Formation of Streamwise Vortices around an Oscillating Circular Cylinder

Abstract

The formation of streamwise vortices in an unsteady viscous flow around an oscillating circular cylinder is investigated by numerical simulation based on the finite volume solution of the three-dimensional Navier-Stokes equations. The small disturbance given to the initial velocity field is amplified due to the instability of the oscillating flow to form the three-dimensional flow structure. In the formation process the streamwise vorticity component increases with its fluctuation with half the oscillation period. The development of the three-dimensional structure is much slower than that of the two-dimensional structure in the plane normal to the cylinder axis. It is found that on both sides of the cylinder the raws of pairs of the counter-rotating streamwise vortices place in alternating arrangement. The streamwise vortices are also formed along the center plane of the flow in front of and behind the cylinder, each of which has the opposite rotation of streamwise vortices on the sides of the cylinder at the same section. Tracing the vortex lines in the residual field suggests that these structures of streamwise vortices should be locally closed in the vicinity of the cylinder.