Experiments on Bubble Generation by a Hydrofoil Moving beneath the Water Surface for Reducing Ship Drag

摘要

We have invented two types of hydrofoil bubble generator for drag reduction of ship that can reduce the energy for air bubble generation on the ship hull. Their fundamental process of air entrainment and subsequent bubble generation by the hydrofoil facility are described by a simple fluid dynamic model. We experimentally determined the critical velocity of the bubble generation and the relationship between air volume flow rate and the hydrofoil velocity. The magnitude of the negative pressure produced above the hydrofoil, which is a driving force of the air entrainment, depends on the shape of the hydrofoil, gap ratio (normalized depth of the hydrofoil), Reynolds number, Froude number, and angle of attack. Recent applications of the drag-reduction technology with air bubbles to a ship save about 10%-15% of the total energy consumption of the ship. The device works as a self-priming pump when the draft of the ship is shallow (< ~5 m) as predicted by the theory. For ships of deeper draft, the device needs the assistance of an air compressor. Because the magnitude of the negative pressure above the hydrofoil depends on the flow condition around the hydrofoil, proper operation of compressors is necessary. We also show experimental results on optimization of hydrofoils to enhance the hydrofoil performance of air entrainment and air bubble generation.