Deceleration efficiencies of shrub windbreaks in a wind tunnel

Wu, Xiaoxu; Zou, Xueyong; Zhou, Na; Zhang, Chunlai; Shi, Sha

doi:10.1016/j.aeolia.2014.10.004

Cited by 51 publications

(49 citation statements)

References 21 publications

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“…The model of Judd et al [44] for individual plants has subsequently been supported by wind tunnel data (e.g., [59][60][61][62]) and modelling experiments [63]. However, few studies have sought to examine the turbulent wake structures generated by dryland plants in the field.…”

Section: Introduction: the Drylands Contextmentioning

confidence: 95%

“…New approaches to resolve the partition of wind momentum fluxes over heterogeneous land surfaces (see Section 3.2.1) may provide opportunities to reduce the uncertainty in sediment deposition schemes. Theoretical calculations [57,58] and experimental measurements [46,54,62,63,79,90] suggest that protective wakes downwind of individual vegetation elements extend to approximately 7-10 h (where h is the height of the element). However, this can vary significantly depending on plant porosity, pliability and configuration.…”

Section: Trapping Of Windborne Sedimentmentioning

confidence: 99%

“…Liu et al [72] hypothesised that this resulted from reduced diameter influences, whereby vegetation clumping decreases the total wake spread, thus reducing the protective effects of wake interference and allowing wind to flow through unimpeded. Wind tunnel studies [62,95] suggest that multiple-row barriers reduce wind velocities more efficiently than single-row barriers, at least in the immediate lee. This effect is likely due to the additional turbulence generated by a sequence of fences compared to an isolated barrier [101].…”

Section: Trapping Of Windborne Sedimentmentioning

confidence: 99%

“…This effect is likely due to the additional turbulence generated by a sequence of fences compared to an isolated barrier [101]. Wu et al [62] also showed that using multiple vegetation species can provide optimal sheltering effects. There is little research on the impact of vegetation patch configuration at the meso scale, with the exception of some scaled-down wind tunnel experiments [60,78,79] and modelling studies of forest edges [81][82][83].…”

Section: Trapping Of Windborne Sedimentmentioning

confidence: 99%

“…The difference in velocity recovery length has been attributed to a lack of flow moving laterally around a patch [77] or a fence where the obstacle width is assumed to be infinite [63,86,99,103], in contrast to isolated vegetation elements where faster-moving airflow mixes with slower-moving airflow in the wake via counter-rotating vortices [59]. shown to be significantly longer than in the case of single vegetation elements [46,54,62,64,77,81,83,91,98,102]. The difference in velocity recovery length has been attributed to a lack of flow moving laterally around a patch [77] or a fence where the obstacle width is assumed to be infinite [63,86,99,103], in contrast to isolated vegetation elements where faster-moving airflow mixes with slower-moving airflow in the wake via counter-rotating vortices [59].…”

Section: Trapping Of Windborne Sedimentmentioning

confidence: 99%

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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: Introduction: the Drylands Contextmentioning

confidence: 95%

Section: Trapping Of Windborne Sedimentmentioning

confidence: 99%

Section: Trapping Of Windborne Sedimentmentioning

confidence: 99%

Section: Trapping Of Windborne Sedimentmentioning

confidence: 99%

Section: Trapping Of Windborne Sedimentmentioning

confidence: 99%

See 3 more Smart Citations

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

Mayaud

Webb

2017

Land

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A coupled vegetation/sediment transport model for dryland environments

2017

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Dryland regions are characterized by patchy vegetation, erodible surfaces, and erosive aeolian processes. Understanding how these constituent factors interact and shape landscape evolution is critical for managing potential environmental and anthropogenic impacts in drylands. However, modeling wind erosion on partially vegetated surfaces is a complex problem that has remained challenging for researchers. We present the new, coupled cellular automaton Vegetation and Sediment TrAnsport (ViSTA) model, which is designed to address fundamental questions about the development of arid and semiarid landscapes in a spatially explicit way. The technical aspects of the ViSTA model are described, including a new method for directly imposing oblique wind and transport directions onto a cell‐based domain. Verification tests for the model are reported, including stable state solutions, the impact of drought and fire stress, wake flow dynamics, temporal scaling issues, and the impact of feedbacks between sediment movement and vegetation growth on landscape morphology. The model is then used to simulate an equilibrium nebkha dune field, and the resultant bed forms are shown to have very similar size and spacing characteristics to nebkhas observed in the Skeleton Coast, Namibia. The ViSTA model is a versatile geomorphological tool that could be used to predict threshold‐related transitions in a range of dryland ecogeomorphic systems.

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Characterizing turbulent wind flow around dryland vegetation

Mayaud

Wiggs

Bailey

2016

Earth Surf Processes Landf

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Wind flow has been studied in situations where it encounters porous and solid windbreaks, but there has been a lack of research exploring turbulent wind dynamics around and in the lee of real vegetation elements. In dryland contexts, sparse vegetation plays an important role in modulating both the erosivity of the wind and the erodibility of surfaces. Therefore, understanding the interactions between wind and vegetation is key for improving wind erosion modelling in desert landscapes. In this study, turbulent wind flow around three typical dryland vegetation elements (a grass clump, a shrub, and a tree) was examined in Namibia using high-frequency (10 Hz) sonic anemometry. Spatial variations in mean wind velocity, as well as Reynolds stresses and coherent turbulent structures in the flow, were compared and related to the porosities and configurations of the study elements. A shelter parameter, originally proposed by Gandemer (1979, Journal of Wind Engineering and Industrial Aerodynamic 4: 371-389), was derived to describe the combined impact of the different elements on the energy and variability of horizontal wind flow. Wind velocity was reduced by 70% in the immediate lee of the grass and 40% in the lee of the shrub, but velocity recovered exponentially to equilibrium over the same relative distance in both cases (~9 element heights downwind). Quadrant analysis of the highfrequency wind flow data revealed that the grass clump induced a small recirculation zone in its lee, whereas the shrub did not. Also, higher Reynolds shear stress Àu'w' À Á and higher 'flow positivity magnitude' [ratio of Q1 (outward interaction) and Q4 (sweep) quadrants to Q2 (ejection) and Q3 (inward interaction) quadrants] was generally observed in the wake of the grass. These differences arose because the porosity of the grass clump (53%) was lower than the porosity of the shrub (69%), and thus bleed flow through the shrub was more significant. The bluff-body behaviour of the grass resulted in a more intense and more extensive sheltering effect than the shrub, which implies that overall sediment transport potential is lower in the wake of the grass. The tree displayed a different wake structure to the grass and shrub, owing to the elevation of its crown. A 'bottom gap' effect was observed, whereby wind velocities increased possibly due to streamline compression in the gap between the ground and the underside of the tree crown. Differences in flow momentum between the bottom gap and the low-pressure leeward region of the crown are a probable explanation for the formation of a large recirculation vortex. The bottom gap effect led to decreased sheltering up to three tree heights downwind, but the surface became increasingly protected by the frontal impact of the crown over a further eight tree heights downwind (~30 m). The extraction of momentum from the air by the tree therefore resulted in a far more extensive sheltering effect compared to the grass and shrub. This study represents an important investigation of the impact of dif...

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Deceleration efficiencies of shrub windbreaks in a wind tunnel

Cited by 51 publications

References 21 publications

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

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

A coupled vegetation/sediment transport model for dryland environments

Characterizing turbulent wind flow around dryland vegetation

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