The Effect of Nozzle Length and Exhaust Plume on Transonic and Supersonic Axisymmetric Base Flows

Abstract

A combined PIV and Schlieren measurements have been carried out in the transonic-supersonic wind tunnel (TST-27) to investigate the effect of exhaust plume and the variation in nozzle length on the flow topology and mean pressure distribution on the wake of axisymmetric backward facing step model at freestream Mach numbers of 0.76 and 2.20, respectively. Four different nozzle length configurations with and without the presence of a supersonic exhaust plume have been tested. Testing with different nozzle length configuration resulted in flow cases where the shear layer reattachment occurred on the nozzle (solid reattachment), on the flow downstream of the nozzle (fluidic reattachment), and intermittently on the nozzle and on the flow (hybrid reattachment). A qualitative identification of the effect of exhaust plume and the variation in nozzle length on the flow topology on the wake of axisymmetric backward facing step model, at the above mentioned conditions, is successfully described by means of Schlieren visualization. The topological flow features for the subsonic and supersonic flow cases has been identified (i.e shock waves, Prandtl-Meyer expansion fan, boundary layer, separated shear layer, recompression and plume shocks). On the other hand, a quantitative identification of the effect of exhaust plume and the variation in nozzle length on the flow topology and mean pressure distribution on the wake of axisymmetric backward facing step model, at the above mentioned conditions, is successfully done by means of planar PIV. The planar PIV measurements in the wake region of the model provided detailed information of the mean flow field properties (i.e mean velocity, turbulent kinetic energy, Reynolds stresses and reverse flow). Using these mean flow field properties data, the mean pressure distribution is reconstructed based on the momentum equation.It has been shown that an increase in nozzle length and the presence of an exhaust plume caused an increase in mean reattachment length at freestream Mach number of 2.20, while no significant change in mean reattachment length was noticed at freestream Mach number of 0.76. Significantly higher turbulent kinetic energy levels have been observed for L/D = 1.8 cases where solid reattachment occurred, at freestream Mach number of 2.20. In contrast to supersonic flow cases, the flow cases at freestream Mach number of 0.76 showed a significantly lower turbulent kinetic energy levels. Comparisons of flow cases with a long nozzle without a plume and flow cases with a short nozzle but with a plume suggest that the presence of the plume cannot accurately be modeled by replacing the plume with a solid geometry. From the pressure results it is observed that the location of the low-pressure region downstream of the base remained unchanged for different flow cases with and without exhaust plume and for different nozzle lengths. Furthermore, it has been shown that an increase in nozzle length leads to higher local pressure at the nozzle exit and hence results in a less under-expanded for the supersonic flow cases or more over-expanded plume for the subsonic cases.