Free shear layer instability due to probes in rotating source—sink flows

Abstract

Experiments with a rotating source-sink annulus have shown that hot-wire probe supports can give rise to low-wavenumber, azimuthally propagating disturbances. These have been observed by a number of workers, and interpreted as Ekman boundary-layer instabilities and inertial eigenmodes of the annulus. The waves are associated with the wake downstream from the probe support; and a simplified model of the wake instability in cylindrical coordinates is presented. This model has a number of features in common with the observed disturbances. Extensive data have been obtained on wave frequencies and magnitudes, wavenumbers and phase relationships, and on wake structure and onset of the disturbances. The results of hot-wire anemometry experiments in an annulus are discussed in light of the present findings. It is concluded that the wave motions interpreted as type-II (class-A) Ekman instabilities and inertial eigenmodes by Tatro & Mollo-Christensen (1967) were probe-associated disturbances, while the boundary-layer waves observed by Caldwell & Van Atta (1970) were type-II disturbances, similar to those observed by Faller & Kaylor (1966a) using dye techniques.