ABSTRAKT

Experimental results on the reaction of the near wall turbulence and drag to a localized time periodical blowing are reported. The injection velocity is periodical and dissymmetric in time, with a rapid acceleration phase followed by a slow deceleration one. The flow is relaminarized during 70% of the oscillation period mainly during the deceleration phase. The latter maintains stable the vorticity layer induced by the blowing and prevents its roll-up contrarily to a sinusoidal-time periodical blowing. Thus, a time mean drag reduction of 55 % is obtained at 100 wall units downstream of the blowing slot and this is 37% larger than the drag reduction obtained by a steady blowing with the same time mean severity parameter. Direct numerical simulations results confirm these observations, in particular the near wall activity is considerably reduced in the case of local blowing periodical and dissymmetric in time.