PIV studies of coherent structures generated at the end of a stack of parallel plates in a standing wave acoustic field

Oscillating flow near the end of a stack of parallel plates placed in a standing wave resonator is investigated using particle image velocimetry (PIV). The Reynolds number, Re _d , based on the plate thickness and the velocity amplitude at the entrance to the stack, is controlled by varying the acoustic excitation (so-called drive ratio) and by using two configurations of the stacks. As the Reynolds number changes, a range of distinct flow patterns is reported for the fluid being ejected from the stack. Symmetrical and asymmetrical vortex shedding phenomena are shown and two distinct modes of generating “vortex streets” are identified.     

Flow-structure interactions of two parallel inverted flags with small separation distances in a water tunnel

In this study, the dynamics and flow fields of two parallel inverted flags are investigated using particle image velocimetry technology. The separation distance between two flags is less than two times the length of the flag, and the length ratio of these two flags is considered in the investigation. The results show that for the dynamic behaviours of two identical flags with a larger separation distance, the anti-phase and in-phase modes occur successively in the periodic oscillation as the flow velocity increases. The anti-phase and in-phase oscillations occur according to the formation position of the low-pressure and recirculation areas at different flow velocities. Moreover, a novel coupled flapping mode is observed at smaller separation distances: the contact anti-phase flapping mode, in which one flag oscillates with a large symmetric amplitude, and the other flag oscillates with a single-side large amplitude. As the separation distance further decreases, the in-phase mode appears for a larger range of flow velocity values, to avoid contact for the largest possible amplitude oscillation. Finally, as the length ratio decreases to 0.75, the oscillation frequency of the shorter flag becomes twice that of the longer flag, causing the in-phase and anti-phase oscillations to occur simultaneously in one cycle (i.e., the multi-phase flapping state). Interestingly, the two flags oscillate out of phase in the flapping apart process to avoid contact at a higher flow velocity. In general, the lower amplitude of the longer flag and two contact flags relative to that of an isolated flag clearly indicates the importance of two equal-length and non-contact flags for energy harvesting.     

Unsteady Couette flow in a rotating system

The unsteady Couette flow between two infinite horizontal plates induced by the non-torsional oscillations of one of the plates in a rotating system under the boundary layer approximations is investigated. An exact solution of the governing equations has been obtained by using Laplace transform technique. It is shown that when the oscillating plate situated at an infinite distance from stationary plate then the problem reduces to the unsteady boundary layer problem in a rotating system with non-torsional oscillations of the free-stream velocity.     

Visualization and structure of confined, milliscale, unsteady impinging slot jets and associated vortices

Flow characteristics of confined, laminar milliscale slot jets are investigated from visualizations, as they impinge upon a flat target plate, with a fully developed velocity profile at the nozzle exit. The effects of Reynolds number Re and normalized nozzle-to-plate distance H/B are considered for a nozzle width B of 1.0 mm. Transition from a stable symmetric jet to an unsteady oscillating jet is observed as the Reynolds number increases (with H/B constant), where the Reynolds number associated with this transition decreases as the normalized nozzle-to-plate distance H/B increases. Instantaneous visualizations show unsteady lateral distortions of jet columns at experimental conditions corresponding to the presence of continuous sinusoidal oscillations, intermittent oscillating motion of the jet column, and jet flow fluctuation/flapping motion. Also apparent in flow visualization sequences are smoke signatures associated with instantaneous vortex structures, which form as secondary flows develop in fluid which, initially, is just adjacent to and within the jet column. Associated jet and vortex structural changes are described as different modes of unsteadiness are present, including characterization of jet column unsteadiness using jet column oscillation frequency, and lateral and streamwise extents of jet distortion.     

隔离板对流向振荡圆柱尾流旋涡脱落的抑制

用隔离板对直径为D, 沿流向振荡的圆柱后涡脱落进行抑制. 隔离板放于圆柱尾流中心线上,控制参数包括隔离板长度L/D以及隔离板前缘到柱体振荡中心的距离G/D. 实验的雷诺数范围Re=V_∞D/v=1.01×10⁴～1.69×10⁴,柱体折减振频范围f_eD/V_∞=0～0.03, 柱体振幅固定为A/D=0.2. 风洞烟线显示和热线测量结果表明:当 G/D位于一个有效区域内时,可有效抑制振荡柱体尾流的旋涡脱落. 该有效区的大小随着隔离板板长的增大而增大, 随着Re数和圆柱振荡频率的增大而减小.     

