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Electromagnetic control of flow separation by stationary and time periodic forces
Weier, T.; Mutschke, G.; Gerbeth, G.;
Electromagnetic flow control is a viable option, if the fluid in question is electrically conducting. In the case of liquid metals or
semiconductor melts, this control method is being used on an industrial scale. The application of Lorentz forces to fluids of low
electrical conductivity like seawater and other electrolytes is less common, although first efforts date back to the 1950s [1]. The present talk gives a short overview on separation control with steady wall parallel Lorentz forces [2], while the main part reports on recent activities on time periodic forces. The latter topic reveals certain parallels to current aerodynamic research on separation control by oscillatory blowing. The main motivation to investigate time periodic Lorentz forces is that periodic input of momentum has proven to be around two orders of magnitude more efficient than steady blowing [3].

An electromagnetic body force F results from the vector product of current density j and magnetic induction B. For fluids of low
electrical conductivity (10 S/m) as seawater and other electrolytes, the currents due to the induced voltages are generally very low for magnetic fields of moderate strength (1 T). Consequently, the Lorentz force due to these currents is negligible. In order to obtain current densities large enough for flow control purposes it is therefore necessary to additionally apply an electric field.

There are several distinct features, that make the Lorentz force an attractive actuator: momentum is directly generated in the fluid
without associated mass flux; its frequency response is practically unlimited; virtually any excitation wave form might be realized by
feeding the electrodes with appropriate current. On the other hand, efficiency of momentum generation by weak magnetic and strong electric fields is generally small, since Joule losses dominate in electrically low conducting fluids.

For the experiments described in the following, an arrangement of flush mounted electrodes and permanent magnets producing a streamwise wall parallel Lorentz force has been used. The Lorentz force decays exponentially with the wall distance. The characteristic parameter to describe the Lorentz force action on the suction side flow is the effective momentum coefficient cµ. It relates the rms­value of the momentum injected by the Lorentz force to the freestream dynamic pressure multiplied by the foil area. Since the mechanism of periodic forcing is supposed to be connected to shear layer excitation (see [3]), the actuator should be placed near the separation line. A sketch of the NACA 0015 equipped with permanent magnets and electrodes is given in Fig. 1. Flow visualizations on an inclined flat plate (Fig. 2) show, that the otherwise fully separated flow can be reattached in an averaged sense by a time periodic Lorentz force acting near the leading edge. A characteristic feature of the flow are the vortices moving along the plate. F+ is the excitation frequency nondimensionalized by chord length and freestream velocity. Direct force measurements on the stalled NACA 0015 reveal that the maximum lift gain occurs around F+ 1, the forcing effect decays relatively
rapidly for higher frequencies. A specific lift increase at a constant angle of attack and relativ to the value for the separated flow can be obtained by oscillatory forcing with fractions of the momentum input necessary for steady Lorentz forces. In contrast, an equal increase of the maximum lift gain requires a similar expenditure of momentum for both control methods.

Direct numerical simulations (DNS) at lower Reynolds numbers have been performed by using an well-established spectral element method [4]. The results obtained confirm the experimental findings in a qualitative sense. Figure 3 shows a snapshot of velocity contours (gray) and streamtraces of the controlled flow around an hydrofoil of 30o angle of attack at Re = 500, cµ = ...
Keywords: Lorentzf force, separation control, periodic addition of momentum
  • Lecture (Conference)
    13th European Drag Reduction Meeting,17.06.2004, Aussois, France

Publ.-Id: 6185 - Permalink