Ph.D. (Engg) :Studies on the Mixing Layer Between Supersonic Supersonic Co-flows

Two supersonic streams merging together in a co-flow configuration are encountered in several engineering systems, such as high-speed propulsion devices and supersonic ejectors. The thin mixing layer that develops between the two streams is dominated by compressibility effects and is prone to shock interactions in shock-dominated flows. The convective Mach number is defined relative to dominant large-scale structures in the mixing layer and is typically used to characterise the mixing layer. A key observation from previous studies on canonical supersonic-supersonic mixing layers having zero streamwise pressure gradient (ZPG-ML), which has a significant bearing on system design, is that the growth of the mixing layer is significantly reduced as the convective Mach number increases. In applications, however, streamwise pressure gradients can exist due to the flow topology, but there are very few studies on the effects of the streamwise pressure gradient on the growth of mixing layers (SPG-ML), especially in shock-dominated flows, which motivates this study. Further, there is a need to enhance mixing rates for compact design, which can be carried out using passive geometric modifications, and the effects of techniques such as discrete injection through holes and vortex generators like lobes on SPG-ML are not well studied. We study the mixing layer between supersonic-supersonic coflows in a specially designed supersonic mixing layer experimental facility, and using high-fidelity Large Eddy Simulations carried out using the OpenFOAM framework. The Mach number combinations of the two streams (2.0, 3.0) and (2.5, 3.0), with a typical convective Mach number of 0.23, are investigated. The flow is experimentally examined using high-speed schlieren and wall static pressure measurements. First, the LES framework is validated on existing experimental/DNS computations on ZPG-ML, and the computations are found to simulate the mixing layer characteristics well. The flow topology of the SPG-ML involves the generation of an oblique shock and an expansion fan at the point of confluence, as well as the development of the mixing layer downstream in the presence of a streamwise pressure gradient. The shock further reflects from the wall and impinges on the mixing layer. The wall static pressure profiles obtained from the LES simulations agree well with the experimental wall static pressure measurements. The mixing layer growth rate of the SPG-ML before shock interaction is 15% higher than ZPG-ML. Shock interaction significantly increases the three-dimensionality of the turbulent structures in the mixing layer, particularly in the p resence of high baroclinic torques, and enhances the growth rate. In the current study, the mixing layer is found to curve after the shock interaction, thereby sustaining an increase in the mixing layer growth rate compared to previous studies. Two different techniques of introducing streamwise vortices into the mixing layer are investigated, the first where discrete holes connect the high-pressure side to the low-pressure side, leading to a jet into the supersonic stream, generating counter-rotating vortices. In the second technique, elliptic lobes generate large streamwise vortices. Both techniques are found to increase the mixing layer growth rate before the interaction. Shock interaction is found to break up vortices and promote three-dimensionality in the milder case of the jet through the holes. In the case of lobes, the streamwise vortices are strong enough to retain their connectedness despite getting significantly modified by the shock interaction. These observations have implications for the application of such techniques to enhance mixing in shock-dominated flows. Detailed comparative investigations of different supersonic-supersonic mixing layer configurations are examined using experiments and LES data

Speaker:  PANCHABUDHE LAKHAN MADANJI

Research Supervisor: Srisha Rao M V