Effect of volumetric heat sources on hysteresis phenomena in natural and mixed-mode ventilation

The summer-time cooling efficiency of hybrid buildings depends critically upon exploiting multiple environmental resources to dispose of waste heat. To this end, many previous studies have explored the role of wind, which exerts different static pressures on a building's windward and leeward facades. Here, we consider how this methodology may be extended to the converse problem of winter-time heating wherein hot, buoyant air is purposefully supplied to the interior space using a coupled ventilation scheme. A “blocked” flow regime is desired such that cold air inflow is impeded; to avoid interstitial condensation, the pressure distribution within the building must favor outflow through designated extraction vents. For the idealized geometry considered here, blocked conditions represent a unique solution to the flow equations in well-defined regions of parameter space. The likelihood of blocking may be increased through prudent choice of extraction vent size/orientation depending on the external forcing conditions. A discussion of the inherent tradeoffs associated with multi-season design of hybrid buildings is also presented.     

低层坡屋面房屋风荷载特性风洞试验研究

对低层双坡屋面和四坡屋面建筑进行了风洞试验研究,考虑了屋面形式、屋面坡度、来流方向和挑檐长度等不同因素对屋面风压分布的影响,分析了屋面平均和脉动风压系数的分布特性。结果表明,0°风向角（来流垂直吹向屋脊）、屋面坡度为30°时,迎风屋面屋檐及屋脊附近形成较高负压,迎风屋面风压系数呈环状分布;屋面坡度为15°时,迎风屋面风压系数呈阶梯状分布。屋面体型系数受风向角、屋面坡度和屋面形式的影响较大：0°风向角、双坡屋面模型中,15°屋面坡度迎风屋面体型系数为30°屋面坡度的2.76倍;四坡屋面模型中,15°屋面坡度迎风屋面体型系数为30°屋面坡度的228倍;背风屋面体型系数受屋面坡度的影响较小;0°和45°风向角下,对于15°和30°屋面坡度,当屋面坡度相同,屋面形式由双坡改为四坡时,迎风屋面的体型系数绝对值有所增大,屋面更容易受力破坏,但对背风屋面的影响较小。     

严寒地区住宅建筑冬季通风研究

针对严寒地区节能住宅建筑冬季卫生通风不足的问题，本文应用Fluent软件对安装通风装置的示范住宅建筑的室外风场、室内气流组织及温度分布进行模拟研究并实测室内空气温度场。在严寒地区，为保证适当的卫生通风和气流组织，应加强热压的作用，而尽量避免风压的不利影响。对严寒地区典型的建筑住区不同尺度的外部风环境模拟分析结果表明：除建筑住区的迎风面建筑外，整个建筑住区及建筑子区内风场分布较为均匀，符合严寒地区冬季避免冷风侵入耗热量的建筑节能设计。为避免迎风面建筑较高层住宅的背风向房间出现进风口变为排风口的现象，需加强屋顶排风风口处的引风作用；示范性通风建筑室内的进风角度及安装位置、建筑构件、家具的材料以及家具摆放位置等是影响室内热舒适性和室内气流组织的主要原因。     

Computational analysis of wind driven natural ventilation in buildings

The design of natural ventilation in buildings is often performed by means of computational fluid dynamics (CFD) techniques, whose application is gaining popularity. In the present study, Reynolds averaged Navier–Stokes equation (RANS) approach is applied to wind driven natural ventilation in a cubic building. Two different models are considered, namely the two-equation k–ɛ model and the Renormalization Group (RNG) theory. The velocity and pressure distribution inside and around the building are determined, as well as the ventilation rate, for three different configurations: cross ventilation, single-sided ventilation with an opening on the windward wall and single-sided ventilation with an opening on the leeward wall. The numerical results are compared with experimental data, showing a good agreement, particularly when using RNG. The discrepancy in the determination of the ventilation rate is reasonable and the flow distribution inside the building is properly described when RNG model is used. However, the k–ɛ model fails to determine the correct velocity components near the horizontal surfaces. According to these results, the RNG model can be considered a useful tool for the study of wind driven natural ventilation, especially for the assessment of the ventilation rate and of the air distribution inside a room.     

