scholarly journals New oscillatory instability of a confined cylinder in a flow below the vortex shedding threshold

2011 ◽  
Vol 690 ◽  
pp. 345-365 ◽  
Author(s):  
B. Semin ◽  
A. Decoene ◽  
J.-P. Hulin ◽  
M. L. M. François ◽  
H. Auradou
Keyword(s):  
Reynolds Number ◽  
Vortex Shedding ◽  
Oscillation Amplitude ◽  
Forced Oscillations ◽  
Van Der Pol ◽  
Vortex Induced Vibrations ◽  
Lower Amplitude ◽  
New Type ◽  
Karman Vortex ◽  
Transverse Position

AbstractA new type of flow-induced oscillation is reported for a tethered cylinder confined inside a Hele-Shaw cell (ratio of cylinder diameter to cell aperture, $D/ h= 0. 66$) with its main axis perpendicular to the flow. This instability is studied numerically and experimentally as a function of the Reynolds number $\mathit{Re}$ and of the density ${\rho }_{s} $ of the cylinder. This confinement-induced vibration (CIV) occurs above a critical Reynolds number ${\mathit{Re}}_{c} \ensuremath{\sim} 20$ much lower than for Bénard–Von Kármán vortex shedding behind a fixed cylinder in the same configuration (${\mathit{Re}}_{\mathit{BV K}} = 111$). For low ${\rho }_{s} $ values, CIV persists up to the highest $\mathit{Re}$ value investigated ($\mathit{Re}= 130$). For denser cylinders, these oscillations end abruptly above a second value of $\mathit{Re}$ larger than ${\mathit{Re}}_{c} $ and vortex-induced vibrations (VIV) of lower amplitude appear for $\mathit{Re}\ensuremath{\sim} {\mathit{Re}}_{\mathit{BV K}} $. Close to the first threshold ${\mathit{Re}}_{c} $, the oscillation amplitude variation as $ \mathop{ (\mathit{Re}\ensuremath{-} {\mathit{Re}}_{c} )}\nolimits ^{1/ 2} $ and the lack of hysteresis demonstrate that the process is a supercritical Hopf bifurcation. Using forced oscillations, the transverse position of the cylinder is shown to satisfy a Van der Pol equation. The physical meaning of the stiffness, amplification and total mass coefficients of this equation are discussed from the variations of the pressure field.

Numerical Simulation of Vortex Shedding Past a Circular Cylinder in a Cross-Flow at Low Reynolds Number With Finite Volume-Technique: Part 1 — Forced Oscillations

2007 ◽  
Author(s):  
Antoine Placzek ◽  
Jean-Franc¸ois Sigrist ◽  
Aziz Hamdouni
Keyword(s):  
Numerical Simulation ◽  
Reynolds Number ◽  
Circular Cylinder ◽  
Finite Volume ◽  
Vortex Shedding ◽  
Stokes Equations ◽  
Cross Flow ◽  
Forced Oscillations ◽  
Lift And Drag ◽  
Wake Characteristics

The numerical simulation of the flow past a circular cylinder forced to oscillate transversely to the incident stream is presented here for a fixed Reynolds number equal to 100. The 2D Navier-Stokes equations are solved with a classical Finite Volume Method with an industrial CFD code which has been coupled with a user subroutine to obtain an explicit staggered procedure providing the cylinder displacement. A preliminary work is conducted in order to check the computation of the wake characteristics for Reynolds numbers smaller than 150. The Strouhal frequency fS, the lift and drag coefficients CL and CD are thus controlled among other parameters. The simulations are then performed with forced oscillations f0 for different frequency rations F = f0/fS in [0.50–1.50] and an amplitude A varying between 0.25 and 1.25. The wake characteristics are analysed using the time series of the fluctuating aerodynamic coefficients and their FFT. The frequency content is then linked to the shape of the phase portrait and to the vortex shedding mode. By choosing interesting couples (A,F), different vortex shedding modes have been observed, which are similar to those of the Williamson-Roshko map.

