Measurements of the Tip-Gap Turbulent Flow Structure in a Low-speed Compressor Cascade
Technical rept. for 1 Aug 2001-15 May 2004
VIRGINIA POLYTECHNIC INST AND STATE UNIV BLACKSBURG DEPT OF AEROSPACE AND OCEAN ENGINEERING
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Experimental results are presented from a study of the tip-gap turbulent flow structure in a low-speed linear compressor cascade wind tunnel at Virginia Tech that includes a moving belt system to simulate the relative motion between the tip and the casing. Endwall pressure measurements and surface oil flow visualizations were made on a stationary endwall to obtain some global flow features. A custom-made miniature 3-orthogonal-velocity-component fiber-optic laser-Doppler velocimeter LDV with a 50 micron spherical measurement volume was used to measure all three components of instantaneous velocity within the gap between the endwall and the blade tip, mainly for the stationary wall with 1.65 and 3.30 of chord tip gaps, as well as some initial experiments with the moving wall. The surface skin friction velocity was obtained by using viscous sublayer velocity profiles, which verified the presence of an intense lateral shear layer that was observed from surface oil flow visualizations. All second- and third-order turbulence quantities are presented. Tip gap flows are complex, pressure-driven, unsteady highly skewed three-dimensional turbulent flows. The crossflow velocity normal to the blade chord is nearly uniform in the mid tip-gap and changes substantially from the pressure to suction side due to the local tip pressure loading, which is different from the mid-span pressure loading because of tip leakage vortex influence. Normalized circulation within the tip gap is independent of the gap size. The tip gap flow interacts with the primary flow, separates from the endwall, and rolls up on the suction side to form the tip leakage vortex, which is unsteady as observed from the TKE transport vector and oil flow visualizations. Other than the nearest endwall and blade tip regions, the TKE does not vary much in tip gap. The tip leakage vortex produces high turbulence intensities.
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