Full scale experimental study of single-sided ventilation: Analysis of stack and wind effects

Natural ventilation can contribute to the reduction of the air conditioning demand and to the improvement of thermal comfort in buildings. In this paper, the flow field and the air change rate generated by a simple configuration of natural ventilation, namely single-sided ventilation, are examined experimentally. The experiments are realized in a full scale building exposed to outdoor conditions, using several measurement techniques. The main features of the flow generated by stack and wind effect are examined for different outdoor conditions (temperature difference, wind speed and direction). Finally, measured air change rates are compared to those calculated by existing correlations in order to analyze their applicability to the experimental configuration. Results show that the wind generates turbulence diffusion at the opening, counteracting the stack effect. Moreover, in the case of windward opening, there is an additional effect, namely the effect of mixing layer, which tends to increase the airflow rate. Existing correlations give reasonably good results in the case of windward opening, while in the case of leeward opening they overestimate the airflow rate.     

Numerical simulation of the wind field around different building arrangements

A numerical model with the RNG κ−ε turbulence closure model and a pressure correction algorithm of SIMPLEC is used to examine three different building configuration effects on wind flow. Comparisons of computational results with experimental data have been carried out for the vertical velocity profiles at some measurement points. For the experimental study, the building arrangements were presented by 1:150 scale models and tested in a low-speed wind tunnel. It was found that the wind environment for two improved arrangements with lower interval-to-height ratio is better than that for the reference layout with higher aspect ratio in terms of the natural ventilation. The interference effect is more obvious for two improved arrangements than the reference one. The numerical results also show that changing wind direction from perpendicular to the building facades to a 45°-incidence angle has significant effect on the flow field for different configurations.     

Effectiveness of shelterbelt with a non-uniform density distribution

The ecological project of shelterbelts in oasis-desert ecotone is a main measure to wind-sand damage, which is one of the most serious eco-environment problems in arid regions. The effectiveness of shelterbelts with a non-uniform density distribution, planted perpendicular to the prevailing wind direction, was studied through field observations. The results showed that the main factor influencing wind speed is the density distribution of a particular shelterbelt of a specifically given tree specie. In this study, the shelter effects of a sparse shelterbelt was compared with a dense shelterbelt and it corresponded with most previous reports, namely that dense shelterbelts resulted in a higher wind reduction whilst the sparse shelterbelts have a longer shelter distance at the leeward side of the belt. For shelterbelts with a non-uniform density distribution and wind blowing from high density to low density, wind speed changes will be similar to that of a high-density shelterbelt at both the windward and leeward sides of the shelterbelt. However, when the wind blows from the opposite direction, the flow fields change greatly as compared to an evenly distributed shelterbelt. A combined effect of density manifested, thus providing better shelter effects by balancing the reduction in wind speed with the shelter distance.     

Wind-induced natural ventilation of re-entrant bays in a high-rise building

This paper reports a systematic computational study of wind-induced natural ventilation and pollutant transport of re-entrant bays on a total of 30 generic building models of different building heights and with bays of different dimensions. Mean wind flow around each building model and wind-induced flow inside re-entrant bays are computed. To determine the ventilation efficiency of the bay, the computed flow field is used to disperse a scalar pollutant initially occupying the entire bay at a uniform concentration. The subsequent time decay of pollutant concentration inside the bay is studied and the ventilation efficiency is quantified by the retention time. The results show that wind-induced flow inside the bay, especially on the building side face, is complex and highly three-dimensional. Air exchange rates through the roof opening and vertical side opening are analyzed for each bay and their relationship to the ventilation efficiency is discussed. The bays on the building side faces are much worse ventilated than those on the windward or leeward building face. The deeper the side bay, the worse is the air exchange and ventilation. The building height is found to have a governing effect on the ventilation of the windward and leeward re-entrant bays.     

