A systematic study of turbulent heat transfer over rough walls

TL;DR

This study uses direct numerical simulations to analyze how different rough wall geometries affect turbulent heat transfer and skin friction, revealing that roughness characteristics influence the Reynolds analogy factor and heat transfer augmentation.

Contribution

It provides a systematic analysis of the effects of roughness morphology and scaling on heat transfer and skin friction in turbulent flows, including realistic and artificially generated surfaces.

Findings

Reynolds analogy factor varies with surface slope and density.

Reynolds analogy factor correlates with equivalent sand roughness in inner units.

The factor tends to a plateau at high roughness levels.

Abstract

Direct Numerical Simulations are used to solve turbulent flow and heat transfer over a variety of rough walls in a channel. The wall geometries are exactly resolved in the simulations. The aim is to understand the effect of roughness morphology and its scaling on the augmentation of heat transfer relative to that of skin friction. A number of realistic rough surface maps obtained from the scanning of gas turbine blades and internal combustion engines as well as several artificially generated rough surfaces are examined. In the first part of the paper, effects of statistical surface properties, namely surface slope and roughness density, at constant roughness height are systematically investigated, and it is shown that Reynolds analogy factor (two times Stanton number divided by skin friction coefficient) varies meaningfully but moderately with the surface parameters except for the case with extremely low slope or density where the Reynolds analogy factor grows significantly and tends to that of a smooth wall. In the second part of the paper, the roughness height is varied (independently in both inner and outer units) while the geometrical similarity is maintained. Considering all the simulated cases, it is concluded that Reynolds analogy factor correlates fairly well with the equivalent sand roughness scaled in inner units and asymptotically tends to a plateau.