Evidence of complex flow structures in a converging-diverging nozzle caused by a recessed step at the nozzle throat

Experimental investigations were carried out to assess the influence on the flow field of a small recessed step at the throat section of a planar converging-diverging nozzle operating with dry air and in adapted conditions. The depth of the recessed step is 0.1 mm and the nozzle height at throat is 10.0 mm. Pressure measurements along the nozzle axis and Schlieren visualizations were compared to numerical simulations for both the recessed-step and the clean configurations. The presence of the recessed step results in a local perturba-tion of the pressure profile along the axis close to the throat section, which superimposes to that obtained by the clean configuration immediately downstream of the nozzle throat. Amplitude spectrum analysis of the pressure signal, limited to a maximum frequency of 250 Hz, provide no evidence of the benefficial effects of the recessed-step on the flow stability, at least in the considered frequency range. In flow conditions resulting in the formation of shock waves in the divergent section, the recessed-step geometry was found to produce more stable, tough asymmetric, shock configurations with respect to the clean geometry, possibly as a consequence of flow separation at the step edge.