Influence of Wake Vortices Induced by Elliptic Cylinder on Flat-plate Boundary Layer

Abstract

The present numerical study has been conducted to investigate the unsteady boundary layer characteristics over a flat plate under the influence of wake vortices induced from an elliptic cylinder positioned in the free stream by numerical computations. This flow situation is an idealization of that occurring on turbomachinery blades where unsteady wakes are generated by the preceding row of blades. The major-minor axis ratio of the elliptic cylinder is taken 0.6 and the angle of attack 00 positioning the cylinder at different locations from the plate. The investigation covers a Reynolds number range up to 1000 based on the focal distance of the elliptic cylinder and undisturbed free stream velocity. The Reynolds numbers are relatively low so that in the limiting case of a boundary layer undisturbed by wakes this remained laminar over the full length of the test plate. The time dependent, two dimensional flow is simulated numerically using finite element formulation. Quadratic triangular elements and discontinuous linear triangular elements are used for velocity and pressure interpolation respectively. A finite element package COMSOL Multiphysics is used for the present computation. The development of the flow field up to certain time period is considered. Instantaneous streamlines of the disturbed flow field, instantaneous velocity field, instantaneous boundary layer integral parameters, and skin friction on different plate streamwise locations are presented for all the cases. The wake vortices strongly affect the boundary layer compared to undisturbed flow over flat plate. From the above results, two types of interaction can be obtained, one is the strong wake-boundary layer interaction and another is the weak wake-boundary layer interaction depending on the cylinder to plate relative position. Comparisons are also carried out for circular cylinder having the same height of the elliptic cylinder cross-section, for a particular cylinder to plate relative position and Reynolds number.