Electromagnetic control of separation at hydrofoils

Lorentz forces originating from surface-mounted actuators of permanent magnets and electrodes in weakly conducting fluids like seawater can be used to control flow separation at hydrofoils [1]. As induction effects are typically weak, external currents have to be applied to achieve control. An alternating arrangement of electrodes and magnets is considered which creates a mainly streamwise Lorentz force that is exponentionally decaying in wall-normal direction. As spanwise influences are supposed to be small, the problem is treated as two-dimensional in the following. We investigate control by steady forcing at the suction side and by oscillatory forcing near the leading edge of symmetric foils, mostly in the post-stall regime. Similarities to the onventional method of oscillatory blowing for separation control do exist [2]. The numerical results presented cover direct numerical simulations based on a highly accurate spectral element method in the laminar flow regime in order reveal basic control phenomena as well as simulations using turbulence modelling at higher Reynolds numbers which are closer to possible naval application. Although strong-enough steady control is able to suppress separation completely, appropriate time-periodic control turns out to be more effective for finite lift-enhancement when comparing the energetical effort. Optimum control frequencies are found, typical flow structures are analyzed, and "lock-in" phenomena with the natural shedding process are discussed [3]. The scaling behaviour of the energetical effort is compared with recent experimental results [4]. References: 1. T.Weier, G.Gerbeth, G.Mutschke, O.Lielausis, G.Lammers, "Control of Flow Separation using electromagnetic Forces", Flow, Turbulence and Combustion 71 (2003) 5-17. 2. D.Greenblatt, I.J.Wygnanski, "The control of flow separation by periodic excitation", Prog. Aero. Sci. 36 (2000) 487-545. 3. G.Mutschke, G.Gerbeth, T.Albrecht, R.Grundmann, "Separation Control at Hydrofoils