Smart coatings for drag reduction in yachts

In some sailing conditions, the friction drag of a yacht hull can account for more than half of the total resistance. If the surface of the hull was not rigid but flexible as, for instance, the skin of a dolphin, it would be possible to decrease the friction drag considerably. Compliant walls can decrease the friction drag either by delaying the laminar to turbulent transition, or by interacting with a post-transition boundary layer. In the present work the second of these two mechanisms is explored. We study both actively and passively controlled surfaces with direct numerical simulations. We consider a canonical channel flow, where the boundary layer in the half channel represents a thin section of the hull’s boundary layer. The friction Reynolds number of the boundary layer is Re_tau~180, and we show how the results can be scaled to higher Re_tau. We model the coating as an array of independent tiles, each attached to the hull by a spring and a damper, and free to move only in the streamwise or the spanwise direction. A drag reduction of 25% and 3% is achieved with an active and a passive control, respectively, of the proposed surface.