Vortex Shedding Flow Research Articles

Vortex shedding characteristics of multi-column structure with or without horizontal connections

Vortex flow around multiple columns of finite length is ubiquitous in engineering. The present work focuses on the basic fluid physics in terms of the vortex shedding flow patterns and their dependence on structural configurations and flow parameters. Though widely documented in the literature, there is no consensus on certain aspects of the wake characteristics immediately behind the obstacles for a multi-column structure at a relative high Reynolds number range. A comprehensive set of numerical simulations has been conducted to investigate the flow interactions with four square section shaped columns in a diamond configuration, which is complimented by experiments using particle image velocimetry and force measurements in a physical model with Reynolds numbers varying from 3.7×104 to 6.0×104. Horizontal structural members called pontoons were added near the end of the columns to alter the interactions with the surrounding fluid. This work reveals further insights of the fluid physics including the interactions of the vortex shedding processes due to the multi-columns and pontoons. The pontoons are seen blocking the vortices shed from the free end of the column by pushing the recirculation region further away from the free end of each column. In addition to the vortex shedding period being increased, further examination of the wake region indicates that the vortex street tends to be tidier and more structured by adding the pontoons to a basic multi-column structure. The findings will lead to better understanding in vortex shedding fluid physics and improved design in new offshore structure development such as deep-draft semi-submersibles and tension leg platforms.

Wind-driven pumping flow ventilation of highrise buildings: Effects of upstream building arrangements and opening area ratios

Periodic vortex shedding around a building could play an important role in wind-driven single-sided ventilation especially when two free openings are mounted on the leeward wall, in which case “pumping” flow dominates the natural ventilation. In this paper, we investigated the characteristics of vortex shedding and “pumping” flow affected by the arrangements of upstream buildings and opening area ratio of ports on the downstream target building. Computational fluid dynamics (CFD) simulations have been used to predict the instantaneous and mean flow fields. Numerical results indicate that the strength of “pumping” flow could be intensively weakened by two upstream buildings. Vortex shedding from the inner shear layers dominates the vortex shedding from the target building and constrains that from both upstream buildings except at W/B = 0.5, in which case the gap flow is weak and the St is close to that of a single building. The increase of upstream building length leads to decrease of the vortex shedding frequency at the wake of all buildings and ventilation rate of the downstream building. An increase of opening area ratio on the rear wall of the downstream building will raise the Strouhal number but have no positive correlation with ventilation rate. “Pumping” flow oscillating frequency does not have clear correlation with the ventilation rate. Our study on the wake vortex shedding flow across building clusters could benefit the future green design of urban buildings.

On the vortex-induced vibration of a low mass ratio circular cylinder near a planar boundary

Vortex-induced vibration of a circular cylinder near a plane boundary has been investigated experimentally for 2,500≤ReD≤10,000, 3.0≤U*≤12.0, 0.3≤S*≤8, and m*=5.3, where ReD is the Reynolds number based on cylinder diameter, U* is the reduced velocity, S* is the gap distance between the planar wall and the cylinder, and m* is the system's mass ratio. The experimental data includes simultaneous amplitude response and wake measurements via time-resolved planar particle image velocimetry (PIV). From experimental raw data, frequency content of the oscillations as well as wake velocity and vorticity fields are analyzed. A new vortex shedding flow regime map is proposed, illustrating dominant vortex shedding patterns in the wake across the investigated non-dimensional parameter space. The quantitative relationship between the cylinder's motion and instantaneous vortex dynamics is shown.

