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

Vortices, oriented in the streamwise direction, play an important role in g enerating Reynolds shear stresses (and turbulence) for flow over a smooth w all. Many of these have been observed to originate from tiny streamwise vor tices, located in the immediate vicinity of the wall. In this paper we iden tify a different process that forms about 30 percent of the streamwise vort ices at locations, away from the wall, in the middle of the viscous wall la yer. This is accomplished by examining the changes, with time, of the turbu lent field obtained from a direct numerical simulation of turbulent flow in a channel. Streamwise vortices create a shear layer by pumping low momentu m fluid from the wall. One or more small spanwise vortices are formed at th e 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 fo rmed by a single streamwise vortex rather than a pair of counter-rotating v ortices. (C) 2000 American Institute of Physics. [S1070-6631(00)51611-0].