Electromagnetic excitation of a backward facing step flow


Electromagnetic excitation of a backward facing step flow

Weier, T.; Albrecht, T.; Wittwer, S.; Metzkes, H.; Stiller, J.; Gerbeth, G.

We consider the flow over a backward facing step under time periodic electromagnetic forcing. Velocity fields have been obtained by time resolved Particle Image Velocimetry in an electrolyte channel at a step height Reynolds number of 1875.
Growth rates of the shear layer instability fit reasonably well to the classical values of Michalke (1965). As expected, excitation with the most amplified frequency results in a minimal reattachment length xr. For moderate excitation amplitudes (characterized here by the interaction parameter N, i.e. the ratio of electromagnetic to inertial forces), forcing with frequencies outside the range of amplified disturbances has no influence on the reattachment length. This behavior is essential in explaining our findings for non sinusoidal excitation. In that case and for a fixed excitation frequency, neither the rms nor the peak value of N is able to collapse recirculation length reductions for different wave forms. However, using only the amplitude of the fundamental sine wave of the applied wave form in N rearranges the data in such a way that they follow a single line in fair approximation. The harmonics of all investigated wave forms have frequencies at least three times higher than the fundamental one and are beyond the range of amplification. In contrast, for separation control on an airfoil (Weier and Gerbeth, 2004) and larger interaction parameters, the peak value of N was found to determine control efficacy, regardless of the wave form.
Proper orthogonal decomposition of the flow fields shows dominant shear layer modes in case of the unforced flow. Under forcing, the time coefficients of the most energetic modes seem to be related to the flapping frequency of the reattachment region.

Keywords: backward facing step; Lorentz force; flow control

  • Lecture (Conference)
    9th European Fluid Mechanics Conference (EFMC9), 09.-13.09.2012, Rom, Italien

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Publ.-Id: 17352