Study of two supersonic streamwise vortex interactions in a Mach 2.5 flow: Merging and no merging configurations

This work presents the detailed analysis of the flow fields resulting from two supersonic vortex interaction modes in a Mach 2.5 cold flow. The vortex interactions were selected beforehand by means of a reduced order modeling tool to obtain merging of the co-rotating structures in two counter-rotating vortex pairs in one case and to prevent their merging in the second. To experimentally target the flow physics of interest, expansion ramps of the same height but different width and mutual distance were placed on the surface of a strut injector. The resulting flow fields have been characterized using stereoscopic particle image velocimetry surveys conducted at selected streamwise planes. Accurate measurements of the velocity and vorticity fields, strain rates, circulation, and the rate of change of the area of each vorticity patch in the surveyed flow highlight the differences between merging and non-merging scenarios as well as the role of viscosity and turbulent diffusion in the resulting vortex dynamics. In particular, it is observed that, for the high Reynolds numbers typical of these supersonic flows and the associated time scales, an inviscid, non-diffusive formulation can be used for a reduced-order description that allows to capture the relevant dynamics related to the morphological distortion of the interacting streamwise vorticity patches introduced into the supersonic flow. The distribution of the strain rates in the flows resulting from the selected vortex configurations suggests that a faster mixing process may be expected in the merging case. However, the volumetric entrainment measurements show that the configuration in which merging was not attained yields a much larger entrainment of surrounding fluid compared to the merging case.