Unstable flow past cavities is grouped into fluid-dynamic, fluid-resonant, and fluid-elastic categories. Fluid-dynamic oscillations are attributable to instability of the cavity shear layer and are enhanced through a feedback mechanism. Fluid-resonant oscillations are governed by resonance conditions associated with compressibility or freesurface wave phenomena. Fluid-elastic oscillations are primarily controlled by the elastic displacements of a solid boundary. Experimental information and theoretical models for the frequencies and amplitudes of each of these categories of oscillations as well as a combination of them are discussed in detail. Effects of arrangements of cavities in groups and variations in cavity shape (rectangular, circular, triangular, whistle-type, Helmholtz-type) are reviewed. Means of attenuation of oscillations via geometrical modifications and mass addition are summarized. Finally, the effects of cavity oscillations on the time-mean cavity pressure, drag, and shear-layer characteristics are discussed.

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The fluctuating lift and drag acting on a long, rigidly supported circular cylinder placed symmetrically in a narrow rectangular duct were investigated for various blockage percentages over a wide range of Reynolds numbers around the critical value. The data obtained permit a full assessment of the effect of confinement on the mean-drag coefficient, the root-mean-square values of both the drag and the lift fluctuations, the Strouhal number of the dominant vortex shedding, and the Reynolds number marking transition from laminar to turbulent flow separation. Besides experimental information on a subject on which little is known so far, the paper provides a basis for the deduction of better correction procedures concerning the effects of blockage.