Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

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.

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_eθ=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_eθ=0.03 has only a very small effect on the flow. Keeping the excitation frequency at St_eθ=0.012 and increasing the forcing amplitude leads to earlier vortex roll up, larger vortices, and shorter reattachment lengths.