Oblique Wind Research Articles

Approximating dispersion from a finite line source

We examine the basis of the approximation suggested by Calder [1973. On estimating air pollution concentrations from a highway in an oblique wind. Atmospheric Environment 7, 863–868], Luhar and Patil [1989. A general finite line source model for vehicular pollution prediction. Atmospheric Environment 23, 555–562], and Esplin [1995. Approximate explicit solution to the general line source problem. Atmospheric Environment 29, 1459–1463] for the concentration downwind of a finite line source when the wind blows at an angle to the source. The Luhar–Esplin (LE) approximation is equivalent to replacing the varying source–receptor distances in the integral representing the concentration caused by the line source by an effective distance along the wind direction between the receptor and the line source. This approximation performs poorly when the wind approaches a direction parallel to the line source. It can be improved by retaining the effective-distance approximation only for evaluation of vertical diffusion and approximating the resulting line source integral with the assumption of small cross-wind distance. The resulting Horst–Venkatram approximation has much smaller errors than those of the LE approximation for the range of receptor locations considered in this paper.

Design Wind Loads on Gable-Ended Low-Rise Buildings with Moderate and Steep Roof Slopes

Wind tunnel model studies were carried out to determine the external wind pressure distributions on large, low-rise buildings with roof pitches of 15° and 35°. The studies showed a significant increase in the magnitude of the mean and peak negative pressure coefficients with an increase in length to span aspect ratio, on the downwind roof and leeward wall, near the windward gable-end of buildings with a steep 35° pitch roof, for oblique approach winds. These oblique approach winds also generate large design wind load effects on the frames near the gable-end. There is little variation in mean and peak pressure coefficients with aspect ratio, across the frame of the building with a moderate 15° pitch roof. AS/NZS 1170.2, which specifies increased suction pressures with aspect ratio on the downwind roof slope and leeward wall of steep pitch roofbuildings compared to the previous edition of the Standard, provides satisfactory design wind load effects on frames near the gable-end. However, some design load effects on frames near the gable-end of the 15° pitch roof building are underestimated by AS/NZS 1170.2.

Wind effects on a large cantilevered flat roof: loading characteristics and strategy of reduction

Mean and extreme pressure distributions on a large cantilevered flat roof model are measured in a boundary layer wind tunnel. The largest peak suction values are observed from pressure taps beneath conical "delta-wing type" corner vortices that occur for oblique winds, then the characteristics and causes of the local peak suctions are discussed in detail. Power spectra of fluctuating wind pressures measured from some typical taps located at the roof edges under different wind directions are presented, and coherence functions of fluctuating pressures are also obtained. Based on these results, it is verified that the peak suctions are highly correlated with the conical vortices. Furthermore, according to the characteristics of wind loads on the roof, an aerodynamic solution to minimize the peak suctions by venting the leading edges and the corners of the roof is recommended. The experimental results show that the suggested strategy can effectively control the generation of the conical vortices and make a reduction of 50% in mean pressures and 25% in extreme local pressures at wind sensitive locations on the roof.

Experimental and numerical study of wind pressures on irregular-plan shapes

This paper presents the results of a program of wind tunnel model tests on pressure distributions for irregular-plan shapes (L- and U-shaped models). The experiments were carried out in a closed-circuit wind tunnel and a multi-channel pressure measurement system was used to measure mean values of loads on 1:100 scale models. The same tests were carried out on a cube-shaped model as an experimental validation. The effectiveness of the model shape in changing the surface pressure distributions is assessed over an extended range of wind directions. The experimental data for the L- and U-shaped models showed different wall pressure distributions from those expected for single rectangular blocks. Furthermore, a Computational Fluid Dynamics (CFD) code was used to illustrate some particular cases and to provide a better understanding of the flow patterns around these irregular-plan models and of the pressure distributions induced on their faces. Computed pressure coefficients have also been compared with wind tunnel results for normal and oblique wind incidence. A general good agreement has been found for normal wind incidence whereas some differences have occurred for other directions.

建物周辺気流におけるCFD解析の予測精度に関する研究 : 風向の変化を伴う2棟建物モデル周辺気流を対象として

The wind tunnel experiment for two rectangular buildings model with the scale ratio of 2:1:l in parallel arrangement with oblique wind angles was carried out. The experimental database was used for validation for the prediction accuracy of CFD technique applied for this flowfield. The special attention was paid to the velocity distribution in strong wind region near ground surface for the validation. The CFD analysis with two turbulence models, i.e., the standard k-ε model and the modified k-ε model, was conducted in order to clarify the prediction accuracy of CFD methods in comparison with the experimental database.

