Control of Karman Vortex Street behind a Thin Airfoil at Low Reynolds Number

Shohei Takagi,Yasufumi Konishi,Masato Asai,Shigeru Obayashi

doi:10.4236/jfcmv.2016.43011

Abstract

To control the Karman vortex street formed behind a thin airfoil at a certain incidence, a control cylinder was placed at the suction side apart from the airfoil in the downstream region. Both smoke visualization and hot-wire measurements confirmed that the Karman vortex street was completely suppressed. The mechanism for suppression of the vortex street was examined with a hot-wire survey.

Highlights

Downstream of a thin flat plate placed in a low-turbulence uniform flow, the basic flow is usually hypothesized to be parallel, where the wake is destabilized by the so-called convective instability
The present paper aims at experimentally suppressing the Karman vortex street behind a thin airfoil model making use of a thin secondary cylinder as a control device
Control of the Karman vortex street formed behind a thin airfoil at a certain incidence, the influence of a control cylinder placed well apart from the airfoil in the downstream region was studied

Summary

Introduction

Downstream of a thin flat plate placed in a low-turbulence uniform flow, the basic flow is usually hypothesized to be parallel, where the wake is destabilized by the so-called convective instability. The complex ray theory [4] shows that the frequency selection of the global mode may be explained from the viewpoint of a logarithmic singularity, which corresponds to a turning point in the global-instability theory and is relevant to the recirculation region in the near wake of the model It was observed in a water channel experiment [5] that a Karman-vortex street behind a circular cylinder at a. Acquired outputs are linearized making use of King’s law V 2 = (Tw − Ta) ⋅ A + B U , where V and U are the bridge output voltage and local velocity, Tw and Ta are the operating temperature of the hot-wire sensor and air temperature in freestream, and A and B are calibration constants. The hot-wire support stem with a diameter of 6 mm inserted through the slitted side wall was mounted on a 3-D traversing mechanism, which is managed under the LabVIEW 2010 software together with a dataacquisition system supplied by National Instruments

Smoke-Wire Method and Flow Visualization

Profiles of Mean and Fluctuating Velocities

Control of Karman Vortex Street and Flow Visualization

Effect of Control Cylinder

Conclusion