Cloud structuring of microbubbles ascending along a vertical wall

Abstract To elucidate the motion of spatiotemporally developing flow structures driven by densely concentrated microbubbles, we experimentally investigated the distribution and motion of microbubbles rising along a vertical plane wall. We performed optical measurements and an ultrasonic velocity profiling to determine the distribution and motion of microbubbles. We found that the microbubbles form clouds (fluid blobs containing rich microbubbles) with a three-dimensional structure, the spatial patterns of which become more periodic in the streamwise direction than in the spanwise direction. In addition, owing to the coalescence of neighboring microbubble clouds caused by the hydrodynamic interaction between them, the separation length of the buoyant force distribution of microbubbles increases with increasing distance from the bottom of the tank. According to the Reynolds number of the microbubbles cloud presented here, we conclude that the unsteady motion of microbubble clouds can be analogously modeled as an interacting motion of moving bodies in flow.