Unsteady Fluid Force Acting on In-Line Oscillating Bluff Body in Lock-in Phenomenon

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Vortex-excitation oscillation due to a Karman vortex is generally known as oscillation by flow, wherein oscillation in the direction perpendicular to the flow is induced in the vortex flow. Also, when the natural frequency of a bluff body approaches the shedding frequency of the Karman vortex, the value of the shedding frequency of the Karman vortex becomes similar to that of the natural frequency ; this is called the lock-in phenomenon. Under this lock-in condition, the oscillation amplitude of the bluff body increases rapidly, causing, in some cases, a serious accident. Therefore, to examine countermeasures for the oscillation of an obstacle which generates a vortex, it is important to study the fluid force acting on the obstacle in a lock-in state, in particular, to evaluate the hydrodynamic excitation force. However, most of the reports regarding fluid force acting on a circular cylinder concern cases of fixed support, and to date, there is no report on fluid force acting on a circ … More ular cylinder in an oscillating state, except that by Bishop and Hassan. In this report, 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 x 500 mm x 2000 mm and Reynolds number of 1.9 x 10^4.The cylinder was then forced to oscillate sinusoidally in the streamwise 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-averaged pressure distribution and fluctuating pressure distribution on the circular cylinder were measured for the displacement in the oscillation. Consequently it was found as follows' (1) In this experiment, the lock-in region begins from Sv=0.325, and it leaves from the lock-in state at Sv=0.425 (2) The mean drag and fluctuating drag increase and become maximum in the lock-in region, while the base pressure in the rear surface (θ =180度) of the cylinder becomes low and attains a minimum. Less