Analysis of air layer shape formed by air injection at the bottom of flat plate

Air lubrication drag reduction has been proven to be a workable drag reduction technology in laboratory and for the full ship scales. The bottom air layer shape and coverage are the key factors affecting the drag reduction effect, which cannot be controlled well in actual sea conditions. This study presents the experimental investigation on the air-water mixed flow shape at the lower surface of a flat plate. The air layer shapes are captured by an image system under different air flow rates and velocities. Four distinct air-water mixed flow are classified, referred to as dispersed bubble flow (DBF), mixture flow (MF), air layer flow (ALF) and bifurcation flow (BF). The ALF regime should be considered as the best way for the drag reduction, because the air layer flow is relatively stable and the air layer coverage is large and uniform under flat plate. In addition, the attitude of the plate has a great influence on the air-water mixed flow shape on the lower surface of the flat plate. The tail trim of the plate would suppress the longitudinal diffusion of the air bubbles, and the heel will shift the air-water mixed shape.