Retroreflective shadowgraph of clustered rocket nozzles

The plume produced by a cluster of two large area-ratio thrust optimized parabolic contour nozzles is visualized over a range of nozzle pressure ratios by way of retroreflective shadowgraphy. Both nozzles exhibit free shock separated flow, restricted shock separated flow and an end-effects-regime prior to flowing full. Experimental tests were conducted in the anechoic chamber and high speed flow facility at The University of Texas at Austin. Steady and transient (start-up) operations of the nozzles are studied with the primary focus being the end effects regime. This occurs at a pressure ratio of 37 for these nozzles and is associated with elevated sound levels in the immediate vicinity of the nozzles and vehicle. The shadowgraphy images reveal the formation of turbulent large scale structures, on the order of the nozzle diameter, during the end effects regime. These large scale structures are driven by the intermittent opening of the last trapped annular separation bubble with the ambient which grow rapidly within the first two nozzle diameters. Radon transforms of the spatially resolved shadowgraphy images are then used to characterize the statistical behavior of the acoustic wavefronts that reside within the hydrodynamic periphery of the nozzle flow during fixed operations of the nozzle cluster. Acoustic loads are assessed by synchronizing an eighth-inch microphone with the shadowgraphy images. The findings reveal basic principles to understand the sources of most intense vibro-acoustic loads during the end effects regime.