Vortex-induced vibrations of an elastically mounted sphere with three degrees of freedom at Re = 300: hysteresis and vortex shedding modes

2011 ◽  
Vol 686 ◽  
pp. 426-450 ◽  
Author(s):  
Suresh Behara ◽  
Iman Borazjani ◽  
Fotis Sotiropoulos
Keyword(s):  
Vortex Shedding ◽  
Distinct Type ◽  
Transverse Plane ◽  
Circular Path ◽  
Hairpin Vortices ◽  
Elastic Supports ◽  
Linear Path ◽  
Vortex Induced Vibrations ◽  
Lower Amplitude ◽  
Lock In

AbstractFluid–structure interaction (FSI) simulations are carried out to investigate vortex-induced vibrations of a sphere, mounted on elastic supports in all three spatial directions. The reduced velocity (${U}^{\ast } $) is systematically varied in the range ${U}^{\ast } = 4\text{{\ndash}} 9$, while the Reynolds number and reduced mass are held fixed at $\mathit{Re}= 300$ and ${m}^{\ast } = 2$, respectively. In the lock-in regime, two distinct branches are observed in the response curve, each corresponding to a distinct type of vortex shedding, namely, hairpin and spiral vortices. While shedding of hairpin vortices has been observed in several previous investigations of stationary and vibrating spheres, the shedding of intertwined, longitudinal spiral vortices in the wake of a vibrating sphere is reported herein for the first time. When the wake is in the hairpin shedding mode, the sphere moves along a linear path in the transverse plane, while when spiral vortices are shed, the sphere vibrates along a circular orbit. In the spiral mode branch, the simulations reveal hysteresis in the response amplitude at the beginning of the lock-in regime. Lower-amplitude vibrations are found as the sphere sheds hairpin vortices for increasing ${U}^{\ast } $ up until the beginning of the synchronization regime. On the other hand, higher-amplitude oscillations persist for the spiral mode as ${U}^{\ast } $ is decreased from the point of the start of the synchronization. The hairpin mode is found to be unstable for the value of reduced velocity where the spiral and hairpin solution branches merge together. When this point is approached along the hairpin solution branch, the sphere naturally transitions from shedding hairpin vortices and moving along a linear path to shedding spiral vortices and moving along a circular path in the transverse plane. The spiral mode was not observed in the work of Horowitz & Williamson (J. Fluid Mech., vol. 651, 2010, pp. 251–294), who studied experimentally the vibration modes of a freely rising or falling sphere and only reported zigzag vibrations. Our results suggest that this apparent discrepancy between experiments and simulations should be attributed to the fact that, for the range of governing parameters considered in the simulations, the elastic supports act to suppress streamwise vibrations, thus subjecting the sphere to a nearly axisymmetric elasticity constraint and enabling it to vibrate transversely along a circular path.

scholarly journals A parametric study of energy extraction from vortex-induced vibrations

2018 ◽  
Vol 42 (4) ◽  
pp. 359-369
Author(s):  
Olivier Paré-Lambert ◽  
Mathieu Olivier
Keyword(s):  
Reynolds Number ◽  
Vortex Shedding ◽  
Maximum Efficiency ◽  
Mass Ratio ◽  
Parametric Space ◽  
Turbine Performance ◽  
Vortex Induced Vibrations ◽  
Mass Ratios ◽  
Low Mass ◽  
Wake Modes

This paper presents a parametric investigation of an oscillating-cylinder turbine concept based on vortex-induced vibrations. The parametric space includes four parameters: the Reynolds number, the mass ratio, the dimensionless stiffness, and the dimensionless damping. The damping–stiffness space is explored for four different mass ratios at a fixed Reynolds number of 200. Also, the influence of the parameters on the amplitude of cylinder displacement and on the efficiency of power harnessing is discussed. Vortex-shedding patterns observed within the parametric space are investigated. The 2S, 2P, and C(2S) wake modes are observed and are related to turbine performance. Preliminary results show a maximum efficiency of 10.6%, which is obtained with low mass ratios.

