Displacements of the flow stability limit under rotational rate modulation

Possibilities of controlling the stability limit position in the spherical Couette flow are experimentally investigated. The rotational rate of the inner sphere periodically varies with respect to a nonzero average value, and the outer sphere remains motionless. The same as with steady-state rotation, the instability in the form of traveling azimuthal waves is caused by an increase in the average rotational rate. The flow velocity is measured by a laser Doppler anemometer. It is shown that the flow can be both destabilized and stabilized, depending on the modulation amplitude, when the modulation frequency approaches the eigenfrequency of the linear mode.