Vortex induced breakup of free surfaces in microgravity environments

Abstract

Organized vorticity with circulation Γand length scale ι can move through a liquid otherwise at rest and approach a free surface. The dominant parameter for such interactions is the Froude number, Fr = Γ/( gl 3) 0.5. Earthbound experiments, which are limited to Fr<10, have shown that the ensuing collisions result in substantial deformations of the free surface. The same experiments, carried out under microgravity conditions, can reach values 1000 times larger, suggesting that such waves will be dramatically enhanced, and that surface breaking is likely. Computational studies show that the effect of surface tension σ, expressed through the Weber number We=VρΓ/σ, cannot provide sufficient attenuation of the vortex-induced waves to avoid wave breaking even when Fr is only O(10). This poses distinct problems for the processing of liquids in space manufacturing scenarios. Surface waves may be controlled, or at least suppressed, by means of a contaminant layer of an immiscible liquid thinly dispersed over the free surface. The resulting surface elasticity, which does not depend on gravity, may provide a mechanism for controlling surface waves in a microgravity environment. The present paper explores these aspects of the effects of the microgravity environment on space materials processing and suggests a self-contained experiment which can be carried out in a drop tower or in space.