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THE ELECTROMAGNETICALLY FORCED FLOW OVER A BACKWARD-FACING STEP

Tom Weier
Magneto-Hydrodynamics Division, Forschungszentrum Dresden-Rossendorf Bautzner Landstr. 128, D-01328 Dresden, Germany

Thomas Albrecht
Department of Magnetohydrodynamics Helmholtz-Zentrum Dresden-Rossendorf Bautzner Landstr. 400, 01328 Dresden, Germany; Department of Mechanical and Aerospace Engineering Monash University VIC 3800, Australia

Gunter Gerbeth
MHD Department, Institute of Safety Research, Forschungszentrum Dresden-Rossendorf P.O. Box 51 01 19, D-01314 Dresden, Germany

Sebastian Wittwer
Institut fur Stromungsmechanik Technische Universitat Dresden 01062 Dresden, Germany

Hans Metzkes
Institut fur Stromungsmechanik Technische Universitat Dresden 01062 Dresden, Germany

Jorg Stiller
Technische Universitat Dresden Institute of Fluid Mechanics 01062 Dresden, Germany

Аннотация

The flow over a backward-facing step is a prototype of a separating and reattaching shear flow and has therefore received a considerable amount of interest. We focus here on the excitation of the separated shear layer since it is often understood as the basic mechanism in active flow control. Forcing frequencies and amplitudes are obviously major parameters of influence, but different signal forms can have a profound impact as well, albeit the physical mechanism behind the latter is still not fully understood in the case of airfoils.
Electromagnetic body forces offer a simple and direct way to provide excitation by different wave forms. Particle image velocimetry measurements have been performed in a free surface electrolyte channel. We will discuss spatial amplification rates in the unforced shear layer, which show a fair agreement with results obtained by others in free shear layers. Compared to the natural flow, forcing near the optimal excitation frequency St = 0.012 leads to a much earlier vortex roll-up and, consequently, the reattachment length is reduced. For the first subharmonic of the optimal excitation frequency, vortex roll up starts later but produces larger vortices. Excitation with the relatively high frequency of St = 0.03 has only a very small effect on the flow. Keeping the excitation frequency at St = 0.012 and increasing the forcing amplitude leads to earlier vortex roll up, larger vortices, and shorter reattachment lengths.
Using different wave forms to excite the shear layer at the most amplified frequency, the reattachment length is determined by the amplitude of the fundamental only.