Oblique Wind Research Articles

Numerical simulations of wind-driven rain on building facades under various oblique winds based on Eulerian multiphase model

Wind-driven rain (WDR) is an important factor affecting the hygric behavior and durability of building facades. In recent years, CFD simulations of WDR impinging on building facades almost have been focusing on isolated buildings for wind directions perpendicular to the facades. Very few efforts have been made towards study of complicated WDR on buildings considering group effect and the effect of varying wind direction. By virtue of the WDR simulation approach based on the Eulerian multiphase model, this paper presented a validation study of CFD simulations of WDR for an array of low-rise building models provided by recently field measurements. The simulation results suggested that CFD simulations can provide fairly accurate estimates of WDR for complex situations by the present method. And simulations of WDR are performed for a single building and building complex, analyzing the characteristics of catch-ratio distribution with various wind directions and focusing on the impact of group effect and varying wind direction on WDR. The differences of WDR distribution on the building facades are presented and discussed by comparing the results of the single with those of the complex. The conclusions in this paper can provide references for further research of WDR and design of engineering.

Numerical simulations of wind-driven rain on an array of low-rise cubic buildings and validation by field measurements

The relation between wind-driven rain (WDR) and its potential negative effects on the hygrothermal performance and durability of building facades can be better understood by the correct estimation of the spatial and temporal distribution of the WDR intensity. Computational Fluid Dynamics (CFD) simulations with Eulerian Multiphase (EM) modeling are used to obtain accurate spatial and temporal information on WDR. The EM model has the advantage of predicting the WDR intensity on all surfaces of a complex geometry within the domain at once. There is a lack of numerical studies on the WDR intensity in generic and idealized multi-building configurations. In this paper, WDR intensities on an array of 9 low-rise cubic building models for wind from three different wind directions are estimated numerically using the EM model including the turbulent dispersion of raindrops. The numerical results are validated by comparing the calculated catch ratio values with data from field measurements in Dübendorf, Switzerland after two rain events with different characteristics. The CFD simulations successfully estimate the WDR intensities at the positions of 18 WDR gauges for both rain events. The influence of turbulent dispersion is found to be lower than 3% for both rain events. It is found that, for oblique wind directions, even though the maximum WDR intensity on the facades is lower, the whole building is exposed to up to 57% more WDR.

Quantitative ventilation assessments of idealized urban canopy layers with various urban layouts and the same building packing density

This paper investigates urban canopy layers (UCLs) ventilation under neutral atmospheric condition with the same building area density (λp = 0.25) and frontal area density (λf = 0.25) but various urban sizes, building height variations, overall urban forms and wind directions. Turbulent airflows are first predicted by CFD simulations with standard k-ε model evaluated by wind tunnel data. Then air change rates per hour (ACH) and canopy purging flow rate (PFR) are numerically analyzed to quantify the rate of air exchange and the net ventilation capacity induced by mean flows and turbulence. With a parallel approaching wind (θ = 0°), the velocity ratio first decreases in the adjustment region, followed by the fully-developed region where the flow reaches a balance. Although the flow quantities macroscopically keep constant, however ACH decreases and overall UCL ventilation becomes worse if urban size rises from 390 m to 5 km. Theoretically if urban size is infinite, ACH may reach a minimum value depending on local roof ventilation, and it rises from 1.7 to 7.5 if the standard deviation of building height variations increases (0%–83.3%). Overall UCL ventilation capacity (PFR) with a square overall urban form (Lx = Ly = 390 m) is better as θ = 0° than oblique winds (θ = 15°, 30°, 45°), and it exceeds that of a staggered urban form under all wind directions (θ = 0°–45°), but is less than that of a rectangular urban form (Lx = 570 m, Ly = 270 m) under most wind directions (θ = 30°–90°). Further investigations are still required to quantify the net ventilation efficiency induced by mean flows and turbulence.

