Air lubrication to reduce hull skin friction is an idea that originated, more than a century ago. There are few implementations of this concept, and there are even fewer systematic investigations at high Reynolds number. To address this, a series of experiments was performed at the W. B. Morgan Large Cavitation Channel that examined the drag reducing effects of partial cavity drag reduction (PCDR) and air layer drag reduction (ALDR) at high Reynolds number. Three different experimental setups were employed. A flush injection slot was used to generate a continuous air layer along the bottom of a 12 m long flat plate. In addition, air layers were generated via flush injection behind a 12.7 mm step. These two configurations were used to investigate ALDR with a series of experiments that included a slow increase in the volumetric flux of air injected at free-stream speeds to 15.3 ms~(-1). These results indicated that.there is a distinct threshold above which the air injected through the slot coalesces into a continuous layer of air. In addition, once ALDR was established: friction drag reduction in excess of 80% was observed over the entire smooth model for speeds to 15.3 ms~(-1); the critical volumetric flux of air required to achieve ALDR was observed to be approximately proportional to the square of the free-stream speed; slightly higher injection rates were required for ALDR as the surface tension was decreased; stable air layers were formed at free-stream speeds to 12.5 ms~(-1) with the surface fully (sand-grain) roughened. (though approximately 50% greater volumetric air flux was required); and its stability was sensitive to the inflow conditions. Sensitivity to the inflow conditions can be mitigated by employing a small faired step that helps create a fixed separation line. Furthermore, to investigate the possibility of reducing pumping costs, a deep plenum shape was implemented on the lower surface of the model. The goal of this plenum was to capture a recirculating air flow to reduce the pumping requirements (i.e. PCDR). The plenum was designed using linear gravity wave theory. It was formed by an abrupt step near the model's leading edge and a long sloping re-attachment region downstream. Air was injected from the aft face of the step to create a cavity approximately 17.8 cm deep. Friction loads, air flow, and cavity pressure were measured for a range of air fluxes and speeds near the cavity design speed of 3.4 ms'1. Cavities were shown to be stable with respect to large changes in air flux and slow perturbations in tunnel speed and pressure. Stable cavities were produced that reduced the skin drag by more than 95% over the extent of the cavity, including the cavity closure.
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