Turbulent Taylor–Couette flow over riblets: drag reduction and the effect of bulk fluid rotation

A Taylor–Couette facility was used to measure the drag reduction of a riblet surface on the inner cylinder. The drag on the surfaces of the inner and outer cylinders is determined from the measured torque when the cylinders are in exact counter-rotation. The three velocity components in the instantaneous flow field were obtained by tomographic PIV and indicate that the friction coefficients are strongly influenced by the flow regimes and structures. The riblet surface changes the friction at the inner-cylinder wall, which generates an average bulk fluid rotation. A simple model is proposed to distinguish drag changes due to the rotation effect and the riblet effect, as a function of the measured drag change w/ ₖ, ₀ and shear Reynolds number Re ₒ. An uncorrected maximum drag reduction of 5. 3 % was found at Re ₒ=4. 7 10⁴ that corresponds to riblet spacing Reynolds number s^+=14. For these conditions, the model predicts an azimuthal bulk velocity shift of 1. 4 %, which is confirmed by PIV measurements. This shift indicates a drag change due to a rotation effect of −1. 9 %, resulting in a net maximum drag reduction of 3. 4 %. The results correspond well with earlier reported results and demonstrate that the Taylor–Couette facility is a suitable and accurate measurement tool to characterize the drag performance of surfaces.