Numerical Study of Unsteady Three-Dimensional Flow Separation from a Cyllnder

Abstract

Three-dimensional separation in unsteady flows when the vortex shedding occurs from a cylinder is investigated using the finite-difference solution of the incompressible Navier-Stokes equation. The diameter of the cylinder model is varied sinusoidally in the spanwise direction. The computed velocity field adjacent to the cylinder surface is analyzed by the phase-space theory to detect and classify critical points on the surface. The flow pattern of the three-dimensional separation becomes quite complicated and surprisingly changes in time as the large-scale vortices are shed from the alternate side of the cylinder. The nodal and saddle points move along the circumferential lines where the cylinder diameter is maximum or minimum, and the separation and reattachment lines wind accordingly. The focal points also appear, move, and disappear along the separation line, and as a result, the topology of the flow pattern changes in a drastic manner.