A study of interunit dispersion around multistory buildings with single-sided ventilation under different wind directions

This study examines the interunit dispersion characteristics in and around multistory buildings under wind-induced single-sided ventilation conditions using computational fluid dynamics (CFD) method, under the hypothesis that infectious respiratory aerosols exhausted from a unit can reenter into another unit in a same building through opened windows. The effect of balconies on the interunit dispersion pattern is considered. The RNG k − ε model and the two-layer near-wall model are employed to establish the coupled indoor and outdoor airflow field, and the tracer gas technique is adopted to simulate pollutant dispersion. Reentry ratios from each unit to other units under prevailing wind directions are quantified and the possible interunit dispersion routes are then revealed. It is found that many reentry ratios appear to reach around 10.0%, suggesting that the interunit dispersion is an important pollutant transmission route. The interunit dispersion pattern is highly dependent on the incident wind direction and the fact whether the building has protrusive envelope features. On average, the strongest dispersion occurs on the windward wall of the buildings under oblique wind direction, owing to high ACH (air change per hour) values and unidirectional spread routes. Except under a normal incident wind, the presence of balconies intensifies the interunit dispersion by forming dispersion channels to increase the reentry ratios.

How to Parametrize Urban-Canopy Drag to Reproduce Wind-Direction Effects Within the Canopy

The mean wind direction within an urban canopy changes with height when the incoming flow is not orthogonal to obstacle faces. This wind-turning effect is induced by complex processes and its modelling in urban-canopy (UC) parametrizations is difficult. Here we focus on the analysis of the spatially-averaged flow properties over an aligned array of cubes and their variation with incoming wind direction. For this purpose, Reynolds-averaged Navier–Stokes simulations previously compared, for a reduced number of incident wind directions, against direct numerical simulation results are used. The drag formulation of a UC parametrization is modified and different drag coefficients are tested in order to reproduce the wind-turning effect within the canopy for oblique wind directions. The simulations carried out for a UC parametrization in one-dimensional mode indicate that a height-dependent drag coefficient is needed to capture this effect.

Pollutant Concentrations in Street Canyons of Different Aspect Ratio with Avenues of Trees for Various Wind Directions

This study summarizes the effects of avenues of trees in urban street canyons on traffic pollutant dispersion. We describe various wind-tunnel experiments with different tree-avenue models in combination with variations in street-canyon aspect ratio W/H (with W the street-canyon width and H the building height) and approaching wind direction. Compared to tree-free street canyons, in general, higher pollutant concentrations are found. Avenues of trees do not suppress canyon vortices, although the air ventilation in canyons is hindered significantly. For a perpendicular wind direction, increases in wall-average and wall-maximum concentrations at the leeward canyon wall and decreases in wall-average concentrations at the windward wall are found. For oblique and perpendicular wind directions, increases at both canyon walls are obtained. The strongest effects of avenues of trees on traffic pollutant dispersion are observed for oblique wind directions for which also the largest concentrations at the canyon walls are found. Thus, the prevailing assumption that attributes the most harmful dispersion conditions to a perpendicular wind direction does not hold for street canyons with avenues of trees. Furthermore, following dimensional analysis, an estimate of the normalized wall-maximum traffic pollutant concentration in street canyons with avenues of trees is derived.

Modelling Street-Scale Flow and Dispersion in Realistic Winds—Towards Coupling with Mesoscale Meteorological Models

Further to our previous work—simulations of flow and dispersion in an oblique wind over the DAPPLE site (Xie and Castro, Atmos Environ 43:2174–2185, 2009)—large-eddy simulations of flows and dispersion over the same site in a wind perpendicular to Marylebone Road and the windward surfaces of most of the buildings were performed. The DAPPLE site is located at the intersection of Marylebone Road and Gloucester Place in central London. In order to investigate the effects of wind direction on flows and dispersion, the velocity and scalar fields in the perpendicular wind were compared with those in the oblique wind. Furthermore, realistic wind conditions measured on the BT Tower at 190 m above street level were processed and used to drive the numerical simulations of flows and dispersion at the DAPPLE site. This leads to significant predictive improvements of the dispersion compared with field measurements, which provides validation and confidence for coupling mesoscale meteorological models, e.g. the UK Met Office’s Unified Model and the NCAR’s Weather Research & Forecasting Model, with the street-scale large-eddy simulation of urban environments.

