EFFECTS OF THE FREE-STREAM DENSITY RATIO ON FREE AND FORCED SPATIALLYDEVELOPING SHEAR LAYERS

The effects of the free-stream density ratio on the evolution of the i ncompressible, high Reynolds and Froude number, confined mixing layer are investigated numerically. Two-dimensional simulations of the spati ally developing flow with and without external forcing arp obtained us ing the Lagrangian transport element method. Results indicate that a n onunity density ratio alters the flow characteristics significantly. I n the unforced flow, it increases the layer growth as the slow stream becomes denser, biases the speed of both the Linear instability waves and the rollup eddies toward that of the denser stream, and modifies e ntrainment in favor of the dense fluid. These results, which are in ag reement with experimental and analytical evidence, are analyzed in ter ms of the evolution of the vorticity field and, in particular, of the action of the mechanism of baroclinic vorticity generation. It is foun d that this mechanism creates vorticity of opposite signs across each eddy,which, through simple kinematical arguments, is linked to the alt eration of the eddy speed and the modification of the local entrainmen t patterns. High-amplitude external forcing modifies the growth behavi or of the layer while leaving its entrainment characteristics and the eddy speeds unaffected. In this case the layer growth is no longer mon otonically varying with the free-stream density ratio. Instead, it is a strong function of the momentum ratio, reaching a minimum at a momen tum ratio of unity and increasing more significantly for higher values of this parameter. Enhancement of the layer growth via forcing occurs only when the momentum ratio is substantially different from unity. I t is found that the forced layer growth characteristics are related to the layer orientation, which is also a function of the momentum ratio . Using this fact and basic principles, a simple analytical model is d erived to explain the numerical results. It is suggested that the unfo rced flow behaves differently due to its initial instability character istics that are bypassed when forcing is present. (C) 1995 American In stitute of Physics.