Abstract
We measured mean values of drags CD, rms values of lifts CL, and phase angles φL 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 StC ranging from O.O to 0.5. Force measurements show that there exists a peak in CD and CL curves as StC increases. The CD 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 CL 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 CD and CL 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 StC. Based on visualization, the flow features of vortex excitation and low-speed instability are discussed.
