Vortex Shedding Modes Research Articles

Effect of array submergence on flow and coherent structures through and around a circular array of rigid vertical cylinders

Flow past a submerged array of rigid cylinders is more complex compared to the limiting case of an emerged array because part of the flow approaching the array is advected over it and the mean-flow three-dimensionality is increased inside and around the array. For sufficiently high submergence, the flow moving over the top of the array generates a vertical separated shear layer (SSL) and modifies the structure of the wake flow. The case of a circular array of diameter D containing solid cylinders of diameter d (=0.03D) and height hp placed in a flat-bed open channel of depth h = 0.56D is investigated. Detached eddy simulations that resolve the flow past the individual cylinders are conducted at a Reynolds number ReD = 37 500 for two solid volume fractions (SVF) of the array region (SVF = Nd2/D2 = 0.09 and SVF = 0.23 corresponding to aD = 3.9 and 9.6, where N is the number of cylinders in the array and aD is the nondimensional frontal area per unit volume for the array) and several values of the relative height of the cylinders (hp/h = 0.25, 0.5, 0.75, and 1). Results are also compared with the limiting case of a solid cylinder (SVF = 1). The strong weakening of the antisymmetric vortex-shedding mode observed for submerged cases with hp/h ≤ 0.75 is related to the flow component advected over the array and the formation of a U-shaped vortex behind the array, which impedes the interactions of the two lateral (horizontal) SSLs forming on the sides of the array. For sufficiently high SVFs and high array submergence, the U-shaped vortex penetrates inside the array, which means that fluid and particles from the near wake can enter the array region. The decrease in hp/h reduces the coherence of the horseshoe vortex forming in front of the array, the length of the steady wake region, and the Strouhal number associated with the antisymmetric shedding mode. Simulation results show that billow vortices have a much reduced capacity to entrain and carry sediments in the wake of the array even for relatively low array submergences (e.g., for hp/h = 0.75) compared to hp/h = 1. The decrease in the mean streamwise drag coefficient for the cylinders in the array, C¯d, with the decrease in hp/h, is nearly linear for hp/h > 0.25. The rate of decay of C¯d with the decrease in hp/h increases with the SVF. Using the simulation results, the paper also discusses how changes in the flow structure triggered by increased array submergence affect nutrient and sediment transport inside and around vegetated patches in natural erodible channels.

Acoustics-driven vortex dynamics in channel branches with round intersections: Flow mode transition and three-dimensionality

A combined experimental and large eddy simulation study was conducted to investigate acoustics-driven vortex dynamics inside channel branches with round intersections. The underlying flow mode transition and intensified flow three-dimensionality, which are closely related to the Coanda effect at round intersections, were comprehensively demonstrated. A dynamic pressure transducer array was first used to establish the relationship of the excited acoustic pressure pulsations to the channel-branch intersection radius (r) and the mainstream Reynolds number. In complementary simulations, three configurations with r/D = 0, 0.2, and 0.4 (where D is the short edge of the side-branch) were selected for demonstration. First, the simulated results were well validated in terms of acoustic pressure pulsations and phase-dependent flow fields. Subsequent analysis of the time-averaged and statistical flow characteristics revealed the existence of significantly intensified flow fluctuations inside the round channel branches having r/D = 0.2 and 0.4. Next, the proper orthogonal decomposition analysis was conducted to extract the dominant flow modes and to identify the energy transition from the streamwise vortex-shedding mode to vertical flow-oscillation mode. To this end, the influence of flow-mode transition on the phase-dependent flow fields was further investigated. The intensified branch-flow streaks resulted in a channel flow transition from synchronous convection of co-rotating vortex pairs into alternating convection of a single large-scale vortex, yielding a stronger flapping motion of the mainstream flow. Finally, the intensified flow three-dimensionality, presented by the essential spanwise Reynolds shear stresses inside the round channel branches, was found to relate to the strong turbulent mixing process caused by the flapping mainstream flow and the vertical branch flow oscillation. These findings are of great significance for industrial pipeline design and optimization.

