Drag reduction at low and high Reynolds numbers.

Abstract

Reduction of energy wasted by friction drag is studied at Low and High Reynolds Numbers. Replacing a high viscosity liquid with low viscosity air or vapor in the boundary layer adjacent to a wall significantly reduces the friction drag. Two different drag reduction problems are studied: open clutch plates (low Re) and fast Navy ships (high Re). For the open clutch geometry, the relative motion of the friction and separator plates causes viscous shear stresses in the transmission fluid passing through the 100 microns gap. This results in a drag torque on both the disks that wastes energy and decreases fuel economy. Air presence next to a plate reduces friction drag due to the low viscosity. Air is not injected by any external means but introduced in the domain from the surrounding casing. CFD analysis and experiments show that surface modification of the clutch plates helps aeration incipience at low rotation rates thus reducing the drag and increasing the overall automobile efficiency by 3--4%. A two-phase lubrication model is presented that predicts aeration in clutches given the surface conditions and other clutch parameters. This model combines the contact line condition with aeration prediction techniques. For fast Navy ships, injecting air or partial cavitation of water behind an appendage such as a wedge or backward facing step on the ship hull results in significant drag reduction. Air injection should provide economic benefits compared to the drag it reduces to be successful. At high Re, slight unsteadiness or break-up of this air film will lead to large unsteady loads. Linear stability analysis of air-water films is conducted to obtain suitable operating regimes for Froude number, viscosity and density ratios.