The fluid mechanics of natural ventilation—displacement ventilation by buoyancy-driven flows assisted by wind

This paper describes the fluid mechanics of natural ventilation by the combined effects of buoyancy and wind. Attention is restricted to transient draining flows in a space containing buoyant fluid, when the wind and buoyancy forces reinforce one another. The flows have been studied theoretically and the results compared with small-scale laboratory experiments. Connections between the enclosure and the surrounding fluid are with high-level and low-level openings on both windward and leeward faces. Dense fluid enters through windward openings at low levels and displaces the lighter fluid within the enclosure through high-level, leeward openings. A strong, stable stratification develops in this case and a displacement flow is maintained for a range of Froude numbers. The rate at which the enclosure drains increases as the wind-induced pressure drop between the inlet and outlet is increased and as the density difference between the exterior and interior environment is increased. A major result of this work is the identification of the form of the nonlinear relationship between the buoyancy and wind effects. It is shown that there is a Pythagorean relationship between the combined buoyancy and wind-driven velocity and the velocities which are produced by buoyancy and wind forces acting in isolation. This study has particular relevance to understanding and predicting the air flow in a building which is night cooled by natural ventilation, and to the flushing of gas from a building after a leak.     

Experimental investigation and CFD analysis of cross-ventilated flow through single room detached house model

Cross-ventilation is a complicated flow problem and difficult to control because wind exhibits a large degree of variation. The paper focuses on three items: a) to clarify and understand some of the basic characteristics of airflow as the influence of the opening size on the windward vortex and the leeward wake; b) to explore what information about the flow above the ground can be retrieved from pressure measurements on the ground; and c) to explore the accuracy of CFD. To meet these objectives, wind tunnel tests and CFD analyses were carried out. The studied object was a detached- house model provided with two openings. The size of these openings was changed in a wide range from narrow cracks to large openings. In the experiments, pressure measurements on the ground and PIV measurements were made. The internal flow was visualized with the sand erosion method. Pressure measurement on a floor surface is a relatively easy and an inexpensive method. In this experiment, the windward and leeward areas in particular were investigated to understand flow pattern and to confirm correspondence between flow pattern and recorded pressure on the ground. Those measurements show the difference in flow at different size openings in terms of the vortex on the windward side and the wake. When the size of the opening exceeded a certain value the near wake on the leeward side disappeared and on the windward side the vortex disappeared. The pressure distribution, flow pattern, and velocity profile are shown and compared between measurement and CFD.     

台风 “鲇鱼”作用下厦门沿海某超高层建筑的风场和风压特性实测研究

为研究我国沿海地区超高层建筑的风场和风压特性,在2010年台风“鲇鱼”登陆前后对厦门沿海某超高层建筑的风场和建筑表面风压进行了同步监测。通过对实测风场和风压数据的深入分析表明：沿海地区超高层建筑风场的湍流度随风速增大变化平稳,阵风因子随湍流度的增大而增大;实测脉动风速功率谱密度与von Karman谱吻合较好;建筑各面内测点之间的瞬时风压、平均风压、平均风压系数和极值风压系数具有较强的相关性;实测平均风压和平均风压系数在迎风面较大,在背风面非常小;当风从角部吹向建筑时,随着风向角的变化,两迎风面的平均风压系数随着平均风速的增大变化规律相反;两背风面的平均风压系数随着平均风速的增大逐渐减小;迎风面的极值风压系数随着风向角的变化正负波动较大,背风面的极值风压系数分布较为均匀;迎风面的脉动风压系数较大且变化较大,背风面的脉动风压系数非常小且变化平稳;建筑各面的极值风压系数和脉动风压系数的幅值随着风速的增大逐渐减小。     

Experimental and CFD study of ventilation flow rate of a Monodraught™ windcatcher

Monodraught™ windcatchers are commercial natural ventilation devices, which are primarily driven by wind to produce both extract and supply air flow. The measurement of the net flow rate (extract minus supply) of a Monodraught™ windcatcher ABS 550 for various wind speeds and directions is introduced. The ventilation measurement system uses a cone flow meter and a blower fan. CFD standard k − ? turbulence model is employed to calculate the flow rate. The situation using a blower fan is considered in modelling and the effect of the manometer sensitivity is also discussed. The comparison has indicated a good agreement between measurement and simulation. CFD modelling of the windcatcher is then carried out for the situation of outdoor far field wind. At the same nominal wind speed, the calculated extract flow rate of the windcatcher in a far field wind is roughly twice that for the situation using a blower fan, the wind direction has a small effect on the extract flow rate. The extract and supply flow rates are also calculated for various room pressure due to various wall openings and installation on a flat roof or a pitched roof. The contribution of the buoyancy effect on the flow rates is discussed in simulation.     

