Control of Flow Separation from a Hydrofoil Using Lorentz Forces

Electromagnetic body (Lorentz) forces originating from a system of permanent magnets and electrodes are used to control the flow of a weakly electrically conducting NaCl solution around a NACA-0017-like hydrofoil. The orientation of the Lorentz force is always parallel to the surface. The effect of a suction-side, streamwise Lorentz force is quantified by means of force balance measurements for chord-length Reynolds numbers of 3 to 8x 10^4. Depending on the angle of attack, two different effects are observed. (1) At small angles of incidence, a moderate increase in lift due to additional circulation is observed. Simultaneously, a decrease in the drag of the hydrofoil is caused by the added momentum. (2) At higher angles of attack, where the unforced hydrofoil would normally stall, a more pronounced lift increase - by 90% at Re=3 10^4 - and a corresponding drag reduction are observed due to separation prevention. The physical tendencies found in the experiments are confirmed by direct numerical simulations at lower Reynolds numbers.