Effect of surface roughness on vortex-induced vibrations of a freely vibrating cylinder near a stationary plane wall

The effects of surface roughness of the vortex-induced vibration (VIV) responses of a circular cylinder near a stationary plane wall were studied using the two-dimensional unsteady Reynolds-averaged-Navier-Stokes equations and the shear stress transport k−ω model coupling with a fourth-order Runge-Kutta method. A smooth cylinder and rough cylinders with three different degrees of surface roughness were selected for the study. The VIV response amplitude, structural vibration frequency, lock-in region, vortex shedding flow pattern, Strouhal number and hydrodynamic coefficient for cylinders with different degrees of surface roughness were compared. The numerical results show that for a smooth cylinder and a cylinder with small surface roughness, the reduced velocity range can be separated into four parts based on the VIV amplitude: an initial branch, an upper branch, a lower branch and a desynchronization region. However, for cylinders with intermediate and large surface roughnesses, the upper branch is absent, leaving only three branches. The lock-in phenomenon can apparently be found for all surface roughnesses, but the width of the lock-in region is not very sensitive to the variation of the surface roughness. As surface roughness increases, the Strouhal number has an increasing tendency; however, the mean drag coefficient has a decreasing tendency.

Passive control of the onset of vortex shedding in flow past a circular cylinder using slit

This study investigates the passive flow control phenomena over a two-dimensional circular cylinder using numerical simulations in the laminar regime. The aim is to explore one of the passive control techniques, which involves the introduction of a slit to the geometry of the cylinder. The two parameters, slit width ratio S/D (slit width/diameter) and slit angle (measured with respect to the incoming flow direction), play an essential role in determining the trend of critical Reynolds number (Rec). Most of the analysis invokes flow visualization and saturation amplitude methods to obtain the critical Reynolds number (indicative of the onset of vortex shedding) for different cases. Furthermore, Hopf bifurcation analysis using the Stuart-Landau equation and global stability analysis confirm the accuracy and consistency in the predicted solutions. The additional amount of flow through the slit increases the pressure downstream of the cylinder, which consequently leads to an increase in Rec of the modified cylinder. The critical Reynolds number increases with S/D of the modified cylinder at 0° slit angle (as an additional amount of flow grows with S/D). The critical Reynolds number shows an increasing trend with the slit angle in the range of S/D = 0.05–0.15 as the fluctuation intensity reduces with the slit angle in this range. For S/D = 0.15–0.25, the extra amount of flow through the slit induces instability in the wake, which causes a decrease in Rec with the slit angle. A correlation is obtained, which estimates the critical Reynolds number for a given S/D and slit angle.

Numerical Simulation of Laminar Vortex-Shedding Flow around a very Slender and Rectangular Bluff Body

Numerical Simulation of Laminar Vortex-Shedding Flow around a very Slender and Rectangular Bluff Body

Kelvin–Helmholtz Instability and Alfvénic Vortex Shedding in Solar Eruptions

Abstract We report on a three-dimensional MHD numerical experiment of a small-scale coronal mass ejection (CME)-like eruption propagating though a nonmagnetized solar atmosphere. We find that the Kelvin–Helmholtz instability (KHI) develops at various but specific locations at the boundary layer between the erupting field and the background atmosphere, depending on the relative angle between the velocity and magnetic field. KHI develops at the front and at two of the four sides of the eruption. KHI is suppressed at the other two sides of the eruption. We also find the development of Alfvénic vortex shedding flows at the wake of the developing CME due to the 3D geometry of the field. Forward modeling reveals that the observational detectability of the KHI in solar eruptions is confined to a narrow ≈10° range when observing off-limb, and therefore its occurrence could be underestimated due to projection effects. The new findings can have significant implications for observations, for heating, and for particle acceleration by turbulence from flow-driven instabilities associated with solar eruptions of all scales.

