Effect of thin and rough layers of porous materials on drag

Coating is one of the key technologies used for reducing drag in marine applications. Newly developed paints containing porous hydrogel polymers reduce drag. The present work investigates the effects of the thickness and roughness of such porous coatings by using direct numerical simulations of turbulent channel flow. Smooth permeable layers with different thicknesses have been simulated. It was found that smooth thin layers of thickness equal to or less than 20% of the fluid region cannot affect drag significantly, and the resultant changes in drag were below 1%. Next, to study the effect of roughness on the flow, two types of rectilinear rough surfaces were simulated in both permeable and impermeable states: (i) isotropic roughness with equal lengths in the streamwise and spanwise directions and random height, and (ii) anisotropic roughness with streamwise length equal to ten times the spanwise length and random height. In the impermeable state, both roughness patterns increased drag, with 33% and 3% increases with isotropic roughness and with anisotropic roughness, respectively. In the permeable state, drag increased further by 41.9% and by 9% with isotropic and anisotropic permeable roughness, respectively.