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arXiv · 2312.05168

Turbulent flow of Newtonian and non-Newtonian fluids in rough channels: Part I: Roughness effect on Newtonian fluids

C. Narayanan·S. Nauer·J. -S. Singh·R. Belt·T. Palermo·D. Lakehal
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Abstract

Direct Numerical Simulations (DNS) of turbulent channel flow at a shear Reynolds number of $Re_{*}=360$ for Newtonian and Herschel-Bulkley fluids in smooth and rough channels has been performed. The rough surface was made of irregular undulations modeled with the immersed surface method. The rough surface was such that the ratio of the channel half-height to the RMS roughness height is 48, and the RMS and the maximum crest and trough heights are 7.5 and 23 wall units, respectively. This part, limited to Newtonian turbulent flows, has demonstrated the generic nature of the rough surface that is ideal for studying non-Newtonian flows in practical applications. The results confirm that turbulence in the outer layer is not directly affected by the rough surface. The roughness effects on the turbulence quantities are confined to the layer between 0 and 25 wall units; beyond which, the profiles collapse with those for smooth pipes. In the roughness sublayer, the streamwise normal Reynolds stress is reduced while the spanwise and wall-normal components are increased. The Reynolds shear stress increases significantly, causing a proportional increase in turbulence production near the wall affecting the dominant terms in the turbulent kinetic energy budget. The friction factor using the Colebrook-White correlation calculated by specifying the sand-grain roughness as 7.5 wall units predicts the friction velocity and the bulk velocity accurately. The streaky structures that exist near smooth walls were observed to be broken by the roughness elements, leading to a denser population of coherent structures near the wall, which increase the velocity fluctuations. The coherent structures developed in the roughness layer do not seem to penetrate in to the outer layer, and no evidence could be found as to the direct impact of the roughness layer on the outer one.

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C. Narayanan, S. Nauer, J. -S. Singh, R. Belt, T. Palermo, D. Lakehal. 2023-12-08. Turbulent flow of Newtonian and non-Newtonian fluids in rough channels: Part I: Roughness effect on Newtonian fluids. https://arxiv.org/abs/2312.05168

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