Electromagnetic control of separation at hydrofoils

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 of alternating stripes of permanent magnets and externally fed electrodes 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. The results are based on direct numerical simulations in the laminar flow regime in order to reveal basic control phenomena as well as on simulations using turbulence modelling at higher Reynolds numbers which are closer to possible naval application.
By applying a strong enough steady control, separation can always be completely suppressed. The scaling behaviour of the maximum lift gain Delta C_L^{max} in the turbulent regime nicely agrees with experimental results. - Oscillatory forcing always has to compete with the natural shedding process, lock-in behavior may occur. Lift-optimum control for strong amplitudes is found in a frequency band around the natural shedding frequency. In terms of the momentum coefficient describing the control effort, appropriate excitation frequencies in relation to the natural vortex shedding frequency allow for a more effective lift control than steady forcing for small lift gains; for large lift enhancement the energetic effort seems to approach the level of steady control.