Distinctions between single and twin impinging jet dynamics

Impinging jets are characterized by an acoustic feedback resonance capable of generating intense tones. This investigation examines changes in single impinging jet (SIJ) dynamics when another jet is added alongside to form a dual impinging jet (DIJ) arrangement of interest in vertical takeoff and landing applications. The emphasis is on the hydrodynamic and acoustic coupling in the region between the jets, which affects aircraft surface loading. Well-resolved large eddy simulations of SIJ and DIJ are employed with under-expanded Mach 1.27 jets; the nozzle exits are placed 4 diameters from the ground plane and, for the DIJ, separated 4.3 diameters from each other to mimic ongoing experiments. Three different SIJ feedback harmonics of the fundamental frequency are identified using two-point space-time correlations. Using spectral proper orthogonal decomposition, these tones are classified as either asymmetric or axisymmetric modes in the SIJ. Each individual jet in the DIJ configuration also exhibits these nominal tones. However, differences are observed on the inboard sides between the jets, where coupling effects engender an azimuthally localized Kelvin-Helmholtz instability and impingement mechanism. The global coupling between the two jets manifests as counter-rotating helical modes, which reinforce the lowest of the three identified SIJ impinging tones.