Modification of a circular cylinder wake with synthetic jet: Vortex shedding modes and mechanism

2014 ◽  
Vol 43 ◽  
pp. 14-32 ◽  
Author(s):  
Li-Hao Feng ◽  
Jin-Jun Wang
Keyword(s):  
Circular Cylinder ◽  
Vortex Shedding ◽  
Synthetic Jet ◽  
Cylinder Wake ◽  
Vortex Shedding Modes

POD ANALYSIS OF THREE-DIMENSIONAL HARMONICALLY FORCED WAKE FLOW OF A CIRCULAR CYLINDER

2015 ◽  
Vol 39 (4) ◽  
pp. 789-803 ◽  
Author(s):  
Negar Nabatian ◽  
Xiaofei Xu ◽  
Njuki Mureithi
Keyword(s):  
Circular Cylinder ◽  
Vortex Shedding ◽  
Transverse Velocity ◽  
Lift Coefficient ◽  
Oscillation Amplitude ◽  
Three Dimensional ◽  
Wake Flow ◽  
Cylinder Wake ◽  
Pod Analysis ◽  
Vortex Shedding Modes

A 3D numerical simulation of a circular cylinder wake is presented in this paper. The cylinder is harmonically forced in the stream-wise direction. The objective of the present work is to investigate the effect of the oscillation amplitude on the secondary transition of the wake. The frequency of the lift force is then linked to the form of the vortex shedding mode. The relation between these vortex shedding modes using POD analysis of the transverse velocity and the unsteady lift coefficient of 3D simulation is in good agreement with the 2D model. Results show that the 3D spanwise effect, which can change the wake structure, is suppressed at Re = 200 by streamwise oscillation of the cylinder. Thus the 2D analysis can effectively model the temporal instability of the wake flow.

Vortex-induced vibrations of a long flexible circular cylinder

1993 ◽  
Vol 250 ◽  
pp. 481-508 ◽  
Author(s):  
D. Brika ◽  
A. Laneville
Keyword(s):  
Circular Cylinder ◽  
Flow Velocity ◽  
Vortex Shedding ◽  
Abrupt Change ◽  
Velocity Range ◽  
End Effects ◽  
Synchronization Region ◽  
Vortex Induced Vibrations ◽  
Set Up ◽  
Vortex Shedding Modes

In an experimental study of the vortex-induced oscillations of a long flexible circular cylinder, the observed stationary amplitudes describe an hysteresis loop partially different from earlier studies. Each branch of the loop is associated with a vortex shedding mode and, as a jump from one branch to the other occurs, the phase difference between the cylinder displacement and the vortex shedding undergoes an abrupt change. The critical flow velocities at which the jump occurs concur with the flow visualization observations of Williamson & Roshko (1988) on the vortex shedding modes near the fundamental synchronization region. Impulsive regimes, obtained at a given flow velocity with the cylinder initially at rest or pre-excited, and progressive regimes resulting from a variation of the flow velocity, are examined. The occurrence of bifurcations is detected for a flow velocity range in the case of the impulsive regimes. The coordinates of the bifurcations define a boundary between two vortex shedding modes, a boundary that verifies the critical curve obtained by Williamson & Roshko (1988). The experimental set-up of this study simulates half the wavelength of a vibrating cable, eliminates the end effects present in oscillating rigid cylinder set-up and has one of the lowest damping ratios reported for the study of this phenomenon.

