The Influence of External Modulation on the Stability of Azimuthal Taylor-Couette Flow: AN Experimental Investigation.

The stability of azimuthal Taylor-Couette flow against the bifurcation to Taylor vortex flow, when the cylinders' angular velocities are sinusoidally modulated, has been experimentally studied. An infrared photodetection system was used to detect the onset of secondary flow. The results are compared to the stability limit in the standard experiment in which the inner cylinder rotates at constant angular velocity and the outer cylinder is stationary. External modulation can have a profound effect and can either stabilize or destabilize the basic flow depending on the specific boundary conditions. The results of two experiments are given. In the first, the inner cylinder's angular velocity was modulated around a non-zero mean and the outer cylinder was stationary. In this case the flow was destabilized except at high frequencies of modulation in which case the stability limit was unaffected by the modulation. These results are in qualitative agreement with the theoretical predictions of Carmi and Tustaniwskyj (1981). In the second experiment the inner cylinder rotated at constant angular velocity and the outer cylinder modulated about zero mean. In this case modulation had very little effect on the stability limit of the basic flow at very high and very low modulation frequencies. At intermediate frequencies, however, the effect was quite pronounced. Stability increased monotonically with the ratio of the amplitude of the outer cylinder angular velocity to the inner cylinder angular velocity. The stabilization was dramatic when this ratio exceeded one to one. These results qualitatively conflict with the results of Tustaniwskyj and Carmi (1980) and Carmi and Tustaniwskyj (1981). An explanation is given of why the experimental results are reasonable. Finally, a scaling has been employed to show that this modulation is dynamically equivalent to a constant counter-rotation of the cylinders, if the angular velocity of the outer cylinder in the constant rotation experiment is equal to the mean of the magnitude of the outer cylinder angular velocity in the modulation experiment.