Some Perspectives on Wall Turbulence: Vortex Spatial Organization and High Reynolds Number Scaling Laws

Abstract-
In this seminar recent developments in the study of turbulent boundary layers will be presented. First, a stereoscopic PIV investigation will be described where all three instantaneous components of the velocity field are measured in streamwise-spanwise planes. Datasets were obtained in the logarithmic layer and beyond. The vector fields in the log layer revealed signatures consistent with vortex trains or packets similar to those proposed by Adrian and co-workers. Groups of legs of hairpin vortices appeared to be coherently arranged along the streamwise direction. These regions also generated substantial Reynolds shear stress, sometimes as high as 40 times . A feature extraction algorithm was developed to automate the identification and characterization of these packets of hairpin vortices. In the log region, identified patches contributed approximately 30% to while occupying only 4% of the total area. The significant contributions to Reynolds shear stress indicate that this packet structure is an integral part of the turbulence transport mechanism. Beyond the log layer, the spatial organization into packets is seen to break down. These findings are consistent with attached eddy model calculations, which will also be described briefly.

The second part of the seminar will describe experiments at very high Reynolds number conducted on the salt flats of Western Utah. These data are used to test similarity scaling laws of the turbulence intensities. New formulations will be presented that explain various anomalies in the literature including the "mixed scaling" proposal of Degraaf and Eaton.

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