Some Observations on the Mechanism of Entrainment

Abstract

The turbulent entrainment process has been variously described as due to the effect of the small scale turbulence at the interface (Corrsin and Kistler, 1955; Townsend, 1966), the action of mixing jets which engulf volumes of fluid in bulk (Grant, 1958) and, most recently, a process of vortex pairing (Winant and Browand, 1974). A simple visualization study undertaken to examine the fundamental mechanism directly will be described in the present article. This experiment was conducted in the wake of a sphere towed through a water-filled trough, so that the “free stream” was actually motionless fluid. It was thus possible to mark a volume element of the free stream with dye, and follow its motion as it was injested by the wake. The common technique of injecting dye behind the model was used to make the wake itself visible. Motion pictures of the mixing process were made for subsequent study. Figure 1 is a tracing from this filmstrip.