Numerical study of flow-induced oscillations of two rigid plates elastically hinged at the two ends of a stationary plate in a cross-flow

The flow-induced oscillation (FIO) of bluff bodies is commonly encountered in the fluid structure interaction (FSI) problems. In this study, we use an unstructured moving grid strategy and simulate the FIO of two rigid plates, which are elastically hinged at the two ends of a fixed flat plate in a cross-flow. We use a hybrid finite-element-volume (FEV) method in an arbitrary Lagrangian–Eulerian (ALE) framework to study FIO of the two hinged plates. The current simulations are carried out for wide ranges of flow Reynolds number (50–175), spring stiffness coefficient, and the two hinged plates' moment of inertia magnitudes. The influences of these parameters are investigated on the magnitudes of maximum deflection angle, the amplitude of oscillation, the total lift and drag coefficients, and so on. The study is also carried out in the transition period to describe the in-phase and out-of-phase angular oscillations occurring for the two elastically hinged plates with respect to each other. After the transition period, the two hinged plates eventually arrive to a similar periodic oscillation; however, with some phase lags. We find that the achieved phase lag is equal to the phase lag between the two pairs of flow vortices, which are alternatively shed into the flow from the upper and lower hinged plates. Similar to past FIO problems, the current model also exhibits two important lock-in and phase-switch FSI phenomena; however, in angular directions. There is a phase jump of approximately 170° between the aerodynamic lift coefficient and angular oscillations of hinged plates, which nearly occurs in the middle of lock-in region. Indeed, our literature review shows that this is the first time to report the phase-switch phenomenon in angular oscillations of three-element bluff bodies in a FSI problem.     

An experimental study on the coherent structures and chaotic phenomena in the axisymmetric countercurrent shear flow

The coherent structures and the chaotic phenomena in the transition of the axisymmetric countercurrent mixing shear flow were investigated experimentally. Two kinds of self-excited oscillation modes could exist in the axisymmetric countercurrent mixing shear flow. One is the shear layer self-excited oscillation mode corresponding to the high Reynolds number regime and the other is the jet column self-excited oscillation mode corresponding to the low Reynolds number regime in the case of the velocity ratio ranging from 1 to 1.5. Analyzing the auto-power spectrum, self-correlation-function and three dimensional reconstructed phase trajectory, the route to chaos through three Hopf bifurcations intercepted by an intermittence of the dynamical system corresponding to the axisymmetric countercurrent mixing shear flow was discovered when the velocity ratio is equal to 1.32.     

Combined action of transverse oscillations and uniform cross-flow on vortex formation and pattern of a circular cylinder

This paper attempts to study the roles of lateral cylinder oscillations and a uniform cross-flow in the vortex formation and wake modes of an oscillating circular cylinder. A circular cylinder is given lateral oscillations of varying amplitudes (between 0.28 and 1.42 cylinder-diameters) in a slow uniform flow stream (Reynolds number=284) to produce the 2S, 2P and P+S wake modes. Detailed flow information is obtained with time-resolved particle-image velocimetry and the phase-locked averaging techniques. In the 2S and 2P mode, the flow speeds relative to the cylinder movement are less than the uniform flow velocity and it is found that initial formation of a vortex is caused by shear-layer separation of the uniform flow on the cylinder. Subsequent development of the shear-layer vortices is affected by the lateral cylinder movement. At small cylinder oscillation amplitudes, vortices are shed in synchronization with the cylinder movement, resulting in the 2S mode. The 2P mode occurs at larger cylinder oscillation amplitudes at which each shear-layer vortex is found to undergo intense stretching and eventual bifurcation into two separate vortices. The P+S mode occurs when the cylinder moving speeds are, for most of the time, higher than the speed of the uniform flow. These situations are found at fast and large-amplitude cylinder oscillations in which the flow relative to the cylinder movement takes over the uniform flow in governing the initial vortex formation. The formation stages of vortices from the cylinder are found to bear close resemblance to those of a vortex street pattern of a cylinder oscillating in an otherwise quiescent fluid at Keulegan–Carpenter numbers around 16. Vortices in the inclined vortex street pattern so formed are then convected downstream by the uniform flow as the vortex pairs in the 2P mode.     