Numerical investigation of vortex-induced vibration for two tandem circular cylinders with different diameters

Vortex-induced vibration of two tandem circular cylinders with different diameters under low Reynolds number (Re = 200) is investigated numerically by solving the uncompressible two-dimensional Navier–Stokes equations. The arbitrary Lagrange–Euler method is used to simulate the movement of mesh. Effects of diameters and gap ratios are considered. Numerical results show that diameter ratios and gap ratios have little effect on maximum vibration amplitude. Four different vortex shedding modes are detected in this study.

Experimental Investigation on the Route to Vortex-acoustic Lock-In Phenomenon in Bluff Body Stabilized Combustors

ABSTRACT We investigate the phenomenon of vortex-acoustic lock-in in a bluff body stabilized Rijke type combustor, which experiences self-excited thermoacoustic oscillations. The focus is to identify and understand the various regions (in airflow rate) occurring during the transition to lock-in. In this regard, an axisymmetric bluff body stabilized burner is used, which sheds Benard–Von Karman vortices. The dynamics of the combustor is monitored through the measurement of unsteady pressure and heat release rate fluctuations. At low airflow rates, peaks associated with both vortex shedding and acoustic modes are observed in the measured spectra. These peaks are far apart. As the airflow rate is increased, vortex shedding frequency increases. At a certain flow rate, it reaches the acoustic frequency. Beyond this flow rate, large amplitude oscillations occur and only one common peak is observed in the spectra. This common peak has the frequency close to the acoustic mode of the duct. Vortex shedding process locks in to this common frequency and the phenomenon is termed as vortex-acoustic lock-in. We observe a series of well-defined regions that appear as the system progresses toward lock-in. In this paper, we made an attempt to understand the characteristics of these regions in a systematic manner. The study will help in developing lower-order models, which capture the essential dynamics of lock-in observed in vortex shedding combustors.

Jump phenomena in vortex-induced vibrations of a circular cylinder at a low Reynolds number

The cross flow-induced vibrations of a circular cylinder at the Reynolds number of 150 are numerically investigated in a systematic manner in terms of a wide range of reduced velocity. The effect of the mass ratio on the cylinder behavior is studied, with three mass ratios, namely, 2, 10, and 50, being considered particularly in detail. The mass ratio is defined as the mass of the cylinder to the mass of the fluid it displaces. A sudden decrease in the vibration amplitude takes place at a certain value of the reduced velocity, accompanied by an abrupt increase in the lift coefficient and the vortex shedding frequency. The vortex shedding frequency at the upper end of the lock-in region is about 0.14 for all the mass ratios, which may mark the lower limit of the vortex shedding frequency at this Reynolds number. The jump phenomena may be ascribed to this limitation. Moreover, the vortex shedding frequency in the non-lock-in region varies slightly with the reduced velocity but is not approaching the Strouhal number for the stationary cylinder at the same Reynolds number. In fact, the frequency rises with the increasing mass ratio and reaches about 0.2 as the mass ratio is larger than 10. Besides, the vortex shedding mode does not remain “2S” for the mass ratio larger than 14 when the reduced velocity is increased to get into the non-lock-in region since the vortex shedding frequency is separate from the cylinder oscillation frequency.

Numerical Investigation on Vortex-Induced Rotations of A Triangular Cylinder Using An Immersed Boundary Method

