Drag Reduction via Large-scale Opposition Flow Control in a High-Reynolds-number Turbulent Boundary Layer

Studies over the past decade have shown that large-scale coherent structures (superstructures) in the logarithmic region of a high-Reynolds-number turbulent boundary layer play an important role in the dynamics of the near-wall turbulence. Consequently, it is hypothesized that the skin-friction drag can potentially be reduced using a ƒow control scheme that targets the large-scale structures. Here we employ a largescale blowing-only opposition ƒow control scheme in an attempt to reduce the turbulence intensity of the large-scales and to deduce how that a‚ects the mean wall-shear stress. Œe study was conducted in a turbulent boundary layer at a friction Reynolds number of Reτ ≈ 14 790. Wall-normal jet ƒows were used as actuators to manipulate the structures aliated with positive wall-shear stress signatures. A reduction of 8% is observed in the pre-multiplied power spectral density of the large-scale streamwise velocity ƒuctuations in the logarithmic region. Additionally, the small-scale streamwise ƒuctuating energy is enhanced by 5% in the logarithmic region suggesting that the jets have introduced additional small scale turbulence into the logarithmic region. A maximum skin-friction drag reduction of 3.5% was achieved at 1.6δ downstream of the actuators, evaluated via hot-€lm shear-stress sensors. Œe relative amplitudes of both the high- and low-speed events have been reduced, which is believed to be a result of the abated counterrotating roll modes that are aliated with these events.