The Effect of Concave and Convex Forebody on the Wake of a Circular Cylinder (original) (raw)

Longitudinal vortex structures in a cylinder wake

Physics of Fluids, 1994

This paper presents the velocity field of the longitudinal vortices found in the wake of a circular cylinder, as measured using digital particle image velocimetry (PIV). Vorticity and circulation of the longitudinal vortices are presented, based on instantaneous velocity distributions in a transverse plane behind the cylinder. Recently, flow visualizations by Wei and Smith,' Williamson,' Welsh et a1.,3 and Bays-Muchmore and Ahmed4 have shown that three-dimensional (3-D) vertical structures develop in the wake of a bluff body. The 3-D vertical structures were found to include pairs of counterrotating longitudinal vortices superimposed on the nominally two-dimensional (2-D) Kirmin vortex street as speculated

Three-dimensional vortex structures in a cylinder wake

1996

The three-dimensionality of the velocity field in the wake of a circular cylinder has excited considerable interest and debate over the past decade. Presented here are experimental results that characterize the underlying vorticity field of such wakes. Using particle image velocimetry (PIV), instantaneous velocity fields were measured and from these the vorticity of the longitudinal vortices lying in the region between Karman vortices was found.

Velocity Perturbations Induced by the Longitudinal Vortices in a Cylinder Wake

Journal of Fluids Engineering, 1996

This paper presents data showing the three-dimensional vortical structures in the near wake region of circular cylinders. The in-plane velocity field was measured using a digital Particle Image Velocimetry (PIV) technique. The vortical structures are found to include inclined counter-rotating longitudinal vortices in the braids joining consecutive Ka´rma´n vortices. A simple vortex-pair model is proposed to estimate velocity perturbation induced by the longitudinal vortices in the near wake region. The perturbation resulting from the longitudinal vortices is shown to induce spanwise velocity modulation and a velocity spike of a nominally two-dimensional vortex street.

On Vorticity Dynamics in Turbulent Wake behind Circular Cylinder

MATEC Web of Conferences

The circular cylinder turbulent wake is studied experimentally using Particle Image Velocimetry (PIV) method. The vorticity component parallel to the cylinder axis is evaluated from measured velocity field. Dynamics of the vorticity field is analysed using the Oscillation Pattern Decomposition (OPD) method. The 3 dominant modes corresponding to the 1st, 2nd and 3rd harmonics of Strouhal frequency are detected and presented. The corresponding topologies are characterized by waves of vortical systems. The results are similar to the dynamics of velocity field, however the procedure is much easier to implement, as only half number of degrees of freedom are active.

Experimental Investigation of 3D Dynamical Effects in a Wake behind Circular Cylinder

MATEC Web of Conferences

A circular cylinder in crossflow is subjected to the study of 3D dynamical structure of its wake. The typical dynamics is characterized by quasi-periodic behaviour called Kármán – Bénard vortex street with the typical frequency in dimensionless form known as Strouhal number. The experimental study relies on stereo Particle Image Velocimetry method, the plane of measurement is perpendicular to the flow in the distance 3.8 cylinder diameters in streamwise direction. Reynolds number was around 5 thousand. The structure and dynamical behaviour of the wake along the cylinder axis is studied in details. On the top of the known streamwise velocity deficit definition of the wake, the streamwise oriented dynamically evolving vortices are detected. For the detailed dynamics examination, the Oscillation Pattern Decomposition method was used. Waves of various structures travelling along the cylinder axis as well as some pulsations were identified. The wake dynamics is characterised by the strea...

Dynamics of the Tip Vortices in the Wake Behind a Circular Cylinder of Finite Length

MATEC Web of Conferences, 2020

The dynamics of the tip vortices in the wake behind a wall-mounted finite-length circular cylinder of the aspect ratio 2 was studied experimentally using time resolved stereo PIV technique. The cylinder was mounted normal to a ground plane and it was subjected to a cross-flow with thin boundary layer developed on the wall, the Reynolds number based on inflow velocity and cylinder diameter was 9.7 thousands. The dynamics of tip vortices were analysed using the POD method applied to the plane perpendicular to the flow close to the cylinder. Besides the decaying power spectrum, slower that the Kolmogorov-type one, the two distinct frequencies were detected on Strouhal numbers 0.09 and 0.15. These frequencies could be linked to the vortical structures dynamics in the wake. The frequency Sh = 0.15 corresponds to predominantly spanwise vortices dynamics with anti-symmetrical patterns with respect to the cylinder axis, while the frequency Sh = 0.09 corresponds to mainly streamwise vortical...

On 3D structure of a circular cylinder wake

Epj Web of Conferences, 2022

The 3D structure of the wake behind a circular cylinder in a cross-flow is studied experimentally using stereo PIV technique, Reynolds number was around 5 thousands. The statistical characteristics along the cylinder axis are effectively constant and thus 2D, however instantaneous flow-field in the wake is fully 3D. Within the wake dynamical flow, the streamwise vortices and spots of streamwise velocity component pulsations are detected.

