Exploration by Shake-the-Box technique of the 3D perturbation induced by a bubble rising in a thin-gap cell

Abstract

From Lagrangian measurements using the Shake-the-Box technique (Schanz et al. in Exp Fluids 57(5):70, 2016) we investigate the velocity field about a single bubble rising in a thin-gap cell and we discuss it in an Eulerian framework of analysis. The gap thickness is equal to 2.85 mm and the bubble-induced perturbation extends over at least 10 cm in the plane of the cell, so that a very flat volume of observation is required. At first validation of the technique is provided during an emptying of the cell. It allows to fix all methodological parameters to ensure accurate measurements and, indirectly, to measure precisely the gap thickness. Then, the velocity field about a confined high-Reynolds number bubble is investigated. This velocity field is discussed in comparison with a previous description obtained by two-dimensional Particle Image Velocimetry (2D2C-PIV) with volume enlightening of the whole cell (Roig et al. in J Fluid Mech 707:444–466, 2012; Filella et al. in J Fluid Mech 778:60–88, 2015). Velocity is first averaged over the gap and compared to the 2D2C-PIV measurement obtained from tomo-reconstructed and projected frontal images. Then a 3D description of the velocity field is presented. It allows to discuss the quality and limitations of the 2D description of the wake with a velocity field integrated across the cell. In particular, the 3D description obtained from STB shows that the flow becomes mainly parallel to the cell plates at a distance of approximately one bubble diameter and that the wake is organised in slices parallel to the plates moving with their own dynamics that may vary their relative in-plane orientations, but all follow the general exponential viscous law of decay.