Numerical study of bubble induced mixing in stratified fluids

Abstract

Direct numerical simulations of up to 146 bubbles rising in unbounded stratified fluids using finite-volume/front-tracking method are performed. Both the bubble dynamics and destratification effects caused by the bubble motion are analyzed. The importance of bubble deformability and bubble Reynolds numbers on the induced background mixing are studied by varying the Eo¨tvo¨s number in the range 1.55 to 4.95 and Reynolds number in the range 25 to 200. Highly deformable, high Reynolds number bubbles undergo path instabilities and give rise to higher levels of mixing. Liquid and bubble velocity fluctuations and pseudo-turbulence caused by the bubble motion are examined and are seen to play an important role in mixing statistics. An increase in turbulent kinetic energy (TKE) levels with void fraction is noted. TKE levels are seen to decrease slightly as the stratification strength is increased, indicating increasing stability and resistance to destratification. The dependence of mixing parameters on the void-fraction of bubbles and stratification strength of the liquid is also presented. An increase in buoyancy flux across pycnoclines is observed as void fraction increases. A similar increase in vertical mass flux is observed for a decrease in stratification strength. The bubble dispersion and wake flow patterns are seen to strongly influence mixing properties.