A numerical study of vortex-induced rotations (VIRs) of an equivalent triangular cylinder, which is free to rotate in the azimuthal direction in a uniform flow, is presented. Based on an immersed boundary method, the numerical model is established, and is verified through the benchmark problem of flow past a freely rotating rectangular body. The computation is performed for a fixed reduced mass of m*=2.0 and the structural stiffness and damping ratio are set to zero. The effects of Reynolds number (Re=25–180) on the characteristics of VIR are studied. It is found that the dynamic response of the triangular cylinder exhibits four distinct modes with increasing Re: a rest position, periodic rotational oscillation, random rotation and autorotation. For the rotational oscillation mode, the cylinder undergoes a periodic vibration around an equilibrium position with one side facing the incoming flow. Since the rotation effect, the outset of vortex shedding from cylinder shifts to a much lower Reynolds number. Further increase in Re leads to 2P and P+S vortex shedding modes besides the typical 2S pattern. Our simulation results also elucidate that the free rotation significantly changes the drag and lift forces. Inspired by these facts, the effect of free rotation on flow-induced vibration of a triangular cylinder in the in-line and transverse directions is investigated. The results show that when the translational vibration is coupled with rotation, the triangular cylinder presents a galloping response instead of vortex-induced vibration (VIV).

Geometry effects on mean wake topology and large-scale coherent structures of wall-mounted prisms

Turbulent wake behind five wall-mounted rectangular prisms is measured by time-resolved particle-image velocimetry with an aim to generalize the effects of prism geometry (in terms of aspect ratio, AR, and side ratio, SR) on the wake characteristics. For the time-averaged wake, the arch-type topology is well supported. With AR increasing from 1 to 4 for the square prisms, a transition process between the “dipole” and “quadrupole” wake patterns is observed. On the other hand, an increase in SR does not seem to greatly influence the mean wake structure behind rectangular prisms unless reattachment occurs on the side walls. The turbulent wake velocity fields are analyzed with a proper orthogonal decomposition method, considering the intrinsic symmetries of vortex shedding activities. In the extraction of dominant low-order flow structures, the low-frequency shift mode and periodic vortex shedding mode pair are always captured as the most important turbulent kinetic energy contributors but with significant nonlinear cross-superposition. A low-pass Gaussian filter is then introduced to achieve separation between these two types of coherent structures via new spatial modes, which maintain the same mode shape patterns. The size of the shift mode vortex is found closely related to the time-averaged wake structure, while the pattern of the Karman vortex shedding mode pair seems completely independent. The two separated pure modes develop continuously and compete with each other along the model height. The present study appears to be the first one to investigate the effects of prism geometry on large-scale coherent wake motions from this perspective.

Effect of incoming shear on unsteady wake in flow past surface mounted polygonal prism

Three-dimensional numerical investigations for flow past surface mounted finite height prisms of different cross sections (circular, square, and triangle with apex and base facing) have been carried out using Open Source Field Operation and Manipulation. A general code has been written to consider the effect of impinging shear flow using GroovyBC and is integrated with the existing solver. The effect of impinging shear at the inlet (shear intensity, K) on three-dimensional vortex structures has been explored using iso-Q surfaces for the varying Reynolds number (Re) ranging from 60 to 200 and fixed aspect ratio equal to 5. Three different vortex shedding regimes have been investigated based on values of Re and K, viz., steady flow, symmetric, and asymmetric modes of vortex shedding. Interwave and intrawave frequency modulations and their effects on wake oscillation have been illustrated using Hilbert spectra of transverse velocity signals in the wake. Effects of K and Re on wake oscillation frequency have been presented in terms of marginal spectra of the velocity signals. The extent of nonlinear fluctuations in the wake has been quantified in terms of “degree of stationarity.” Moreover, different modes with their associated frequencies and growth rates that are responsible for transition from the asymmetric to the symmetric mode have been discussed using the computational technique named “Dynamic Mode Decomposition.” Variation in the mean drag coefficient with the change in values of K and Re has also been reported.

