Drift, shape, and intrinsic destabilization of pulses of traveling-wave convection

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Abstract
I describe experiments on ‘‘pulses’’ of convective traveling waves in an annular geometry, using ethanol-water mixtures with moderate negative separation ratio. In a sufficiently uniform cell at constant Rayleigh number, pulses drift in the direction of propagation of the underlying traveling waves, with no long-term change in velocity. The drift velocity increases with increasing distance ε above onset. In contrast with previous observations of motionless pulses, this result is qualitatively consistent with theories based on a subcritical Ginzburg-Landau equation. The pulse shape is also described in detail. The pulses exhibit a noticeable asymmetry, which decreases as ε is increased. I also describe experiments at high ε in which pulse destruction by convective amplification of traveling-wave fluctuations is suppressed by the existence of multiple pulses. In this case, destabilization takes place by an intrinsic mechanism: Above a certain threshold, the pulse simply expands into the rest of the system, accompanied by a large decrease in wave speed.