TURBULENT DRAG REDUCTION BY NEAR-WALL SPANWISE MOTION
Dr. Pierre Ricco (King's College London, Division of Engineering)
It is known that wall-bounded turbulent flows present a significant attenuation of turbulent activity and a drag reduction effect when modified by spanwise wall oscillations. A new drag-reducing technique, namely streamwise-traveling waves of spanwise wall velocity, is presented, of which the spanwise oscillating wall represent a special case. The skin-friction attenuation is found to depend on both the frequency and the streamwise wavelength of the forcing and four turbulent regimes are identified. They arise because of the comparative magnitudes of the time scales involved, i.e. an equivalent period of the wall motion and the survival time of the near-wall turbulent coherent structures, and of the velocity scales, namely the wave phase speed and the turbulent convection velocity. The drag reduction correlates uniquely with the thickness of the spanwise boundary layer induced by the waves for specified wall forcing conditions.