Fence Height Research Articles

Effects of Fences and Green Zones on the Air Flow and PM2.5 Concentration around a School in a Building-Congested District

We investigated the effects of wall- and tree-type fences on the airflow and fine particular matter (PM2.5) concentration around a school using a computational fluid dynamics (CFD) model. First, we validated the simulated wind speeds and PM2.5 concentrations against measured values, and the results satisfied the recommended criteria of the statistical validation indices used. Then, we evaluated the fence effects for 16 inflow directions by conducting numerical simulations with different fence types and heights. With east–southeasterly inflow, relatively high PM2.5 from the road was transported to the school. However, the wall-type fence prevented the PM2.5 from the road from entering the school, and the PM2.5 concentration decreased significantly downwind of the fence. With east–northeasterly inflow, the horizontal wind speed decreased due to the drag caused by the tree-type fence, resulting in a shift in the flow convergence region. The PM2.5 concentration decreased in the region of strengthened upward flow. This occurred because the number of pollutants transported from the background decreased. A comparison of the two fence types revealed that the effect of the tree-type fence on inbound pollutants was more significant, due to increased upward flows, than the effect of the wall-type fence.

Effect of Gust Wind on Flow over a Wall-Mounted Fence

In this work, the characteristics of incoming and wake flows downstream of wall-mounted fences under wind gust were explored with wind tunnel experiments. A time-resolved particle image velocimetry was used to capture the flow dynamics across two different fence heights. The results show that during the gust period, the wake presents distinct meandering and strong flow mixing. The Probability Density Function distribution of flow velocities indicates that the mixing effect increases with the streamwise distances. Specifically, for locations above the fence top tip, the growth of streamwise distance decreases the footprint of wind gust. However, for locations lower than the fence top tip, the local wind flows exhibit stronger variations before and after wind gust with the growth of downstream distance. Overall, at the same relative streamwise and spanwise locations downstream of fences within the wake region, the higher fence better suppresses the influence of gust wind.

Assessment of the Behavioural Response of Korean Water Deer (Hydropotes inermis argyropus) to Different Fence Heights.

Simple SummaryThe present study aimed to investigate the success rate of fences and classify the behavioural responses of Korean water deer (Hydropotes inermis argyropus) to different fence heights. The dominant behaviours before the deer crossed the fences by performing vertical and running jumps were recession and rest when the fence heights were lower or higher than 1.2 m, respectively. The general threshold (fence height) for discriminating success or failure was 0.9 m; however, we recommend a fence height of 1.5 m, considering the cost and roadkill risk. Placing exit pathways for deer and eliminating possible resting areas outside fences are essential for reducing the number of successful jump attempts.Fences have been widely implemented to reduce the risk of wildlife–vehicle collisions, wildlife disease spread, and crop damage. To manufacture fences, it is imperative to assess the behavioural responses of the target species. Here, we investigated the success rate of fences and classified eight behavioural responses of Korean water deer (Hydropotes inermis argyropus) to different fence heights. We explored the association of 801 behavioural responses and defined a threshold based on 40 events by applying non-metric multidimensional scaling and a binary logistic generalised linear mixed model. With fences lower and higher than 1.2 m, recession and rest were the dominant behaviours, respectively, before the deer crossed the fences by performing vertical and running jumps. Considering all independent events, 0.9 m was the marginal threshold, with highly variable outliers over this value. Placing exit pathways for deer and eliminating possible resting areas outside fences are essential for reducing the number of successful jump attempts. The optimal fence height could differ based on conditional factors; however, we recommend a height of 1.5 m considering the cost and roadkill risk. In conclusion, exploring and classifying the behavioural responses of the target species may be critical for establishing appropriate fence protocols.

