On the layer-to-bubbly flow regime transition in gas lubrication systems

Gas lubrication is a promising strategy for reducing energy losses in industrial and transport applications, including pipeline flows and marine drag reduction. A central challenge is to predict the transition from a continuous gas layer regime (LR) to a dispersed bubbly regime, which strongly influences drag-reduction performance. Despite extensive experimental and numerical efforts, the mechanisms governing this transition remain unclear, and it is still debated whether it requires fully three-dimensional turbulence or can instead be explained by more fundamental and lower-dimensional interfacial dynamics phenomena. Here, we address this question by using two-dimensional volume-of-fluid simulations of the Navier–Stokes equations in a canonical gas-lubricated flow configuration. We show that both the bubbly and continuous-LRs observed in three-dimensional systems also emerge in this reduced setting, with trends consistent with three-dimensional observations. This indicates that intrinsically three-dimensional turbulent mechanisms are not necessary to capture the regime transition, which instead reflects more elementary interfacial and shear-driven processes. Guided by this observation, we interpret the transition as arising from a competition between liquid-driven shear and advection, which act to disrupt the gas layer, and cross-flow gas injection, which sustains it. This balance is controlled by the liquid and gas velocities, Ul and Ug, and their associated destabilizing (tdest) and restoring (trest) characteristic time scales. From this framework, we derive a linear scaling relation between Ug and Ul at the transition, in quantitative agreement with the numerical results.