Mechanism and performance of a hydrofoil bubble generator utilized for bubbly drag reduction ships

Following the success of bubbly drag reduction for marine vessels by hydrofoil bubble generators (Kumagai et al., 2015), we conducted two kinds of experiments to specify and improve its working principle. Firstly, PIV measurement was carried out to elucidate the flow structure around a hydrofoil running beneath a free surface. Froude number dependence of the anti-stall characteristics and cancellation of vertical pressure gradient above the hydrofoil were confirmed, which entrain the atmospheric air into water flow. Secondly, we mounted the hydrofoil to a model ship that runs in a 100-m-towing facility and found three different two-phase flow patterns in realizing bubble generation. Volume flow rate of generated bubbles increased with the ship speed to the power of greater than two, thus wall-occupation fraction of bubbles also increases with the ship speed. Furthermore, hydrofoil drag during bubble generation was measured and used to estimate net power-saving, having proved advantage of the hydrofoil bubble generator for long ships at high speed conditions.