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Giorgio Maria Cavallazzi
Riblets are an already assessed tool for industrial applications to reduce the skin-friction drag by decreasing the mixing of streamwise momentum close to the wall. Their performances are strongly related to the geometry and, in particular, to the dimensions with respect to the flow characteristics. Among all the configurations previously studied, sinusoidal riblets are yet to be fully disclosed because discordant results can be found in literature.
The aim of the work is to develop a Direct Numerical Simulation (DNS) code, written using the CPL language, to model a turbulent incompressible channel flow with sinusoidal riblets on the inner surface. This is done by embedding an analytical correction close to the sharp tips, that allows to decrease the computational requirements to make extensive parametric studies more affordable.
The code is able to predict a Drag Reduction (DR, evaluated as the change in the skin friction coefficient Cf with respect to the one of a reference smooth channel Cf,0 ) with V-shaped straight riblets matching previous experiments with 4.8% drag reduction. Two sets of sinusoidal riblets were considered after the validation: one of them, with a moderate maximum deflection of the wave and a long wavelength, gave 6.4% drag reduction. This result makes the study of sinusoidal riblets worth investigating in the future. Possible improvements in the implementation of the analytical correction in the code are discussed, after an analysis of the DR computed based on other quantities, such as the velocity centreline defect with respect to the smooth channel.