A fast direct numerical simulation method for characterising hydraulic roughness

TL;DR

This paper introduces a rapid direct numerical simulation method using minimal-span channels to accurately characterize hydraulic roughness of surfaces across smooth to fully rough regimes, reducing computational costs.

Contribution

The novel method enables fast, direct characterization of hydraulic roughness from profilometry data using minimal-span channels, bypassing high computational costs of traditional DNS.

Findings

Minimal-span channels capture the Hama roughness function.

Simulations with explicit and modeled roughness yield consistent results.

Method allows high-Reynolds-number DNS at fixed blockage ratios.

Abstract

We describe a fast direct numerical simulation (DNS) method that promises to directly characterise the hydraulic roughness of any given rough surface, from the hydraulically smooth to the fully rough regime. The method circumvents the unfavourable computational cost associated with simulating high-Reynolds-number flows by employing minimal-span channels (Jimenez & Moin 1991). Proof-of-concept simulations demonstrate that flows in minimal-span channels are sufficient for capturing the downward velocity shift, that is, the Hama roughness function, predicted by flows in full-span channels. We consider two sets of simulations, first with modelled roughness imposed by body forces, and second with explicit roughness described by roughness-conforming grids. Owing to the minimal cost, we are able to conduct DNSs with increasing roughness Reynolds numbers while maintaining a fixed blockage ratio, as is typical in full-scale applications. The present method promises a practical, fast and accurate tool for characterising hydraulic resistance directly from profilometry data of rough surfaces.