Abstract
This paper concerns a study of pressure fluctuations beneath hypersonic transitional and turbulent boundary layers and associated acoustic loading on a flat surface. We have employed high-order implicit large eddy simulations in conjunction with the atmospheric (von Kármán) multimode energy spectrum as initial condition, and conducted simulations at Mach 4, 6 and 8 and for different inflow turbulence intensities. The spectral analysis of the pressure fluctuations shows consistent results with the available theoretical, experimental and numerical data for fully turbulent boundary layers. In the transition region the spectrum roll-off diverges from the existing scaling predictions for incompressible, as well as fully-turbulent compressible flows. This study shows that the spectrum in the transition region is governed by different scaling laws. The Mach number has a direct impact on the spectrum for both transitional and fully turbulent flows, especially in the high-frequency region of the spectrum. Increasing the inlet turbulence intensity leads to higher amplitude pressure fluctuations in the mid-to-high-frequency region, faster transition to turbulence, and higher acoustic loading on the solid surface.
Highlights
Pressure fluctuations within supersonic and hypersonic transitional and turbulent boundary layers (TBLs) are a dominant cause of acoustic fatigue that structural elements of an aircraft are exposed to, according to Bull [1]
Contrary to previous studies that focused on single mode or ‘controlled’ transition, this paper focuses on multi-mode perturbations, which comprise a large number of waves imitating the von Karman atmospheric turbulence
The turbulence intensity Tu 1⁄4 3% is comparable to the free-stream turbulence intensity used by Brandt et al [31] who performed numerical simulations that qualitatively reproduced experiments of transition over a flat-plate triggered by grid-generated turbulence
Summary
Pressure fluctuations within supersonic and hypersonic transitional and turbulent boundary layers (TBLs) are a dominant cause of acoustic fatigue that structural elements of an aircraft are exposed to, according to Bull [1]. Based on incompressible data and conventional (noisy flow) hypersonic wind-tunnel measurements an attempt to establish a correlation between transitional and turbulent pressure fluctuations was made [7]. These initial attempts did not accurately predict the transitional pressure fluctuations. To the best of the authors' knowledge studies for the frequency content of pressure fluctuations in attached transitional boundary layers at supersonic and hypersonic speeds have not been carried out. The aim of this work is to present spatial and spectral analysis of pressure fluctuations beneath a transitional hypersonic boundary layer in conjunction with a von Karman atmospheric-like inflow condition. The results are compared with available Direct Numerical Simulations (DNS) and experimental data in the fully turbulent region, and the accuracy of theoretical predictions is investigated
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