Seawater Flow Transition and Separation Control

First efforts on using Lorentz forces to control the flow of electrically low conducting fluids date back to the 1950s [1]. The
present talk reports on recent activities to delay transition in flat plate boundary layers and to control separation at hydrofoils by means of streamwise, wall parallel Lorentz forces. The actuator consists of a strip­like arrangement of permanent magnets and electrodes as proposed by [2].

Direct Numerical Simulations (DNS) by means of a spectral element method investigate the stability of a flat plate boundary layer under the influence of the Lorentz force. The evolution of Tollmien­Schlichting waves and proper 3­D disturbances is reported. A
stabilizing effect of the mean profile modification due to the Lorentz forces applied was found, verifying the assumption of [2].

Separation control at hydrofoils is undertaken by means of steady [3] as well as time periodic Lorentz forces. The latter topic reveals certain parallels to current aerodynamic research on separation control by oscillatory blowing. The main motivation for applying time periodic momentum input is that efficiency can be around two orders of magnitude higher than in case of steady blowing [4]. Since the mechanism of periodic forcing is supposed to be connected to shear layer excitation (see [4]), the Lorentz force actuator is placed in the nose region of the NACA 0015 investigated here. Direct force measurements on the stalled NACA 0015 reveal that the maximum lift gain occurs around a nondimensionalized excitation frequency of F+ 1, decaying rapidly for larger frequencies. For stalled hydrofoils, the same lift increase can be obtained by oscillatory forcing with only fractions of the momentum input necessary for steady forcing. In contrast, an equal increase of the maximum lift gain requires a similar expenditure of momentum for both control methods. DNS at lower Reynolds numbers confirm the experimental findings in a qualitative sense.

[1] Resler, E.L., Sears, W.: The prospects for
magneto­aerodynamics. J. Aero. Sci. 25, 235­245, 1958.

[2] Gailitis, A., Lielausis, O.: On a possibility to reduce the
hydrodynamic resistance of a plate in an electrolyte. Applied
Magnetohydrodynamics. Reports of the Physics Institute, 12, 143­146,
1961 (in Russian).

[3] Weier, T., Gerbeth, G., Mutschke, G., Lielausis, O., Lammers, G.:

Control of Flow Separation Using Electromagnetic Forces. Flow,
Turbulence and Combustion, 71, 5-17, 2003.

[4] Greenblatt, D., Wygnanski, I.J.: The control of flow separation by
periodic excitation. Prog. Aero. Sci., 36, 487­545, 2000.