Experimentelle Untersuchungen zum Rauhigkeitsproblem

Schlichting, Hermann

doi:10.1007/bf02084166

Cited by 337 publications

(181 citation statements)

References 7 publications

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“…Shear stress (or drag) partitioning is traditionally used to quantify the proportion of the wind shear stress or shear velocity ( * ) that is acting on a soil surface, compared to the proportion acting on vegetation elements (e.g., [48,96,115]). The theoretical basis for partitioning comes from the work of Schlichting [116], who proposed that the total drag on a roughened surface ( ) can be expressed as:…”

Section: Effects On Dragmentioning

confidence: 99%

Vegetation in Drylands: Effects on Wind Flow and Aeolian Sediment Transport

Mayaud

Webb

2017

Land

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Drylands are characterised by patchy vegetation, erodible surfaces and erosive aeolian processes. Empirical and modelling studies have shown that vegetation elements provide drag on the overlying airflow, thus affecting wind velocity profiles and altering erosive dynamics on desert surfaces. However, these dynamics are significantly complicated by a variety of factors, including turbulence, and vegetation porosity and pliability effects. This has resulted in some uncertainty about the effect of vegetation on sediment transport in drylands. Here, we review recent progress in our understanding of the effects of dryland vegetation on wind flow and aeolian sediment transport processes. In particular, wind transport models have played a key role in simplifying aeolian processes in partly vegetated landscapes, but a number of key uncertainties and challenges remain. We identify potential future avenues for research that would help to elucidate the roles of vegetation distribution, geometry and scale in shaping the entrainment, transport and redistribution of wind-blown material at multiple scales. Gaps in our collective knowledge must be addressed through a combination of rigorous field, wind tunnel and modelling experiments.

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Section: Effects On Dragmentioning

confidence: 99%

Vegetation in Drylands: Effects on Wind Flow and Aeolian Sediment Transport

Mayaud

Webb

2017

Land

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“…These irregular surfaces are more representative of real-world rough surfaces than traditional rough surfaces constructed from basic elements such as blocks and cones (see e.g. Schlichting 1936; Krogstad et al 2005;Orlandi & Leonardi 2008). Using DNS we are able to measure the flow not only above the rough surfaces but also within the rough surfaces, i.e.…”

mentioning

confidence: 99%

Reynolds-number dependence of the near-wall flow over irregular rough surfaces

2016

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The Reynolds-number dependence of turbulent channel flow over two irregular rough surfaces, based on scans of a graphite and a grit-blasted surface, is studied by direct numerical simulation. The aim is to characterise the changes in the flow in the immediate vicinity of and within the rough surfaces, an area of the flow where it is difficult to obtain experimental measurements. The average roughness heights and spatial correlation of the roughness features of the two surfaces are similar, but the two surfaces have a significant difference in the skewness of their height distributions, with the graphite sample being positively skewed (peak-dominated) and the grit-blasted surface being negatively skewed (valley-dominated). For both cases, numerical simulations were conducted at seven different Reynolds numbers, ranging from Re τ = 90 to Re τ = 720. The positively skewed surface gives rise to higher friction factors than the negatively skewed surface in all cases. For the highest Reynolds numbers, the flow has values of the roughness function U + well in excess of 7 for both surfaces and the bulk flow profile has attained a constant shape across the full height of the channel except for the immediate vicinity of the roughness, which would indicate fully rough flow. However, the mean flow profile within and directly above the rough surface still shows considerable Reynolds-number dependence and the ratio of form to viscous drag continues to increase, which indicates that at least for some types of rough surfaces the flow retains aspects of the transitionally rough regime to values of U + or k + well in excess of the values conventionally assumed for the transitionally to fully rough threshold. This is also reflected in the changes that the near-wall flow undergoes as the Reynolds number increases: the viscous sublayer, within which the surface roughness is initially buried, breaks down and regions of reverse flow intensify. At the highest Reynolds numbers, a layer of near-wall flow is observed to follow the contours of the local surface. The distribution of thickness of this 'blanketing' layer has a mixed scaling, showing that viscous effects are still significant in the near-wall flow.

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“…The equivalent sand-grain roughness height, k s , was first defined by Schlichting (1936) as the size of the sand grains from the experiments of Nikuradse (1933) that gave effectively the same frictional resistance as the roughness under consideration.…”

Section: Introductionmentioning

confidence: 99%

Direct numerical simulation of turbulent channel flow over a surrogate for Nikuradse-type roughness

2017

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A tiled approach to rough surface simulation is used to explore the full range of roughness Reynolds numbers, from the limiting case of hydrodynamic smoothness up to fully rough conditions. The surface is based on a scan of a standard grit-blasted comparator, subsequently low-pass filtered and made spatially periodic. High roughness Reynolds numbers are obtained by increasing the friction Reynolds number of the direct numerical simulations, whereas low roughness Reynolds numbers are obtained by scaling the surface down and tiling to maintain a constant domain size. In both cases, computational requirements on box size, resolution in wall units and resolution per minimum wavelength of the rough surface are maintained. The resulting roughness function behaviour replicates to good accuracy the experiments of Nikuradse (1933 VDI-Forschungsheft, vol. 361), suggesting that the processed grit-blasted surface can serve as a surrogate for his sand-grain roughness, the precise structure of which is undocumented. The present simulations also document a monotonic departure from hydrodynamic smooth-wall results, which is fitted with a geometric relation, the exponent of which is found to be inconsistent with both the Colebrook formula and an earlier theoretical argument based on low-Reynolds-number drag relations.

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Experimentelle Untersuchungen zum Rauhigkeitsproblem

Cited by 337 publications

References 7 publications

Vegetation in Drylands: Effects on Wind Flow and Aeolian Sediment Transport

Vegetation in Drylands: Effects on Wind Flow and Aeolian Sediment Transport

Reynolds-number dependence of the near-wall flow over irregular rough surfaces

Direct numerical simulation of turbulent channel flow over a surrogate for Nikuradse-type roughness

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