Laminar boundary layers over small-scale textured surfaces

Abstract: We develop a model for steady, laminar boundary layers over small-scale textured surfaces. Although the texture is small relative to the boundary-layer thickness, it modifies the flow via a slip length. We use matched asymptotic expansions to simplify the problem, dividing the flow into outer, boundary-layer and inner regions. The far-field behaviour of the inner problem yields a slip boundary condition for the boundary layer. We derive an asymptotic solution valid when the slip length is small. For arbitrary slip lengths, we develop a numerical method combining Chebyshev collocation and finite differences. We apply this framework to canonical small-scale textured surfaces, including superhydrophobic surfaces and riblets, and utilise existing analytical slip formulae. We demonstrate how slip can effect the boundary layer's velocity field, wall shear stress and displacement thickness. Our approach enables computationally inexpensive modelling of small-scale textured surfaces in applications ranging from microfluidics to marine transport.