Vortex shedding patterns in flow past a streamwise oscillating square cylinder at low Reynolds number using dynamic meshing

We present a two-dimensional numerical study for uniform flow past a streamwise oscillating square cylinder at a Reynolds number of 200. To overcome the limitations with an oscillating inlet flow as used in earlier studies, a dynamic meshing feature is used to oscillate the cylinder. A parametric study is carried out by varying amplitude and frequency of cylinder oscillation. Two symmetric modes, named here as S-II-I and S-IV-D, have been found. In S-II-I mode, a pair of vortices are shed symmetrically on each side of the cylinder in one cycle (S-II mode), and in S-IV-D mode, two pairs of vortices of opposite sense are shed on each side of the cylinder. A vortex flapping mode has also been obtained for low to moderate amplitude and frequency ratios. A new mode of vortex shedding termed the “vortex dipole” mode is found and involves the alternate arrangement of vortex pairs unlike the zigzag arrangement of single vortices in a Kármán vortex street. As in most nonlinear oscillators, vortex shedding becomes chaotic when forced sufficiently strongly and is usually associated with nonlinear interactions between competing frequencies. Many modes observed in the current study become chaotic when the peak cylinder velocity becomes comparable with the inlet velocity. The 0-1 test for chaos is applied to the time series of lift coefficient to show that the signals are truly chaotic. We also observe chaos due to mode competition when shedding transitions from an antisymmetric to symmetric modes.

Various approaches to determine active regions in an unstable global mode: application to transonic buffet

The transonic flow field around a supercritical airfoil is investigated. The objective of the present paper is to enhance the understanding of the physical mechanics behind two-dimensional transonic buffet. The paper is composed of two parts. In the first part, a global stability analysis based on the linearized Reynolds-averaged Navier–Stokes equations is performed. A recently developed technique, based on the direct and adjoint unstable global modes, is used to compute the local contribution of the flow to the growth rate and angular frequency of the unstable global mode. The results allow us to identify which zones are directly responsible for the existence of the instability. The technique is firstly used for the vortex-shedding cylinder mode, as a validating case. In the second part, in order to confirm the results of the first part, a selective frequency damping method is locally used in some regions of the flow field. This method consists of applying a low-pass filter on selected zones of the computational domain in order to damp the fluctuations. It allows us to identify which zones are necessary for the persistence of the instability. The two different approaches give the same results: the shock foot is identified as the core of the instability; the shock and the boundary layer downstream of the shock are also necessary zones while damping the fluctuations on the lower surface of the airfoil; and outside the boundary layer between the shock and the trailing edge or above the supersonic zone does not suppress the shock oscillation. A discussion on the several physical models, proposed until now for the buffet phenomenon, and a new model are finally offered in the last section.

Numerical simulations of flows around a dual step cylinder with different diameter ratios at low Reynolds number

Abstract Flows around a dual step cylinder are numerically investigated at a Reynolds number of 200 for three diameter ratios (D/d) of 2, 1.43 and 1.19. Simulation results reveal three distinctive vortex shedding modes: (i) the N-S mode for D/ d = 2 , (ii) the transition mode for D/ d = 1 . 43 , and (iii) the L-S mode for D/ d = 1 . 19 . The vortex shedding frequency transition from the smaller-diameter cylinder to the larger-diameter one, the dominant shedding frequency downstream of the larger cylinder, and the vortex interactions exhibit significant differences in the three vortex shedding modes. As D/ d = 2 , the direct cross-boundary and half-loop connections take place downstream of the step where vortex dislocations are essentially fixed. When decreasing D/d, two new vortex interactions, the double-half-loop and three-half-loop connections, appear with a larger spanwise vortex dislocation occurrence range. In addition, flows in different D/d cases are governed by a distinctive vortex shedding law. A pair of streamwise vortices occur downstream the centroid of the larger cylinder. This reduces the cylinder step effect on the local vortex shedding, absorbing the shear layer of the spanwise N-cell vortices, and making the hydrodynamic force coefficients smaller. Interestingly, these fluid force coefficients further decrease as D/d increases.