Wind Loads and Responses of Two Neighboring Dry Coal Sheds

This paper discusses an investigation into the interference effects on the wind-induced responses of a dry coal shed structure used for depositing coal stacks. Two interference factors, a nearby shed and coal stacks inside the shed, are considered. A wind tunnel test was conducted to obtain wind pressure distributions on the roof surface of the shed, and the results are briefly analyzed. The characteristics of the wind-induced responses of the structure and the effects of interference factors are then illustrated. The paper places emphasis on wind-induced responses and interference factors variations, and points out the situations when the maximum interference effects occur. The effects of an adjacent shed and incident wind direction are more pronounced than the spacing of adjacent sheds and coal stacks inside the shed. The most unfavorable results appear when an oblique wind blows at 45° span-wise with no interfering shed nearby, but when there is low coal stack inside. Values of the interference factor are also slightly higher than 1.0 for the maximum absolute peak responses in all cases investigated. This indicates that interference conditions magnify the peak responses of an isolated roof only minimally.

Computational study on aerodynamic mitigation of wind-induced, large-amplitude vibrations of stay cables with strakes

Modifications of circular cylinder surfaces, such as strakes and helical wires, effectively mitigate Kármán vortex-induced vibrations normal to flow and have been applied to the reduction of large-amplitude vibrations of stay cables in bridges, which occur under wind oblique to a cable with or without rainfall. This aerodynamic control method cannot be fully effective without understanding the behavior of the flow around and the associated forces on oblique cables. To address this issue, flow around a yawed cylinder with various strake patterns was studied using three-dimensional detached eddy simulation (DES) at Reynolds number of 1.4×10 5. Results demonstrated that strake patterns strongly influence the development of flow structures around a yawed cylinder and therefore the associated forces on the cylinder. The results suggest that particular strake patterns can mitigate large-amplitude and low-frequency vibrations of stay cables induced by oblique wind.

Cyclic cross-bedding in the eolian dunes of the Sergi Formation (Upper Jurassic), Recôncavo Basin: Inferences about the wind regime

The Upper Jurassic Sergi Formation displays eolian cyclic cross-bedding, composed of a regular intercalation of grain-flow and wind-ripple packages, forming 1–3 m thick cycles. Each cycle is bound by erosive downwind surfaces, and it comprises a sequence of lower angle, wind-ripple laminae followed by steeper grain-flow strata that built out upon the wind-ripple wedges in the migration direction. The cross-strata mean vectors dip consistently towards SW. These cycles are related to seasonal changes in wind direction. The cyclic stratification pattern suggests an alternation of northeasterly transverse winds responsible for slipface evolution and dune migration, followed by reverse transversal or oblique winds (overall southerly winds). These winds caused dune degradation and reworking and led to the formation of reactivation surfaces and the related deposition of wind-ripple wedges. The wind pattern inferred from the eolian cyclic cross-bedding is consistent with the quantitative atmospheric circulation model that indicates that a monsoonal wind regime operated in the low latitudes of Gondwana during the Late Jurassic.

Barchan dune asymmetry: Observations from Mars and Earth

Barchan dune asymmetry refers to the extension of one barchan limb downwind. It is a common dune form on Earth and also occurs on Mars and Titan. A new classification of barchan limbs is presented where three types of limb morphology are identified: linear, kinked and beaded. These, along with other dune-scale morphological signatures, are used to identify three of the causes of barchan asymmetry on Mars: bi-directional winds, dune collision and the influence of inclined topography. The potential for specific dune asymmetric morphologies to indicate aspects of the formative wind regime on planetary surfaces is shown. For example, the placement of dune limbs can indicate the general direction and relative strength of formative oblique winds; an extreme barchan limb length may indicate a long duration oblique wind; a kinked limb may be evidence of the passage of a storm; beaded limbs may represent surface-wave instabilities caused by an increase in wind energy parallel to the dune. A preliminary application of these signatures finds evidence for bi-modal winds on Mars. However, these and other morphological signatures of wind direction and relative strength should be applied to planetary landforms with caution as more than one process (e.g., bi-modal winds and collision) may be operating together or sequentially on the dunefield. In addition, analysis should be undertaken at the dunefield scale and not on individual dunes. Finally, morphological data should be acquired from similar-scale dunes within a dunefield. In addition to bi-modal wind regimes on Mars, the frequent parallel alignment of the extended barchan limb to the dune suggests that dune collision is also an important cause of asymmetry on Mars. Some of the more complex dunefield patterns result from a combination of dune collision, limb extension and merging with downwind dunes. Dune asymmetric form does not inhibit dune migration in the Namib Desert or on Mars. Data from the Namib suggest that dune migration rates are similar for symmetric and asymmetric dunes. Further modeling and field studies are needed to refine our understanding of the potential range of limb and dune morphologies that can result from specific asymmetry causes.

