著：（比利時(shí)）艾瑞克·特里 譯：徐琴

1 概述：智慧城市中風(fēng)參數(shù)評(píng)估的重要性

在密度逐漸增大的城市中，越來越多高層建筑的出現(xiàn)導(dǎo)致了步行高度上風(fēng)速的增加。這不僅降低了行人舒適感和整個(gè)社區(qū)環(huán)境的吸引力，對(duì)于年長(zhǎng)以及行動(dòng)不便的人群也有潛在風(fēng)險(xiǎn)，因此，降低公共空間的高風(fēng)速非常重要。城市內(nèi)的平均氣溫同樣隨著城市密度的增加而升高，在日間積聚的熱量被困在城市建成環(huán)境的密集邊界內(nèi)，使得夜晚溫度難以降低，埃因霍溫科技大學(xué)（Eindhoven University of Technology）在這一問題上進(jìn)行了深入研究[1-2]。

國(guó)際研究也表明城市中逐漸升高的夜晚溫度可導(dǎo)致死亡率的提升，尤其是65歲以上并患有心血管及呼吸系統(tǒng)疾病的人群。風(fēng)在緩解這一問題上扮演了重要角色，可以通過增強(qiáng)街道通風(fēng)和散熱等方式給城市降溫。

2 智慧城市中公共空間的風(fēng)環(huán)境設(shè)計(jì)準(zhǔn)則

全球很多城市和地方政府已經(jīng)開始意識(shí)到風(fēng)在改善城市環(huán)境中的重要作用。然而風(fēng)參數(shù)評(píng)估大多時(shí)候都被用于研究已然出現(xiàn)的各類問題，如果我們能在設(shè)計(jì)階段就引入風(fēng)參數(shù)評(píng)估，就能預(yù)估單個(gè)建筑項(xiàng)目在建成之后對(duì)風(fēng)環(huán)境的影響。

由于風(fēng)存在于較大尺度的自然環(huán)境中，因而也須在大規(guī)模的城市尺度上進(jìn)行考量。這意味著在研究風(fēng)的影響時(shí)不應(yīng)局限于建筑尺度，而應(yīng)考慮整個(gè)區(qū)域或街區(qū)（圖1）。此外，如果在新開發(fā)項(xiàng)目的早期就把風(fēng)的效應(yīng)納入考慮，可以及時(shí)調(diào)整建筑體量、景觀設(shè)計(jì)和街道建筑的朝向等。當(dāng)各個(gè)建筑的設(shè)計(jì)進(jìn)入后期時(shí)，再對(duì)步行高度的風(fēng)環(huán)境舒適度進(jìn)行最終測(cè)試，通過適當(dāng)增加植被和樹蔭等措施來緩解可能出現(xiàn)的問題。

Actiflow①對(duì)政府部門的建議如下：1）在評(píng)估風(fēng)對(duì)所在城市的影響時(shí)，須同時(shí)考慮陰影、聲音、熱島、流動(dòng)性和空氣污染等； 2）確定城市中的關(guān)鍵區(qū)域，設(shè)想如何在大尺度上應(yīng)對(duì)其所面臨的挑戰(zhàn)（例如開放或密化城市肌理，引入公園或其他景觀元素，確定高層建筑的選址或重要街道朝向等）；3）基于公共空間的本土功能，確定需要達(dá)到的品質(zhì)標(biāo)準(zhǔn)；4）將大尺度上的解決方案及品質(zhì)要求轉(zhuǎn)化為開發(fā)商和建筑師的實(shí)際操作導(dǎo)則，例如可允許的建筑體量、朝向和最大高度，入口及私人戶外空間的位置，如何判斷建筑設(shè)計(jì)達(dá)到所需標(biāo)準(zhǔn)等。

只有當(dāng)良好的導(dǎo)則及規(guī)范實(shí)施之后，開發(fā)商和建筑師才能及時(shí)檢驗(yàn)每項(xiàng)設(shè)計(jì)概念是否遵循了相應(yīng)的規(guī)范。通過在設(shè)計(jì)階段引入風(fēng)參數(shù)評(píng)估，潛在的局部問題更容易在早期識(shí)別，設(shè)計(jì)經(jīng)調(diào)整后的影響也能得到評(píng)估，例如增加挑檐、底座和樹木等（圖2）。當(dāng)設(shè)計(jì)方案確定之后，再進(jìn)行最終的審核來檢驗(yàn)項(xiàng)目是否達(dá)到了政府所設(shè)定的規(guī)范標(biāo)準(zhǔn)。

3 風(fēng)參數(shù)評(píng)估技術(shù)

