Flow Past an Oscillating Cylinder: Effects of Oscillation Mode on Wake Structure

Abstract

We present a computational study of three-dimensional flow past a cylinder forced to oscillate both transversely and in-line with respect to a uniform stream. The cylinder oscillates with a frequency in the in-line direction equal to twice the transverse oscillation frequency, thus following a figure-eight trajectory; for a flow from left to right, the figure-eight is traversed in either a counter-clockwise or a clockwise direction in the upper half plane. Flow simulations were performed for Re = 400 (for which the unforced flow is fully three-dimensional) for different cases, defined in terms of the oscillation mode (counter-clockwise or clockwise motion) and the ratio of transverse oscillation frequency to the natural frequency of vortex shedding (F = 0.8, 0.9, 1.0 and 1.1). The results demonstrate that the effect of cylinder oscillation on the flow structure and forces differs substantially between the counter-clockwise and the clockwise oscillation mode. For the counter-clockwise mode, forcing at low amplitude decreases the flow three-dimensionality, with the wake becoming increasingly three-dimensional for transverse oscillation amplitudes higher than 0.25–0.30 cylinder diameters. For the case of clockwise mode, a strong stabilizing effect is found: the wake becomes two-dimensional for a transverse oscillation amplitude of 0.20 cylinder diameters, and remains so at higher amplitudes.