Vortex shedding in the near wake of a finite cylinder

Vortex shedding of both finite and infinite cylinders have been investigated by means of the Particle Image Velocimetry (PIV) technique and compared. To obtain reliable turbulence statistics 5˙000 images have been taken for each test. Instantaneous and mean velocity fields are measured in many planes along the cylinder length, in order to study the flow evolution between the basis and the end of the cylinder. The Reynolds number investigated (Re = 16˙000) is within the Shear-Layer Transition Regime, indicated by Williamson (1996) for the infinite cylinder. This regime is characterized by the formation of Karman vortices. As regard the finite one, a counter-rotating pair of tip vortices forms at the free end, extends into the wake and interacts in a complex manner with Karman vortex shedding. For the aspect ratio used in this work (AR=8), Kawamura et al. (1984) showed that the Karman vortices are present only in a small region near the cylinder base. In order to detect the Karman vortex shedding, a phase averaged method has been chosen. This has been computed by means of the Proper Orthogonal Decomposition (POD) technique and it has been evidenced that this approach works properly for the infinite cylinder, as well as for some sections near the base of the finite one. As conclusion of this consideration and accordingly with Kawamura et al. (1984), it has been identified the region which shows the Karman vortex shedding for a finite cylinder with AR=8 .

Three-dimensional vorticity patterns of cylinder wakes

Experiments in Fluids, 2009

The vortex organization of cylinder wakes is experimentally studied by time-resolved tomographic Particle Image Velocimetry at Reynolds numbers ranging from 180 to 5,540. Time resolved measurements are performed at Re = 180, 360 and 540, whereas the transitional (Re = 1,080) and turbulent regimes (Re = 5,540) are investigated by snapshots separated in phase by more than p/4. The vortex structure evolution is visualized by the 3D vorticity field, revealing a regular shedding at the lowest Reynolds, whereas at Re [ 500 the Bénard-Kármán vortex street exhibits counter-rotating stream-wise vortex pairs (characteristic of Mode B) dominating the 3D motion. The regime at Re = 360 produces a transitional pattern where the counter-rotating vortex pairs (Mode B), coexist with profoundly distorted shedding of oblique elements forming a chain of rhombus-like vortex cells. In the turbulent flow regime (Re = 5,540) a large increase in the range of flow scales is directly observed with the appearance of Kelvin-Helmholtz type vortices in the separated shear layer consistently with what is abundantly reported in literature. The statistical description of the secondary structures is inferred from a 3D autocorrelation analysis yielding two span-wise wavelengths for the counter-rotating pairs, an inner length given by (twice) the distance between counter-rotating elements and an outer one given by the distance between pairs. The uncertainty analysis of the present tomographic PIV experiments reveals that this approach is suited for the investigation of vortex wakes with a typical error of 2 and 10% on the velocity and vorticity vectors, respectively.

Three-dimensional vorticity measurements in the wake of a yawed circular cylinder

Physics of Fluids, 2010

Using a phase-averaged technique, the dependence of the wake vortical structures on cylinder yaw angle ␣ ͑=0°-45°͒ was investigated by measuring all three-velocity and vorticity components simultaneously using an eight-hot wire vorticity probe in the intermediate region ͑x / d =10͒ of a yawed stationary circular cylinder wake. For all yaw angles, the phase-averaged velocity and vorticity contours display apparent Kármán vortices. It is found that when ␣ Յ 15°, the maximum coherent concentrations of the three vorticity components do not change with ␣. However, when ␣ is increased to 45°, the maximum concentrations of the coherent transverse and spanwise vorticity components decrease by about 33% and 50%, respectively, while that of the streamwise vorticity increases by about 70%, suggesting that the strength of the Kármán vortex shed from the yawed cylinder decreases and the three dimensionality of the flow is enhanced. The maximum coherent concentrations of u and v contours decrease by more than 20% while that of w increases by 100%. Correspondingly, the coherent contributions to the velocity variances ͗u 2 ͘ and ͗v 2 ͘ decrease, while that of ͗w 2 ͘ increases. These results may indicate the generation of the secondary axial vortices in yawed cylinder wakes when ␣ is larger than 15°. The incoherent vorticity contours ͗ xr 2 ͘ are stretched along an axis inclining to the x-axis at an angle ␤ in the range of 60°-25°for ␣ = 0°-45°. The magnitudes of ͱ͗ xr 2 ͘ ‫ء‬ and ͱ͗ yr 2 ͘ ‫ء‬ through the saddle points are comparable to the maximum concentration of the coherent spanwise vorticity z ‫ء‬ at all cylinder yaw angles, supporting the previous speculation that the strength of the riblike structures in the cylinder wake is about the same as that of the spanwise structures, even in the yawed cylinder wakes.