Effect of surface roughness and initial gap on the vortex-induced vibrations of a freely vibrating cylinder in the vicinity of a plane wall

We studied the effects of the surface roughness and initial gap on the responses of vortex-induced vibration (VIV) of a circular cylinder near a stationary plane wall, employing numerical methods. The VIV response amplitudes, lock-in regions, hydrodynamic forces, VIV trajectories and flow fields for three different surface roughnesses and two different initial gaps were systematically compared. The results reveal that the reduced velocity range can be divided into three regions based on the VIV amplitude as pre-lock-in, lock-in, and post-lock-in regions. The width of the lock-in region is not sensitive to the variation of the roughness. The mean drag coefficient has a decreasing tendency with the increased roughness. For a small initial gap, the clockwise wall boundary layer vortices has coalesced with the clockwise vortices shed from the upper side of the cylinder, which further suppresses the shedding of the counter-clock wise vortices from the lower side of the cylinder. The vortex shedding flow pattern displays a weak 2S mode. However, for a large initial gap, there is no coalescing action operating in the wake region and hence most of the vortex shedding flow patterns show an asymmetric 2S mode.

Effects of gap ratio on flow-induced vibration of two rigidly coupled side-by-side cylinders

Vortex-induced vibration of two elastically mounted and rigidly coupled side-by-side circular cylinders in uniform flow is investigated using Direct Numerical Simulations (DNS). The two cylinders are allowed to vibrate in the cross-flow direction only. The aim of this study is to investigate the effect of gap ratio between the two cylinders on the vibration and wake flow. To achieve this, simulations are performed for gap ratios of 0.5, 1 and 3 and a wide range of reduced velocities from 1 to 20. The Reynolds number, the mass ratio and the damping ratio are 1000, 2 and 0, respectively. For small gap ratios of 0.5 and 1, the range of reduced velocity with high vibration amplitude is divided into a lock-in regime and a subharmonic regime. The subharmonic regime is a regime where the vibration frequency is half the dominant frequency of each individual cylinder’s lift coefficient. In the subharmonic regime, the dominant frequency components of the two cylinders do not contribute to the vibration because they are out of phase with each other. In the lock-in and subharmonic regimes, the wake is vortex shedding flow instead of biased gap flow for small gap ratios of G=0.5 and 1. The combination of lock-in and subharmonic regimes at G=0.5 and 1 are wider than the lock-in regime of a single cylinder. The weak interference between the two cylinders make the lock-in regime of G=3 the same as that of a single cylinder. A single vortex street wider than that of a single cylinder is found for G=0.5 and two vortex streets are found for G=3. For G=1, vortex shedding happens but clearly defined vortex street does not exist because strong interaction between the vortices causes very fast dissipation of the vortices.

Effects of polymer/surfactant additives on forced convective heat transfer in vortex shedding flow past a circular cylinder

The unsteady incompressible flow of dilute polymer/surfactant solutions past an unconfined heated circular cylinder is studied to reveal the effects of these additives on convective heat transfer properties in the laminar and transitional vortex shedding regime. Both inelastic and elastic non-Newtonian features are considered and compared using two-dimensional numerical simulations. Inelastic power-law and purely viscoelastic Oldroyd-B as well as Giesekus and FENE-P constitutive models, exhibiting both shear thinning and elastic effects are adopted in the formulation. A finite element based software is used in simulations for the Reynolds number range 80≤Re≤300, power-law index range 0.6≤n≤1 and Weissenberg number Wi=1 for weakly elastic solutions. Heat transfer enhancement is observed under inelastic shear thinning effects through an increase of the local and average Nusselt numbers, however purely viscoelastic effects lead to a decrease in the rate of convective heat transfer for the studied range of parameters. The increase in the Giesekus model mobility factor which is associated with anisotropic Brownian motion effects on polymer/surfactant molecules increases the heat transfer rate, and shifts the location of the maximum local Nusselt number to the front of the cylinder. However, increasing the FENE-P model finite extensibility parameter related to the maximum polymer chain length, decreases the local Nusselt number while the maximum heat transfer rate location moves towards the rear region on the cylinder surface. Temperature gradients decrease in the wake region under elasticity whereas shear thinning leads to an increase in the gradients and to the formation of clustered isotherms.