A study of natural ventilation in a refuge floor

Under the Building Codes of Hong Kong Special Administration Region, the provision of refuge floors has been an indispensable element in high-rise building design since 1996. Wind-induced cross natural ventilation is an important design criterion of a refuge floor since it helps to prevent any smoke entering to become persistent state remained (logging) on the refuge floor. This paper reports a study of refuge floor natural ventilation induced by wind flow around a high-rise building with a refuge floor arriving from different wind incidence angles. The study is based on CFD simulations which are validated by wind tunnel measurements. The refuge floor under investigation has a main services core at the centre and support walls flush with the building walls along two opposite sides. The results reveal that at all wind angles, wind is able to enter the refuge space from the windward side and escape from the leeward side. At some wind angles, wind is found to re-enter the refuge space from the leeward opening and accumulate behind the main services core. Based on the results, we suggest that stairs connecting to the refuge space should be located inside the side corridors formed by the internal and external side walls.     

计算流场尺寸对风压系数影响的研究

计算流体动力学（CFD）技术已成为研究建筑结构风压系数的有效手段之一,但合理的风场构筑理论和构筑方法是准确模拟建筑结构风场的基础,对此研究成果不多。基于CFD软件,建立了单体方形建筑有限元分析模型,通过改变三维流场特征尺寸,研究了单体建筑物在不同流场断面高度、宽度,来流、尾流长度等尺寸效用下结构表面风压特征及风压系数变化规律。结果表明：流场高度主要影响迎风面的风压系数,流场宽度的变化主要影响结构迎风面以及结构两侧的风压系数分布,流场长度的影响集中在迎风面和背风面。研究结果可为此类建筑物合理使用数值风洞提供参考。     

Wind-induced ventilation performances and airflow characteristics in an areaway-attached basement with a single-sided opening

In the present study, we performed both wind tunnel experiments and numerical simulations on a scale model with the focus on wind-driven natural ventilation in an areaway-attached basement with a single-sided opening. In the experiments, the mean value of the effective ventilation rate, purging flow rate (PFR) was measured for nine wind incidence angels based on the homogeneous emission rate method. The experimental results were used to validate two numerical approaches: Reynolds averaged Navier–Stokes (RANS) modeling and large-eddy simulation (LES). The influences of inflow turbulent fluctuations for LES modeling were also examined. The comparisons between the experiment and the numerical simulation indicate that LES can provide more accurate results than RANS and the inflow turbulent fluctuations should be taken into account for LES modeling. Based on LES with the inflow turbulent fluctuations, the mean airflow patterns within and around the areaway-attached basement were further studied for different wind incidence angles to investigate the influence of wind direction on ventilation performance in the areaway space. Furthermore, the relationships between the effective ventilation rate and the kinetic energy in the basement space were analyzed for three wind directions: 0°, 90° and 180°. A close correlation was found between the mean values, whereas the corresponding time variations showed large discrepancies. Finally, we compared the effective ventilation rate obtained using the homogeneous emission rate method and the airflow rates through the opening using two integration procedures. The effective ventilation rates were found lower than the airflow rates through the opening.     

The effect of traditional wind vents called zarceras on the hygrothermal behaviour of underground wine cellars in Spain

This paper studies the hygrothermal behaviour of traditional underground wine cellars in one of the most important wine growing areas of Spain, the Ribera del Duero, paying special attention to the effects of traditional wind vents called zarceras. The purpose is to highlight energy-saving practices that aid the design and renovation of similar constructions. This was carried out by monitoring the temperature and relative humidity inside two wine cellars of similar properties throughout 2007, one with a zarcera and the other without. The results of the analysis show that the existence of the wind vent does not increase the ventilation from May to August. On the other hand, from September to March, the zarcera favours ventilation, affecting both temperature and relative humidity. The interior temperature is more greatly affected by the exterior temperature, giving rise to greater variations, although this difference is of little importance for maturing wine (annual variation 2.5 °C more than the cellar without a zarcera). The interior relative humidity is reduced, reducing the risk of condensation and problems of mould (the cellar without a zarcera reached excessively high values, 97% as an annual average, compared to 91% in the cellar with a zarcera). Thus the existence of zarceras in underground cellars improves the overall hygrothermal behaviour, providing suitable conditions for maturing wine without air conditioning systems, an advisable element in the building of new underground wine cellars.     

