We investigate the deformation of a linear viscoelastic compliant coating in a turbulent flow for a wide range of coating parameters. A one-way coupling model is proposed in which the turbulent surface stresses are expressed as a sum of streamwise-travelling waves with amplitudes determined from the stress spectra of the corresponding flow over a rigid wall. The analytically calculated coating deformation is analysed in terms of the root-mean-square (r.m.s.) surface displacement and the corresponding point frequency spectra. The present study systematically investigates the influence of five coating properties namely density, stiffness, thickness, viscoelasticity and compressibility. The surface displacements increase linearly with the fluid/solid density ratio. They are linearly proportional to the coating thickness for thin coatings, while they become independent of the thickness for thick coatings. Very soft coatings show resonant behaviour, but the displacement for stiffer coatings is proportional to the inverse of the shear modulus. The viscoelastic loss angle has only a significant influence when resonances occur in the coating response, while Poisson's ratio has a minor effect for most cases. The modelled surface displacement is qualitatively compared with recent measurements on the deformation of three different coatings in a turbulent boundary-layer flow. The model predicts the order of magnitude of the surface displacement, and it captures the increase of the coating displacement with the Reynolds number and the coating softness. Finally, we propose a scaling that collapses all the experimental data for the r.m.s. of the vertical surface displacement onto a single curve.

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A significant part of the fuel used for transportation results from the drag in turbulent flows. Techniques for turbulent drag reduction yield associated reductions of the fuel consumption and greenhouse gas emissions, which is desirable from both economic and environmental perspectives (cf. chapter 1). This thesis investigates two passive techniques that could be exploited for the reduction of frictional drag in turbulent flows, namely textured and compliant surfaces. Correspondingly, the aim of the thesis is twofold, namely to explore the drag-reducing potential of riblet-textured surfaces, and to characterize the interaction between time-dependent (possibly turbulent) flows and a compliant wall. The work presented in this thesis was performed as part of the European project SEAFRONT, which aimed at the development of environmentally benign antifouling and dragreducing technologies for the maritime sector… @en

The manufacture and preliminary testing of a drag-reducing riblet texture with fouling-control properties is presented. The commercial fouling-release product Intersleek® 1100SR was modified to manufacture riblet-textured coatings with an embossing technology. Hydrodynamic drag measurements in a Taylor–Couette set-up showed that the modified Intersleek® riblets reduced drag by up to 6% compared to a smooth surface. Barnacle settlement assays demonstrated that the riblets did not substantially reduce the ability of Intersleek® 1100SR to prevent fouling by cyprids of Balanus amphitrite. Diatom adhesion tests revealed significantly higher diatom attachment on the riblet surface compared to smooth Intersleek® 1100SR. However, after exposure to flow, the final cell density was similar to the smooth surface. Statically immersed panels in natural seawater showed an increase of biofilm cover due to the riblets. However, the release of semi-natural biofilms grown in a multi-species biofilm culturing reactor was largely unaffected by the presence of a riblet texture.

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A bird-feather-inspired herringbone riblet texture was investigated for turbulent drag reduction. The texture consists of blade riblets in a converging/diverging or herringbone pattern with spanwise wavelength Λf. The aim is to quantify the drag change for this texture as compared to a smooth wall and to study the underlying mechanisms. To that purpose, direct numerical simulations of turbulent flow in a channel with height Lz were performed. The Fukagata-Iwamoto-Kasagi identity for drag decomposition was extended to textured walls and was used to study the drag change mechanisms. For Λf/Lz ≳ O(10), the herringbone texture behaves similarly to a conventional parallel-riblet texture in yaw: the suppression of turbulent advective transport results in a slight drag reduction of 2%. For Λf/Lz ≲ O(1), the drag increases strongly with a maximum of 73%. This is attributed to enhanced mean and turbulent advection, which results from the strong secondary flow that forms over regions of riblet convergence/divergence. Hence, the employment of convergent/divergent riblets in the texture seems to be detrimental to turbulent drag reduction.@en

The use of drag reducing surface textures is a promising passive method to reduce fuel consumption. Probably most wellknown is the utilisation of shark-skin inspired ridges or riblets parallel to the mean flow. They can reduce drag up to 10%. Recently another bio-inspired texture based on bird flight feather riblets has been proposed. It differs from the standard riblets in two ways. First, the riblets are arranged in a converging/diverging or herringbone pattern. Second, the riblet height or groove depth changes gradually. Drag reductions as high as 20% have been claimed [2]. The objective of the present work is to study the drag reducing properties and mechanisms of this texture. To that purpose Direct Numerical Simulations (DNSs) of turbulent plane channel flow have been performed. Structured roughness has been applied to both walls and several geometric parameters have been varied. Marginal drag reductions on the order of 2.5% and significant drag increases well beyond 100% were found. The latter is attributed to a strong secondary flow that mixes momentum through the whole channel. In future optimization studies we might look for conditions at which secondary motions affect the near-wall cycle of turbulence only.@en