Investigations into the design of sand control fence for Gobi buildings

Wind-blown sand disasters, as an environmental problem, have great impact on human activities. Sand fence has been recognized as an efficiency technology in anti-desertification engineering and is wildly applied to protect highways and railways around the desert. But few people study its protection to Gobi buildings, which may also suffer from sand disaster. To study its protection to Gobi buildings, we systemically investigate the three-dimensional structure of wind field and sand distribution around sand fence for buildings by Computational Fluid Dynamic simulations and wind-tunnel experiments. The dependencies of sand-prevention efficiency on fence height, porosity, inclined slat angle and wind speed are proposed respectively. An optimal height of sand fence is suggested to be 2.0 m−2.5 m, and an optimal porosity is 10%−15%, by synthetically considering the factors of sand prevention efficiency and construction cost; the slat angles of inclined fence is recommended as 135°−150° for maximizing the sand sedimentation on the windward side of fence.

Characteristics of extracted ion beam from a cesium-free negative ion source using sheet plasma.

We report the characteristics of extracted beam current in the test plasma produced by direct current for sheet plasma upgrade producing negative ions by volume production without cesium (Cs) seeding. The negative hydrogen ion beam is extracted by a two-grid extraction system, which is located at the periphery of the sheet plasma. Experimental observations show that (i) negative hydrogen ions are successfully extracted from the sheet plasma by single/multi-aperture grids and (ii) the ratio of the extracted electron current IEG(e) and the hydrogen negative ion current IEG(H-), IEG(e)/Ic(H-), decreases from 8.0 to 2.0 with an increase in the height of the electron fence (HEF), which is a filter that prevents electron diffusion from the extraction region.

Monsoon wave transmission at bamboo fences protecting mangroves in the lower mekong delta

Abstracts This paper is concerned with a study on monsoon wave transmission at bamboo fences protecting newly-planted mangroves in the Lower Mekong Delta (LMD) of Vietnam. For this, new field experiments were carried out in combination with the use of an existing field dataset by Albers et al. (2013). In support of the analysis of the field data, influences of governing parameters on wave transmission are systematically examined using a numerical wave tank (i.e. a RANS-VOF model). It follows that the fence height is relatively more influential than the fence width. Also, high-frequency waves are generally more effectively dissipated by the fences than long-frequency waves. Importantly, the fence porosity is found to affect attenuation of both high- and low-frequency waves drastically, indicating its important role in the quantitative understanding of wave transmission as well as in the fence functional design. An empirical formulation of wave transmission at bamboo fences, needed for the fence functional design, was eventually derived from all the field data. As hinted at by the numerical analysis, the fence freeboard, the fence width and especially the fence porosity are found the major governing parameters.

Fence height control on sand trapping

Abstract Sand fences are widely used to control sand deposition to initiate or stabilize dunes. Previous studies on sand fences are often focused on the porosity control of their trapping efficiency, however, there exists very little research on the fence height effect. We deployed fences with three heights and equal porosity at a field site near Jericoacoara, Brazil, and examined the variation of their topographic profile and trapping efficiency. We identified two stages of dune development around fences. During stage I, dune topographic profile area is proportional to the fence and dune height, and the trapping efficiency decreases substantially with time. During stage II, dunes continue to grow horizontally and the trapping efficiency decreases with time, but at a slower rate compared to stage I. Coarser sand tends to deposit close to the fence. The findings from this study can enhance the understanding of the complex wind-sand-structure interactions and provide insights for future dune recovery applications.

Flow development upstream of a fence

The effect of Reynolds number on the flow development upstream of a rigid, non-porous, static fence is investigated experimentally. The flow field is measured using time-resolved, two-component particle image velocimetry at Reynolds numbers based on fence height of 18000, 36000, and 54000. The results show that a laminar separation bubble forms upstream of the junction vortex at the base of the fence. The mean extent of the bubble decreases with increasing Reynolds number, with mean separation moving downstream and mean reattachment moving upstream. In the aft portion of the bubble, shear layer vortices form and are shed at scaled frequencies and wavelengths that are comparable to laminar separation bubble shedding in low Reynolds number airfoils and flat plates with an imposed adverse pressure gradient. The strong periodicity of the associated coherent structures and the proximity of shear layer roll-up relative to the fence should be taken into consideration in the relevant designs due to potential implications to structural loading. A simple flow separation prediction model combining inviscid fence flow solution with Thwaites’ method is introduced and shows good agreement with the experimental results for the Reynolds number range considered.

