Fluid Force Acting on Two-Dimensional Circular Cylinder in Lock-in Phenomenon

In this paper, we describe the fluid force acting on a two-dimensional oscillating circular cylinder in a lock-in region. The experiment was carried out in an N.P.L blow -down wind tunnel with a working section of 500 mm × 500 mm × 2 000 mm and Reynolds number of 1.9 × 10^4. The cylinder was then forced to oscillate sinusoidally in the lift direction. The power spectrum of the fluctuating velocity in the wake behind the circular cylinder was measured to show the lock-in region in the present experiment. The time-mean pressure distribution and fluctuating pressure distribution on the circular cylinder were measured for the displacement in the oscillation. Consequently, it was found that the mean drag and fluctuating lift increase and become maximum in the lock-in region, while the base pressure at the rear surface (θ=180°) of the cylinder decreases and attains a minimum.