Experimental study of the effects of exhaust plume and nozzle length on transonic and supersonic axisymmetric base flows: An experimental study

PIV measurements have been carried out to study the effect of exhaust plume and nozzle length on the flow topology and mean pressure distribution of axisymmetric base flows at freestream Mach numbers 0.76 and 2.20. Four different nozzle lengths with and without exhaust plume have been tested. The use of different nozzle lengths leads to flow cases in which the shear layer impinges on the model (solid reattachment), on the flow downstream of the model (fluidic reattachment), and intermittently on the model and on the flow (hybrid reattachment). An increase in nozzle length and the presence of an exhaust plume led to an increase in mean reattachment length at Mach 2.20, whereas no significant change in reattachment length was observed at Mach 0.76. The flow cases with the longest nozzles for which solid reattachment occurred showed significantly higher turbulent kinetic energy levels, at Mach 2.20, but significantly lower levels at Mach 0.76. 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 modelled by replacing the plume with a solid geometry. Pressure results showed that the location of the low-pressure region downstream of the base remains rather invariant for different flow cases with and without plume and for different nozzle lengths. An increase in nozzle length leads to higher local pressure at the nozzle exit and therefore results in a less under-expanded or more over-expanded plume.