remains poor. In order to achieve a broadband performance, parallel resonator array is required. This system is shown to reduce system stiffness significantly while the overall mass increase is much smaller than that of a single MPP resonator. Advanced means of reducing system stiffness by electromagnetic forces is discussed, and the physics of a negative dynamic mass is also analyzed.

An introduction of 5.5 m × 4 m anechoic wind tunnel in China Aerodynamics Research and Development Center (CARDC, in Mianyang) and some typical experimental results are presented. A C919 1:14 scaled aircraft model test shows that noise sources are mainly located at the landing gear, high-lift device, flap-wing joint points, slat-fuselage conjunction positions. And a 1:8 scaled high speed train model test shows that main noise sources are located at the first bogie and pantograph position.

In this study, the large eddy simulation (LES) numerical method is applied to simulate the flow filed of a foil with the gap between tip and end plate. The flow field details of three different tip gaps were simulated. The noise source induced by the tip-leakage flow was also predicted by LES method, and then the noise characters of foils with different tip-leakage were calculated by acoustic analogy method using Ffowcs Williams and Hawkings (FW-H) equation. The correlation between noise and vortex was analysed by using the spectral method. This study will give information to rotor tip design.

Measurements of flow induced wall-mounted finite airfoil noise have been taken in an anechoic wind tunnel to examine the influence of camber on tonal noise production. The airfoils have an aspect ratio (ratio of airfoil span, L, to chord, C) of $$L/C = 2$$ and measurements encompass variations in camber of 0–8%. The results include far-field acoustic spectra and sound maps taken with a microphone array that reveal geometry effects on noise production.

In this paper, the flow characteristics and the noise of a subsonic jet are studied by experimental and computational methods. Blind holes are added to the inner wall of a subsonic round nozzle. Measurements of sound pressure signals are combined with large-eddy simulation to study the flow structures and the noise control. The results show that the porous wall of the subsonic nozzle has changed the vortical structures and the shear layer, and then the radiated sound is changed.

Near field screech tone analysis of a typical underexpanded low supersonic circular jet issuing from sonic nozzle have been carried out numerically through solving axisymmetric Navier-Stokes equations directly. Screech tones’s spectral information and the dynamic evolution of their corresponding flow structures and acoustic field are presented. Numerical results indicate that axisymmetric A₁ mode and A₂ mode screech tones are generated at the trailing edges of fourth and third shock-cell respectively. It is also found that screech tone’s generation is associated with the compressive regions outside jet shear layer closely.

In the present paper, the attention was focused on the effect of flow separation, which occurred at upstream of tube banks, on acoustic resonance in In-line tube banks. The flow separation was generated by using the orifice at the upstream of tube banks. We measured the acoustic pressure on the surface of side walls, spectrum, correlation and phase delay of acoustic pressure in the spanwise direction. The height of passage to diameter ratio at the orifice were 0, 2.8, 5.7, 8.6, 11.6, 14.4. When the acoustic resonance of first, second, and third mode occurred, the peak SPLs increased. As the gap velocity increased overall, the acoustic mode number and peak SPLs increased. As height of passage decreased, the peak SPLs decreased and the onset velocity of each mode of acoustic resonance increased. We have also discussed the prediction method to estimate the onset velocity of acoustic resonance with flow separation at upstream of tube banks.

The current beamforming approach for locating moving acoustic sources could not correctly evaluate sources with time-dependent frequency fluctuation. The frequency fluctuation is usually caused by the non-constant relative speed between the solid object and the flow. The source frequency varies with time in a certain frequency range, so that the frequency-domain beamforming approach, which only focuses on a fixed narrow frequency band, cannot focus on the fluctuating frequencies. A frequency compensation method is proposed to smooth the frequency fluctuation. Fluctuating frequencies are squeezed into a narrow frequency band, so that a frequency fluctuating source can be treated as a conventional steady source. The method is then integrated into the frequency-domain beamforming approach for moving sources, and is able to identify moving sources with frequency fluctuation. The integrated method is verified by a simulation and an experiment performed on a rotating source.

In this study, combustion oscillation characteristics of hydrogen-rich fuel were investigated. The experimental results fueled by mixture of natural gas and hydrogen show that hydrogen-rich combustion influences on the oscillating frequencies. In the case of only natural gas, the single oscillating frequency around 350 Hz is obtained, and in the hydrogen-containing fuel case, the double oscillating frequencies around 200 and 400 Hz are measured. However, the latter oscillating frequencies could not be derived from the one-dimensional acoustic analysis. Therefore, to figure out these frequencies, the acoustic impedance was measured experimentally and the oscillating frequencies were re-calculated using the measured acoustic impedance as the acoustic boundary conditions. As a result, the 200, 350, and 400 Hz frequencies could be expressed using the acoustic impedances.

Beamforming is an imaging technique that locates sources by a phased array system. Conventional source model in beamforming is the monopole, and problematic result may appear towards directional sources. As an alternative, a modified beamforming method is proposed in this paper to extend this technique to dipole sources. The method is developed by predefining orientations and selecting direction of maximum output at each scanning point. Simulation and experiment are conducted to illustrate and verify this method, and results are shown to give a proper location of dipole sources.

The mechanism of sound generation by lips is studied for basic understandings of biological sound. The sound is similar to buzzing, and is known as a sound which is generated when the trumpet player blows breath to a mouth piece. The sound and vibration of lips are observed experimentally by using a microphone with a high directivity and a high-speed CCD camera. The flow through the opening of lips are observed by using a smoke visualization. The vibration of the lips model is also observed by a high-speed CCD camera with 1000 fps. The lips model shows interesting motion by jetting the compressed air. The cyclic motion of the opening and closing is observed. The frequency of motion relates the natural frequency of the model. The oscillation causes the pulsating sound that is superimposed higher frequencies. The digon shape of nozzle relates the higher frequency sound.

In this paper, Aeolian tone and the related flow field of a circular cylinder (CC) and a tapered circular cylinder (TC1) are predicted using the numerical simulation for incompressible flow. The number of computational grids is 4 million. The Reynolds number based on diameter at middle span and uniform flow velocity is 2.65 × 104. The Aeolian tone spectra are calculated using the formula of FW-H and sound sources around the cylinder are computed based on Powell-Howe vortex sound theory. As a result, comparing these distributions of the sound source, the vortex sound intensity of TC1 is less than of CC in span wise direction. Aerodynamic sound spectra by the experiment and calculation provided good agreement.