Frictional Drag Reduction: Review and Numerical Simulation of Microbubble Drag Reduction in a Channel Flow

Abstract

The reduction of ship’s resistance is one of the most effective way to reduce emissions, operating costs and to improve EEDI. It is reported that, for slow moving vessels, the frictional drag accounts for as much as 80% of the total drag, thus there is a strong demand for the reduction in the frictional drag. The use of air as a lubricant, known as Micro Bubble Drag Reduction, to reduce that frictional drag is an active research topic. The main focus of authors is to present the current scenario of research carried out worldwide along with numerical simulation of air injection in a rectangular channel. Latest developments in this field suggests that, there is a potential reduction of 80% & 30% reduction in frictional drag in case of flat plates and ships respectively. Review suggests that, MBDR depends on Gas or Air Diffusion which depends on, Bubble size distributions and coalescence and surface tension of liquid, which in turn depends on salinity of water, void fraction, location of injection points, depth of water in which bubbles are injected. Authors are of opinion that, Microbubbles affect the performance of Propeller, which in turn decides net savings in power considering power required to inject Microbubbles. Moreover, 3D numerical investigations into frictional drag reduction by microbubbles were carried out in Star CCM+ on a channel for different flow velocities, different void fraction and for different cross sections of flow at the injection point. This study is the first of its kind in which, variation of coefficient of friction both in longitudinal as well as spanwise direction were studied along with actual localised variation of void fraction at these points. From the study, it is concluded that, since it is a channel flow and as the flow is restricted in confined region, effect of air injection is limited to smaller area in spanwise direction as bubbles were not escaping in spanwise direction.