Characterizing the Roughness in Channel Flows Using Direct Numerical Simulations

: Turbulent flows over bumpy walls are ubiquitous and pose a fundamental challenge to various engineering applications such as coastal boundary layers, drag on ships, hydraulic conveyance networks, and bluff body aerodynamics, to name a few. In this study, we used direct numerical simulations (DNS) along with a direct-forcing immersed boundary method (IBM) to understand the connection between the roughness geometry and the mean flow drag. A bumpy wall was constructed using an array of randomly oriented ellipsoids characterized by the Corey shape factor ( C o ). We found that our results exactly validated the experimental studies by Nikuradse for sand-grain type roughness ( C o ¼ 1 . 0 ). Additionally, we observed that the mean flow drag increased for decreasing C o through an increase in the form-drag contribution and a decrease in the viscous drag. We also developed a relationship between the statistics of the bottom height distribution and the roughness parameter ( z 0 ) that may help explain the spread observed in the drag coefficient predicted when using conventional tools such as the Moody diagram. DOI: 10.1061/JHEND8.HYENG-13666. This work is made available under the terms of the Creative Commons Attribution 4.0 International license, https://creativecommons.org/licenses/by/4.0/.