Direct total skin-friction measurement of a flat plate in zero-pressure-gradient boundary layers

The total skin friction on a flat plate is directly measured by using a towing tank up to Reynolds number Re_L ≃ 10⁷ (or R_θ ≃ 10⁴). Plates of 3.3 and 4.3 m in length are towed in still water, balancing the vertical weight by small flotation devices, and their drag force is measured by a highly sensitive load cell. We have developed a new technique to correct wave-making resistance, pressure resistance and drag on a turbulence simulator. When the measured total drag is converted into local drag, it is found that the local frictional resistance is about 6% smaller than that given by the Kármán–Schoenherr formula. But it is consistent with the values obtained by the floating element technique, oil film interferometry and asymptotic evaluations.