Tip Gap Effects on the Unsteady Behavior of a Tip Leakage Vortex

Abstract

The effects of tip gap on the unsteady behavior of a tip leakage vortex downstream of an idealized axial compressor rotor blade have been investigated in a linear cascade wind tunnel, extending the study of Ma and Devenport. The wind tunnel features a moving end wall to simulate the relative motion between the rotor and casing, and vortex generator pairs attached to the moving end wall that produce an idealized unsteady vortical inflow. Detailed three-component mean velocity and turbulence measurements have been made just downstream of the blade trailing edges for a series of tip gaps, from 0.83 to 3.3% chord, and phase averaged with respect to the relative position of the blades and vortex generator wakes, to reveal the structure of this flow and its dependence on tip gap. Significant fluctuations in the size, strength, structure, and position of the tip leakage vortex are produced by the vortical inflow even though it is one to two orders weaker than the tip leakage vortex. Interestingly, the amplitude of these effects increases with tip gap as the tip vortex strengthens. For small tip gaps, the disturbance to the leakage vortex appears to be a consequence of direct interaction with the inflow vortices. However, for larger tip gaps it is the indirect action of the inflow vortices interfering with the shedding of circulation from the blade tip that appears to be the dominant source of unsteadiness in the leakage vortex.