Wind turbine extreme gust control

AbstractThis paper focuses on the problem of extreme wind gust and direction change recognition (EG&DR) and control (EEC). An extreme wind gust with direction change can lead to large loads on the turbine (causing fatigue) and unnecessary turbine shutdowns by the supervisory system caused by rotor overspeed. The proposed EG&DR algorithm is based on a non‐linear observer (extended Kalman filter) that estimates the oblique wind inflow angle and the blade effective wind speed signals, which are then used by a detection algorithm (cumulative sum test) to recognize extreme events. The non‐linear observer requires that blade root bending moments measurements (in‐plane and out‐of‐plane) are available. Once an extreme event is detected, an EEC algorithm is activated that: (i) tries to prevent the rotor speed from exceeding the overspeed limit by fast collective blade pitching; and (ii) reduces 1p blade loads by means of individual pitch control algorithm, designed in an ℋ∞ optimal control setting. The method is demonstrated on a complex non‐linear test turbine model. Copyright © 2009 John Wiley & Sons, Ltd.

Numerical simulation of dispersion around an isolated cubic building: Comparison of various types of k– ɛ models

Prediction accuracy of flow and dispersion around a cubic building with a flush vent located on its roof was examined using various k– ɛ models, and numerical results were compared with wind-tunnel data. Four types of turbulence models, i.e., the standard k– ɛ model, the RNG k– ɛ model, the k– ɛ model with Launder and Kato modification and the Realizable k– ɛ model were compared in this study. The standard k– ɛ model provided inadequate results for the concentration field, because it could not reproduce the basic flow structure, such as the reverse flow on the roof. However, revised k– ɛ models provided concentrations in better agreement with the experimental data. The effect of an oblique wind angle and vent locations on the prediction accuracy was also investigated. It was confirmed that the prediction accuracy of the velocity field strongly affected that of the concentration field. The RNG model showed general agreement with the experiment, and was the best of the turbulence models tested. However, it becomes clear that the results for all CFD models show poor prediction accuracy of concentration distribution at the side and leeward surfaces of the building since they all underestimate the concentration diffusion on these regions. The concentrations predicted by all CFD models were less diffusive than those of the experiment.

Contamination of Fresh Air Intakes Due to Downwash from a Rooftop Structure

The effect of rooftop structures on the dispersion of building exhaust was evaluated using wind tunnel experiments with an isolated building for four different full-scale equivalent heights varying from 15 to 70 m. The stack was located on the roof of the building downwind of the rooftop structure. In some cases, downwash produced by the rooftop structure caused a significant reduction in dilution (i.e., increased contaminant concentration) compared with results obtained with a flat-roofed model. For the wind normal to the building, the effect of the rooftop structure depended on building height and the crosswind width of the structure. Maximum reduction in dilution occurred for the 15-m building with a wide penthouse. The influence of the rooftop structure on plume behavior for the normal wind diminished as building height increased because of an increase in size of the separated flow region above the roof. On the other hand, for an oblique wind at 45°, the rooftop structure produced low dilution for all building heights. The paper shows that the American Society of Heating, Refrigerating and Air-Conditioning Engineers geometric stack design method and dilution model can lead to an unconservative design.