Wind driven “pumping” fluid flow and turbulent mean oscillation across high-rise building enclosures with multiple naturally ventilated apertures

This paper presents a study on the impact of incoming wind velocity magnitudes and horizontal aperture separations on natural ventilation flows in a single-sided wind-driven naturally ventilated building with two apertures (SS2) on the rear wall or the front wall. Both the velocity fields and the contaminant concentration fields were simulated and investigated. The present study is based on CFD simulations with unsteady Reynolds-averaged Navier-Stokes (URANS) SST k-ω model. A vortex shedding flow mechanism has been identified when the two apertures are mounted either on the rear wall or on the front wall, through which the air flow oscillates at a certain rate. CFD results further demonstrate that the oscillating frequency increases with the incoming wind magnitude and is nearly independent of the horizontal aperture separations. For the same building configuration, the root mean square (r.m.s) of the non-dimensional ventilation flow rate is independent of the incoming wind speed, whereas it decreases when the horizontal aperture separation is reduced. The time-averaged contaminant concentration fields also validated the different ventilation performance of different aperture configurations. This novel ventilation mechanism could be applied to alleviate the poor ventilation performance of buildings with single-sided apertures.

A boundary thickening-based direct forcing immersed boundary method for fully resolved simulation of particle-laden flows

A boundary thickening-based direct forcing (BTDF) immersed boundary (IB) method is proposed for fully resolved simulation of incompressible viscous flows laden with finite size particles. By slightly thickening the boundary thickness, the local communications between the Lagrangian points on the solid boundary and their neighboring fluid Eulerian grids are improved, based on an implicit direct forcing (IDF) approach and a partition-of-unity condition for the regularized delta function. This strategy yields a simple, yet much better imposition of the non-slip and non-penetration boundary conditions than the conventional direct forcing (DF) technique. In particular, the present BTDF method can achieve a numerical accuracy comparable with other representative improved methods, such as multi-direct forcing (MDF), implicit velocity correction (IVC) and the reproducing kernel particle method (RKPM), while its computing cost remains much lower than them and nearly equivalent to the conventional DF method. The dependence of the optimum thickness value of boundary thickening on the form of the different regularized delta functions is also revealed. By coupling the lattice Boltzmann method (LBM) with BTDF-IB, the robustness of the present BTDF IB method is demonstrated using numerical simulations of creeping flow (Re=0.1), steady vortex separating flow (Re=40) and unsteady vortex shedding flow (Re=200) around a circular cylinder. The accuracy and robustness of the present method for moving particle-laden flows are performed on three benchmark cases, such as free sedimentation of single circular cylinder, DKT sedimentation of two particles and Rayleigh-Taylor instability of 504 particles sedimentation in an enclosed cavity. The computational efficiency between the present BTDF method and the MDF method is also compared at last.

Experimental investigation on the noise reduction method of helical cables for a circular cylinder and tandem cylinders

Cylinder noise can be remarkably reduced when the surface is helically wrapped with cables along the cylinder span. In this paper, aeroacoustic and aerodynamic experiments are conducted to reveal the noise reduction mechanism of the helical-cable technique. With the application of helical cables on the single circular cylinder, three-dimensional flow is promoted into the cylinder near wake, leading to the suppression of two-dimensional vortex shedding process and flow fluctuations. Besides, the three-dimensional wake also destroys the in-phase relationship of flow fluctuations along the cylinder span, leading to the reduction of spanwise correlation length and noise radiation efficiency. These two effects together contribute to the cylinder noise reduction. Furthermore, this helical-cable technique is extended to tandem cylinders and the results show that it can also reduce the tandem cylinder noise effectively. Especially for the moderate and large spacing tandem cylinder configurations, the key of the noise reduction is the control on the upstream cylinder.