流向振荡柱体尾流控制研究进展

详细介绍了近几年采用尾部喷射、隔离板和小窄条控制件等3 种方法对流向振荡柱体绕流旋涡脱落的抑制情况. 在研究范围内存在非锁频和3种锁频旋涡脱落模式. 风洞实验表明, 尾部喷射对这4 种模式都有抑制效果,窄条控制件对非锁频和2种锁频模式具有抑制效果, 而隔离板仅对非锁频和1 种锁频模式有效. 在不同流动和振荡条件下找出了每种方法的有效控制区, 研究了减阻和减少脉动力的效果, 并探讨了控制机理.      

Numerical simulation of force and wake mode of an oscillating cylinder

The turbulent flow behind a circular cylinder subjected to forced oscillation is numerically studied at a Reynolds number of 5500 by using three-dimensional Large Eddy Simulations (3-D LES) technique with the Smagorinsky model. The filtered equations are discretised using the finite volume method with an O-type structured grid and a second-order accurate method in both time and space. Firstly, the main wake parameters of a stationary cylinder are examined and compared in the different grid resolutions. Secondly, a transversely oscillating cylinder with a constant amplitude in a uniform flow is investigated. The cylinder oscillation frequency ranges between 0.75 and 0.95 of the natural Kármán frequency, and the excitation amplitude is moderate, 50% of the cylinder diameter. The flow characteristics of an oscillating cylinder are numerically examined and the corresponding wake modes are captured firstly in 3-D LES at Re=5500. A transition between different wake modes is firstly investigated in a set of numerical simulations.     

Wake interference behind two flat plates normal to the flow: A finite-element study

A finite-element model of the Navier-Stokes equations is used for numerical simulation of flow past two normal flat plates arranged side by side at Reynolds number 80 and 160. The results from this simulation indicate that when the gap between the plates is twice the width of a single plate, the individual wakes of the plates behave independently, with the antiphase vortex shedding being dominant. At smaller gap sizes, the in-phase vortex shedding, with strong wake interaction, is favored. The gap flow in those cases becomes biased, with one of the wakes engulfing the other. The direction of the biased flow was found to be switching at irregular intervals, with the time histories of the indicative flow parameters and their power spectra resembling those of a chaotic system.This research was sponsored by NASA-Johnson Space Center under Grant NAG9-449, by NSF under Grant MSM-8796352, and by the U.S. Army under Contract DAAL03-89-C-0038.     

Oscillation of shallow flow past a cavity: Resonant coupling with a gravity wave

Self-excited oscillations of flow past a cavity are generated in a shallow free-surface system. The shear layer past the cavity opening has two basic forms: a separated free-shear flow; and a shear flow along a slotted plate. Instabilities of these classes of shear flows can couple with the fundamental gravity-wave mode of the adjacent cavity. The dimensionless frequencies of both types of oscillations scale on the length of the cavity opening, rather than the gap distance between the slats, i.e., a large-scale instability is always prevalent. A technique of high-image-density particle image velocimetry allows acquisition and interpretation of global, instantaneous images of the flow pattern, including patterns of vorticity and Reynolds stress correlation. Use of a cinema approach provides representations of the timewise evolution of the global, instantaneous flow structure, and thereby definition of the amplitude peaks and phase angles of the coupled fluctuations via auto- and cross-spectral techniques. These methods, along with global, averaged representations of the fluctuating flow field, provide insight into the onset of fully coupled (phase-locked) oscillations of the shear flow past the resonator cavity. The common, as well as the distinctive, features of the resonant-coupled instability of the shear flow past the slotted plate are characterized, relative to the corresponding coupled instability of the free-shear layer. Varying degrees of resonant coupling between the unstable shear layer and the adjacent resonator are attained by variations of the inflow velocity, which yield changes of the predominant oscillation frequency, relative to the resonant frequency of the adjacent cavity. Well-defined, coherent oscillations are indeed attainable for the case of the shear flow along the slotted plate, though their amplitude is significantly mitigated relative to the case of a free-shear layer. The degree of organization of the self-excited, resonant-coupled oscillation and the manner in which it varies with open area ratio and geometry of the plate are interpreted in terms of the flow structure on either side of, and within, the slotted plate; these features are compared with the corresponding structure of the free-shear layer oscillations.     