特高压拉线式悬索塔风洞试验研究

拉线式悬索塔由双立柱支撑结构及拉线悬索系统组成,在概念和设计上均较为简洁。然而由于其高耸特点,因此是风敏感结构。为测试该类结构的风振响应特点,进行了塔线体系气动弹性模型的风洞试验。试验结果表明:在紊流风场中,立柱的顺风向风振响应与横风向属于同一量级;随风速增加,背风侧拉线趋向松弛,背风侧立柱受压减小;而迎风侧拉线则越趋张紧,迎风侧立柱受压增加,因此,需特别关注迎风侧拉线断线及迎风侧立柱受压稳定问题。     

Effect of balconies on ventilation inducing aeromotive force on low-rise buildings

Data on mean wind pressure distribution on opposite walls of a five storey building model with smooth surface and also with balconies mounted thereon have been presented, and the effect of balconies on ventilation inducing wind forces has been discussed. It is shown that provision of balconies alters wind pressure distribution on windward wall but does not introduce significant change in it on the leeward side. Ventilative force across openings located centrally on opposite walls also remains almost unchanged due to the balconies. However, the ventilative forces across windows located at different heights on the windward wall only, get diminished at all the floors near the upstream end for obliquely incident wind, and over the entire wall width at the intermediate floors for the wind incidence at 90 ° angle on the wall. Enhancement in the ventilation inducing wind forces is caused at the top floors on the downstream side for obliquely incident wind, and near the ends of the wall at the ground and top floors for the wind impinging at right angle on the wall.     

Wind conditions and ventilation in high-rise long street models

We regarded high-rise cities as obstacles and channels to wind. We first studied wind conditions and ventilations in idealized high-rise long street models experimentally and numerically with a constant street width (W = 30 mm), variable street heights (H = 2 W, 2.5W, 3W, 4W), variable street lengths (L = 47.4W, 79W, 333W, 667W) and a parallel approaching wind. The flow rates penetrating into windward entries are a little larger than the reference flow rate in the far upstream free flow through the same area with windward entries in all models. The stream-wise velocity decreases along the street as some air leaves upwardly across street roofs. Near the leeward entry, there is a downward flow which brings some air into the street and results in an accelerating process. In the neighborhood scale long streets (L = 47.4W and 79W), wind in taller streets is stronger and the ventilation is better than a lower one. For the city scale long streets (L = 333W and 667W), a constant flow region exists where the vertical velocity is zero and the stream-wise velocity remains constant. In such regions, turbulent fluctuations across the street roof are more important to air exchange than vertical mean flows. In a taller street, the process to establish the constant flow conditions is longer and the normalized balanced horizontal flow rate is smaller than those in a lower street. In the city scale long streets, the turbulence exchange rate can be 5–10 times greater than the mean flow rate.     

Flow reattachment on the roof of a 6 m cube

Five ultrasonic anemometers are used to measure flow velocities above the roof of a 6 m cube and at a reference point upstream. Various analysis techniques are applied to the data in order to illustrate the differences between: the mean reattachment position when the mean wind is normal to the windward face (0°), which is at x/h≈0.6; the median instantaneous reattachment position at x/h=0.66 and the reattachment position that would occur if the wind direction was held at a constant 0°, which is at x/h=0.75 and is also the position of zero conditionally averaged u velocity at instants when the v velocity is zero. It is also shown that the flow is highly unsteady and that the reattachment length varies from negligible separation to no reattachment, which occurs for 20% of the time. Some of these variations are related to fluctuations in the onset wind speed and direction, but they are also influenced by the dynamic response of the separation vortex system. The formation and shedding of vortices means that certain frequencies in the turbulence spectrum, around 1 Hz, are slightly amplified, whereas frequencies above 10 Hz are filtered out as a result of the inertia of the vortex system. The effects of reattachment length on the pressure distribution are briefly considered but it is shown that these do not account for the differences between the Silsoe field data and typical wind-tunnel results. It is suggested that the pressure differences may be related to Reynolds number, but it appears that this is not associated with changes in reattachment length.