Pressure field estimation from ultrasound Doppler velocity profiler for vortex-shedding flows

A novel algorithm of pressure field estimation based on ultrasound velocity profiler (UVP) is developed. The method consists of UVP measurement of velocity distribution in fluid flows and numerical analysis of the measured data using fluid dynamics equations. We introduce equation of continuity, incompressible Navier-Stokes equation and proper orthogonal decomposition (POD) into the basic algorithm, so that pressure field of space-time two-dimensional unsteady fluid flow is fully reconstructed. Since UVP is based on ultrasound Doppler principle, the local instantaneous pressure distribution is obtained non-intrusively. The performance of an algorithm is evaluated for vortex shedding flow behind a circular cylinder at Re = 1000. Considering the specification of UVP, the optimal method of experimental data conversion to pressure information is proposed. We have found that the one-dimensional velocity measurement by UVP upon Taylor's frozen hypothesis is suitable for evaluation of pressure field in wake of the cylinder. The present algorithm is also demonstrated for opaque fluid flows by considering vortex flow in milk.

Suppression of vortex shedding in flow around a square cylinder using control cylinder

Abstract The closeness or matching of the self-sustained vortex shedding excitation with the structural frequency of the body may cause resonance resulting in structural failures. The present study aims at the suppression of vortex shedding using a passive method, by placing a small control cylinder in the near wake of the main cylinder. Square control cylinders of sides either 20%, 25% or 30% of the main cylinder are employed to change the dynamics of the flow. Direct numerical simulation (DNS) of two-dimensional, unsteady, incompressible flow past a square cylinder at Reynolds numbers of 75, 100 and 125 has been carried out. The Navier–Stokes equations are solved using Marker and Cell (MAC) method with the second-order spatial and temporal discretizations. Computed results indicate that there exists an explicit spatial domain present on the edges of the separating shear layers in the near wake within which the placement of the control cylinder guarantees complete suppression. The mechanism of the suppression may be attributed to an effective increase in vorticity diffusion of the separating shear layer. Also, changes in the control cylinder size and Reynolds number have a considerable effect on the vortex shedding suppression. A remarkable drag reduction of the main cylinder is also achieved for partial as well as complete suppression. The study also highlights that the partial suppression of vortex shedding results in significant reductions in Strouhal number, and fluctuating lift and drag forces. The control behavior of the flow past a square cylinder surprisingly resembles that of the flow past a circular cylinder.

Computational Analysis of Strouhal Number Variation in Vortex Meters

Vortex meters are used to measure the flow rate by measuring the vortex shedding frequency behind a trapezoidal (or triangular)-shaped body. In the current work, analysis of various parameters affecting the performance of vortex flowmeters is carried out, using computational flow simulation. The linear relation between vortex shedding frequency and flow velocity is verified. The variation of Strouhal number with shedder angle, length and width is investigated.

Numerical investigation of different aspect ratios for flow past three inline rectangular cylinders

In this paper, numerical investigations are carried out to study the effect of aspect ratios (AR = length/height) for flow past three rectangular cylinders aligned inline at different gap spacings (g) using lattice Boltzmann method. The aspect ratio is varied between 0.25 and 3, gap spacing is taken in the range from 0.5 to 7 and the Reynolds number is fixed at 150. With the help of present numerical investigation, it is found that the variation of forces with different AR and g is governed by six distinct flow patterns: (1) shear layer reattachment flow; (2) steady flow; (3) quasi-steady flow; (4) strongly interactive vortex shedding flow; (5) two-row vortex street flow; and (6) critical flow pattern. It is observed that the development of vortices, interferences of the free shear layers and the wake vortices distribution of three cylinders play a vital role in the variation of shedding frequencies and fluid forces. Consequently, the cylinders placed at downstream usually experience serious fluctuating forces due to unsteady wakes generated by the upstream cylinders. Also, the upstream cylinder normally experiences larger mean drag force as compared to the downstream ones. The sudden jumps in mean drag coefficients and Strouhal numbers with their maximum and minimum values are identified, respectively. The upstream and middle cylinders force coefficients strongly affected due to critical aspect ratios at different fixed gap spacings. It is observed that the wake transition at early stages significantly changes by changing the aspect ratios for fixed gap spacings. The results also show that the middle and downstream cylinders experience negative drag force for some aspect ratios and gap spacings. Furthermore, the dependence of the critical gap spacing on the aspect ratios is also discussed.

