Observations of the formation of streamwise vortices by rotation of arch vortices

Abstract

Vortices, oriented in the streamwise direction, play an important role in generating Reynolds shear stresses (and turbulence) for flow over a smooth wall. Many of these have been observed to originate from tiny streamwise vortices, located in the immediate vicinity of the wall. In this paper we identify a different process that forms about 30 percent of the streamwise vortices at locations, away from the wall, in the middle of the viscous wall layer. This is accomplished by examining the changes, with time, of the turbulent field obtained from a direct numerical simulation of turbulent flow in a channel. Streamwise vortices create a shear layer by pumping low momentum fluid from the wall. One or more small spanwise vortices are formed at the top of this layer. These grow in size and rotate in the direction of flow. Previous investigators have suggested that spanwise vortices could have a direct role in the formation of streamwise vortices. This paper describes, in detail, a process by which this is accomplished. Of particular interest is the need to recognize that the shear layer is asymmetric since it is formed by a single streamwise vortex rather than a pair of counter-rotating vortices.