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Boundary layer control by means of wall parallel Lorentz forces

Weier, T.; Fey, U.; Gerbeth, G.; Mutschke, G.; Lielausis, O.; Platacis, E.
Lorentz forces can be used to control the near wall flow of low conducting liquids like sea-water. To achieve force densities strong enough to influence the flow, both magnetic and electric fields have to be applied to the fluid. Here the wall parallel Lorentz force in streamwise direction was generated by means of periodically arranged electrodes and permanent magnet strips of alternating polarity and magnetization direction, respectively. In a first approximation the resulting force is independent of the spanwise coordinate and decays exponentially with the wall distance. Such a force distribution acts as a source of momentum to the flow. It gives the possibility to compensate for viscous losses and to counteract adverse pressure gradients, thereby stabilizing the boundary layer and preventing separation.
Experimental results on the control of a flat plate boundary layer in a sodium chloride solution up to Re=9·105 will be given. LDA measurements show the effect of the Lorentz force on the boundary layer profile. At moderate force strength the mean velocity profiles are characterized by momentum thicknesses smaller than in the unforced case, at high enough Hartmann numbers a wall jet develops. Additionally, a turbulent, but practically non growing boundary layer has been observed for a special combination of Reynolds and Hartmann number. The fluctuating streamwise velocity component is slightly damped due to the accelerating action of the Lorentz force. Force balance measurements on the controlled flat plate show a reduction of the total drag by up to 80% compared to the uncontrolled case. The sole reason for this dramatic drag reduction is the momentum gain caused by the Lorentz force. From the velocity profiles one can conclude on a skin friction increase in the forced cases. However, the momentum gain overcomes the skin friction increase.
The effect of a suction-side, streamwise Lorentz force on a NACA-0017-like hydrofoil is quantified by means of force balance measurements for chord-length Reynolds numbers of 3 to 8·104. 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 (90% at Re=3·104) and a corresponding drag reduction are observed due to separation prevention.
Direct numerical simulations at low Reynolds numbers confirm the physical tendencies of the experiments.

  • Lecture (Conference)
    4th International Conference "MHD at dawn of 3rd Millennium", Presqu'ile de Giens, France, September 18-22, 2000

Publ.-Id: 3215