Aerodynamic Forces Acting on an Oscillating Rectangular Cylinder and the Aeroelastic Instabilities at Moderate Reynolds Numbers (Experiments)

Abstract

We measured mean values of drags C_D, rms values of lifts C_L, and phase angles φ_L of lifts relative to body displacements on a rectangular cylinder. Cylinders with different side ratios of B/H=l, 2 and 3 were forced to oscillate transversely in a uniform flow at an amplitude of 14% of the shorter side H. The experiments were conducted in a water tunnel, with the forced oscillation Strouhal number St_C ranging from O.O to 0.5. Force measurements show that there exists a peak in C_D and C_L curves as St_C increases. The C_D values of the oscillating square cylinder after the peak are constant and much smaller than the values before the peak, while those of rectangular cylinders with B/H=2 and 3 are almost unchanged after the peak. The C_L values after the peak reach minimum values and then increase gradually due to flow inertia. The low-speed instability is aerodynamically featured by sharp decreases of C_D and C_L values after exceeding their peaks. Furthermore, flow visualization shows that the wake flow of an oscillating rectangular cylinder goes through three stages, i.e., full separation, alternate reattachment and full reattachment, with increasing St_C. Based on visualization, the flow features of vortex excitation and low-speed instability are discussed.