Entrainment in Free Turbulent Flows

Abstract

By definition, free turbulent flows are bounded on at least one side by ``ambient'' fluid in irrotational motion. Since the fluid within the turbulent flow is in complex rotational motion, there must be viscous diffusion of vorticity into the ambient fluid, setting it into turbulent motion. For simple flows, such as two-dimensional wakes, jets and boundary layers in zero pressure gradient, entrainment of ambient fluid transfers mean flow energy to the turbulent motion and provides energy for the Reynolds stresses and for the viscous dissipation of energy on small scales.