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CFD and CAA Analysis of Single Stream Isothermal Jets with Noise Suppression Devices

Since the 50’s, with the appearance of the turbojet engines, the jet noise is being exhaustively studied since it is one of the most important source of aircraft noise. Many attempts have been made to reduce the jet noise, including higher by-pass turbofan engines. Chevron nozzles also have been used by the industry to try to reduce the jet noise with a low performance and weight penalty. This work shows a computation procedure to assess how this noise suppression devices impact on both fluid dynamics and acoustics of single isothermal jets. Towards this goal, different chevron nozzles, with 6, 8 and 12 lobes have been analyzed. The calculation procedure is based on a Reynolds Average Navier-Stokes calculation, followed by a stochastic noise generation and radiation method, resulting in a relatively fast noise calculation procedure. The calculation procedure has predicted the expected fluid dynamic and acoustic behavior for chevron nozzles, e.g., shortening the potential core length, high frequency noise increase and low frequency noise attenuation. The parametric study of the number of lobes has shown that this parameter impacts the mixing region. The presence of chevrons tends to split the noise source mechanism into two distinct regions, one close to the nozzle exit and the other farther downstream, near x/D j =10. The results show that this relatively simple and quick analysis reproduced important parameters in designing new nozzles and can be used as a way to better understand the influence of chevrons.

Revisiting the role of anisotropy for jet noise modelling

The present work describes a study about the influence of anisotropy in a RANS based prediction scheme for noise prediction of subsonic single jet flows. This task was motivated by an ongoing research to improve the spectral predictions obtained by using Lighthill Acoustic Analogy. The numerical approach employed is originally based on the MGBK technique. Nevertheless, an improved energy-transfer time scale (TET) has been utilized in order to enhance the spectrum shape prediction. In the present model, the anisotropy effect on the noise prediction is indirectly coupled with the aerodynamic flow calculation. However, in order to unveil the actual contribution of the anisotropy, the coupling with the aerodynamic calculation has been performed through the computation of turbulence quantities as the stress tensor components by using a Reynolds Stress Turbulence Model (RSTM). The numerical results permit to identify the effect of the anisotropy parameters and its effect in the final spectrum shape prediction. This approach has indicated some limitations and some potential ways to improve the current methodology towards jet noise prediction of subsonic single flows.