On the interaction of compliant coatings with boundary-layer flows

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Abstract
The interactions of compliant coatings with laminar, transitional and turbulent boundary layers are investigated. A 2 m long flat plate is towed in the range of speeds of 20–140 cm/s in an 18 m water channel using a carriage riding on an oil film. Isotropic and anistropic compliant coatings are used to cover about 20% of the working Plexiglas surface. The compliant material used is a viscoelastic plastisol gel produced by heating a mixture of polyvinyl chloride resin, a plasticizer and a stabilizer, and allowing them to gel. The shear modulus of rigidity of the plastisol was varied by changing the percentage of PVC in the mix. Anisotropy is introduced by placing the plastisol on a rubber surface having longitudinal grooves scaled with the low-speed streaks in the turbulent boundary layer. The most pronounced effect of the surface compliance in a turbulent boundary layer is a hydroelastic instability in the form of a spanwise wave structure on the compliant surface. The compliant-surface deformation was measured using a novel remote optical technique. The onset speed of the hydroelastic instability waves depends on the thickness and the modulus of rigidity of the plastisol. Their wavelength, wave speed and amplitude are found to depend on these plastisol parameters as well as on the towing speed. In a laminar boundary layer with freestream speeds of over twice the corresponding onset velocity for the turbulent case, no similar instability is observed.