Responses of three-dimensional flow to variations in the angle of incident wind and profile form of dunes: Greenwich Dunes, Prince Edward Island, Canada

This study reports the responses of three-dimensional near-surface airflow over a vegetated foredune to variations in the conditions of incident flow during an 8-h experiment. Two parallel measurement transects were established on morphologically different dune profiles: i) a taller, concave–convex West foredune transect with 0.5-m high, densely vegetated (45%), seaward incipient foredune, and ii) a shorter, concave-straight East foredune transect with lower, sparsely vegetated (14%) seaward incipient foredune. Five stations on each transect from the incipient dune to the crest were equipped with ultrasonic anemometers at 0.6 and 1.65 m height and logged at 1 Hz. Incident conditions were recorded from a 4-m tower over a flat beach. Winds increased from 6 m s − 1 to > 20 m s − 1 and were generally obliquely onshore (ENE, 73°). Three sub-events and the population of 10-minute averages of key properties of flow ( U, W, S, CV U) from all sample locations on the East transect ( n = 235) are examined to identify location- and profile-specific responses over 52° of the incident direction of flow (from 11 to 63° onshore). Topographic steering and forcing cause major deviations in the properties and vectors of near-surface flow from the regional wind. Topographic forcing on the concave-straight dune profile increases wind speed and steadiness toward the crest, with speed-up values to 65% in the backshore. Wind speed and steadiness of flow are least responsive to changes in incident angle in the backshore because of stagnation of flow and are most responsive at the lower stoss under pronounced streamline compression. On the steeper concave–convex profile, speed and steadiness decrease toward the crest because of stagnation of flow at the toe and flow expansion at the slope inflection point on the lower stoss. Net downward vertical velocity occurs over both profiles, increases toward the crest, and reflects enhanced turbulent momentum conveyance toward the surface. All of these flow responses are enhanced with faster speeds of incident flow and/or more onshore winds. Significant onshore steering of near-surface vectors of flow (to 37°) occurs and is greatest closer to the surface and during highly oblique winds (~ 15° onshore). Therefore, even subtle effects of streamline compression and amplification of flow under alongshore conditions effectively steer flow and sand transport toward the dune. As topographic forcing and steering cause significant, three-dimensional deviations in near-surface properties of flow, most regional-scale and/or two-dimensional models of dune process-response dynamics are insufficient for characterizing coastal and desert dune sediment budgets and morphodynamics. In particular, deflection of sand transport vectors with greater fetch distances than those derived from regional winds may occur. Coincident flow, transport and morphological response data are required to better quantitatively model these processes.

On the validity of numerical wind-driven rain simulation on a rectangular low-rise building under various oblique winds

Wind-driven rain (WDR) is one of the most important boundary conditions for hygrothermal building envelope analysis. Although Computational Fluid Dynamics (CFD) simulation of WDR on building facades has been applied intensively in the past decade, validation is still quite limited, and most previous validation efforts have focused either on wind directions perpendicular to the facade or on buildings of complex geometry. This paper addresses CFD simulations of WDR on the west facade of a simple, rectangular low-rise test building for various oblique winds and CFD validation by comparing the simulation results with full-scale measurements. It is shown that overall, fairly accurate results can be obtained, but that the numerical simulations can significantly underestimate the WDR amounts near the downwind edge of the facade when the wind direction is increasingly oblique. These discrepancies are at least partly attributed to the very small impact angles of the raindrops at these facade positions and the resulting inaccuracies in the numerical model.

高盛土に対応した新型防雪柵の開発とその視程障害緩和効果について

In cold and snowy regions, road traffic is prone to hindrance from snowstorm-induced poor visibility and snowdrifts. To mitigate these hindrances, snow fences have been installed along roads. In recent years, roads have often been constructed on high embankments, as is often seen on high-standard national highways. For roads on high embankments, collector snow fences are installed on the slopes of embankments to mitigate poor visibility from blowing snow and to secure the effective road width. In such cases, the top of the snow fence does not extend much above the road surface, which gives the fence less ability to mitigate poor visibility than for collector snow fences installed on roads without embankments. We has developed an advanced snow fence, whose performance on roads on embankments is superior to that of conventional snow collector fences. Advanced snow fences are 7 m in height. To reduce the wind pressure acting on the fence, the metal slats in the upper section are highly porous. The top third consists of mesh panel with 80 % porosity, the middle third consists of perforated slats with 30 % porosity, and the bottom third consists of non-perforated slats with 0 % porosity. The fences have proven superior to conventional snow fences in improving visibility under oblique winds.

