Direct numerical simulation of mixing enhancement in supersonic shear flows

Direct numerical simulations of supersonic mixing layers and jets have been carried out to study the scalar mixing characteristics of supersonic shear flows. A precursor two dimensional mixing layer study, including a splitter plate with zero thickness under supersonic conditions, showed no upstream propagation of flow information. This is in contrast with subsonic cases in which a feedback mechanism is observed and leads to self-sustained vortical structures. Simulations of mixing layers and jets were carried out for (i) planar inflow, for (ii) flow with spanwise sinusoidal variation and for (iii) the spanwise sinusoidal variation with a forced streamwise vortex. Only the latter (iii) case showed enhanced mixing, this is observed from the simulations, which were run up to a near self-similar state as seen in the collapse of the mean streamwise velocity profiles into a common form for all three cases. The streamwise vortex cases showed a scalar probability density function (pdf) with a preferred mixed fluid composition, compared with planar and sinusoidal cases. Finally, a case study was made with a practical injector geometry at two inflow Reynolds numbers of Re = 600 and Re = 1200 respectively. The results showed that rapid mixing occurred immediately downstream of the injector. This case study shows a highly diffusive flow, which is seen in the small change of the mean scalar profile obtained across the mixed region. With an increase in inflow Reynolds number, there is an increase in the three dimensionality of the flow, seen in the higher lateral and spanwise turbulence intensities when compared with the lower Reynolds number case