STREAMWISE-TRAVELINGWAVES OF SPANWISE WALL VELOCITY IN A PIPE FLOW: EXPERIMENTALLY ASSESSING THE TURBULENT DRAG REDUCTION

Abstract. We report the first experimental assessment of the capabilities of an active, open-loop technique for drag reduction in turbulent wall flows that has been recently discovered (Quadrio et al., J. Fluid Mech., 2009). The technique consists in generating waves of spanwise velocity that travel in the streamwise direction at the wall. A proof-of-principle experiment has been devised to assess drag reduction by the traveling waves. Although the available DNS data have been collected for the plane channel flow, in the present reserach we employ a circular pipe, where the sinusoidal spatial modulation of the azimuthal wall velocity is realized by subdividing an axial portion of the pipe into 60 thin slabs, which are driven by 6 independent shafts. Different speeds of nearby slabs provide, although in a discrete setting, the desired spatial variation of transversal wall velocity. The drag is measured through the pressure drop across the active pipe length. Measurements are made particularly difficult by the tiny amount of pressure drop implied by the experimental conditions, that must be selected in order to obtain a large viscous length scale. The results confirm the possibility of achieving large reductions of the friction drag in the turbulent regime. In our initial tests described in this paper, we have measured up to 26