Scientific Article

Presence and properties of acoustic peaks near the nozzle of impinging rocket jets

Univ Lyon, École Centrale de Lyon, INSA Lyon, Université Claude Bernard Lyon I, CNRS, Laboratoire de Mécanique des Fluides et d’Acoustique, UMR 5509, 69134 Écully, France

^* Corresponding author: mathieu.vare@ec-lyon.fr

Received: 25 March 2022
Accepted: 8 August 2022

Abstract

The presence and properties of acoustic peaks near the nozzle of impinging rocket jets have been investigated. Four jets at a Mach number of 3.1 impinging on a plate at a distance L = 15r₀, 20r₀, 25r₀ and 30r₀ from the nozzle, where r₀ is the nozzle radius, have been computed using large-eddy simulations. In all cases, upstream-travelling pressure waves are generated by the jet impingement on the plate, with amplitudes decreasing with the nozzle-to-plate distance. The near-nozzle pressure spectra contain peaks, at frequencies not varying much with this distance. For L ≥ 20r₀, the spectra are dominated by a low-frequency peak, whereas two additional high-frequency peaks emerge for L = 15r₀. The low-frequency peak is associated with the azimuthal mode n_θ = 0, whereas the two other ones are due to strong components for modes n_θ ≥ 1. As for near-nozzle tones for free and impinging jets at lower Mach numbers, the peak frequencies fall close to the frequency bands of the upstream-propagating guided jet waves, showing a link between the peaks and the latter waves. Regarding the peak levels, they do not change significantly with the nozzle-to-plate distance for the low-frequency peak, but they decrease by 1.5 to 18 dB as the distance increases for the other peaks. Finally, for L ≥ 20r₀, the near-nozzle peak frequency is close to that of the strongest shear-layer structures, indicating a connexion between the upstream noise and these structures. For L = 15r₀, a shock-leakage mechanism of a near-plate shock is found to generate the upstream noise.