CFD simulation of the wind field over a terrain with sand fences: Critical spacing for the wind shear velocity

Abstract Sand fences are often erected to reduce wind speed, prevent aeolian soil focus our investigation on the behaviour of the near-surface wind shear velocity, which is defined here using a simple model based on wind speed values at height 20 % of the fence height. Our simulations reveal novel insights about the three-dimensional structure of the turbulent wind flow between fences that are important for developing a theoretical description of this flow. We find that the area of soil associated with values of near-surface wind shear velocity that are below the minimal threshold for sand transport has two regimes, depending on the spacing L x between the fences. When L x is smaller than a critical value L xc , the wake zones associated with each fence are inter-connected (regime A), while these wake zones appear separated from each other (regime B) when L x exceeds this critical value of spacing. The system undergoes a second order phase transition at L x = L xc , with the cross-wind width of the protected zone scaling with 1 - L x / L xc β in regime A, with β ≈ 0.32 . Moreover, we also perform a comparative investigation of arrays of constant and multiple fence heights on the basis of the two-dimensional shear velocity field obtained from our simulations. Our findings have implication for a better understanding of aeolian transport in the presence of sand fences, as well as to develop optimization strategies for measures to protect soils from wind erosion.

Numerical simulation and optimization study of surface pressure and flow field around a triangular prism behind a porous fence

The flow structure around upright porous fences and a triangular prism was numerically investigated by using the RNG k-e turbulence model. The numerical method was validated by comparing the simulations with the experimental data, and then the effects of porosity and height of the wind fence on the flow field around the prism model and surface pressures were studied. The aim of this paper is to achieve the best shelter effect of a porous fence on the triangular prism located behind the fence with the height of 0.5h and 1.0h respectively, where h is the height of the prism. The gradient algorithm was adopted as the optimization algorithm to determine the optimum porosity of the wind fence. Three objective functions were considered to minimize (1) the sum of the absolute values of the mean pressure coefficients on the prism surface; (2) the total turbulence kinetic energy around the prism model, and (3) the total values of the peak velocity ratios involving velocity and turbulence. The numerical results showed that a porous fence with the height of about 0.5h–1.0h seems to be most effective in reducing the absolute value of the mean pressure coefficient on the model surface. Additionally, all three objective functions were capable of finding an optimal shelter performance of porous fences on the prism with surface pressure or flow characteristics considerations.

Riparian and Upland Afforestation: Improving Success by Excluding Deer from Small Areas with Low Fencing

We propose that small patches of land protected by low (1.2-m tall) fence is a viable approach for restoring and/or conserving forests in riparian or upland areas with high white-tailed deer (Odocoileus virginianus Zimmerman) populations. We also propose that this new approach may be advantageous because, unlike tree shelters, fenced areas protect shrubs just as well as trees in afforestation projects. Multi-year field experiments were performed involving deer exclosures where fence height and fenced area were manipulated in upland and lowland pasture and forest settings. Results show that low fencing significantly increased seedling survival and growth relative to unfenced areas when fenced areas were small (i.e., less than or equal to ∼100 m2). Moreover, tall fencing (1.8 or 2.3 m) did not provide significantly greater protection than shorter (1.2-m) fencing when the fenced areas were kept small. We propose that creating small patches of deer-free pasture or forest habitat will greatly support new approaches to afforestation such as applied nucleation. Finally, we show that low-stature fencing may also have application for protecting larger areas (≥0.8 ha) if the fencing is deployed around the perimeter as two low fences erected in parallel and with a small (3-m) space between them. We conclude that low fencing can result in levels of survival and growth of both trees and shrubs suitable to meet the success criteria for afforestation projects funded by the US federal government.