Time-resolved velocity field measurements of separated flow in front of a vertical fence

 An experimental investigation of the velocity fields of the turbulent shear flow in front of a vertical fence was performed using a hybrid particle tracking velocimetry technique. The vertical fence, with an aspect ratio of 10, was embedded in a thin laminar boundary layer. The Reynolds number of the flow based on the fence height was about 5,000. The measured instantaneous velocity fields and flow visualization results revealed a complex but organized flow structure in front of the fence. The stagnation point detached from the surface of the fence, increasing the streamline curvature near the fence. Time resolved turbulence statistics were obtained and compared with those of a 3D junction flow. Received: 23 August 2000/Accepted: 1 February 2001     

Flow field of self-excited rotationally oscillating equilateral triangular cylinder

This paper studies the flow field of a particular fluid-structure interaction phenomenon—the continuous angular oscillation of a centrally pivoted equilateral triangular cylinder (prism), under uniform two-dimensional incompressible flow. Dye flow visualization of a 30 cm long and 10 cm wide cylinder in a two-dimensional water tunnel was conducted. Under a uniform incoming flow of 7.5 cm/s, the cylinder oscillated continuously after an initial perturbation. On the windward side of the cylinder, a vortex was formed at the sharp edges of the cylinder during the initial phase, whereas on the leeward side, the flow stayed attached. The phase-averaged particle image velocimetry (PIV) measurements are also presented. PIV results show the interchange of flow patterns from that over a flat plate to flow past a sharp edge and vice versa.     

Experimental investigation of flow over a backward-facing step in proximity to a flexible wall

Turbulent flow between a flexible wall and a solid surface containing a backward-facing step (BFS) was investigated using digital particle image velocimetry and high-speed photography. Stationary sheet of paper under tension was positioned above the solid surface in proximity to the BFS. The incoming air flow emerged from a planar nozzle that was located in the solid wall upstream of the BFS. Flows corresponding to two values of the Reynolds number (3,000 and 3,600) based on the step height and the maximum flow velocity at the step location were characterized in terms of patterns of time-averaged velocity, out-of-plane vorticity, streamline topology, and turbulence statistics. In addition, paper sheet oscillation was characterized using high-speed photography. For the control case of a solid upper wall with the geometry that represented the time-averaged paper profile, hydrodynamic frequencies were characterized using unsteady pressure measurements. Frequencies of the natural vibration modes of the paper sheet were well separated from the hydrodynamic frequencies corresponding to the oscillations of the shear layer downstream of the BFS. As the inflow velocity increased, the paper sheet was pulled closer to the solid surface, which resulted in increased confinement of the incoming jet. The flow reattachment length calculated on the basis of time-averaged flow patterns increased with the increasing Reynolds number.     

A numerical study on the flow patterns of two oscillating cylinders

Flow around two oscillating cylinders in a side-by-side arrangement at Reynolds number (Re)=185 is simulated using the immersed boundary method. The purpose of this study is to investigate the combined effects of the gap between the two cylinders and their oscillation in the flow. The cylinders oscillate transversely to a uniform cross-flow with a prescribed sinusoidal function in the opposite direction, with the oscillation amplitude equal to 20% of the cylinder diameter. The gap between the two cylinders and the oscillating frequency are chosen as major variables for the parametric study to investigate their influence on the flow pattern. The ratio of mean gap distance between the two oscillating cylinders to the cylinder diameter is chosen to be 0.6, 1.0, 1.4, and 1.8, and the ratio of oscillating frequencies to the natural vortex shedding frequency of a fixed cylinder is 0.8, 1.0, and 1.2. Wake patterns and the drag and lift coefficients are described and compared with those from a single oscillating cylinder and two stationary cylinders. The wake patterns of two oscillating cylinders can be explained by flow mechanisms of two stationary cylinders, a single oscillating cylinder, and their combinations, and are in agreement with classifications of flow over two stationary cylinders presented in previous studies. In the case of two oscillating cylinders, the modulation phenomenon appears from a lower excitation frequency than in a single oscillating cylinder. Generally, oscillating cylinders have higher drag and root-mean-square (r.m.s.) values of drag coefficients than stationary cylinders.     

