Wake of a compressor cascade with tip gap, Part 1: Mean flow and turbulence structure

The purpose of this work is to understand the structure of the vortex-dominated endwall flows found in aircraft engine fans and compressors. Our approach is to model the flow using a linear cascade where detailed turbulence measurements can be made and the relative motion between blade tip and casing can be simulated. The cascade consists of eight 4% thick modified circular-are blades and operates at a chord Reynolds number of 3.88 x 10(5) with a thin inlet boundary layer and 12.5 deg of turning. We present baseline results for the tip-leakage flow with stationary endwall. Three component velocity and turbulence measurements are used to reveal the evolution of the flow as a function of distance downstream of the blades for a tip gap of 1.6% chord and as a function of tip gap from 0.8 to 3.3%. Overall, these measurements reveal much of the structure of a tip-leakage vortex wake, the manner of its decay and mechanisms of turbulence production, and its relationship to the two-dimensional parts of the blade wakes. Although the vortex is a region of coherent rotating motion, we find that its dynamics are dominated by the streamwise mean-velocity deficit it produces. Mean velocities associated with the deficit are more than twice as strong as those associated with the rotating motion and decay more slowly with downstream distance. Turbulence kinetic energy in the vortex is produced almost entirely by velocity gradients associated with the deficit. Turbulent activity is thus centered in an are-shaped region above the vortex center where these gradients are large.