Control of deep cavity tones using a spanwise cylinder at low-subsonic speeds

Abstract

Paper presented at the 6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 30 June - 2 July, 2008.

Deep cavity configuration at subsonic velocity could be found in many industrial processes, ranging from windows and sunroofs in automobiles and over space between two consecutive train vehicles. These cavities may induce aeroacoustic couplings between the cavity shear layer oscillations and the acoustic modes of the installation. This aero-acoustic coupling can leads to serious damages of vehicles due to resonance of high pressure fluctuation level around the cavity. The study of deep cavity at low velocity presents a great practical interest to suppress acoustic noise. In spite of numerous studies devoted to the cavity and its control, very few of them relate to the deep cavity configuration at low velocity. The focus of the present study is to apply a passive control to the case of the deep cavity flow at relatively low velocities. A Detailed Experimental study of flow over a deep cavity was conducted towards understanding the attenuation of tones using a spanwise cylinder. The cavity length-to-depth aspect ratio is L/H = 0.2. Single hot-wire measurements characterized the incident turbulent boundary. A “no control” cavity was compared with a similar configuration using a cylinder on the leading edge of the cavity. Parametric changes of the spanwise cylinder such as the distance from the wall are studied. Maximum control across the range of studied velocities occurs for a particular position of the spanwise cylinder. Reductions in sound pressure levels (SPL) of up to 36 dB were obtained. Moreover, a shaped cylinder was also studied and shows that the attenuation of tones is not due to high frequency pulsing as suggested in literature, but to an increase of the cavity shear layer thickness due to change in the mean axial velocity profiles.