城市環(huán)境中的風(fēng)參數(shù)評(píng)估技術(shù)通常有3種：1）實(shí)景測(cè)量；2）風(fēng)洞實(shí)驗(yàn)；3）CFD計(jì)算機(jī)模擬（也稱CFD計(jì)算機(jī)流體力學(xué)）。

3.1 實(shí)景測(cè)量

測(cè)量實(shí)時(shí)風(fēng)速和風(fēng)向的方法可用于評(píng)估某個(gè)已知的狀況或問題。然而要得到統(tǒng)計(jì)學(xué)上可靠的數(shù)據(jù)通常需要監(jiān)測(cè)較長(zhǎng)時(shí)間（一般來說為幾個(gè)月），這樣才能記錄到每個(gè)可能產(chǎn)生的風(fēng)況。實(shí)景測(cè)量的另一個(gè)局限性在于每次只能在部分固定場(chǎng)景下收集數(shù)據(jù)，因而對(duì)于特定區(qū)域的整體風(fēng)環(huán)境缺乏完整了解。最大的挑戰(zhàn)還是很難將風(fēng)的瞬時(shí)測(cè)量數(shù)據(jù)與國(guó)家氣象局已知的不受干擾的風(fēng)況聯(lián)系起來。實(shí)景測(cè)量方法有助于識(shí)別潛在的問題或挑戰(zhàn)，但不建議用作完整的城市風(fēng)參數(shù)評(píng)估，尤其不適用于評(píng)估未來的狀況（例如城市中未建成的新開發(fā)項(xiàng)目）。

3.2 風(fēng)洞實(shí)驗(yàn)

與實(shí)景測(cè)量不同的是，風(fēng)洞實(shí)驗(yàn)是將城市局部的等比縮小模型置于有特定氣象邊界的風(fēng)環(huán)境下監(jiān)測(cè)。風(fēng)洞可以模擬真實(shí)的風(fēng)況，包括多種不同的風(fēng)向、風(fēng)的動(dòng)態(tài)變化等。如果想要了解某個(gè)單體建筑或植被的效果，只需在多次試驗(yàn)中增加或移除模型中的某些元素。

然而風(fēng)洞實(shí)驗(yàn)與實(shí)景測(cè)量一樣，收集到的數(shù)據(jù)僅來自模型中安裝有壓力探頭的位置，而不同位置之間的壓力變化卻不得而知。同時(shí)由于測(cè)量工具對(duì)于所使用的縮小模型來說體量較大，使得模型局部的風(fēng)效應(yīng)難以測(cè)量，例如高層建筑的陽臺(tái)或私人戶外區(qū)域。

3.3 CFD計(jì)算機(jī)模擬

除了做實(shí)體的城市模型，計(jì)算機(jī)也能通過構(gòu)造3D的城市虛擬模型來計(jì)算風(fēng)效應(yīng) （圖3）。風(fēng)在整個(gè)城市范圍內(nèi)的表現(xiàn)和風(fēng)向由專業(yè)的軟件模擬。同樣的，也可以通過在3D模型里添加或移除建筑單體或植被后，對(duì)其作用進(jìn)行評(píng)估。不過，電腦模擬實(shí)驗(yàn)的運(yùn)行時(shí)間一般都比風(fēng)洞實(shí)驗(yàn)稍長(zhǎng)。

CFD的優(yōu)點(diǎn)是軟件能在完整的城市環(huán)境中顯示出局部的風(fēng)速和風(fēng)向，這意味著任何值得注意或意料之外的風(fēng)況變化都能被捕捉到，而且由于電腦模型尺度沒被縮小，即使是陽臺(tái)或其他私人區(qū)域周邊細(xì)微的空氣流動(dòng)也可被研究。

在評(píng)估城市平均風(fēng)速或風(fēng)環(huán)境設(shè)計(jì)方面，CFD計(jì)算機(jī)模擬只要是被CFD領(lǐng)域的專業(yè)人士操作，在很大程度上都被證實(shí)是可靠的。但在分析風(fēng)的動(dòng)態(tài)特征例如狂風(fēng)等案例中，CFD方法仍處在開發(fā)當(dāng)中，具有很大的改善提升空間。

4 荷蘭的風(fēng)參數(shù)評(píng)估

將戶外空間的風(fēng)數(shù)據(jù)進(jìn)行分析和分類的方式多種多樣。部分國(guó)家有自己的國(guó)家標(biāo)準(zhǔn)或是參考風(fēng)參數(shù)評(píng)估相關(guān)的科學(xué)文獻(xiàn)，也有一些國(guó)家還沒有完善的官方標(biāo)準(zhǔn)。