Advanced application of a sublayer fence probe in highly instationary turbomachinery flows: observations on prestall instabilities

The present paper introduces a novel approach for measuring near wall flow velocities by utilizing a sublayer surface fence probe. Hereby, a difference in static pressure builds up over a microscopic obstacle within the viscous sublayer. An analytical model of the angular dependent pressure difference is employed to derive information about the flow direction. Furthermore, a computational preston tube approach has been used to calibrate the surface fence probe with regard to a flow velocity to be assigned to half of the fence height. Through the use of a sophisticated analyzing algorithm the flow direction and its velocity can be determined as a function of time. As a proof of concept measurements were conducted within the radial gap of an annular compressor rig yielding both mean and time resolved near wall flow fields. Former are in very good compliance with oil flow visualizations proving the methods accuracy. The experimental results give unprecedented insights into an unsteady flow phenomenon that arises when an axial compressor is operating close to its stalling limit. The presented technique allows for investigating turbomachinery areas which formerly were observable only by computational means.Graphical

Computational investigation on secondary flows in a linear turbine cascade with tapered dual fence

The focus of the present work is to minimize the secondary flow losses inside a linear turbine cascade by means of a novel design of streamwise dual fence. The leading edge and trailing edge of the fences have been modified so as to reduce the total pressure loss coefficient in the passage. The study has been carried out computationally based on RANS simulations with SST turbulence model. Numerous simulations have been undertaken with single fence and dual fence models and compared with the base case model. The dual fence model with tapered trailing edge exhibits significant loss reduction compared to the base case. A suitable fence height ratio (FHR) has been identified for the dual fence model. The FHR = 2 configuration reduces the secondary flow kinetic energy by 78 % within the blade passage and it reduces the exit angle deviation significantly throughout the span. Detailed flow field analysis has been carried out to understand the physical mechanism behind the loss reduction with dual fence models. It is observed that fence-1 breaks the pressure side leg of the horse shoe vortex at the beginning of their formation itself. The radial penetration of the suction side leg of the horse shoe vortex is restrained by fence-2. These combined effects prevent the formation and mixing of two prominent loss core regions thereby avoiding the accumulation of low energy fluid near the suction side of blade.

Establishing factors of building vulnerability towards tsunami hazard

The objective of this article is to establish the attributes (factors) of building vulnerability towards tsunami hazard, particularly for areas in Padang, Indonesia, as well as those areas with the same characteristics. Within this framework, this study informs the progress towards a more systematic approach to factor development for a Relative Vulnerability Index (RVI) score for assessing the building vulnerability towards tsunami hazard. The RVI score is used to predict tsunami risk zones on building vulnerability. The analysis was conducted to develop the factors that influence the building vulnerability. Developing factors of the building vulnerability to tsunami hazard is a key aspect for assessing the RVI, as their result may misguide local decision-makers to act adequately concerning the local risk if they are based on an inappropriate set of factors. These factors were classified under three distinct groups: 1) internal factors; 2) external factors and 3) water inundation factor. The selection of factors are based on the building and the location characteristics. Therefore, the factors that may influence the risk were listed. Based on the kinds of literature and opinion of practitioners/experts, several imperative factors that affect the risk were identified and studied. The factors were tabulated and were sorted from highest to lowest numbers. Based on the tabulation of these factors, the summary of analysis results for internal factors are: construction material; the number of stories; foundation type; date of the building; preservation condition; ground floor hydrodynamic; shape and orientation of the building. Furthermore, based on the tabulation results for external data factors and tsunami inundation factor, the external factors are: movable object; building row; height of concrete fence; natural barrier; distance from the coastline; distance from the river; obstacle islands and gap between building attributes and water inundation factor is tsunami inundation depth.

