Effect of Shocks on Mixing and Thrust in ...

The flow in the divergent part of a highly over-expanded nozzle suffers strong instability due to formation of shock waves which causes mixing enhancement in the flow and destabilizes the shear layer adjacent to it. The code FLUENT has been used to simulate the flow of two different viscous fluids (air and hydrogen) through five different Convergent-Divergent (C-D) nozzles of varying area ratio and exit angles. 2-D steady state RANS equation has been simulated for both inviscid and viscous flow. SSTKW (Shear Stress Transport k-ω) turbulence model has been invoked to capture the viscous flow phenomena efficiently. The converged solution shows basic viscous flow phenomenon like lambda shock for NPR≥1.32, asymmetric lambda shock for NPR≥1.40 and flow separation for NPR≥1.32. In addition, turbulent kinetic energy increases sharply with the shock and asymmetric flow separation. Separated flow reattaches at the smaller leg of the lambda shock. However, it continues to be separated till the exit at the larger leg side of the lambda shock for NPR≥1.65 (for nozzles with αe>0°). Mixing efficiency at the exit of the nozzle rises with increase in NPR for NPR≤1.72. But this increase in mixing efficiency could not continue for higher NPRs. The thrust for computed viscous predictions decreases with NPR upto NPR=1.8 and then starts increasing. The computed results are generally in trends with the experiments as far as shock; in addition, aftershock and mixing are concerned.