Wind tunnel study on the influence of different parapets on the roof pressure distribution of low-rise buildings

High suction loads appear on roofs of low-height buildings. The use of parapets with appropriate height at the roof edges alleviates these loads. The performance of six parapet configurations to decrease the suction loads induced on roofs by oblique winds has been studied in a low speed wind tunnel. The studied parapet configurations include vertical wall parapets, either solid or porous, and cantilevered parapets formed by a small horizontal roof close to the building roof. Low-height parapets with a medium porosity and cantilevered parapets are more efficient than solid parapets to reduce the wind suctions generated on the roofs by conical vortices.

The influence of Helmholtz resonance on internal pressures in a low-rise building

This paper deals with an investigation of the phenomenon of Helmholtz resonance under oblique wind flow, and an examination of the applicability of the quasi-steady approach to internal pressures in buildings with a dominant opening. Studies on a 1:50 scale model of the Texas Tech University (TTU) test building in a boundary layer simulation show that ‘Helmholtz resonance under oblique wind flow’ produces an extremely strong response in internal pressure fluctuations, in comparison with that obtained under normal onset flow. It is verified that ‘eddy dynamics over the opening’ rather than ‘freestream turbulence’ is responsible for the intense excitation at oblique flow angles, implying that even if the Helmholtz resonance frequency were to be in the tail of the freestream turbulence spectrum, severe excitation would still be possible.Experimental measurements of internal pressures for a range of opening situations also reveal that the quasi-steady approach is inapplicable in the prediction of peak internal pressures. Furthermore, it is demonstrated that while the provisions of the Australian/New Zealand wind loading code—AS/NZS1170.2:2002, which is based upon the quasi-steady method, is adequate as far as mean internal pressures are concerned, it however underpredicts peak internal pressures in some situations. In particular, for the range of situations studied, measurements indicated that peak pressures were up to 25% higher than the AS/NZS1170.2:2002 provisions, in the case of openings in the positive pressure and sidewall regions. It is also shown that for openings located in the sidewall region, peak internal pressures could be just as extremely positive as it can be negative. It is suggested that in the calculation of internal pressures, the AS/NZS1170.2:2002 provide for the use of local pressure factors K l, that are at present applied only to external pressure calculations. Secondly, the code should provide for internal pressure coefficients to be both negative and positive, when openings are located in sidewall regions. Finally, in order to account for the effects of additional fluctuations arising from Helmholtz resonance oscillations, the possibility of the use of an internal pressure factor K i should be explored.

Field Evaluation of Windbreak Protection for Orchards

In wind tunnel research, experiments are carried out with invariable wind speed and wind direction. These ideal circumstances are never encountered in the field. To evaluate the effect of wind barriers in a practical situation a field experiment was set up. A half-standard tree orchard was selected for that purpose. The orchard was protected against excessive winds mainly by a natural barrier of alder bushes combined with a synthetic screen, although alder bushes only and conifers were used as well on some places. The methodology to determine wind speed reduction in full-scale experiments consisted of measuring average wind speed leeward of a barrier during a given time interval simultaneously with the average reference wind speed and the mean wind direction. From these measurements, wind speed reduction and incidence angle of wind direction with the barrier could be derived. In spite of the fact that the results of on-field measurements are not directly comparable with those of wind tunnel research, because of different materials, similar trends were found. An overall wind speed reduction of about 40% at a height of 1 m between the fruit trees was obtained relative to the reference wind speed at a height of 4 m while at the height of 2 m the overall wind speed reduction decreases to 20%. Wind speed reduction is mostly small at a height of 3 m and non-existing at 4 m. Furthermore, the three-dimensional natural barriers in the field experiment act differently than the two-dimensional artificial screens of the wind tunnel research due to the thickness and suppleness of the natural barrier, especially with oblique wind. The intensity of wind speed also affects the wind speed reduction: a higher wind speed results in a smaller wind speed reduction leeward of the natural barrier compared to a lower wind speed. No substantial differences were observed between the three types of barriers. Although measurements were performed in winter to avoid damage during the flowering and fruit bearing season, an appropriate picture of the sheltering effect of the barriers on the orchard could be constituted in spite of the fact that alder bushes and fruit trees had no leaves.