Reducing peak pressures under the saddle at thoracic vertebrae 10-13 is associated with alteration in jump kinematics

There is little information about horse-saddle interaction at take-off for a fence, although there is potential that this could have an influence on performance. It was hypothesised that (1) maximum peak pressure under the saddle would occur in the phase of maximum thoracolumbar flexion prior to hindlimb take-off; and (2) limb and trunk kinematics at take-off over the fence would be affected by reducing peak pressure at Thoracic vertebrae (T)10-13 at the point in the stride where peak pressures occur. The peak pressures under the usual saddle (Saddle S) and a saddle modified to reduce peak pressures at T10-13 (Saddle F) were measured during approach and take-off over a 1.30 m upright fence in 12 elite jumping horses. The timing of peak pressures was determined by comparison with simultaneous video data. Shoulder, carpal flexion angle and trunk angle to the horizontal at hindlimb take-off, take-off distance from the fence and fetlock height above the fence were determined using high speed motion analysis. Peak pressures under the saddle at T10-13 and kinematic data were compared between Saddles S and F. Maximum peak pressures occurred at forelimb vertical, during hindlimb protraction, consistent with thoracolumbar ventroflexion. Saddle F was associated with significantly lower peak pressures at T10-13, greater shoulder and carpal flexion, a steeper trunk angle, and higher fetlock height above the fence than Saddle S. Forelimb take-off distance from the fence was not different between saddles, but hindlimbs were significantly closer to the fence with Saddle F, indicating potential increase in ventroflexion through the thoracolumbosacral region. These findings suggest that reducing peak pressures under the saddle at T10-13 are associated with altered kinematics during the approach and take-off over a fence, which may have a positive effect on jumping performance.

Effects of a Fence on Pollutant Dispersion in a Boundary Layer Exposed to a Rural-to-Urban Transition

Simultaneous particle-image velocimetry and laser-induced fluorescence combined with large-eddy simulations are used to investigate the flow and pollutant dispersion behaviour in a rural-to-urban roughness transition. The urban roughness is characterized by an array of cubical obstacles in an aligned arrangement. A plane fence is added one obstacle height h upstream of the urban roughness elements, with three different fence heights considered. A smooth-wall turbulent boundary layer with a depth of 10h is used as the approaching flow, and a passive tracer is released from a uniform line source 1h upstream of the fence. A shear layer is formed at the top of the fence, which increases in strength for the higher fence cases, resulting in a deeper internal boundary layer (IBL). It is found that the mean flow for the rural-to-urban transition can be described by means of a mixing-length model provided that the transitional effects are accounted for. The mixing-length formulation for sparse urban canopies, as found in the literature, is extended to take into account the blockage effect in dense canopies. Additionally, the average mean concentration field is found to scale with the IBL depth and the bulk velocity in the IBL.

Flow Resistance Coefficients of Porous Brush Seal as a Function of Pressure Load

Developments in brush seal analyses tools have been covering advanced flow and structural analyses since brush seals are applied at elevated pressure loads, temperatures, surface speeds, and transients. Brush seals have dynamic flow and structural behaviors that need to be investigated in detail in order to estimate final leakage output and service life. Bristles move, bend, and form a grift matrix depending on pressure load. The level of pressure load determines the tightness of the bristle pack, and thus, the leakage. In the computational fluid dynamics (CFD) analyses of this work, the bristle pack is treated as a porous medium. Based on brush seal test data, the flow resistance coefficients (FRC) for the porous bristle pack are calibrated as a function of pressure load. A circular seal is tested in a static test rig under various pressure loads at room temperature. The FRC calibration is based on test leakage and literature-based axial pressure distribution on the rotor surface and radial pressure distribution over the backing plate. The anisotropic FRC are treated as spatial dependent in axisymmetrical coordinates. The fence height region and the upper region of bristle pack have different FRC since the upper region is supported by backing plate, while bristles are free to move and bend at the fence height region. The FRC are found to be almost linearly dependent on the pressure load for investigated conditions. The blow-down is also calculated by incorporating test leakage and calibrated FRC.

