Air Lubrication on a Flat Plate in a Steady Water Stream

Air injection underneath the surface of objects immersed in water is known to effect drag reduction benefits (Ceccio in Annu Rev Fluid Mech 42:183–203, 2010 [1]). In the present work, we quantify the drag reduced upon air injection underneath the surface of an immersed flat plate through air vents. The flat plate considered in the computational studies is 330 mm long and 800 mm wide, having a thickness of 8 mm, and it is fully immersed in a steady water stream flowing at constant velocity. The four vents, six vents, eight vents geometries have vent diameter of 15 mm, while the circular vent configuration has a 15 mm wide air vent slot. We utilize the Volume of Fluid (VoF) method for the simulation of the two-phase air–water flow underneath the surface of a flat plate after validating our computational methodology. The velocity, pressure, air volume fraction contours together with the velocity vectors are observed at various water stream velocities, and air injection rates are presented for several air vent geometries. There is a reduction in the drag coefficient, cd with the increase in the air injection rate, m* at all Reynolds numbers, Re for all the four vents, six vents, eight vents and circular vent geometries and the magnitude of the drag coefficient for all the vent configurations is observed not to vary much in their magnitudes. From the computed values of the drag coefficient, we arrive at a power-law equation for the drag coefficient, cd in terms of the non-dimensional operating parameters, m* and Re based on general linear least squares method.