Influence of superhydrophobic surfaces on the laminar-to-turbulent transition in a channel flow. *

Tailoring bio-mimetic rough surfaces researchers are accessing new approaches reducing drag in wall bounded shear flows. Among them Underwater SuperHydrophobic Surfaces (U-SHS) have proven to be capable of dramatically reduce skin friction of an overlying liquid turbulent flow, providing a stable, lubricating layer of gas bubbles trapped within the surface's nano-sculptures. As long as a specific set of geometrical and thermodynamical conditions are ensured, wetting transition is avoided and the no-slip boundary condition at the wall is relaxed; this so called 'Lotus effect' is typically achieved when the length scale of U-SHS roughnesses is several order of magnitudes smaller than the overlying flow, bringing out both experimental and numerical challenges.

In this framework we want to study, by means of numerical simulations, the influence of U-SHS in a closed channel, following the complete evolution from laminar, to transitional and fully developed turbulent flow.

We report the results of transition over U-SHS taking into account the dynamics of each microscopic liquid-gas free-surface by means of a fully coupled fluid-structure solver and show that U-SHS can triple transition time to turbulence.