Study of heat shielding qualities of a brick wall corner with additional insulation on the brick

This work presents the theoretical investigations results for the brick wall thermal protection properties exterior corner with the insulated layer SUPERROCK mineral wool. As the outer corner of the enclosing structures has lower thermal protection properties than the other part of the wall, it needs additional insulation. Two variants of additional insulation were considered in the work, in the deepened outer surface and in the middle of the brick wall. In this case, the additional heater was not continuous, but divided into two parts and shifted from the corner. The length and volume of additional insulation for different variants were determined. The optimal variant of the proposed schemes was determined by the minimum volume of the heater and compared with the existing ones. The volume of additional insulation is set at 1 meter of the fence height. For the proposed variant of the location for the heater in the brick wall middle, its thickness in the brick cavity, the length and offset from the corner magnitude were determined. For a location-variant of the heater in the deepened on the outer surface of the fence bricklayer, the length of the additional insulation and its displacement from the corner were determined. The thickness of additional insulation was taken 130 mm (width of the brick and seam between the bricks). Studies were carried out for the first temperature zone.

Wind tunnel test on the effect of metal net fences on sand flux in a Gobi Desert, China

The Lanzhou-Xinjiang High-speed Railway runs through an expansive windy area in a Gobi Desert, and sand-blocking fences were built to protect the railway from destruction by wind-blown sand. However, the shielding effect of the sand-blocking fence is below the expectation. In this study, effects of metal net fences with porosities of 0.5 and 0.7 were tested in a wind tunnel to determine the effectiveness of the employed two kinds of fences in reducing wind velocity and restraining wind-blown sand. Specifically, the horizontal wind velocities and sediment flux densities above the gravel surface were measured under different free-stream wind velocities for the following conditions: no fence at all, single fence with a porosity of 0.5, single fence with a porosity of 0.7, double fences with a porosity of 0.5, and double fences with a porosity of 0.7. Experimental results showed that the horizontal wind velocity was more significantly decreased by the fence with a porosity of 0.5, especially for the double fences. The horizontal wind velocity decreased approximately 65% at a distance of 3.25 m (i.e., 13H, where H denotes the fence height) downwind the double fences, and no reverse flow or vortex was observed on the leeward side. The sediment flux density decreased exponentially with height above the gravel surface downwind in all tested fences. The reduction percentage of total sediment flux density was higher for the fence with a porosity of 0.5 than for the fence with a porosity of 0.7, especially for the double fences. Furthermore, the decreasing percentage of total sediment flux density decreased with increasing free-stream wind velocity. The results suggest that compared with metal net fence with a porosity of 0.7, the metal net fence with a porosity of 0.5 is more effective for controlling wind-blown sand in the expansive windy area where the Lanzhou-Xinjiang High-speed Railway runs through.

Experimental study of ventilated cavity flow over a 3-D wall-mounted fence

Ventilated cavity flow over a fixed height 3-D wall-mounted fence is experimentally investigated in a cavitation tunnel for a range of free-stream conditions. The impact of 3-D effects on cavity topology is examined, along with the dependence of the cavitation number and drag on the volumetric flow rate coefficient, fence height based Froude number and vapour pressure based cavitation number. Three different flow regimes are identified throughout the range of cavitation numbers for a particular free-stream condition. Generally, the cavity has a typical re-entrant jet closure the intensity of which is found to increase linearly with increasing Froude number. This increase in re-entrant jet intensity causes an increase in drag with Froude number for constant volumetric flow rate coefficient. At low Froude numbers the closure mechanism transitions from a single to a split re-entrant jet. The parameters used to characterize the cavity topology show a linear dependence on Froude number irrespective of the closure mode. The cavity topology and drag are found to be independent of vapour pressure based cavitation number.