Effects of liquid turbulent boundary layer spanwise organisation on air lubrication

Abstract

The effect of the streamwise velocity structuring on air lubrication is experimentally assessed in a flat plate turbulent boundary layer. High-speed, planar particle image velocimetry is performed on a wall-parallel plane within the logarithmic region upstream of the air injection, concurrently with high-speed shadowgraphy of the air phase immediately downstream of it. Three different air phase regimes are evaluated (bubbly, transitional, air layer), all deeply embedded in the log region and for a constant liquid freestream velocity. Air bubbles formed downstream of low-speed streaks are found to be larger and to convect much slower than those formed downstream of high-speed events. The spanwise variation of air coverage and its time evolution follow the spanwise structuring and evolution of log-region residing streaks for both the bubbly and transitional regimes, with low-speed streaks promoting air coverage and high-speed ones inhibiting it, a correlation stronger closer to the injector but remaining significant for large downstream distances (x>4δ). High-speed streaks are also shown to be largely responsible for liquid pockets developing close to the injector in the air layer regime, locally breaking its spanwise continuity.