Dynamics of Flow Around a Cylinder Oscillating In-Line for Low Reynolds Numbers

2010 ◽  
Author(s):  
L. Baranyi ◽  
K. Huynh ◽  
N. W. Mureithi
Keyword(s):  
Reynolds Number ◽  
Oscillation Amplitude ◽  
Wake Flow ◽  
Computational Domain ◽  
Cylinder Wake ◽  
Mean Values ◽  
Reynolds Number Flow ◽  
Lower Amplitude ◽  
Low Reynolds ◽  
Wake Modes

This study builds on an earlier study of low-Reynolds number flow about a cylinder forced to oscillate in-line with the main flow, which found vortex switches at some oscillation amplitude values. Here we extend the Reynolds number domain to Re = 60–350, utilize a computational domain characterized by R2/R1 = 360, and do computations at two frequency ratios of f/St0 = 0.8 and 0.9. Computations were carried out using a thoroughly tested finite-difference code. Some results were compared with those obtained by Ansys CFX, and good agreement was found. When plotted against oscillation amplitude, rms and time-mean values of force coefficients revealed a shift toward lower amplitude with higher Re. Findings for the effect of frequency ratio are similar. Where vortex switches occurred, a pre- and post-jump analysis is carried out. POD analysis of the cylinder wake flow field is employed to reveal the detailed wake dynamics as the forcing parameters are varied. The analysis provides further details on the transition of the dominant wake modes in response to the symmetry breaking bifurcation underlying the vortex switches observed in the simulations.

scholarly journals Numerical Computations of Vortex Formation Length in Flow Past an Elliptical Cylinder

Fluids ◽  
2020 ◽  
Vol 5 (3) ◽  
pp. 157
Author(s):  
Matthew Karlson ◽  
Bogdan G. Nita ◽  
Ashwin Vaidya
Keyword(s):  
Reynolds Number ◽  
Aspect Ratio ◽  
Vortex Shedding ◽  
Critical Reynolds Number ◽  
Vortex Formation ◽  
Project Based Learning ◽  
Unsteady Regime ◽  
Karman Vortex ◽  
Elliptical Cylinders ◽  
Asymmetric Configuration

We examine two dimensional properties of vortex shedding past elliptical cylinders through numerical simulations. Specifically, we investigate the vortex formation length in the Reynolds number regime 10 to 100 for elliptical bodies of aspect ratio in the range 0.4 to 1.4. Our computations reveal that in the steady flow regime, the change in the vortex length follows a linear profile with respect to the Reynolds number, while in the unsteady regime, the time averaged vortex length decreases in an exponential manner with increasing Reynolds number. The transition in profile is used to identify the critical Reynolds number which marks the bifurcation of the Karman vortex from steady symmetric to the unsteady, asymmetric configuration. Additionally, relationships between the vortex length and aspect ratio are also explored. The work presented here is an example of a module that can be used in a project based learning course on computational fluid dynamics.

Characteristics of Aerodynamic Sound Radiated From Two Finned Cylinders

2014 ◽  
Author(s):  
Hiromitsu Hamakawa ◽  
Hiroki Matsuoka ◽  
Kazuki Hosokai ◽  
Eiichi Nishida ◽  
Eru Kurihara
Keyword(s):  
Reynolds Number ◽  
Vortex Shedding ◽  
Transverse Direction ◽  
Flow Direction ◽  
Cross Flow ◽  
Near Wake ◽  
Aerodynamic Sound ◽  
Spacing Ratio ◽  
Staggered Arrangement ◽  
Karman Vortex

In the present paper the attention is focused on the characteristics of aerodynamic sound radiated from two finned cylinders with tandem and staggered arrangement exposed to cross-flow. We measured the spectrum of SPL and flow velocity for the cylinder spacing ratios ranged from 0 to 1.05 in the transverse direction and the ratios from 1.24 to 6.8 in the flow direction at Reynolds number of 1.0×105−1.9×105. As a result, we found that the peak SPL and Strouhal number of vortex shedding for two finned cylinders depend on the cylinder spacing ratios as well as those for bare cylinders. The peak SPL of the spectrum varied complexly with the tube spacing ratio. The peak levels of SPL for tandem finned cylinders were approximately 8 dB lower than that for the tandem bare cylinders. At the cylinder spacing ratio of 1.24 in the flow direction, the peak SPL for two finned cylinders at the cylinder spacing ratio of 0.72 in the transverse direction was about 8 dB larger than that for tandem finned cylinders. The peak SPL depended on the spanwise correlation length of the Karman vortex formed in the near wake of the downstream of two finned cylinders.

scholarly journals The Effect of Slat Opening on Vortex Shedding Behind a Circular Cylinder

2019 ◽  
Vol 8 (4) ◽  
pp. 6879-6885
Keyword(s):  
Reynolds Number ◽  
Circular Cylinder ◽  
Governing Equation ◽  
Vortex Shedding ◽  
Vortex Formation ◽  
Bluff Bodies ◽  
Vortex Induced Vibrations ◽  
Better Than ◽  
Laminar Model