Numerical investigation of wake flow regimes behind a high-speed rotating circular cylinder in steady flow

Flow around a high-speed rotating circular cylinder for$Re\leqslant 500$is investigated numerically. The Reynolds number is defined as$UD/\unicode[STIX]{x1D708}$with$U$,$D$and$\unicode[STIX]{x1D708}$being the free-stream flow velocity, the diameter of the cylinder and the kinematic viscosity of the fluid, respectively. The aim of this study is to investigate the effect of a high rotation rate on the wake flow for a range of Reynolds numbers. Simulations are performed for Reynolds numbers of 100, 150, 200, 250 and 500 and a wide range of rotation rates from 1.6 to 6 with an increment of 0.2. Rotation rate is the ratio of the rotational speed of the cylinder surface to the incoming fluid velocity. A systematic study is performed to investigate the effect of rotation rate on the flow transition to different flow regimes. It is found that there is a transition from a two-dimensional vortex shedding mode to no vortex shedding mode when the rotation rate is increased beyond a critical value for Reynolds numbers between 100 and 200. Further increase in rotation rate results in a transition to three-dimensional flow which is characterized by the presence of finger-shaped (FV) vortices that elongate in the wake of the cylinder and very weak ring-shaped vortices (RV) that wrap the surface of the cylinder. The no vortex shedding mode is not observed at Reynolds numbers greater than or equal to 250 since the flow remains three-dimensional. As the rotation rate is increased further, the occurrence frequency and size of the ring-shaped vortices increases and the flow is dominated by RVs. The RVs become bigger in size and the flow becomes chaotic with increasing rotation rate. A detailed analysis of the flow structures shows that the vortices always exist in pairs and the strength of separated shear layers increases with the increase of rotation rate. A map of flow regimes on a plane of Reynolds number and rotation rate is presented.

On the mechanism of symmetric vortex shedding

In this paper, we study the vortex-shedding mechanism of a square cylinder subjected to a mean flow with a superimposed pulsatile flow, under symmetric vortex shedding conditions. The near-body vortical events are interpreted in accordance with the two-dimensional vorticity transport equation, and the interactions between different vortical regions are quantified by circulation balance for a control volume in the near wake. Strong wake counter flow that splits into two branches, with one branch considerably disrupting the boundary layer while other branch cuts off supply of vorticity to the shear layer, is found to be an essential feature of symmetric vortex shedding. To further clarify the roles of correlated terms contributing to the vorticity generation and transport in the wake, the Reynolds number was varied while keeping other excitation parameters constant. This results in the intensity of symmetric vortex shedding increasing with Reynolds number. The increase in wake counter-flow strength with increased Reynolds number cannot be explained either by a relative velocity between the mean and pulsatile flow components or relative acceleration between bluff body and fluid. Based on the results it is inferred that the brief rolling up of the shear layer to form a wake vortex during a part of the pulsation cycle, together with the associated unsteady tangential pressure gradient around the body, contribute to the stronger wake counter flow at higher Reynolds numbers. At lower pulsation amplitudes, the tendency of the fluid in the wake to form strong counter flow can interact significantly with the transverse entrainment flow to alter the natural vortex-shedding mechanism, to give different vortex-shedding modes.

Vortex-induced vibration of a cooled circular cylinder

We numerically investigate vortex-induced vibration of a cooled circular cylinder in the presence of thermal buoyancy. We employ an in-house fluid-structure interaction solver based on a sharp-interface immersed boundary method. The cylinder is elastically mounted and is free to vibrate transversely to the flow direction. The surface of the cylinder is prescribed at a temperature lower than that of the fluid, and the gravity is aligned opposite to the flow direction. Numerical simulations are carried out for the following parameters: Reynolds number, Re = 150, Prandtl number, Pr = 7.1, Richardson number, Ri = [−1, 0], mass ratio, m = 2, and reduced velocity, UR = [3, 20]. The oscillation amplitude of the cylinder is larger in the presence of the thermal buoyancy for 4 < UR < 15. The amplitude is maximum at UR = 11 and is around ≈1D* in the presence of the thermal buoyancy, where D* is the diameter of the cylinder. However, this amplitude is ≈0.6D* at UR = 4 in the absence of the thermal buoyancy. The lock-in (synchronization) region is obtained for a wider range of UR in the presence of the thermal buoyancy. In the presence of the thermal buoyancy, along with a dominating vortex shedding frequency, we obtain multiple weak as well as strong even and odd harmonics along with subharmonics of the fundamental frequency in the lift signal. However, the secondary frequencies are limited to only a weak third harmonic of the fundamental frequency in the absence of the thermal buoyancy. We observe elongated as well as wider vortices and isotherms in the presence of the thermal buoyancy although the vortex shedding mode remains “2S.” Our results show that there exists a critical minimum absolute value of Ri in order to achieve the lock-in and this value increases with UR.

