On the impact of frontal solidity on turbulent boundary layers over irregular bio-inspired surfaces

The impact of bio-inspired topographies on the mean flow statistics is studied via direct numerical simulations of spatially developing turbulent boundary layers with zero pressure gradient and validated against a peer experimental study. Calcareous biofouling topographies similar to those found on the hull of naval vessels are selected as study cases comprising barnacle- and tubeworm-type organisms both at one fixed planar solidity and two different frontal solidities. Topographies are synthesized via an in-house surface synthesis algorithm in order to mimic actual biofouled topographies. Focus is given on the effect of the frontal solidity, which is investigated by performing a parametric study where only the latter is altered while the rest of the main topographical statistics are kept the same. Strong correlation is observed between the frontal solidity and the roughness function, $\Delta U^{+}$, the boundary layer growth rate and the pressure forces exerted on the surfaces. However, that correlation was found to be more dominant in the barnacle-type topographies. All surfaces induce strong near-wall flow separation, while its characteristics are investigated through a histogram-based analysis of the surface forces.