A comparative investigation of the TTU pressure envelope -Numerical versus laboratory and full scale results

Wind tunnel pressure measurements and numerical simulations based on the Reynolds Stress Model (RSM) are compared with full and model scale data in the flow area of impingement, separation and wake for and wind azimuth angles. The phase averaged fluctuating pressures simulated by the RSM model are combined with modelling of the small scale, random pressure field to produce the total, instantaneous pressures. Time averaged, rsm and peak pressure coefficients are consequently calculated. This numerical approach predicts slightly better the pressure field on the roof of the TTU (Texas Tech University) building when compared to the wind tunnel experimental results. However, it shows a deviation from both experimental data sets in the impingement and wake regions. The limitations of the RSM model in resolving the intermittent flow field associated with the corner vortex formation are discussed. Also, correlations between the largest roof suctions and the corner vortex phenomena are observed. It is inferred that the intermittency and short duration of this vortex switching might be related to both the wind tunnel and numerical simulation under-prediction of the peak roof suctions for oblique wind directions.

A novel approach for wind tunnel modelling of transmission lines

This paper is concerned with the behaviour of transmission line cables in severe boundary layer winds. It examines the effect of the scale of turbulence on the response of a line-like structure (cable model) through wind tunnel tests and the comparison of wind tunnel tests with theoretical predictions made through the statistical method using influence lines. Consistency with theory allowed the development of a new modelling approach to conductor systems using a distorted horizontal length scale (span wise) to accommodate these systems in the wind tunnel. Transverse and oblique wind incidences were tested. From the results obtained in the experimental work, it is apparent that the new modelling approach to conductor systems in wind tunnels is a valid technique. It is necessary, however, that a correction be made in the values of the variance of the response measured in the distorted model.

Flow visualization of conical vortices on flat roofs with simultaneous surface pressure measurement

Wind tunnel and full-scale pressure studies of flow over low-rise buildings have repeatedly shown that on the roof, the largest mean and peak suction values are observed for taps beneath the conical “delta-wing type” corner vortices that occur for oblique winds. To better understand the flow mechanism which produces these negative pressure coefficients, a flow visualization study of conical vortex behaviour was performed in the wind tunnels of Colorado State University (CSU) and at full scale at Texas Tech University (TTU). The mean position and size of the vortices as a function of wind direction is presented. Pressures were also simultaneously measured beneath the vortex visualization plane in the wind tunnel for the worst case wind directions. These pressure profiles were correlated with the digitally enhanced images of the vortex flow. The greatest suction was found to follow directly beneath the moving vortex core. For smooth flow, the magnitude of the suction beneath the core was seen to vary inversely with the vortex size, but no relationship between vortex size and suction could be seen for turbulent flow.

Wind load effects and equivalent pressures on low-rise house roofs

Wind loads on the roof of a typical low-rise house are determined by carrying out a wind tunnel model study. The region near the windward gable end are subjected to the largest wind loads. The large suction pressures near the ridge for oblique approach winds exceed data given in AS1170.2. The mean and fluctuating wind pressures and load effects on the roof tributary supported by the second truss from the windward gable end (i.e. truss subjected to the largest wind loads) are analysed in detail. Smaller load effects are generally obtained by applying the covariance integration method with wind tunnel data, compared with values derived from using AS1170.2 data. The equivalent static pressure distributions obtained from the load–response-correlation method are generally smaller than the peak pressures derived from AS1170.2, except near the ridge for oblique approach winds.

Aeolian transport of sediment on a beach during and after rainfall, Wildwood, NJ, USA

A field investigation was conducted 9 March 1994 on a dissipative beach to compare wind characteristics and aeolian transport during and after a light rain. Winds blew alongshore during the monitoring period. Moisture content of surface sediment on the backbeach was above 7% during rain and 4% four hours after rainfall ceased. Trapping rates, measured using a cylindrical vertical trap, were 14.1 kg m −1 h −1 during rainfall and 140.2 kg m −1 h −1 for a period beginning 3 h 33 min after rainfall ceased. Results indicate that light rain, resulting in bulk surface moisture values over 7%, is not sufficient to eliminate transport, but the rate of transport is lower than can occur on the beach three to four hours following rain. The amount of sand trapped at Wildwood during rainfall is relatively high when compared to many previous studies on beaches in the absence of rain. The great length of the beach as a source area, combined with parallel or oblique winds are significant in increasing rates of sediment transport.

Pressures on a scale model of the Texas Tech Building

Measurements of wind pressures on a one-tenth scale model of the Texas Tech Building in a large wind tunnel are described. The mean and RMS pressure coefficients obtained from the model for critical corner tappings for oblique wind directions, are in excellent agreement with the corresponding full-scale values. The largest minimum pressure coefficients for these tappings are approximately 20% less in magnitude than the corresponding full-scale values. The increased Reynolds number for the present test series is believed to play a significant part in the greatly improved agreement obtained.

