Zero-pressure-gradient boundary layers over high-skewness surfaces are studied via direct numerical simulations (DNS) in computational domains of up to $135\delta$ long with friction Reynolds numbers up to ${\text{Re}}_{\tau }\approx 4500$. Monoculture and multiculture biofouling-type surfaces were utilized at a planar solidity of approximately ${\lambda }_p\approx 40\%$. It is found that both the evolution of the main boundary layer quantities, as well as of the total drag, scale with the mean roughness height, $\overline{h}$, regardless of the roughness element shape and distribution. The same conclusions are made with respect to the roughness function $\mathrm{\Delta }{U}^{+}$. The outer-layer similarity is also investigated under different scalings and verified in both the first- and second-order turbulence statistics. Furthermore, the roughness impact on the flow dynamics in both a quantitative and qualitative manner is also explored by means of quadrant analysis. Finally, an attempt is made toward adapting the Schlichting's boundary layer equations for rough walls through a height-based occupancy ratio, that successfully captures most of the DNS-expected boundary layer evolutions.

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• Roughness
• Three-dimensional turbulence
• Turbulent boundary layers