Simple and two-phase supersonic jets impinging on plates

Abstract

Fluid dynamic phenomena occurring during the impingement of a supersonic free jet on a plate have been studied both, experimentally and numerically. This happened under conditions typical of the cold spray deposition process. Therefore, also particle-laden jets were considered. Under certain conditions a recirculation bubble is formed in the shock layer ahead of the plate. Numerical computations of this flow that have been performed with the DLR TAU-code, are compared with shadowgraphs. Experimental results obtained with a newly developed multi-exposure method combined with synchronized pressure measurements, show that the formation of recirculation bubbles is typically connected with self-excited flow oscillations. Based on a spectral analysis, different mechanisms of self-excited oscillations are discussed and compared with related phenomena reported in the literature. The importance of flow oscillations on the behaviour of a particle cloud that is formed ahead of the plate in the case of two-phase flows, will also be explained. Finally, the emission of light that is sometimes observed when the metallic particles impinge on the metal plate with a high velocity, is traced to mechanoluminescence.