Effects of Freestream Turbulence on Cavity Tone and Sound Source

Hiroshi Yokoyama,Hiroshi Odawara,Akiyoshi Iida

doi:10.1155/2016/7347106

Hiroshi Yokoyama, Hiroshi Odawara + Show 1 more

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https://doi.org/10.1155/2016/7347106

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Abstract

To clarify the effects of freestream turbulence on cavity tones, flow and acoustic fields were directly predicted for cavity flows with various intensities of freestream turbulence. The freestream Mach number was 0.09 and the Reynolds number based on the cavity length was 4.0 × 104. The depth-to-length ratio of the cavity,D/L, was 0.5 and 2.5, where the acoustic resonance of a depth-mode occurs forD/L= 2.5. The incoming boundary layer was laminar. The results for the intensity of freestream turbulence of Tu = 2.3% revealed that the reduced level of cavity tones in a cavity flow with acoustic resonance(D/L=2.5)was greater than that without acoustic resonance(D/L=0.5). To clarify the reason for this, the sound source based on Lighthill’s acoustic analogy was computed, and the contributions of the intensity and spanwise coherence of the sound source to the reduction of the cavity tone were estimated. As a result, the effects of the reduction of spanwise coherence on the cavity tone were greater in the cavity flow with acoustic resonance than in that without resonance, while the effects of the intensity were comparable for both flows.

Highlights

Flows over open cavities such as the sunroofs of automobiles and landing gear configurations of airplanes often generate self-sustained oscillations and intense tonal sound
Direct simulations of flow and acoustic field were carried out for cavity flows with and without acoustic resonance (D/L = 0.5 and 2.5) under various freestream turbulent conditions to clarify the effects of freestream turbulence on the cavity tone
The freestream Mach number was M = 0.09 and the Reynolds number based on the cavity length was ReL = 4.0 × 104

Summary

Introduction

Flows over open cavities such as the sunroofs of automobiles and landing gear configurations of airplanes often generate self-sustained oscillations and intense tonal sound. Sarohia [2] measured velocity fluctuations around cavities for laminar flows over shallow cavities (depth-to-length ratio of cavities D/L < 0.35 and freestream Mach number M < 0.07). He found that instability in the shear layer of the cavities was amplified by self-sustained oscillations. A cavity flow without acoustic resonance does not generate selfsustained oscillations or intense tonal sound in cavity flows at low Mach numbers in the turbulent boundary layer. Direct numerical simulations were performed to investigate the three-dimensional stability of cavity flows [4]

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