Electromagnetic Control of Separation at Hydrofoils

Abstract

Lorentz forces originating from surface-mounted actuators of permanent magnets and electrodes in weakly conducting fluids like seawater provide a convenient tool for separation control at hydrofoils. A well-known actuator design is considered which creates a mainly streamwise Lorentz force that is exponentially decaying in wall-normal direction. Separation control by steady forcing at the suction side and by oscillatory forcing near the leading edge of a symmetric foil is investigated numerically, mostly in the post-stall regime. Direct numerical simulations are performed in the laminar flow regime in order to reveal basic control phenomena as well as simulations using turbulence modelling at higher Reynolds numbers which are closer to possible naval applications. Strong enough steady control s capable of suppressing separation completely, and the scaling behaviour of the maximum lift gain $\Delta C_L^{max}$ in the turbulent regime is found to agree nicely with experimental results. As oscillatory forcing always has to compete with natural shedding, lock-in behavior is detected, and lift-optimum control at strong control is found in a frequency band around the natural shedding frequency. In terms of the momentum coefficient describing the control effort, appropriate excitation allows for a more effective lift control than steady forcing for small lift gains; for large lift enhancement the effort seems to approach the level of steady control.