Numerical Investigation of Non-uniform Density Mixing Layers

Turbulent flow is one of the most important phenomena of fluid mechanics that has been the subject of extensive experimental and numerical investigation, due to their simplicity, while highly relevant in engineering applications. Reproduction of flow structure dynamics and entrainment mechanism using numerical simulations are extremely challenging. Accurate prediction of both small- and large- scale structures will produce the correct mixing process, similar to a real flow.In this study Large Eddy Simulations of high Reynolds number, three-dimensional, spatially developing mixing layers are performed. The investigation includes isothermal and exothermic reacting mixing layers with diffusion flames up to 900.8K above ambient. In the isothermal mixing layer, the high and the low density ratio simulations show a dominant engulfment entrainment mechanism and a nibbling entrainment mechanism, respectively. Only the simulations employing a physically correlated inflow condition develop spatially stationary, streamwise orientated structures; these structures are an integral part of the turbulence vortex structure that leads to the correct mixing behaviour. The large-scale, spanwise orientated, vortex structure spacing is shown to vary with the density ratio and velocity ratio parameter and heat release. The Large Eddy Simulations employing a physically correlated inflow condition capture all the salient flow features of the reference experiments. These simulations produce excellent mean flow statistics with the reference experiments, including the temperature rise and product formation. Below the adiabatic flame-temperature, 553.8K, an engulfment mechanism dominates; while above 600.3K, the fluid mixing is done by the nibbling mechanism. The mixing mechanism is insensitive to the equivalence ratio. An increasing inlet fluctuation magnitude changes the non-marching p.d.f. at low levels to a hybrid and marching p.d.f. at medium and high levels, respectively. A relationship for the mixing mechanism evolution is established based on turbulent kinetic energy Reynolds number, Rekmax.