Numerical Simulation of Vortex Shedding Past a Circular Cylinder in a Cross-Flow at Low Reynolds Number With Finite Volume Technique: Part 2 — Flow-Induced Vibration

2007 ◽  
Author(s):  
Antoine Placzek ◽  
Jean-Franc¸ois Sigrist ◽  
Aziz Hamdouni
Keyword(s):  
Circular Cylinder ◽  
Vortex Shedding ◽  
Time Integration ◽  
Cross Flow ◽  
Forced Oscillations ◽  
Turbulent Regime ◽  
Flow Induced Vibration ◽  
Coupling Procedure ◽  
Vortex Shedding Modes ◽  
Good Agreement

This paper is the sequel of the work exposed in a companion publication dealing with forced oscillations of a circular cylinder in a cross-flow. In the present study, oscillations of the cylinder are now directly induced by the vortex shedding process in the wake and therefore, the former model used for forced oscillations has been modified to take into account the effects of the flow in order to predict the displacement of the cylinder. The time integration of the cylinder motion is performed with an explicit staggered algorithm whose numerical damping is low. In the first part of the paper, the performances of the coupling procedure are evaluated in the case of a cylinder oscillating in a confined configuration for a viscous flow. Amplitude and frequency responses of the cylinder in a cross-flow are then investigated for different reduced velocities U* ranging from 3 to about 15. The results show a very good agreement at Re = 100 and the vortex shedding modes have also been related to the frequency response observed. Finally, some perspectives for further simulations in the turbulent regime (at Re = 1000) with structural damping are presented.

Locked-on vortex shedding modes from a rotationally oscillating circular cylinder

2017 ◽  
Vol 146 ◽  
pp. 324-338 ◽  
Author(s):  
H.V.R. Mittal ◽  
Qasem M. Al-Mdallal ◽  
Rajendra K. Ray
Keyword(s):  
Circular Cylinder ◽  
Vortex Shedding ◽  
Vortex Shedding Modes

Flow about a circular cylinder between parallel walls

2001 ◽  
Vol 440 ◽  
pp. 1-25 ◽  
Author(s):  
LUIGINO ZOVATTO ◽  
GIANNI PEDRIZZETTI
Keyword(s):  
Circular Cylinder ◽  
Vortex Shedding ◽  
Vortex Dynamics ◽  
Lift Force ◽  
The Body ◽  
Cylinder Wake ◽  
Regime Changes ◽  
Wall Boundary Layer ◽  
Karman Street ◽  
The Mean

The flow about a body placed inside a channel differs from its unbounded counterpart because of the effects of wall confinement, shear in the incoming velocity profile, and separation of vorticity from the channel walls. The case of a circular cylinder placed between two parallel walls is here studied numerically with a finite element method based on the vorticity–streamfunction formulation for values of the Reynolds number consistent with a two-dimensional assumption.The transition from steady flow to a periodic vortex shedding regime has been analysed: transition is delayed as the body approaches one wall because the interaction between the cylinder wake and the wall boundary layer vorticity constrains the separating shear layer, reducing its oscillations. The results confirm previous observations of the inhibition of vortex shedding for a body placed near one wall. The unsteady vortex shedding regime changes, from a pattern similar to the von Kármán street (with some differences) when the body is in about the centre of the channel, to a single row of same-sign vortices as the body approaches one wall. The separated vortex dynamics leading to this topological modification is presented.The mean drag coefficients, once they have been normalized properly, are comparable when the cylinder is placed at different distances from one wall, down to gaps less than one cylinder diameter. At smaller gaps the body behaves similarly to a surface-mounted obstacle. The lift force is given by a repulsive component plus an attractive one. The former, well known from literature, is given by the deviation of the wake behind the body. Evidence of the latter, which is a consequence of the shear in front of the body, is given.

Vortex dynamics for flow over a circular cylinder in proximity to a wall

2017 ◽  
Vol 812 ◽  
pp. 698-720 ◽  
Author(s):  
Guo-Sheng He ◽  
Jin-Jun Wang ◽  
Chong Pan ◽  
Li-Hao Feng ◽  
Qi Gao ◽  
...  
Keyword(s):  
Circular Cylinder ◽  
Shear Layer ◽  
Flat Plate ◽  
Vortex Shedding ◽  
Vortex Dynamics ◽  
Wake Vortex ◽  
Secondary Vortex ◽  
Cylinder Wake ◽  
Gap Ratio ◽  
Shedding Frequency

