We develop a new methodology to examine the conditional and unconditional vertical velocity induced by high-Reynolds-number bubbles rising in a uniform flow, at low to moderate void fraction α (up to 15%). These statistics provide a local description of the perturbation of the liquid velocity around a test bubble in the swarm. In particular, the attenuation of the length of the wakes with increasing void fraction is measured for a large range of void fraction. The strong attenuation of the wakes is related to wake intermingling mechanisms. The methodology also enables a definition of the interstitial liquid flow. The velocity of the fluid averaged over all the interstitial volume far away from the bubbles is introduced. It is a useful concept, in particular to define the relative velocity, or for drift models. Our experimental results allow a discussion of the predictions of irrotational drift models. For low void fraction (α≤2%), potential flow models provide practical estimates of the interstitial velocity field. At higher void fractions, the effect of vorticity is important. A simple phenomenological model is proposed to include the effect of the flow generated by the bubble wakes.
In this experimental investigation we analyze the nature of the primary instability of a bubbly mixing layer. We consider upward flows that develop in a vertical channel of finite dimensions, with bubbles injected on one side of the mixing layer at the inlet. The induced buoyancy effect generates longitudinal accelerations, which are at the origin of self-excited large-scale oscillations under certain conditions. We provide experimental evidence that these oscillations are global modes, and we develop a simple model able to predict the conditions of appearance of these global modes. The model includes as essential mechanisms the buoyancy and a transverse mass transfer due to liquid miscibility in the mixing layer.
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