Drag reduction regimes in air lubrication

Air lubrication regimes were studied using simultaneous drag force measurements and multi-plane imaging to characterise the regimes and identify the governing mechanisms of drag reduction. Bubbly, transitional and air layer regimes are identified over a large range of free-stream velocities ( upper U Subscript normal infinity U ∞ $U_{\infty }$ ), air flow rates ( upper Q Subscript italic air Q air $Q_{\textit{air}}$ ) and Froude-depth numbers ( italic Fr Subscript d Fr d $\textit{Fr}_d$ ). For the lowest upper U Subscript normal infinity U ∞ $U_{\infty }$ , drag reduction lags significantly behind the non-wetted area coverage in all cases and no simple correlation exists. Within the bubbly regime, a drag increase is found for low upper U Subscript normal infinity U ∞ $U_{\infty }$ with large, slow-moving bubbles forming a single layer over the plate height. For higher velocities, bubbles become smaller and disperse vertically, while the drag starts decreasing. For higher upper Q Subscript italic air Q air $Q_{\textit{air}}$ , irrespective of upper U Subscript normal infinity U ∞ $U_{\infty }$ , air patches start to form (transitional regime) and drag monotonically decreases, with the onset of the air layer regime at 60 % drag reduction. A new sc