Effect of large-scale vortical structure on mixing and spreading of shear layer is numerically investigated. Two strut configurations namely Straight & Tapered strut at two convective Mach numbers (Mc = 1.4 & 0.37) for two jet heights (0.6 & 1mm) are investigated. Hydrogen jet is injected through a two-dimensional slot in oncoming coflow at Mach 2. Excellent agreement between simulated and experimental data is witnessed, whereas the instantaneous data reveal the presence of various large-scale structures in the flow field. From the instantaneous field, it becomes apparent that both the geometries have different vortical breakdown locations. It is also noticed that an early onset of vortex breakdown manifests itself into the mixing layer thickness enhancement, effect of which is reflected in overall mixing characteristics. It becomes evident that the shear strength plays an important role in the near field mixing. The higher shear strength promotes the generation of large vortices. The analysis shows that SS-0.6 case offers highest mixing efficiency being dominated by relatively large-scale structures. Eigenmodes obtained through Proper Orthogonal Decomposition (POD) confirm the presence of dominating structures and shed light into the series of events involved in vortex pairing/merging and breakdown. Dynamic modal decomposition (DMD) also strengthens the observation made through the POD. I. INTRODUCTIONThe very notion of atmospheric flight at hypervelocity regime may appear fascinating and appealing but there are certain intrinsic challenges that need to be addressed. The concept of Scramjet was conceived as an air-breathing propulsion system, which is capable of propelling in high Mach number regime. But before the concept is realized certain questions need to be answered related to the design and technological issues. Although the proposed design may appear simple but achieving a stable and sustainable combustion efficiently at these speeds is an engineering marvel. The recent interest in Scramjet has promoted research towards the understanding of supersonic mixing phenomenon. The residence time being exceptionally short and need for efficient combustor has motivated research community to explore the underlying physics governing the mixing in these flow regimes.Various injector configurations have been explored and studied, most popular being normal, oblique and parallel each of them is having their own pros and cons, while the detailed discussion can be found in [1][2][3] . Due to the simplicity of the ____________________________

The present work reports on the flow physics of turbulent supersonic flow over backward facing step (BFS) at Mach 2 using LES methodology where the dynamic Smagorinsky model is used for SGS modeling, while POD is invoked to identify the coherent structures present in the flow. The mean data obtained through the computations is in good agreement with the experimental measurements, while the iso-surfaces of Q-criterion at different time instants show the complex flow structures. The presence of counter rotating vortex pair in the shear layer along with the complex shock wave/boundary layer interaction leading to the separation of boundary layer is also evident from the contours of both Q and the modulus of vorticity. Further, the POD analysis reveals the presence of coherent structures, where the first and second modes confirm the vortical structures near the step as well as along the shear layer in the downstream region; while the second, third and fourth modes confirm the presence of vortices along the shear layer due to Kelvin-Helmholtz (K-H) instability. Moreover, POD as well as frequency analysis is extended at different planes to extract the detailed flow features.

The strut based injector has been found to be one of the most promising injector designs for supersonic combustor, offering enhanced mixing of fuel and air. The mixing and flow field characteristics of the straight (SS) & tapered strut (TS), with fixed ramp angle and height at freestream Mach number 2 in conjunction with fuel injection at Mach 2.3 have been investigated numerically and reported. In the present investigation, hydrogen (H2) and ethylene (C2H4) are injected in oncoming supersonic flow from the back of the strut, where jet to freestream momentum ratio is maintained at 0.79 and 0.69 for H2 & C2H4, respectively. The predicted wall static pressure and species mole fractions at various downstream locations are compared with the experimental data for TS case with 0.6 mm jet diameter and found to be in good agreement. Further, the effect of jet diameter and strut geometry on the near field mixing in strut ramp configuration is discussed for both the fuel. The numerical results are assessed based on various parameters for the performance evaluation of different strut ramp configurations. The SS configuration for both the injectant is found to be an optimum candidate, also it is observed that for higher jet diameter larger combustor length is required to achieve satisfactory near field mixing.

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The present study primarily focuses on the effect of jet spacing and strut geometry on the evolution and structure of the largescale vortices which play a key role in mixing characteristics in turbulent supersonic flows. Numerically simulated results corresponding to varying parameters such as strut geometry and jet spacing (Xn=nDj such that n=2, 3 & 5) for square jet of height Dj=0.6 mm is presented in the current study, while the work also investigates the presence of the local quasi-twodimensionality for the X2(2Dj) jet spacing; however, the same is not true for higher jet spacing. Further, the tapered strut (TS) section is modified to the straight strut (SS) for investigation, where the remarkable difference in flow physics is unfolded between the two configurations for similar jet spacing (X2: 2Dj). The instantaneous density and vorticity contours reveal the structures of varying scales undergoing different evolution for the different configurations. The effect of local spanwise rollers is clearly manifested in the mixing efficiency and the jet spreading rate. SS configuration exhibits excellent near field mixing behavior amongst all the arrangements. However, in case of TS cases, only X2(2Dj) configuration performs better due to the presence of local spanwise rollers. The qualitative and quantitative analysis reveals that near-field mixing is strongly affected by the two-dimensional rollers, while the early onset of wake mode is another crucial parameter to have improved mixing. Modal decomposition performed for SS arrangement sheds light into the spatial and temporal coherence of the structures, where the most dominant structures are found to be the Von-Karman street vortices in the wake region.