Shallow cavity flow tones: transformation from large- to small-scale modes

The generation of flow tones in an axisymmetric cavity is addressed for the case where the inflow is fully turbulent and the acoustic wavelength is much longer than the cavity length. Emphasis is on the nature of flow tones in relation to variations of cavity length. For a sufficiently long cavity, the fully evolved axisymmetric instability yields a large-scale mode. When the cavity length is decreased to progressively smaller values, the mode of oscillation transforms from the large-scale mode to a small-scale mode; it occurs at small values of cavity length, and scales on the momentum thickness of the separating shear layer. At a minimum value of cavity length, distinct oscillations do not occur. The existence of pronounced oscillations, i.e., flow tones, is also related to the dimensionless cavity depth. Oscillations are suppressed when the cavity depth is sufficiently small relative to the momentum thickness of the inflow boundary layer.