The UWO contribution to the NIST aerodynamic database for wind loads on low buildings: Part 3. Internal pressures

Wind tunnel tests of generic low buildings have been conducted at the University of Western Ontario for contribution to the National Institute on Standards and Technology (NIST) aerodynamic database. Part 1 provided the archiving format and basic aerodynamic data. In Part 2, the data of external pressures were compared with existing wind load provisions for low buildings. This paper, Part 3, deals with an investigation of wind-induced internal pressures of low-rise buildings with realistic dominant opening and leakage scenarios. Data from one building model with four different opening sizes were compared with numerical simulations. The existing theory, using the unsteady orifice discharge equation, works well for the building models used in this study, given the external pressures near the openings, irrespective of shifts of wind direction and upstream terrain. Numerical simulations can capture the temporal variations of the internal pressure fluctuations, as well as mean values. The internal pressure fluctuations for the building with leakage (nominally sealed building) are attenuated as they pass through the openings, while mean values are consistent with spatially averaged external pressures. Internal pressure resonance occurs for the dominant opening (3.3% open ratio) with building leakage. Effects of oblique wind angles on internal pressure dynamics are not significant, at least for the openings in the centre of wall, as is the case herein. Peak internal pressures occur for a wind direction normal to the wall with a dominant opening. Measured internal pressure coefficients are compared with current wind load provisions. Some peak values were found to exceed the recommended design values for the dominant windward wall opening cases.

A comparison between CFD and Network models for predicting wind-driven ventilation in buildings

The aim of this paper is to compare the implementation of computational fluid dynamics (CFD) and Network models for airflow rate estimation in buildings. The CFD software used is Fluent 5.5. Comparison between the predicted and simulated airflow rate is suggested as a validation method of the implemented CFD code, while the common practice is to compare CFD outputs to wind tunnel or full-scale measurements. This could be useful for studies that have no access to laboratory or full-scale testing facilities. Results obtained from testing a number of cases have been compared and analysed, considering normal and oblique wind directions. The comparison held between mathematical and CFD results generally showed a good agreement, which seems to justify the use of CFD code for predicting natural ventilation in buildings.

Aeolian sediment transport on a human‐altered foredune

AbstractChanges in wind speed and sediment transport are evaluated at a gap and adjacent crest of a 2 to 3 m high, 40 m wide foredune built by sand fences and vegetation plantings on a wide, nourished fine sand beach at Ocean City, New Jersey. Anemometer masts, cylindrical sand traps and erosion pins were placed on the beach and dune during two obliquely onshore wind events in February and March 2003. Results reveal that: (1) changes in the alongshore continuity of the beach and dune system can act as boundaries to aeolian transport when winds blow at an angle to the shoreline; (2) oblique winds blowing across poorly vegetated patches in the dune increase the potential for creating an irregular crest elevation; (3) transport rates and deflation rates can be greater within the foredune than on the beach, if the dune surface is poorly vegetated and the beach has not had time to dry following tidal inundation; (4) frozen ground does not prevent surface deflation; and (5) remnant sand fences and fresh storm wrack have great local but temporary effect on transport rates. Temporal and spatial differences due to sand fences and wrack, changes in sediment availability due to time‐dependent differences in surface moisture and frozen ground, combined with complex topography and patchy vegetation make it difficult to specify cause–effect relationships. Effects of individual roughness elements on the beach and dune on wind flow and sediment transport can be quantified at specific locations at the event scale, but extrapolation of each event to longer temporal and spatial scales remains qualitative. Copyright © 2006 John Wiley & Sons, Ltd.