Computation of turbulent channel flow with variable spacing riblets

It is known that longitudinal ribs manufactured in a flat surface act to reduce (reduces) turbulent skin-friction drag, thus providing a moderate drag reduction of 4 to 8%. In this paper, nonconventional variable blades are used instead of uniform grooves. Numerical simulation of a turbulent flow over blade-covered surface is performed for three cases: uniform, increasing and decreasing spacing blades. All riblets have rectangular cross-section as well as the same wetted area. Results show that drag reduction is strongly depending on the spacing variation, showing a benefit over uniform riblets for regressive spacing, and an opposite trend for the progressive one. Different nature of the flow over the three cases of blades' surfaces is revealed by looking at transverse planes cross flow motion, mean streamwise velocity and Reynolds stresses. Results show cross flow turbulence intensity reduced for regressive riblets compared to progressive and uniform ones.