Aero-Acoustic Coupling Inside Large Deep Cavities at Low-Subsonic Speeds

Aero-acoustic coupling inside a deep cavity is present in many industrial processes. This investigation focuses on the pressure amplitude response, within two deep cavities characterized by their length over depth ratios (⁠$L/H=0.2$ and 0.41), as a function of freestream velocities of a $2×2m2$ wind tunnel. Convection velocity of instabilities was measured along the shear layer, using velocity cross-correlations. Experiments have shown that in deep cavity for low Mach numbers, oscillations of discrete frequencies can be produced. These oscillations appear when the freestream velocity becomes higher than a minimum value. Oscillations start at $L/θ0=10$ and 21 for $L/H=0.2$ and 0.41, respectively. The highest sound pressure level inside a deep cavity is localized at the cavity floor. A quite different behavior of the convection velocity was observed between oscillating and nonoscillating shear-layer modes. The hydrodynamic mode of the cavity shear layer is well predicted by the Rossiter model (1964, “Wind Tunnel Experiments on the Flow Over Rectangular Cavities at Subsonic and Transonic Speeds,” Aeronautical Research Council Reports and Memo No. 3438) when measured convection velocity is used and the empirical time delay is neglected. For $L/H=0.2$⁠, only the first Rossiter mode is present. For $L/H=0.41$⁠, both the first and the second modes are detected with the second mode being the strongest.