Swirling Flow Effects on the Aeroacoustic Signature of an Aerospike Nozzle
Authors: Golliard Thomas , Mihaescu Mihai 
Published: August 2024

Abstract
Supersonic nozzles are not always operated at design conditions. This leads to the formation of shock-cell structures and potentially higher Sound Pressure Levels as compared with the design conditions. Additionally, the total pressure, temperature and velocity distributions at the nozzle inlet plane are characterized by strong inhomogeneities, conditions dictated by the operating regime of the turbine or combustion chamber. In particular, a swirling flow motion can be induced by these components. Using flow control to induce a swirling motion to the jet can also act as a noise suppression technology. While homogeneous inflow conditions are well documented for a large range of supersonic nozzles, data on the aeroacoustic signature of supersonic swirling jets is scarce.
Implicit Large Eddy Simulations are deployed to simulate the swirling flow of a non-ideally expanded three-dimensional, axisymmetric aerospike nozzle at a Nozzle Pressure Ratio (NPR) = 3 and Temperature Ratio (TR) = 1. Three swirl numbers are considered. Near-field acoustic analyses are completed by far-field acoustic computations based on the Ffowcs Williams-Hawkings (FWH) equation. The swirling flow cases are compared with the baseline case without swirling. The lowest swirl number considered (𝒮 = 0.10) leads to a shortening of the potential core of the jet and a reduced shock cell count. At higher swirl numbers, the circular shock-cell structure is eliminated. Moreover, the shock cell length in the annular part of the jet is increased compared to the baseline case. Because of the additional swirling component, the flow reattaches further downstream on the aerospike bluff body. Two-point space-time cross-correlations of pressure data acquired in the annular shear layer indicate an enhancement of the azimuthal modes, leading to helical acoustic wave propagation in upstream direction. The corresponding Strouhal number increases with increasing swirl number. Furthermore, two-point space-time cross-correlations of pressure data acquired in the circular jet shear layer downstream of the aerospike bluff body show that swirling suppresses screech tones. Power spectral density (PSD) of the radial velocity at monitoring points at the location of the separation bubble and shocks in the vicinity of the nozzle trailing edge allows to identify the oscillation modes of the annular shock-cell structure. The lengthening of the separation bubble leads to a reduced Strouhal number of the radial oscillation mode. Moreover, the passage of vortical structures through the shock cells leads to Broadband-Shock Associated Noise (BB-SAN) whose central frequency depends on the shock cell length and on the convection velocity of vortical structures in the shear layer. The latter decreases with increasing swirl number.
The far-field spectra display mixing noise at low Strouhal numbers for the baseline case and the lowest swirl number, as well as BBSAN. High Sound Pressure Levels (SPL) are detected in agreement with the BBSAN central frequencies. Moreover, high SPL are obtained at the radial oscillation frequencies of the annular shock-cell structure and at the Strouhal number of the azimuthal modes. The global SPL decreases while the nozzle thrust remains at 99 % of the thrust of the baseline case for the low swirl numbers.