Abstract:
In the present work, a numerical computation of shock train phenomena inside a gas dynamic flow passage is studied. The flow passage is of rectangular cross-section with a diverging angle of 1.3°. Reynolds Averaged Navier-Stokes (RANS) equations with Explicit Algebraic Reynolds Stress Model (EARSM) turbulence model are initially implemented for steady computation. The results of this computation are compared with the experiment at the same flow conditions. After that, the results from steady computation are taken as the initial conditions for the unsteady computation using Large Eddy Simulation (LES). LES with a sub grid scale (SGS) model shows an unsteady effect. Shock train structure, Mach no. distribution and separation characteristics are analysed for three Nozzle Pressure Ratios (NPRs). At lower NPR (1.6) the overall strength of the shock train structure is found to be low compared to other two NPR (1.8 and 2.0) cases. The flow through nozzle with high NPR (2.0) is found to be more prone to separate and the length of separation bubble for this case is larger compared to other two cases. The back and forth movement of shock train without any external excitations (self-excited oscillation) is well observed. The distribution of frequency of the shock train oscillation is computed and analysed inside the gas dynamic flow passage. From Fast Fourier Transfer (FFT) analysis, the principle frequency of pressure fluctuation is found to be different at different positions. At the downstream side of the shock train, high frequency fluctuation is found to be the dominant one. Moreover, the shock train structure is found to be three dimensional. It is observed that towards the sidewall both the strength of the shock system and length of the shock train start to decrease. At the second part of this study, effect of heating on shock train structure is investigated. Three different “wall to recovery temperature ratio” is considered. It is found that as the ratio increases,that means more heating is applied, the strength of the shock system decreases but the length of the shock system increases. It is also found that wall heating has effect on the pressure fluctuating characteristics as well.
