Near-wall turbulence-bubbles interactions in a channel flow at Reτ = 400: A DNS investigation

This paper presents DNS results of a turbulent water flow in a channel at Reτ = 400 laden with 0.25 mm diameter air bubbles clustered near the wall (maximum void fraction of α = 8% at y+ ∼ 20). The bubbles were resolved using the Level Set approach built within the CFD/CMFD code TransAT. The fluids properties (air and water) were kept real, including density, viscosity, and surface tension coefficient. The aim of this work is to understand the effects of the bubbles on near-wall turbulence, paving the way towards convective wall-boiling flow studies. The interactions between the gas bubbles and the water stream were studied through an in-depth analysis of the turbulence statistics. The near-wall flow is overall affected by the bubbles, which act like roughness elements during the early phase, prior to their departure from the wall. The average profiles are clearly altered by the bubbles dynamics near the wall, which somewhat contrasts with the findings from earlier similar studies (Lu and Tryggvason, 2013). The shape of the bubbles measured as the apparent to initial diameter ratio is found to change by a factor of two at least, in particular at the later raising-stages when the bubbles burst out from the boundary layer. The clustering of the bubbles seems to be primarily localized in the zone populated by high-speed streaks, and independent from their size. More importantly, the bubbly flow seems to differ from the single-phase flow in terms of turbulent stress distribution and energy exchange, in that all the stress components seem to be increased in the region very close to the wall, by up to 40%. The decay in the energy spectra near the wall was found to be significantly slower for the bubbly flow than for a single-phase flow, which confirms that the bubbles increase the energy at smaller scales. The coherent structures in the boundary layer are broken by the bubbles, which d disrupts the formation of long structures, reducing the streamwise integral length-scale.