Homage to Bob Brodkey at 85: Ejections, Sweeps and Reynolds Shear Stress Generation

Authors: James M. Wallace and James H. Duncan
Institution: University of Maryland

Almost 50 years ago Bob Brodkey and his student, Corino, conceived of and carried out a visualization experiment for the very near wall region of a turbulent pipe flow that, together with the turbulent boundary layer visualization of Kline et al., excited the turbulence research community. Using a high-speed movie camera mounted on a lathe bed that recorded magnified images in a frame of reference moving with the flow, they observed the motions of submicron particles in the sub-layer, buffer-layer and lower part of the log-layer. Surprisingly, these motions were not nearly so locally random as was the general view of turbulence at the time. Rather, connected regions of the near wall flow decelerated and then erupted away from the wall in what they called "ejections". These decelerated motions were followed by larger scale connected motions toward the wall from above that they called "sweeps". They estimated that ejections accounted for 70% of the Reynolds shear stress at Re_d = 20,000 while only occurring about 18% of the time. This fluid dynamics video shows short sequences, at three Reynolds numbers, from the original Corino and Brodkey visual study. In the following decades, it inspired numerous laboratory and simulation studies aimed at unraveling the structure of bounded turbulent flows and understanding the turbulent transport processes within them.