Wavy riblets for turbulent drag reduction

On the influence of wavy riblets on the slip behaviour of viscous fluids

“…The V -shape riblet, associated with the smallest friction coefficient, corresponds to the highest velocity of the flow; on the opposite, the U -shape and the blade riblets, whose friction coefficients at µ = 1 are 4 to 6 times larger, have a stronger slowing effect on the flow. This significant difference between the V -shape riblet on the one hand, and the Ushape and blade riblets on the other hand, can be put in perspective with the well-known experimental results on straight riblets, that demonstrate the efficiency of thin protrusions in the reduction of the drag for turbulent flows (see [25] and the references therein). In our study, such fast and well-localized variations of the boundary give rise to a high L 1 norm of the derivative ∂ 1 Ψ 1 ; combined with sinusoidal oscillations of the boundary along the streamwise direction, they generate high friction coefficients M µ .…”

“…The corresponding devices are called wavy riblets; they consist in straight riblets that are deflected sinusoidally in the spanwise direction, as a function of the streamwise position. Wavy riblets were studied in [25], both numerically and experimentally, in the case of a turbulent flow; however, the authors were not able to conclude regarding the efficiency of the device.…”

Can large-scale oblique undulations on a solid wall reduce the turbulent drag?

“…5,6,11,14 However, a practical, cost-effective implementation has yet to be achieved, mostly hindered by the small optimal spacing required (about 15 µm in cruise-speed conditions) and stringent tolerances on the sharpness of the crests. More complex variants, such as sinusoidal riblets, were studied by Bannier, 4 Kramer et al, 21 and Peet et al, 27 but despite attempts to optimise the geometry, Bannier 4 showed that conventional (straight) riblets appear to be as effective within the uncertainty margins. a) s.ghebali14@imperial.ac.uk b) s.chernyshenko@imperial.ac.uk c) mike.leschziner@imperial.ac.uk…”

Scaling of turbulent structures in riblet channels up to Re τ ≈ 550

“…To our knowledge, no similar set of simulations can be found in the literature. In fact, all previous DNSs with riblets had been conducted at Re τ ≈ 180, [7][8][9][10][11][12] and even DNSs of other types of roughness at Re τ of order 500 have only begun to appear in recent years. [13][14][15][16] In principle, the modulation with Re τ should be small in the case of riblets, because they appear to interact only with the near-wall structures, 17,18 so the effect of their presence would be restricted to the buffer layer.…”