Dynamics of nearly spherical bubbles in a turbulent channel upflow

The dynamics of bubbles in upflow, in a vertical channel, is examined using direct numerical simulations (DNS), where both the flow and the bubbles are fully resolved. Two cases are simulated. In one case all the bubbles are of the same size and sufficiently small so they remain nearly spherical. In the second case, some of the small bubbles are coalesced into one large bubble. In both cases lift forces drive small bubbles to the wall, removing bubbles from the channel interior until the two-phase mixture is in hydrostatic equilibrium, and forming a bubble-rich wall layer. The same evolution has been seen in earlier DNS of bubbly upflows, but here the friction Reynolds number is higher (Re+ 250). In addition to showing that the overall structure persists at higher Reynolds numbers, we show that the bubbles in the wall layer form clusters. The mechanism responsible for the clustering is explained and how bubbles move into and out of the wall layer is examined. The dynamics of the bubbles in the channel core is also compared with results obtained in fully periodic domains and found to be similar. The presence of the large bubble disrupts the wall layer slightly, but does not change the overall picture much, for the parameters examined here.