# Contrasts Between Momentum and Scalar Transport Over Very Rough Surfaces

## TL;DR

This study uses large-eddy simulations to compare momentum and scalar transport over rough surfaces, revealing significant differences in dispersive fluxes and the influence of surface geometry on flow characteristics.

## Contribution

It provides new insights into the contrasting behaviors of momentum and scalar transport, especially dispersive fluxes, over very rough surfaces in turbulent flows.

## Key findings

- Dispersive fluxes are a significant part of total fluxes within roughness sublayers.
- Turbulence transports momentum more efficiently than scalars, but dispersive fluxes show the opposite trend.
- Surface geometry, especially frontal density, strongly affects fluxes and flow regimes.

## Abstract

Large-eddy simulations are conducted to contrast momentum and passive scalar transport over large, three-dimensional roughness elements in a turbulent channel flow. Special attention is given to the dispersive fluxes, which are shown to be a significant fraction of the total fluxes within the roughness sublayers. Based on point-wise quadrant analysis, the turbulent components of the transport of momentum and scalars are found to be similar in general, albeit with increasing dissimilarity for roughnesses with low frontal blockage. However, strong dissimilarity is noted between the dispersive momentum and scalar fluxes, especially below the top of the roughness elements. In general, turbulence is found to transport momentum more efficiently than scalars, while the reverse applies for the dispersive contributions. The effects of varying surface geometries, measured by the frontal density, can be pronounced on both turbulent and dispersive fluxes. Increasing frontal density induces a general transition in the flow from a rough boundary-layer type to a mixed-layer-like type. This transition results in an increase in the efficiency of turbulent momentum transport, but the reverse occurs for scalars due to reduced contributions from large scale motions in the roughness sublayers. This study highlights the need for distinct parameterizations of the dispersive scalar fluxes and the importance of considering the contrasts between momentum and scalar transport for turbulent flows over very rough surfaces.