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EFFECTS OF FLOW SCALE ON THE INTERMITTENT PROPERTIES OF A SINGLE STREAM SHEAR LAYER

Aren M. Hellum
Department of Mechanical Engineering Michigan State University East Lansing, Ml 48824 USA

John F. Foss
Department of Mechanical Engineering Michigan State University East Lansing, Ml 48824 USA

Abstrakt

A four-sensor vorticity (ωz) probe has been used to characterize the external intermittency in a self-preserving single-stream shear layer at two downstream locations: x/θo = 200, 500. The algorithm to define intermittency state as "on" (the mixed, "interior" fluid of the shear layer) or "off" (the exterior fluid of the sheared region's adjacent potential streams) is based on the "activity level" of the ωz(t) time series. This discrimination technique leads to a intermittency time series, I(t), with well-defined 1, 0 states for the interior and exterior states, respectively. This time series allows conditional statistics for the velocity components to be acquired. Relatively minor differences are observed for the statistical values at the two downstream locations, except for a distinctly lower mean intermittency at x/θo = 500. A distinctive result, revealed by the joint histogram of u', v' for the inactive fluid, is the clear evidence for separate "wedges" of inactive fluid from both the high-speed and low-speed sides of the shear layer. Direct comparisons are made to the 1970 results of Wygnanski and Fiedler.