Experimental And Numerical Study Of Axis-symmetrical Supersonic Mixing Layer

Axisymmetric supersonic mixing layers present widely in combined propulsionengines and supersonic injectors. A design method for non-conventional cross sectionsupersonic nozzle is presented. An axisymmetric supersonic wind tunnel facilitatingnon-contact optical measurements is developed. Based on experimental measurementsimplemented in the wind tunnel, and numerical simulations as well, characteristics ofaxisymmetric supersonic mixing layer, including the growth rate, coherent structuresand velocity distribution are analyzed.The conformation of an axisymmetric supersonic mixing layer wind tunnel isdesigned, which has a circular mixing layer situated in a square test section so as tocover the shortage of conventional axisymmetric wind tunnels with circular outer wallsthat are not capable for non-contact optical measurements such as schlieren or NPLS.The components of the wind tunnel are designed. Major factors are analyzed aiming atimproving the flowfield quality of the wind tunnel.Based on the flowfield of a supersonic annular nozzle, streamline tracing method isadopted for supersonic non-conventional cross section nozzle design. Given the initialcharacteristic line and the distribution of the Mach number at one wall or streamline asboundary conditions, the annular nozzle is attained by iterations via method ofcharacteristic curves. Non-conventional cross section nozzle is designed by tracing thestreamlines of the annular nozzle. Numerical simulations demonstrate the uniformity ofthe flowfield at the exit of the non-conventional nozzle, which can meet therequirements of the experiments.Numerical simulations are implemented for axisymmetric supersonic mixing layersfor different radiuses and turbulent intensity. A new definition for mixing layerthickness is put forward for analyzing. Simulation results show that the mixing radiusminimally affects the thickness of the mixing layer. The increase of turbulent intensitysignificantly favors the mixing efficiency.Based on NPLS measurements, the coherent structures and velocity distribution forthe supersonic mixing layer, with a mixing radius of41mm and a convective Machnumber of0.18, is studied in details. Results indicate that the flow pattern of theboundary layer considerably affects the flowfield structure of the axisymmetricsupersonic mixing layer. Turbulent boundary layer inhibits apparent K-H vortexes. Thegrowth rate of the mixing layer shows a slow-fast-slow pattern instead of linear growth.The wake of boundary layer slows down the movements of coherent structures.NPLS and PIV measurements are applied for further study of the velocitydistributions of supersonic mixing layer. The instantaneous, mean and fluctuationvelocity distributions are analyzed, as well as the vorticity distribution. It is illustratedthat, for mixing layers with low convective Mach number, the low speed region inducedby the wake of a thick boundary layer can sustain spatially for a long distance.