The electromagnetically forced flow over a backward-facing step

Tom Weier, Thomas Albrecht, Gunter Gerbeth, Sebastian Wittwer, Hans Metzkes, Jorg Stiller

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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 Steθ = 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 Steθ = 0.03 has only a very small effect on the flow. Keeping the excitation frequency at Steθ = 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.
Original languageEnglish
Title of host publicationProceedings of the 7th International Symposium on Turbulence and Shear Flow Phenomena (TSFP-7)
Subtitle of host publication28 July to 31 July 2011, Ottawa Convention Centre, Ottawa, Canada
PublisherBegell House
Number of pages6
Publication statusPublished - 2011
Externally publishedYes
EventTurbulence and Shear Flow Phenomena 2011 - Ottawa Convention Centre, Ottawa, Canada
Duration: 28 Jul 201131 Jul 2011
Conference number: 7th


ConferenceTurbulence and Shear Flow Phenomena 2011
Abbreviated titleTSFP-7

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