Low-frequency dynamics of the flow around a finite-length square cylinder

Fluctuating pressure on the side faces, lee-ward face and free end of a finite-length square cylinder is measured simultaneously, together with the velocity in its free-end shear flow, to reveal their inherent connection. The width (d) of the tested model is 40 mm, and the aspect ratio is 5. All experiments are conducted in a low-speed wind tunnel at an oncoming flow velocity (U∞) of 10 m/s. The corresponding Reynolds number based on U∞ and d is 27,000. It is found that, the fluctuating pressure on the side faces and free end has remarkable low-frequency instability with the frequency of about 1/10 of the well-known spanwise Karman vortex shedding. Besides, a high-frequency component, corresponding to Karman vortex shedding, is also found in the fluctuating pressure on cylinder side faces. The low-frequency instability is in phase over the entire cylinder span, while the high-frequency component tends to be out of phase, especially at the lower part of the cylinder. The free-end shear flow flaps at the low frequency and strongly correlates with the modes of spanwise vortex shedding. Particularly, when the free-end shear flow flaps to its lower position, alternate spanwise vortex shedding with high pressure fluctuation occurs; when it flaps to its upper position, the occurrence probability of co-shedding with low pressure fluctuation increases significantly especially near the cylinder free end.

Vortex-induced vibrations of dual-step cylinders with different diameter ratios in laminar flows

Vortex-induced vibrations of dual-step cylinders in laminar flows at the Reynolds number of 200 based on a larger diameter are investigated through three-dimensional direct numerical simulations. Four larger-to-smaller diameter ratios (D/d) of 4.0, 2.0, 1.43, and 1.19 are considered for the cylinder with a low mass ratio of 2. Numerical results reveal that D/d significantly influences vibration responses and the vortex shedding process. The case with D/d = 1.43 can effectively reduce vibration amplitudes. The wake vortices are shed in cellular patterns along the cylinder span, with a distinct frequency for each cell. A direct and “X-shaped” connection of wake vortex-shedding modes occurs between different vortex cells, and a loop connection appears between the same cell vortices. A new wake pattern entitled the “out-of-phase vortex shedding” is found downstream of the smaller cylinder at D/d = 2.0. This is manifested by the wake vortices not being shed at a fixed frequency and intensity. The essential reason for this phenomenon lies in a non-lock-in condition between the structural vibration and vortex shedding frequencies of the smaller cylinder. As D/d decreases, a coherence between vortex cells is enhanced, leading to a disappearance of the “out-of-phase vortex shedding.”

Numerical simulation of the two-degree-of-freedom vortex induced vibration of a circular cylinder at Reynolds number of O(104∼105)

Vortex induced vibration (VIV) is of practical interest since it can result in the fatigue damage of structures, so has been a highly researched topic during the past few decades. The present study employs the unsteady Reynolds-Averaged-Navier–Stokes (URANS) equations combined with the shear-stress transport (SST) k-ω turbulence model to carry out the numerical simulation of the two-degree-of-freedom (2DOF) VIV at Reynolds number of O(104∼105). The present numerical method is validated through the comparison of the numerically and experimentally obtained response amplitude, lock-in frequency and hydrodynamic force, and the obtained vortex shedding modes are demonstrated. Based on the numerical model, this study parametrically investigates the effect of the Reynolds number, mass-damping ratio on the 2DOF VIV. The results indicate that the changes of the Reynolds number in the range of 104∼105 seem to have little effect on the VIV, and the maximum non-dimensional amplitude of the cross-flow (CF) VIV keeps close to 1.5. The in-line (IL) VIV is more sensitive to the Reynolds number than CF VIV. Additionally, with the increase of the mass-damping ratio, the peak of the CF non-dimensional amplitude in 2DOF VIV decreases, and its difference with that in 1DOF VIV decreases.