Add-on devices are widely used as one of the means of suppressing vortex induced vibrations from bluff bodies. The present study numerically investigates flow over a circular cylinder attached by an axial slat. The axial slat were of uniform and non-uniform openings of 67% and 44% porosity. The governing equation was solved using viscous-laminar model at Reynolds number, Re=300. It was found that the presence of the axial slats significantly suppressed vortex shedding behind the circular cylinder. The non-uniform slats showed longer vortex formation length with lower drag, in comparison to that of the uniform slats. In addition, the slats with 67% porosity of both uniform and non-uniform openings suppressed vortex better than that of 44% porosity slats, indicated by the longer vortex formation length and weaker intensity of vortices.

Effect of Helical Strakes Around a Finned Tube on Aeolian Tone

2011 ◽  
Author(s):  
Hiromitsu Hamakawa ◽  
Yuki Ito ◽  
Ryunosuke Kamo ◽  
Eiichi Nishida
Keyword(s):  
Reynolds Number ◽  
Turbulence Intensity ◽  
Vortex Shedding ◽  
Large Diameter ◽  
Small Diameter ◽  
Finned Tube ◽  
Spanwise Direction ◽  
Helical Strakes ◽  
Karman Vortex ◽  
High Reynolds

In the present paper, the characteristics of vortex shedding and Aeolian tone radiated from a finned tube with helical strakes were experimentally investigated. The helical strakes are mounted spirally around a serrated finned tube surface. We measured the turbulence intensity in the wake, velocity fluctuation spectrum, the coherence of velocity fluctuations in the spanwise direction and SPL spectrum. The Aeolian tone radiated from the finned tube with helical strakes at high Reynolds number was smaller than the case of no helical strakes. The helical stakes were effective to reduce the Aeolian tone radiated from the finned tube. And the existence of helical strakes of large diameter caused the decrease of the periodicity and the turbulence intensity in the wake of the finned tube. The coherent scale of Karman vortex in the spanwise direction is smaller than that of a finned tube without helical strakes. However, the effect of helical strakes of small diameter on vortex shedding depended on the Reynolds number. The Karman vortex was clearly formed even in the case of helical strakes of small diameter at low Reynolds number. The existence of helical strakes of small diameter around a finned tube caused the increase of the periodicity of vortex shedding from a finned tube. The coherent scale of Karman vortex in the spanwise direction was larger than that of a finned tube without helical strakes.

scholarly journals Vortex-induced vibration analysis of viscoelastically mounted rigid circular cylinders in cross-flow at a moderate Reynolds number

2017 ◽  
Vol 24 (13) ◽  
pp. 2688-2700 ◽  
Author(s):  
A.M.G. de Lima ◽  
B.S.C. da Cunha ◽  
A.R. da Silva ◽  
L.F.F. Rodovalho
Keyword(s):  
Reynolds Number ◽  
Vortex Shedding ◽  
Circular Cylinders ◽  
Engineering Practice ◽  
Viscoelastic Materials ◽  
Engineering Structures ◽  
Viscoelastic System ◽  
Vortex Induced Vibration ◽  
Vortex Induced Vibrations ◽  
Moderate Reynolds Number

The vortex-induced vibrations may have disastrous effects in engineering practice, affecting significantly the durability, reliability and safety of engineering structures. This is a reason for which a great deal of effort has been dedicated to the proposition of control strategies to deal with the vortex-induced vibration problem. However, few works have proposed the use of viscoelastic materials to suppress the vibrations induced by vortex shedding, which motivates the present study. Here, the immersed boundary method combined with the virtual physical model is used to investigate the dynamics of a viscoelastically-mounted rigid cylinder in a fluid flow under transverse oscillations induced by vortex shedding. A straightforward time-domain modeling procedure of immersed viscoelastic system by using a four-parameter fractional derivative model is proposed. After the theoretical aspects, numerical tests are performed to investigate the vortex-induced oscillations and flow characteristics of the immersed viscoelastic system at Reynolds number 10,000 for a range of reduced velocity and temperature for two values of mass ratios. The results demonstrate the interest in using viscoelastic materials to mitigate the vortex-induced vibrations.

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.

Sign in / Sign up

Export Citation Format

Share Document