Actiflow所處的荷蘭在歷史上就跟風(fēng)有很深的淵源。16世紀(jì)時(shí)荷蘭艦隊(duì)乘風(fēng)遠(yuǎn)洋給國(guó)家?guī)Щ亓素?cái)富（圖4），17世紀(jì)時(shí)建造了最負(fù)盛名的荷蘭風(fēng)車，將土地從海平面下爭(zhēng)奪回來（圖5）。從那以后，傳統(tǒng)風(fēng)車就成為荷蘭的象征。

4.1 風(fēng)參數(shù)評(píng)估的荷蘭標(biāo)準(zhǔn)

毋庸置疑，荷蘭是世界上最先提出分析、量化和分類步行環(huán)境風(fēng)舒適度標(biāo)準(zhǔn)的國(guó)家之一。迄今為止Actiflow基 于NEN8100標(biāo) 準(zhǔn)在風(fēng)參數(shù)評(píng)估領(lǐng)域已經(jīng)積累了約15年經(jīng)驗(yàn)，該標(biāo)準(zhǔn)也同時(shí)被周邊多個(gè)西歐和北歐國(guó)家所 參考。

荷蘭NEN8100標(biāo)準(zhǔn)描述了2種評(píng)估步行環(huán)境風(fēng)舒適度的方法：風(fēng)洞實(shí)驗(yàn)以及CFD計(jì)算機(jī)模擬。測(cè)量或模擬結(jié)果將與地區(qū)風(fēng)力數(shù)據(jù)相結(jié)合，從而形成地區(qū)風(fēng)力等級(jí)示意圖。風(fēng)阻及危險(xiǎn)等級(jí)分別由風(fēng)速超過5 m/s或 15 m/s的概率決定。從功能性角度來說，不同的風(fēng)力等級(jí)代表特定區(qū)位是否適宜“通行”，“散步”或者“久留”（圖6、表1）。

表1 根據(jù)NEN 8100定義的風(fēng)阻等級(jí)Tab. 1 Definition of wind hindrance class according to NEN8100

根據(jù)NEN8100標(biāo)準(zhǔn)，風(fēng)危險(xiǎn)度也需被評(píng)估。為了確定風(fēng)的危險(xiǎn)程度，NEN8100標(biāo)準(zhǔn)參考的是風(fēng)速超過15m/s的概率（表2）。風(fēng)的危險(xiǎn)程度被分為2類：“有限風(fēng)險(xiǎn)”和“危險(xiǎn)”。值得注意的是判定風(fēng)危險(xiǎn)度的超出概率數(shù)值比判定風(fēng)舒適度的要小很多。這項(xiàng)標(biāo)準(zhǔn)并不會(huì)明確指出風(fēng)力等級(jí)是否可接受，而只是提出能將風(fēng)的舒適度和危險(xiǎn)度進(jìn)行分類的方法。在特定的城市環(huán)境或建筑周邊所允許的風(fēng)力等級(jí)最終仍由政府決定。

表2 根據(jù)NEN8100定義的風(fēng)危險(xiǎn)度等級(jí)Tab. 2 Definition of wind danger class according to NEN8100

4.2 Actiflow應(yīng)用荷蘭標(biāo)準(zhǔn)的案例

Actiflow每年開展近100項(xiàng)研究，向政府、建筑師和開發(fā)商提供城市環(huán)境中風(fēng)設(shè)計(jì)方面的建議。以下展示的就是其中一項(xiàng)實(shí)際案例，該項(xiàng)目位于阿姆斯特丹，目標(biāo)是為Amstel III地區(qū)的房地產(chǎn)開發(fā)商制定相關(guān)導(dǎo)則。

4.2.1 背景介紹

Amstel III位于阿姆斯特丹東南，占地約1 km2。現(xiàn)主要為辦公區(qū)域，場(chǎng)地內(nèi)的平均建筑高度在25 m左右，多為方形。阿姆斯特丹政府希望將該地區(qū)改造為集居住、辦公和休閑為一體的更具活力的城區(qū)。到2027年，將建成大約5 000所住宅以及商店和辦公場(chǎng)所。同樣的，政府也希望為該地區(qū)提供舒適的戶外環(huán)境。為了實(shí)現(xiàn)這一目標(biāo)，地區(qū)內(nèi)將增加相當(dāng)數(shù)量的建筑體，也就意味著更多的高層建筑和更密集的城市肌理（圖7）。

在規(guī)劃的早期階段，政府想知道新增的建筑體量是否會(huì)導(dǎo)致風(fēng)環(huán)境、聲音以及日照方面的問題。此外，政府也希望為地區(qū)內(nèi)的房地產(chǎn)開發(fā)提供明確指導(dǎo)。通過與荷蘭公司Cauberg Huygen合作，Actiflow模擬并分析了物理體量并以此向政府部門提出建議。該項(xiàng)目的獨(dú)特之處在于我們?cè)谳^早期便介入，項(xiàng)目的大尺度以及對(duì)多種物理體量的綜合模擬。