Intermittency in free vibration of a cylinder beyond the laminar regime

Vortex-induced vibration of a circular cylinder that is free to move in the transverse ($Y$) and streamwise ($X$) directions is investigated at subcritical Reynolds numbers ($1500\lesssim Re\lesssim 9000$) via three-dimensional (3-D) numerical simulations. The mass ratio of the system for all the simulations is $m^{\ast }=10$. It is observed that while some of the characteristics associated with the $XY$-oscillation are similar to those of the $Y$-only oscillation (in line with the observations made by Jauvtis & Williamson (J. Fluid Mech., vol. 509, 2004, pp. 23–62)), notable differences exist between the two systems with respect to the transition between the branches of the cylinder response in the lock-in regime. The flow regime between the initial and lower branch is characterized by intermittent switching in the cylinder response, aerodynamic coefficients and modes of vortex shedding. Similar to the regime of laminar flow, the system exhibits a hysteretic response near the lower- and higher-$Re$ end of the lock-in regime. The frequency spectrum of time history of the cylinder response shows that the most dominant frequency in the streamwise oscillation on the initial branch is the same as that of the transverse oscillation.

Three-dimensional vortex-induced vibration of a circular cylinder at subcritical Reynolds numbers with low-[formula omitted] correction

Reynolds-Averaged Navier-Stokes (RANS) method equipped with the shear stress transport (SST)K−ω model is widely used to simulate the vortex-induced vibration (VIV) of elastically-mounted rigid cylinders subjected to fluid flow. Previous studies show that this method is very difficult to capture the maximum response of the cylinder occurring in the upper regime. Moreover, previous numerical studies by using this method mainly focus on the two-dimensional (2-D) simulations. In reality, VIV is a three-dimensional (3-D) phenomenon, the 3-D effect must then be incorporated in the numerical simulations. To improve the accuracy of the RANS method with SSTK−ω, the low-Reynolds numbers (Re) correction technique is applied to the numerical model in the present study and the 3-D VIV responses of an elastically-mounted cylinder subjected to the subcritical fluid flow regime are numerically investigated. The two-way coupled Fluid-Structure Interaction (FSI) framework is developed and computational fluid dynamics (CFD) analyses are performed for a range of Reynolds numbers Re=2000−12000. Numerical results show that the present method can lead to more accurate VIV response estimations compared to the previous numerical studies. Moreover, the formation of the wake vortex shedding modes and its transition are well captured by this method.

Effects of shear intensity and aspect ratio on three-dimensional wake characteristics of flow past surface mounted circular cylinder

Three-dimensional numerical computations have been carried out for flow past a surface mounted finite height circular cylinder using Open Source Field Operation and Manipulation. Flow field characteristics have been investigated for fixed Reynolds number equal to 300 and varying aspect ratios (AR being the ratio of height to diameter of the cylinder) from 1 to 5. The effect of nonuniform flow (linear shear) with shear intensity (K) ranging from 0 to 0.3 on flow field characteristics has been examined using iso-Q surfaces and streamline plots. Three different flow regimes are reported based on the values of AR and K: steady flow, symmetric mode, and antisymmetric mode of vortex shedding. The formation of critical points (impingement point, source, and spiral node) has been explained using the time-averaged flow field in the longitudinal plane of symmetry. Surface flow topology has been presented with the help of limiting streamlines on the surface of the cylinder. Variation in mean drag coefficient with a change in K has also been reported. Unsteady periodic and aperiodic wake flows have been characterized using the Hilbert spectra of the transverse velocity signal at a point in the wake. The extent of nonlinear interaction in the wake and its influence on frequency distribution have been analyzed using marginal spectra and quantified in terms of degree of stationarity.