On Some Aspects of Unsteady Aerodynamics and Vortex Induced Oscillations of Elliptic Cylinders at Subcritical Reynolds Number

Abstract

The paper presents results of an extensive test program aimed at better understanding of unsteady aerodynamics and vortex induced oscillations of a family of two dimensional elliptic cylinders in the Reynolds number range of 3 × 104 – 105 . In the beginning, results on Strouhal number variation with cylinder eccentricity and angle of attack are presented which can be used to predict critical resonant velocity once the structural properties are identified. This is followed by the data on the fluctuating pressure at the surface of the cylinders which suggest their three dimensional character and significant dependence on the Reynolds number. However, the unsteady, sectional lift coefficient, obtained through integration of the pressure data, is virtually independent of the spanwise location. The correlation of phase between unsteady pressure signals along the span showed parabolic character of the vortexline at zero angle of attack of the model, but the vortexline tends to be straight and its alignment with the model improves as the angle of attack increases. On the other hand, the presence of end gaps (between the model and tunnel walls) results in a parabolic vortexline at all angles of attack. During the vortex excited oscillations, the familiar ‘capture’ phenomenon is exhibited where the vortex shedding frequency is controlled by the cylinder vibration over a finite range of wind velocity. Detailed measurements of cylinder and Strouhal frequencies, displacement amplitude and phase angle as functions of wind velocity, cylinder eccentricity and angle of attack provide information which should prove useful during design of such structural members.