Strong Resonance in Forced Oscillatory Convection

Abstract

We report results of an experimental study of the flow regimes observed when time-periodic convection is forced with an external frequency. The physical system is a layer of mercury heated from below. When the temperature difference across the layer exceeds a critical value ΔT _ο, the oscillatory instability of convection rolls leads to a time-periodic regime at a “natural” frequency f _o. The layer of mercury is rotated with a periodic angular velocity about a vertical axis at frequency f _e, and amplitude A _e. The different flow regimes which result from the competition between the two oscillators are studied in the two-parameter space (A _e, f _e). In addition to the well documented locked (f _e/f _o = n/p, n and p integers) and quasiperiodic (f _e/f _o irrational) states, we observe a new flow regime where the natural oscillator is completely inhibited by the external one. We study the transition between the different regimes and show that most of the experimental results can be interpreted within normal form analysis.