▪ AbstractWe review the experimental evidence on turbulent flows over rough walls. Two parameters are important: the roughness Reynolds number ks+, which measures the effect of the roughness on the buffer layer, and the ratio of the boundary layer thickness to the roughness height, which determines whether a logarithmic layer survives. The behavior of transitionally rough surfaces with low ks+ depends a lot on their geometry. Riblets and other drag-reducing cases belong to this regime. In flows with δ/k ≲ 50, the effect of the roughness extends across the boundary layer, and is also variable. There is little left of the original wall-flow dynamics in these flows, which can perhaps be better described as flows over obstacles. We also review the evidence for the phenomenon of d-roughness. The theoretical arguments are sound, but the experimental evidence is inconclusive. Finally, we discuss some ideas on how rough walls can be modeled without the detailed computation of the flow around the roughness element...

Turbulent flows over rough walls

A fluid is a substance in which the constituent molecules are free to move relative to each other Conversely, in a solid, the relative positions of molecules remain essentially fixed under non-destructive conditions of temperature and pressure While these definitions classify matter into fluids and solids, the fluids subdivide further into liquid and gases

Introduction of fluid mechanics

https://www.wto.org/english/tratop_e/envir_e/wksp_envir_nov12_e/3floriankern_sprusussexuniversity_e.pdf

Carbon Capture and Storage

Wall-bounded turbulent flows at high Reynolds numbers have become an increasingly active area of research in recent years. Many challenges remain in theory, scaling, physical understanding, experimental techniques, and numerical simulations. In this paper we distill the salient advances of recent origin, particularly those that challenge textbook orthodoxy. Some of the outstanding questions, such as the extent of the logarithmic overlap layer, the universality or otherwise of the principal model parameters such as the von Karman “constant,” the parametrization of roughness effects, and the scaling of mean flow and Reynolds stresses, are highlighted. Research avenues that may provide answers to these questions, notably the improvement of measuring techniques and the construction of new facilities, are identified. We also highlight aspects where differences of opinion persist, with the expectation that this discussion might mark the beginning of their resolution.

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Wall-bounded turbulent flows at high Reynolds numbers: Recent advances and key issues

In the last two decades the problem of wave height damping due to bottom friction has received increasing attention among near-shore oceanographers. This fact is reflected in the wealth of papers on the subject; the list of references given herein presents a minor selection only. This paper is an attempt to re-evaluate and systematize the many observations and the rather few detailed measurements of the phenomenon. In nature the wave boundary layer will always be rough turbulent. This is not necessarily the case in a hydraulic model. The aim is therefore to make it possible to determine the proper flow regime for a pure short-period wave motion over a given bed. Values for the wave friction factor and the wave boundary layer thickness are also proposed. The main results of the study are presented in three diagrams giving flow regimes, friction factors and boundary layer thicknesses. Flow parameters are a-jjj/k and RE = U-|j_ ai~/v, a-]m and U-|m being maximum bottom amplitude and velocity according to first order potential wave theory, k is the Nikuradse roughness parameter.

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C F Colebrook

Papers

Turbulent flow in pipes, with particular reference to the transition region between the smooth and rough pipe laws.

The reduction of carrying capacity of pipes with age.

Turbulent flow in pipes: a historic speculation.