SOME ASPECTS OF THE LIFT FORCE ON A SPHERICAL BUBBLE

Several situations in which a spherical bubble experiences a lift forc e are examined, especially through the use of computational results ob tained by solving the full Navier-Stokes equations. The lift force is computed over a wide range of Reynolds number for the case of pure she ar flaw, pure strain and solid body rotation. Using these results, the validity of asymptotic solutions derived in the limit of low Reynolds number or inviscid flow is discussed. A general expression of the lif t force valid for low to moderate shears is proposed. It is shown that for such shears, the lift force in a complex flow can be predicted by superposing the results obtained in pure strain flow and solid body r otation flow. Finally the interaction force experienced by two bubbles rising side-by-side is studied. The computational results reveal that , at variance with the predictions of potential theory, the sign of th is force changes when the Reynolds number or the separation distance b etween the bubbles decreases below a critical value. All these results are discussed in terms of vorticity. The respective role played by th e vorticity generated at the bubble surface and by the one that is eve ntually present in the unperturbed flow is emphasized.