The impact of surface texture on the hydrodynamics of aquatic locomotion

Abstract

Fish have evolved over millions of years, achieving a level of swimming efficiency that far surpasses current human-engineered capabilities. A prevalent feature in many species is skin scales. Scholars posit that this surface texture may offer hydrodynamic advantages, although the mechanisms remain poorly understood. This thesis investigates the effect of roughness on aquatic locomotion. Utilising a combination of numerical simulations and innovative analysis methods, we investigate the intricate interplay between low-parameter, egg-carton-type surface roughness and self-propelled swimming. Our findings indicate that both the roughness wavelength and kinematics are instrumental in shaping the flow structures and power requirements. We observe that scaling the roughness wavelength with the boundary-layer thickness significantly enhances flow mixing, without a proportional increase in forces. Furthermore, the boundary layer of a swimming foil displays fundamentally unstable spatial structures, directly attributable to the swimming motion. This suggests that actuation at the wall could be an effective mechanism to stabilise the boundary layer. Additionally, we probe a parameter space concerning potential stabilising roughness shapes. We demonstrate that surface roughness can be used to improve swimming performance and that variable roughness functions could potentially outperform fixed small-scale roughness, given proper tuning. Our results reveal that the interaction between roughness and kinematics is complex and nonlinear, suggesting that roughness studies on static shapes do not transfer directly to unsteady swimmers.

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Identifiers

ORCID for Jonah Massey: orcid.org/0000-0002-2893-955X

ORCID for Gabriel Weymouth: orcid.org/0000-0001-5080-5016

ORCID for Bharathram Ganapathisubramani: orcid.org/0000-0001-9817-0486

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