Session: 36-03 Fan and Engine Noise

Paper Number: 101947

101947 - Computational Aeroacoustics for a Cold, Non-Ideally Expanded Aerospike Nozzle 

In supersonic aerospace applications, aerospike nozzles have been subject of growing interest. These devices lead to enhanced thrust performance compared to conventional nozzles due to continuous altitude adaption and improved thrust vector control. However, supersonic non-ideal jets are known to generate high levels of noise. The aeroacoustic behaviour of circular and rectangular nozzles has been largely discussed whereas data on the aeroacoustic behaviour of aerospike nozzles is scarce. For further industrial development, the identification of the noise generation mechanisms in such configurations is necessary.

This study sheds light on the main noise components of a cold aerospike nozzle. Implicit Large Eddy Simulations (ILES) allow us to simulate the flow of the cold aerospike at a Nozzle Pressure Ratio (NPR) = 3. For far-field acoustic computation, the Ffowcs- Williams Hawkings (FWH) equation is applied. A mesh sensitivity study is first performed to assess the reliability of the method. Then, the configuration is analyzed in terms of near-field instantaneous and time-averaged flow characteristics. The annular shock structure on the aerospike’s bluff body displays three main shock-cells of length L/D = 0.29 each. Downstream of the bluff body, a second expansion process takes place and leads to the emergence of a circular shock-cell structure with a shock-cell length of L/D = 0.60. It is of crucial interest to characterise the features of the shock-cell structures. The interaction between the vortical flow structures in the shear layers and the shocks generate Broadband Shock-Associated Noise (BBSAN). In order to enhance understanding of the noise generation mechanism for this configuration, several analysis are performed. Two-point cross-correlations of data acquired in monitoring points located along axial lines in the shear layers are used for quantifying the upstream propagating waves associated to a strong tonal component at a Strouhal number St = 1.25. This strong tonal component is known as screech. It is generated by a feedback mechanism between the coherent fluid flow structures propagating downstream in the jet shear layer and the upstream propagating acoustic waves generated at the same frequency by vortex-shock interactions, waves that are interacting with the nozzle lip and excite shear layer instabilities at the frequency of screech. The Overall Sound Pressure Levels (OASPL) in the near-field at upstream angles also display a strong tonal component related to the screech noise at the same Strouhal number. This component is also visible at upstream angles in the far-field spectra computed by means of the FWH equation. The far-field spectra show mixing noise as well as Broadband Shock-Associated Noise, related to the interaction between the convected vortices in the shear layers and the shock-cell structure. The observed noise generation mechanisms are similar to those in circular jets, although the characteristic shock-cell length differs due to the annular configuration.

Presenting Author: Thomas Golliard KTH Royal Institute of Technology

Presenting Author Biography: Thomas Golliard is a PhD student at KTH Royal Institute of Technology in Stockholm. He holds a M.Sc. in Aerospace Engineering from the Technical University of Munich (TUM). His research focuses on computational aeroacoustics and large eddy simulations of supersonic jets.

Authors:

Thomas Golliard KTH Royal Institute of Technology
Mihai Mihaescu KTH Royal Institute of Technology