Effect of exit geometry of blowing air on friction drag of an underwater plate

Air layer drag reduction (ALDR) is one of the important techniques for reducing the frictional resistance of ships and underwater vehicles. The air injection configuration directly influences air layer coverage on the surface and hence drag reduction. Several shapes of air exit have been investigated in the past, including narrow slots, regular-shaped holes, porous media, and arrays/combinations of them. A new exit shape, i.e. modified Nekomimi, is proposed and investigated numerically for ALDR of an underwater flat plate. The work is inspired by air film cooling technology where it is desirable to have an air layer segregating wall from main gas flow. The Reynolds number examined is 6970–22890, based on local turbulent boundary layer thickness, for the incoming water flow or 700–5550, based on the equivalent exit diameter of blowing air. Results show that the Nekomimi exit produces a larger and more even air layer coverage, as indicated by the air volume fraction on the plate, accounting for a more pronounced drag reduction and net energy saving than a circular exit with the same area or spanwise dimension. Multi-Nekomimi-exit arrangement is also investigated to obtain a larger air-layer coverage on the plate, showing good promise in engineering applications.