Computational Aeroacoustics of Supersonic Jets exhausting twin and aerospike nozzle systems

QC260223

Abstract

This study investigates noise generation in supersonic jets with shock-cell structures in aerospike and twin-nozzle configurations using a computational aeroacoustic framework. Twin-nozzles offer simplified airframe integration compared to circular nozzles. However, supersonic jets exhausting twin nozzle systems might exhibit coupled oscillations resulting in elevated sound pressure levels. Aerospike nozzles offer altitude-adapting capabilities and potential improved thrust vector control. Moreover, their axisymmetric shape makes them ideal candidates for their integration in Rotating Detonation Engines (RDEs). Most research on aerospike nozzles has addressed their design and integration within RDEs, while comparatively less attention has been paid to jet behaviour and the associated acoustic signature. Moreover, the effect of high temperature, swirling, and RDE-like boundary conditions on the noise generation mechanisms still warrants further investigation. Although twin-nozzle systems have been widely studied, twin square nozzles remain comparatively underexplored. Non ideally-expanded jets exhibit a shock-cell structures whose interaction with downstream-convecting vortical structures constitutes the primary source of sound. To investigate the noise generation mechanisms, we conduct Large Eddy Simulations (LES) coupled with an aeroacoustic far-field calculation based on the Ffowcs Williams-Hawkings (FW-H) equation at the aforementioned boundary conditions. The coupling mechanisms of the twin-nozzle jets are examined through an analysis of coherent structures based on Doak’s Momentum Potential Theory (MPT), and the numerical predictions show good agreement with the experimental data. The aerospike nozzle jet exhibits two shock-cell structures with distinct spacings: an annular structure around the aerospike, and a second structure forming downstream. High temperatures result in a shorter jet potential core and higher convection velocities of vortical structures, promoting Mach-wave radiation. Swirling boundary conditions shorten, and may even suppress, the downstream shock-cell structure. The screech generation mechanisms are investigated by deriving novel dispersion relations that describe the guided-jet modes (GJM) capable of closing the resonance loop in swirling circular and annular jets. Good agreement is found between the analytical models and the upstream-propagating GJM in the LES calculations. Elevated temperatures promote coupling between the screech resonance loops, a feature not observed under cold conditions. Finally, LES with RDC-like inlet conditions indicates that the oblique shock dominates the aeroacoustic signature. This study presents the first high-fidelity simulations combined with aeroacoustic characterization of supersonic aerospike nozzle jets. It specifically examines RDE-relevant boundary conditions, namely swirling flow and elevated temperatures. Established jet aeroacoustic models are applied and extended to capture the sound generation mechanisms associated with the annular geometry and unique flow conditions. Future work will investigate noise-reduction strategies, including chevrons and co-flow, to mitigate the oblique shock effects in the idealised RDC configuration.

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