Numerical Simulation of 3-D Free Shear Layers

Abstract

The spatially-developing, plane free shear layer generated by the turbulent mixing of coflowing fluid streams is geometrically simple. This simple flow configuration is important in mixing processes and is encountered in many other engineering applications. The existence of large coherent structures in the turbulent mixing layer is confirmed recently by many experimental investigations. The large wave-like structures, which are embedded in a background of small scale turbulence, seem to dominate the mixing layer. The mean flow is controlled by the well defined, organized, coherent structures that do not vanish when small scale turbulence appears at high Reynolds numbers. It has been realized that those large structures that are dynamically significant, are indeed an essential feature of the turbulent mixing layer. Coherent structures are responsible for transport of mass, heat, and momentum in the mixing layer. Entrainment is mostly due to large scale engulfment of irrotational fluids by the coherent structures, but the coherent structure itself is not necessarily highly energetic. Vortex pairing phenomenon between two neighboring vortices is observed, which is responsible for the linear growth of the natural shear layer.