On shelter efficiency of shelterbelts in oblique wind

The changes of shelter effects in oblique flows are studied by numerical simulations. The simulated results show that horizontal profiles of wind speed and the location of the minimum wind speed (i.e., maximum wind reduction) move toward the shelterbelt when approach flows depart from the normal, and wind speed may exceed the undisturbed wind speed in the middle lee because of the channeling effect of shelterbelts. With increasing wind incidence angle (IA), the minimum wind speed may decrease or increase, and the rate of decrease in the shelter distance may be faster or slower than cos(IA), both depending on the height of observation and the density and width of shelterbelts. The mean wind speed reduction over 30 H ( H is shelterbelt height) leeward shows similar characteristics to the shelter distance. The change of shelter effects in oblique flows may result from (i) change of effective shelterbelt density, (ii) different efficiencies in reducing wind speed in directions perpendicular and parallel to the belt, and (iii) change of horizontal wind direction as the flow recovers to the undisturbed direction. The relative importance of each factor, which depends on the height of observation and the density and width of shelterbelts, determines the variation of shelter effects. The simulations produced all the observed qualitative characteristics of shelter effects and for those previous reports giving measured values, the model produced results that are in good agreement.

Oblique wind waves generated by the instability of wind blowing over water

The growth rates of gravity waves are computed from linear, inviscid stability theory for wind velocity profiles that are representative of the mean flow in a turbulent boundary layer. The energy transfer to the waves is largely concentrated in an angle (to the wind) interval that broadens with increasing wind speed and narrows with increasing wavelength. At sufficiently high wind speeds and sufficiently short wavelengths, the waves of maximum growth rate propagate at an oblique angle to the wind. The connection with bimodal directional distributions of observed spectra is discussed.

Approximate explicit solution to the general line source problem

A computationally efficient approximate solution to the general line source problem of finite length and oblique wind angle is presented. The solution is valid for wind angles greater than 15°; for wind angles below this value a point-source approximation is presented.

Numerical evaluation of wind pressures on flat roofs with the k- ε model

The results of an investigation regarding the numerical evaluation of wind pressures on flat roofs by using the Reynolds averaged Navier-Stokes equations and the standard k- ε turbulence model are presented. A low and a taller building model are considered and wind blowing from different directions is assumed. Data are presented for roof edges and corners. Experimental measurements are also carried out in a boundary layer wind tunnel and results are compared with the computed data. Over most of the roof surface area, the comparisons are good for normal wind conditions and the low building model. However, the present approach does not seem to yield adequate representations of available experimental data in the high vorticity region near the windward roof edges of the building, particularly for oblique wind conditions. The influence of proximity of grid points to the roof surface is examined in detail.

The role of wind direction in eolian transport on a narrow sandy beach

AbstractComparison of eolian transport during five high‐velocity wind events over a 29 day period on a narrow estuarine beach in Delaware Bay, New Jersey, USA, reveals the temporal variability of transport, due to changes in direction of wind approach. Mean wind speed measured 6 m above the dune crest for the five events ranged from 8·5 to 15·9 ms−1. Mean wind direction was oblique to the shoreline (63° from shore‐normal) during one event but was within 14° of shore‐normal during the other events.Eolian transport is greatest during low tide and rising tide, when the beach source area is widest and when drying of surface sediments occurs. The quantity of sediment caught in a vertical trap for the five events varied from a total of 0·07 to 113·73 kgm−1. Differences in temperature, relative humidity and moisture and salt content of surficial sediments were slight. Mean grain sizes ranged from 0·33 to 0·58 mm, causing slight differences in threshold shear velocity, but shear velocities exceeded the threshold required for transport during all events.Beach width, measured normal to the shoreline, varied from 15·5 to 18·0 m; beach slope differed by 0·5°. The oblique wind during one event created a source width nearly double the width during other days. Beach slope, measured in the direction of the wind, was less than half as steep as the slope measured normal to the shoreline. The amount of sand trapped during the oblique wind was over 20 times greater than any other event, even those with higher shear velocities. The ability of the beach surface to supply grains to the air stream is limited on narrow beaches, but increased source width, due to oblique wind approach, can partially overcome limitations of surface conditions on the beach.

Numerical simulation of wind-induced pressures on buildings of various geometries

The paper presents the numerical prediction of turbulent wind pressures on the surfaces of various buildings with sharp corners of right angles for different wind directions. The computer program developed for this purpose solves the time-averaged Navier-Stokes equations by using the k-ε turbulence model for closure. The computational procedure is applicable to any arbitrarily-shaped building composed of two rectangular blocks. Computed pressures have been compared with experimentally obtained values for a stepped-roof building (L-shaped in cross-section). Good agreement is found for normal wind directions while differences exist in oblique wind cases.