Numerical simulation of flow past two circular cylinders in cruciform arrangement

Flow past two circular cylinders in cruciform arrangement is simulated by direct numerical simulations for Reynolds numbers ranging from 100 to 500. The study is aimed at investigating the local flow pattern near the gap between the two cylinders, the global vortex shedding flow in the wake of the cylinders and their effects on the force coefficients of the two cylinders. The three identified local flow patterns near the gap: trail vortex (TV), necklace vortex (NV) and vortex shedding in the gap (SG) agree with those found by flow visualization in experimental studies. As for the global wake flow, two modes of vortex shedding are identified: K mode with inclined wake vortices and P mode where the wake vortices are parallel to the cylinders. The K mode occurs when the gap is slightly greater than the boundary gap between the NV and SG. It also coexists with the SG gap flow pattern if the Reynolds number is very small ($Re=100$). The flow pattern affects the force coefficient. The K mode increases the mean drag coefficient and the standard deviation of the lift coefficient at the centre of the upstream cylinder because the wake vortices converge towards the centre. The mean drag coefficient and standard deviation of the lift coefficient of the downstream cylinder decreases because of the shedding effect from the upstream cylinder.

Numerical analysis of underwater flow past columnar projectile with different cross-sections at high Reynolds numbers

Based on Detached Eddy Simulation (DES) technique, the flow around a columnar projectile with different cross-section shapes in the supercritical and extremely supercritical region is simulated by the Fluent. The cross-section of the projectile is regular polygon, which number of edges is 4, 6, 8, 10, 12, 24 and ∞, where ∞ means a circle. The vortex shedding pattern and flow field characteristics are analyzed at Reynolds number 2.5×105 to 2×107. Regarding circular cylinder projectile, when the flow velocity changes from 25 m/s to 200 m/s, the average drag coefficient decreases, and the St Number increases. Regarding regular polygon, when the number of edges for polygon changes from 4 to ∞ at flow velocity 50 m/s, the average drag coefficient decreases, and the St Number increases. The average lift coefficient is almost equal to zero and does not change with the flow velocity and the cross-section. The pressure coefficient Cp of 4-prism, 6-prism, 8-prism, 12-prism and 24-prism has multiple local minimum values at the polygon vertices of the cross section. According to the spectrum analysis, the vortex shedding frequency of 4-prism, 24-prism and cylindrical is single and fixed, so the projectile may cause resonance and deviates from a predetermined trajectory. But for the 6-prism and 8-prism and 12-prism, the cl and cd is multi-periodic vibration. So, considering the flow induced structural vibrations, drag, the power of shrapnel and manufacturing cost, the 8-prism are better choices for cluster warhead underwater in engineering design.

Experimental investigation on flow past nine cylinders in a square configuration

An experimental investigation on flow past nine cylinders in a square configuration was carried out using the particle image velocimetry technique and load cell in a water channel. The center-to-center spacing ratio L/D was in the range of 1.5–3.0 and the Reynolds number Re was varied from 1500 to 5000. The effects of spacing ratio and Reynolds number on the instantaneous time-averaged flow fields and force coefficients are investigated. The results show that three distinct flow regimes are categorized with variation of the spacing ratios and Reynolds numbers, namely, shielding flow regime, transition flow regime and vortex shedding flow regime. Depending on the interferences of shear layers around the nine cylinders, each flow regime is further divided into two types of flow patterns. An interesting feature of bistable flow pattern with different flow modes is observed at small spacing ratio L/D = 1.5. The non-dimensional vortex shedding frequencies appear to be more associated with the individual shear layers rather than the multiple cylinders. Moreover, force analysis, streamline topologies and Reynolds stress contours are presented to elucidate the effects of spacing ratio and Reynolds number on the complex wake interference among the nine cylinders. The flow characteristics and force coefficients are found to be more sensitive to L/D rather than Re.