Kelvin-Helmholtz Instability Modulation via Shock-Boundary Layer Interaction in Supersonic Plug Nozzles

We investigate the instability mechanisms responsible for sound generation in a series of well-validated large eddy simulations (LES) of compressible jets. The databases include a Mach 0.9 jet issuing from a contoured nozzle, and two Mach 1.5 jets from external plug nozzles with different plug geometries. We apply Doak's momentum potential theory (MPT) to decompose the momentum density field into its hydrodynamic and acoustic components, followed by spectral proper orthogonal decomposition (SPOD) to extract correlated hydrodynamic–acoustic mode pairs as a function of frequency. For the Mach 0.9 jet, we recover classical Kelvin–Helmholtz (K-H) wavepackets, that exhibit consistent Gaussian spatial growth and decay envelope at each frequency. In contrast, the plug nozzle jets show significant deviation from this behavior. While the K-H structures are still present, they are strongly modulated by shock–boundary layer interactions (SBLI) on the plug surface. A critical downstream location is identified beyond which the K-H wavepacket structure becomes modulated by flow structures originating on the plug face. This modulation results in interference between the two hydrodynamic families, causing distinct signatures on the radiated acoustic field. Unlike conventional jets, this interaction mechanism is unique to the SBLI dynamics of plug nozzles and offers a plausible explanation for observed variations in their noise characteristics.