Response of the near wall turbulence to localized unsteady blowing periodical and dissymetric in time
Résumé
The active control of the near wall turbulence by suboptimal or optimal strategies is plausible, but it requires unfeasibly dense distribution of the sensors and actuators at the wall. One possible way to remedy to this situation is to go through dual control methods. The latter consist of exciting the near wall turbulence locally to render it more predictable and controllable. Such a strategy has been used by Tardu (1998, 2001) who investigated the reaction of the near wall turbulence to a space localized oscillating blowing sinusoidal in time with small severity. He showed that the flow is partly relaminarized during the oscillation cycle until roughly 50 wall units downstream of the slot. This is due to the creation of a positive spanwise vorticity layer that dilutes the negative vorticity existing near the wall. The relaminarized phase is unstable. The diffusion of the induced vorticity layer is constrained into the low buffer layer with limited effect of turbulent mixing, when the frequency of the oscillating blowing is large. Consequently the vorticity concentrates, becomes compact and rolls-up into a coherent spanwise vortex at approximately the beginning of the low buffer layer. It increases the wall shear stress as it is convected downstream, resulting in an important drag penalty compared with steady blowing with the same mean severity.
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