Shear Layer Dynamics in a Supersonic Rectangular Multistream Nozzle with an Aft-Deck

Large-eddy simulations are employed to analyze the flowfield generated by a multistream propulsion system, comprised of a core Mach 1.6 single-sided expansion ramp stream separated by a splitter plate from a sonic stream evolving on an aft-deck. The emphasis is on deriving a fundamental understanding of three-dimensional unsteadiness due to the different interacting free and wall-bounded shear layers. Simulation results on several grids compare well to data from a companion experiment, including particle image velocimetry measurements, time-resolved Schlieren images, and deck pressure spectra, thereby confirming simulation fidelity for the primary physics of interest. Even at design conditions, the asymmetry induced by the single-sided expansion and the aft-deck results in a complex three-dimensional shock train, whose interactions with the bounding shear layers yield many of the principal observations, including local separation of boundary layers developing on the expansion ramp and aft-deck, as well as plume unsteadiness. The mean transverse deflection of the plume, whether upward or downward, is shown to be related to the nature of the shock train components at the aft-deck trailing edge. As the plume develops downstream, three-dimensional characteristics become prominent, including corner effects and asymmetric entrainment. Spectral analyses and space-time correlations demonstrate that the signature of a shedding instability initiated at the splitter plate trailing edge permeates the entire flowfield, and influences the development of the shock train, deck pressure, and shear layers.