Numerical experiments on spatially developing mixing layers using large eddy simulation
The turbulent mixing layer has been studied extensively in many experimental undertakings. The instability that initially drives the layer produces vortex structures, and the layer grows by their successive pairings. The transition to turbulence, triggered by such a pairing, marks a change in the characteristics of the layer, with quasi-two-dimensional structures present in the turbulent flow. Experimental evidence points to a different mechanism of growth for these structures, but its details have yet to be determined. In this project a Large Eddy Simulation code has been used to simulate spatially developing mixing layers to a high degree of accuracy. Code validation has been performed with a series of test cases relevant to mixing layer flow. The transition to turbulence in mixing layers is simulated using two different inflow conditions, and excellent agreement in predicting the mean transition location against the reference experiment is found when a physically realistic inflow is applied. The mechanism of transition observed in simulations is the same as determined in previous experimental studies. Three subgrid scale models have been used to test the sensitivity of the flow to the modelling procedure. Comparisons are made between two- and three-dimensional mixing layers, demonstrating that two-dimensional simulations are wholly insufficient to capture the essential physics of the real flow. The three-dimensional simulations also show many of the features found in the real pre-transition flow. Finally, the post-transition mixing layer is studied in detail. A fundamental change in the evolutionary nature of the layer is reported, with the coherent structures present in the post-transition region interacting in a different manner to the pre-transition vortices. The post-transition structures grow in a continuous, linear fashion over their lifetime, and interact solely as a result of their growth.