Application of a Virtual-Boundary Method for the Numerical Study of Oscillations Developing Behind a Cylinder Near A Plane Wall

The conditions of onset and the character of the oscillations developing behind a circular cylinder located above a plane wall (screen) in a flow with a velocity profile of the boundary layer type are studied numerically. The dependence of the critical Reynolds number (at which a steady flow regime in the wake behind the cylinder is replaced by an oscillatory regime) on the cylinder-wall gap and the free-stream boundary layer thickness is found.     

Visualization of three-dimensional flow in a four-roll mill

Flow visualization has been used to study the motion of a Newtonian fluid in a four-roll mill. Observations of the effect of increasing Reynolds number on the evolution of steady three-dimensional flow between the rollers are reported. The flow visualization experiments have been conducted for two values of the aspect ratio (defined as the ratio of roller length to gap width between rollers). The rollers have been constrained to rotate in a manner which generates an approximately two-dimensional pure extensional flow near the center of the apparatus at low speeds. Steady, symmetrically positioned vortices grow near the top and bottom walls of the container as the Reynolds number is increased up to a critcal value. An increase in Reynolds number beyond this critical value results in a loss of symmetry in the flow pattern. The nature of this transition is strongly dependent upon the aspect ratio.     

Aeolian tones of a plate cascade

A model for the aeroacoustic resonance effects (aeolian tones) excited around a plate cascade in a gas flow is suggested. Methods of calculating the frequencies of natural acoustic oscillations near the cascade are developed. The effect of the cascade geometry and the Mach number of the main flow on the frequencies, abundance, and modes of the natural oscillations is investigated. Anomalous acoustic oscillations near a cyclic plate cascade are shown to exist and are studied. It is shown that there always exist no less than two natural oscillation frequencies in the gas flow near any nontrivial cyclic plate cascade. It has been found that the natural oscillation frequencies can be combined in bundles such that in the case where the number of plates in a period is large the frequencies pertaining to each bundle occupy a certain interval with arbitrary density. The natural oscillations are classified with respect to the form of the eigenfunctions; the classification is based on the theory of representations of groups of locally plane symmetries of the cyclic plate cascade in the solution space. The correctness of the proposed model of the aeroacoustic resonance effects (aeolian tones) excited near a plate cascade in a gas flow is supported by a comparison with the available experimental and theoretical data. On the basis of the investigation performed, some previously unknown physical phenomena are predicted. Thus, the existence of frequency zones or main-flow Mach number ranges on which aeroacoustic resonance phenomena exist near a cyclic cascade with a large number of plates in a period is proved; it is shown that for certain frequencies of the natural oscillations near the cyclic plate cascade the resonance oscillations may be localized in the vicinity of the source; and the existence of narrow-band wave packets slowly propagating along the cascade is demonstrated. Novosibirsk, e-mail: sukhinin@hydro.nsc.ru. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 2, pp. 171–186, March–April, 2000.     

A numerical investigation of the near-wake structure in the variable frequency forced oscillation of a circular cylinder

The near-wake structure of a uniform flow past a circular cylinder undergoing a constant-amplitude transverse forced oscillation is studied numerically using a 2-D large eddy simulation (LES) calculation with a Reynolds number range from 500 to 8000. Two effects are considered: First, a comparison is made between the wake structures of periodic and nonperiodic forced oscillations of the cylinder. This was done to emphasize the importance of wake-structure differences of a periodic forced oscillation and a self-excited oscillation of a circular cylinder with the latter being characterized as a nonperiodic forced oscillation. The nonperiodic constant-amplitude forced oscillations were obtained by modulating the frequency of the periodically oscillating cylinder. The differences in the vortex-shedding behavior were made evident by analyzing the vorticity field in the entire wake domain. Second, the effect of changes in the moderate values of the Reynolds number for constant and variable frequency oscillation was investigated. Significant effects on the vortex-shedding patterns in the near wake were observed for both aspects of this study.