ON THE ENTRAINMENT PROCESS IN PLANE TURBULENT WAKES

An experimental investigation of the entrainment process in the near regions of two plane turbulent wakes, generated by a porous body and a solid body, has been performed by means of hot-wire anemometry, digital data acquisition, and a pattern recognition technique. A fine-scale turbulence-activity indicator function, based on the envelope of the second derivative of the velocities, is incorporated in the pattern recognition technique for the purpose of studying the entrainment process, and an isocontour map that serves to depict the latter is obtained. The results indicate that although the coherent structures pertaining to the two wake flows are qualitatively the same, the entrainment processes are fundamentally different. With respect to the solid-body wake, as has been widely accepted, the entrainment is due largely to the engulfment action of Kármán-like coherent structures; as a consequence, the entrainment per se is concentrated near the center of the wake. In contrast, with respect to the porous-body wake, the entrainment is due mainly to the action of small-scale eddies, with length scales that are significantly smaller than those of the Kármán- like structures, causing the entrainment to be concentrated near the edge of the flow. © 1995 American Institute of Physics.