Bubble-induced agitation and microstructure in uniform bubbly flows at small to moderate particle Reynolds numbers

The bubble-induced agitation has been quantified in dilute and homogeneous bubbly flows. To characterize the microstructure, experimental techniques have been developed that provide the average perturbed liquid velocity and the pair density. For particulate Reynolds numbers Re-p about unity and contaminated bubbles, the liquid agitation is shown to increase as alpha /Re-p where alpha is the local void fraction. Although in partial agreement with the scaling proposed by Koch (Phys. Fluids, 1993), the pair-density distribution does not exhibit any deficit. On the opposite, experiments at Re-p=O(10) and clean bubbles reveal a strong deficit in the rear of test bubbles as well as a moderate accumulation both in a horizontal plane and all along the deficit zone. The extent of the deficit slowly decreases with the void fraction, roughly as alpha (-0.3 +/-0.1). It is shown that the agitation originates from this near wake region, the structure of which strongly differs from the one due to a single inclusion both in terms of its lateral extent and of the perturbed velocity decay rate. These features are related to the extra momentum sources induced by the microstructure surrounding the test bubble. Accordingly, the resulting velocity variance, that has been found independent of the vessel dimension and of the base flow velocity, is significantly higher than Koch's predictions. Finally, a comparison is achieved with available data and models, and an explanation is suggested for the evolution of the bubble-induced agitation at moderate Re-p. (C) 2001 American Institute of Physics.