The dynamics of vortical structures in flow over a circular cylinder in the vicinity of a flat plate is investigated using particle image velocimetry (PIV). The cylinder is placed above the flat plate with its axis parallel to the wall and normal to the flow direction. The Reynolds number $Re_{D}$ based on the cylinder diameter $D$ is 1072 and the gap $G$ between the cylinder and the flat plate is varied from gap-to-diameter ratio $G/D=0$ to $G/D=3.0$. The flow statistics and vortex dynamics are strongly dependent on the gap ratio $G/D$. Statistics show that as the cylinder comes close to the wall ($G/D\leqslant 2.0$), the cylinder wake becomes more and more asymmetric and a boundary layer separation is induced on the flat plate downstream of the cylinder. The wake vortex shedding frequency increases with decreasing $G/D$ until a critical gap ratio (about $G/D=0.25$) below which the vortex shedding is irregular. The deflection of the gap flow away from the wall and its following interaction with the upper shear layer may be the cause of the higher shedding frequency. The vortex dynamics is investigated based on the phase-averaged flow field and virtual dye visualization in the instantaneous PIV velocity field. It is revealed that when the cylinder is close to the wall ($G/D=2.0$), the cylinder wake vortices can periodically induce secondary spanwise vortices near the wall. As the cylinder approaches the wall ($G/D=1.0$) the secondary vortex can directly interact with the lower wake vortex, and a further approaching of the cylinder ($G/D=0.5$) can result in more complex interactions among the secondary vortex, the lower wake vortex and the upper wake vortex. The breakdown of vortices into filamentary debris during vortex interactions is clearly revealed by the coloured virtual dye visualizations. For $G/D<0.25$, the lower shear layer is strongly inhibited and only the upper shear layer can shed vortices. Investigation of the vortex formation, evolution and interaction in the flow promotes the understanding of the flow physics for different gap ratios.

Evolution of unstable system.

2015 ◽  
Vol 9 (3) ◽  
pp. 2487-2502 ◽  
Author(s):  
Igor V. Lebed
Keyword(s):  
Reynolds Number ◽  
Vortex Shedding ◽  
Stokes Flow ◽  
Stable Solution ◽  
The State ◽  
Unstable System ◽  
Unstable Flow ◽  
Unstable Solutions ◽  
Vortex Shedding Modes ◽  
Hydrodynamics Equations

Scenario of appearance and development of instability in problem of a flow around a solid sphere at rest is discussed. The scenario was created by solutions to the multimoment hydrodynamics equations, which were applied to investigate the unstable phenomena. These solutions allow interpreting Stokes flow, periodic pulsations of the recirculating zone in the wake behind the sphere, the phenomenon of vortex shedding observed experimentally. In accordance with the scenario, system loses its stability when entropy outflow through surface confining the system cannot be compensated by entropy produced within the system. The system does not find a new stable position after losing its stability, that is, the system remains further unstable. As Reynolds number grows, one unstable flow regime is replaced by another. The replacement is governed tendency of the system to discover fastest path to depart from the state of statistical equilibrium. This striving, however, does not lead the system to disintegration. Periodically, reverse solutions to the multimoment hydrodynamics equations change the nature of evolution and guide the unstable system in a highly unlikely direction. In case of unstable system, unlikely path meets the direction of approaching the state of statistical equilibrium. Such behavior of the system contradicts the scenario created by solutions to the classic hydrodynamics equations. Unstable solutions to the classic hydrodynamics equations are not fairly prolonged along time to interpret experiment. Stable solutions satisfactorily reproduce all observed stable medium states. As Reynolds number grows one stable solution is replaced by another. They are, however, incapable of reproducing any of unstable regimes recorded experimentally. In particular, stable solutions to the classic hydrodynamics equations cannot put anything in correspondence to any of observed vortex shedding modes. In accordance with our interpretation, the reason for this isthe classic hydrodynamics equations themselves.

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