# Transfer of mass and momentum at rough and porous surfaces

## TL;DR

This paper develops a framework to characterize mass and momentum transfer at rough and porous surfaces, introducing measures like transpiration length and resistance coefficients, validated through canonical flow problems.

## Contribution

It introduces a set of effective boundary conditions for non-smooth surfaces, linking slip, transpiration, and pressure jump to measurable surface properties.

## Key findings

- Transpiration velocity arises from slip variations and mass conservation.
- Normal momentum transfer causes a pressure jump characterized by resistance coefficients.
- Effective boundary conditions are validated on canonical flow problems.

## Abstract

The surface texture of materials plays a critical role in wettability, turbulence and transport phenomena. In order to design surfaces for these applications, it is desirable to characterise non-smooth and porous materials by their ability to exchange mass and momentum with flowing fluids. While the underlying physics of the tangential (slip) velocity at a fluid-solid interface is well understood, the importance and treatment of normal (transpiration) velocity and normal stress is unclear. We show that, when slip velocity varies at an interface above the texture, a non-zero transpiration velocity arises from mass conservation. The ability of a given surface texture to accommodate for a normal velocity of this kind is quantified by a transpiration length. We further demonstrate that normal momentum transfer gives rise to a pressure jump. For a porous material, the pressure jump can be characterised by so called resistance coefficients. By solving five Stokes problems, the introduced measures of slip, transpiration and resistance can be determined for any anisotropic non-smooth surface consisting of regularly repeating geometric patterns. The proposed conditions are a subset of effective boundary conditions derived from formal multi-scale expansion. We validate and demonstrate the physical significance of the effective conditions on two canonical problems -- a lid-driven cavity and a turbulent channel flow, both with non-smooth bottom surfaces.