# Comparison between super-hydrophobic, liquid infused and rough surfaces: a DNS study

## TL;DR

This study uses DNS to compare the effects of super-hydrophobic, liquid infused, and rough textured surfaces on fluid flow and drag, revealing turbulence structure is mainly influenced by total shear rather than surface type.

## Contribution

It provides a detailed DNS-based comparison of different textured surfaces and their impact on drag and turbulence, considering various fluid interface conditions.

## Key findings

- Drag correlates with wall-normal velocity fluctuations across surfaces.
- Turbulence structure is dominated by total shear, not surface type.
- Interface effects are minimal when fluids have the same viscosity.

## Abstract

Direct Numerical Simulations of two superposed fluids in a channel with a textured surface on the lower wall have been carried out. A parametric study varying the viscosity ratio between the two fluids has been performed to mimic both {\bf idealised} super hydrophobic and liquid infused surfaces and assess its effect on the frictional, form and total drag for three different textured geometries: longitudinal square bars, transversal square bars and staggered cubes. The interface between the two fluids is assumed to be slippery in the streamwise and spanwise directions and not deformable in the vertical direction, corresponding to the ideal case of infinite surface tension. To identify the role of the fluid-fluid interface, an extra set of simulations with a single fluid has been carried out and compared to the results obtained with two fluids of same viscosity separated by the interface. The drag and the maximum wall-normal velocity fluctuations were found to be highly correlated for all the surface configurations, whether they reduce or increase the drag. This implies that the structure of the near-wall turbulence is dominated by the total shear and not by the local boundary condition of super-hydrophobic, liquid--infused or rough surfaces.