An experimental and computational investigation on the effect of transonic flow in hypersonic wind tunnel nozzles, including filtered Rayleigh scattering measurements

The transonic flow region in axisymmetric hypersonic wind tunnel nozzles was investigated computationally and experimentally to probe the effects of throat geometry on the nozzle outflow. Comparison between circle arc throat sections and constant area sections were detailed. An Euler flow solver was used to model the flowfield computationally with a variety of geometries. A scaled duplicate of the 12" hypersonic wind tunnel nozzle was carved for the experimental phase and a special purpose blow-down test facility constructed. Pressure measurements were obtained throughout the entire nozzle flowfield. A Filtered Rayleigh Scattering technique was used in the throat region to determine flow velocity non-obtrusively. The optical technique employed a molecular filter to discriminate laser light scattered from the oxygen molecules in the gas stream. Computational and experimental measurements compared favorably in all cases tested, though experimental errors inherent in the measurement techniques limited the precision of the comparison.; Results showed outflow disturbances can be traced to the nozzle throat region. Subtle contour differences in the throat region cause changes in throat pressure waves which are reflected and magnified near the centerline. Circle arc throats produce a conical pressure wave at the throat exit which focuses to a centerline normal shock, or "Mach disk". Total pressure gradients across this structure and pressure wave reflections from the initial wave structure account for much of the outflow disturbance. Straight throat nozzles also create a conical shock structure, but off the nozzle inlet. The expansion wave at the exit of the throat can be used proactively to diminish the effect of the initial Mach disk, thus decreasing the overall centerline peak. Improvement over either type of nozzle may be gained by a continuous curvature throat where inlet and exit geometry is carefully contoured to minimize wave generation.