Particle image velocimetry study of the impinging height effect on an overexpanded supersonic impinging free jet at Ma = 1.754. Part 1: global coherent structure

Results of an experimental study of an overexpanded axisymmetric supersonic impinging jet at Mach 1.754 with particle image velocimetry (PIV) are presented in this article. The focus of Part I is on the influence of the impinging height on the global coherent flow structure, while in Part II the detailed velocity magnitude, both axial and radial, as well as the position variation of the plate shock will be presented. The PIV results clearly demonstrate the formation of the stagnation bubble as well as the presence of the recirculating flow in it. When the impinging height reaches five times the nozzle exit diameter, the stagnation bubble disappears, which is thought to be the result of the increase in the thickness of the shear layer of the main jet and consequently the decrease in the difference of the stagnation pressure between the centre of the stagnation zone and the impinging point of the main jet shear layer. The comparison between instantaneous and mean PIV measurements demonstrates that there exists highly unsteady behaviour of the impinging jet. The shock wave structure and the unsteady entraining vortex of the nozzle jet and the wall jet are all clearly recorded by PIV. All these results are crucial to understand the effects of the impinging height on the global flowfield coherent structure of the overexpanded axisymmetric supersonic jet impingement.