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 (

), 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 scaling of the associated critical

upper Q Subscript italic air Q air $Q_{\textit{air}}$

is proposed, combining the air exit velocity, the liquid velocity close to the air layer and

italic Fr Subscript d Fr d $\textit{Fr}_d$

. For a further increase of

upper Q Subscript italic air Q air $Q_{\textit{air}}$

and a low

upper U Subscript normal infinity U ∞ $U_{\infty }$

, a thicker and smoother air layer is formed with even lower drag; for higher

upper U Subscript normal infinity U ∞ $U_{\infty }$

, marginal differences are observed. The air layer morphology is significantly altered, however, depending on

italic Fr Subscript d Fr d $\textit{Fr}_d$

: for

italic Fr Subscript d Baseline greater than 0.7 Fr d > 0.7 $\textit{Fr}_d\gt 0.7$

, it is unbounded, extending beyond the current test section length, and for subcritical conditions (deep water regime,

italic Fr Subscript d Baseline less than 0.61 Fr d < 0.61 $\textit{Fr}_d\lt 0.61$

) a closure is formed and the air layer transitions to a cavity of a specific length.