Excitation of Cavity Resonance by Air Flow

Abstract

The excitation of cavity resonance by air flow past the aperture has been experimentally investigated. Rectangular apertures of different aspect ratios were located in the ceiling of a wind tunnel. A large wooden box whose volume could be changed was mounted above to provide the cavity. Intensity levels and frequency spectra were observed as a function of air speed. For any one aperture and volume, several regimes of sharp oscillation appeared in turn as the air speed was increased. Each began at an air speed conforming to nearly the same Strouhal number, S = Nd/iU, where N is the natural frequency of one of the modes of resonance (Helmholtz or organ-pipe type of resonance), U is the free stream velocity, d is the width of aperture in direction of flow, and i is an integer taking the values 1 or 2. For each regime the natural frequency of oscillation shifted upward as the air speed was increased because of a decrease in the equivalent mass reactance of the air in the opening. The oscillation died out abruptly with further increase in air speed, reaching a final frequency asymptotically. There appeared to be a strong interaction between fluctuations in the flow past the edges of the aperture and oscillations of the cavity.