M Mahak

Far-field noise prediction for jets using large-eddy simulation and Ffowcs Williams–Hawkings method

Large-eddy simulations are performed for hot and cold jets with and without a flight stream. The acoustic and flight stream Mach numbers are 0.875 and 0.3, respectively. The temperature ratios for the hot and cold jets are 2.7 and 1.0, respectively. The mean flow field results are in good agreement with the measurements. The Ffowcs Williams–Hawkings equation is used to predict far-field noise. Several axisymmetric Ffowcs Williams–Hawkings surfaces at increasing radial distances are used. They show that the surfaces closer to the jet can be affected by the hydrodynamic pressure. It is important to close the Ffowcs Williams–Hawkings surfaces at the ends to account for all the acoustic signals emanating from the jet. In this work, 11 end discs are used at the downstream end of the Ffowcs Williams–Hawkings surface. It is found that the simple averaging processes to cancel hydrodynamic sound at the end discs are insufficient for slowly decaying jets. In such cases, a partially closed disc can be a better choice. To remove hydrodynamic signals, a filtering scheme for the end discs is suggested. For slowly decaying jets, this gives better results.

Far Field Noise Prediction for Subsonic Hot and Cold Jets Using Large-Eddy Simulation

Large eddy simulations are performed for hot and cold single stream jets with an acoustic Mach number of (Ma = Vj/a∞ = 0.875). The temperature ratio (Tj/T∞) for the hot jet is 2.7 and for the cold jet it is 1.0. Grids with 34 million points are used. The simulation results for the flow field are in encouraging agreement with the mean velocity and Reynolds stress measurements. The Ffowcs Williams-Hawkings (FW-H) equation is used to predict the far-field noise. In this study four cylindrical FW-H surfaces around the jet at various radial distances from the centreline are used. The FW-H surfaces are closed at the downstream end with multiple endplates. These endplates are at x = 25.0D – 30.0D with Δ = 0.5D apart. It is shown that surfaces close to jet get affected with pseudo sound. To avoid pseudo sound, surfaces must be placed in the irrotational region. To account for all the acoustic signals end plates are necessary. However, a simple averaging process to cancel pseudo sound at the end plates is not sufficient.Copyright

Cost-effective hybrid RANS-LES type method for jet turbulence and noise prediction

Jets at higher Reynolds numbers have a high concentration of energy in small scales in the nozzle vicinity. This is challenging for large-eddy simulation, potentially placing severe demands on grid density. To circumvent this, we propose a novel procedure based on well-known Reynolds number (Re) independent of jets. We reduce the jet Re while rescaling the boundary layer properties to maintain incoming boundary layer thickness consistent with high Re jet. The simulations are carried out using hybrid large-eddy simulation type of approach which is incorporated by using near-wall turbulence model with modified properties. No subgrid scale model is used in these simulations. Hence, they effectively become numerical large-eddy simulation with Reynolds-averaged Navier–Stokes covering the full boundary layer region. The noise post-processing is carried out using the Ffowcs-Williams-Hawking approach. The simulations are made for Mach numbers (M) of 0.75 and 0.875 (cold and hot). The results for the overall sound pressure level are observed to be within 2–3% of the measurements, and directivity of sound is also captured accurately for both the cases. Hence, the low Re simulations can be more beneficial in saving time and cost while providing reasonably accurate results.

Cost-Effective Hybrid RANS-NLES Method for Jet Turbulence and Noise Prediction

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