Computational modeling & scaling of plume induced flow separation

Abstract

This dissertation investigated the flow behavior of converging-diverging nozzle numerically. Results of a study exploring the use of saw-tooth surface modifications to improve supersonic micronozzle performance are presented. For certain configurations, the saw-tooth wall modification considered in this study produced an effective "velocity slip" along the surface of the nozzle, thus reducing the influence of viscous forces and enhancing nozzle performance. A parametric study examined how variations in the geometric scale and configuration of the individual "saw teeth" affected flow behavior, specific impulse, and thrust coefficient. By increasing the number of saw teeth, a point was reached where viscous forces damped out the impact that the surface modifications had on the overall flow field behavior. Under certain conditions, the saw-tooth surface modification was shown to increase nozzle performance. The flow structure of the exhaust from a converging-diverging nozzle into supersonic flow field has been investigated for both stead-state and transient analyses. The results of this study are presented, with particular emphasis on plume induced flow separation. The CD nozzles with straight afterbody configurations, having throat diameters on millimeter to centimeter were examined. The parametric studies involving freestream Mach number, chamber-to-ambient pressure ratio and the geometric scaling effects was conducted to explore the impact these parameters had on base-flow/plume interactions and plume induced flow separation (PIFS). The plume induced shockwave motion as well as the "hot zone" caused by the base-flow/plume interaction were investigated by considering freestream Mach and pressure ratio . The 2-equation k-ε Re-Normalization Group (RNG) model and k-ω Shear Stress Transportation (SST) model were utilized for turbulence calculation. Grid independent computational models were validated by comparison with previously published data. Correlations showing the relation between the boundaries of the recirculation zone when PIFS is present as well as the plume induced shock motion with respect to freestream Mach and pressure ratio are presented. Values of these parameters when PIFS and base-flow/plume interaction occur are discussed as a function of geometry.