4.2.2 結(jié)果

我們首先根據(jù)NEN8100開展了風(fēng)環(huán)境分析（圖8、表1）?？梢钥闯?，一方面，有幾處等級(jí)D和E的重點(diǎn)區(qū)域應(yīng)注意在公共空間中避免出現(xiàn)。造成這些問題區(qū)域的關(guān)鍵原因在于現(xiàn)狀街道朝向。荷蘭的主導(dǎo)風(fēng)向?yàn)槲髂舷颍撎幋罅拷值蓝际俏髂稀獤|北向，該方向的風(fēng)力更易沿街聚集，從而在建筑轉(zhuǎn)角處引發(fā)問題。次要原因是新增的高層建筑。從原有較低建筑上方經(jīng)過的風(fēng)碰到高層建筑立面后，由于氣流向下的效應(yīng)對(duì)步行高度形成風(fēng)阻。另一方面，地區(qū)內(nèi)有相當(dāng)多等級(jí)A和B的區(qū)域非常適合設(shè)置戶外座椅和戶外活動(dòng)。

在任何區(qū)域都應(yīng)盡力避免風(fēng)所造成的危險(xiǎn)。如圖9、表2所示，共有2處風(fēng)危險(xiǎn)程度較高并需要緩解的重點(diǎn)區(qū)域。除此之外，圖9也顯示了有限風(fēng)險(xiǎn)的其他區(qū)域。在咨詢阿姆斯特丹政府之后，決定對(duì)這些區(qū)域也采取緩解措施。因?yàn)檫@些有限風(fēng)險(xiǎn)的區(qū)域同時(shí)也是風(fēng)舒適度等級(jí)D和E的區(qū)域，局部措施應(yīng)當(dāng)提高風(fēng)舒適度同時(shí)減小風(fēng)危險(xiǎn)度。

4.2.3 建議

根據(jù)風(fēng)環(huán)境分析結(jié)果，推薦政府采取以下措施：1）依照風(fēng)等級(jí)示意圖所示，盡可能地在相應(yīng)區(qū)域設(shè)計(jì)公共空間；2）盡可能多地使用喬木、灌木及其他植被化解風(fēng)力影響。

這些措施在一定程度上能改善狀況，卻較難提升整個(gè)地區(qū)的風(fēng)舒適度。因此，也針對(duì)該地區(qū)的房地產(chǎn)開發(fā)商制定了相應(yīng)導(dǎo)則：由于地區(qū)內(nèi)的主導(dǎo)風(fēng)向是西南向，建議在Amstel III的西南部設(shè)置低層建筑，然后往東北方向逐漸增加最高建筑高度；重要建筑物應(yīng)當(dāng)置于基座之上或者設(shè)計(jì)挑檐以減小氣流向下效應(yīng)對(duì)步行高度的影響；為了減小西南方向的風(fēng)力聚集，建筑群最好以不規(guī)則形式排列，而非方形網(wǎng)格形式。

基于Actiflow的這項(xiàng)研究，阿姆斯特丹市政府為Amstel III地區(qū)的房地產(chǎn)開發(fā)商制定了開發(fā)導(dǎo)則和要求。各個(gè)開發(fā)商有義務(wù)在設(shè)計(jì)早期向政府證明其建筑設(shè)計(jì)滿足了風(fēng)舒適度的相關(guān)要求。

注釋：

① Actiflow是荷蘭一家專注于風(fēng)、室內(nèi)氣候、空氣動(dòng)力學(xué)和氣流設(shè)計(jì)的咨詢公司，在過去15年，向政府、建筑師、開發(fā)商和工程師提供有關(guān)風(fēng)環(huán)境舒適度和安全性的意見。不僅如此，Actiflow在建筑物理方面提供的服務(wù)還包括陰影研究、城市熱島效應(yīng)評(píng)估、室內(nèi)氣候及火災(zāi)中的排煙散熱等。所有這些研究都建立在復(fù)雜的氣流模擬技術(shù)之上。該項(xiàng)技術(shù)也被Actiflow用于新產(chǎn)品和系統(tǒng)的研發(fā)，同樣的，Actiflow也涉足汽車、運(yùn)動(dòng)和風(fēng)力機(jī)空氣動(dòng)力學(xué)，以及HVAC系統(tǒng)的開發(fā)、食品加工設(shè)備和室內(nèi)跳傘的風(fēng)洞開發(fā)等。更多關(guān)于Actiflow的信息請(qǐng)?jiān)L問網(wǎng)站：www.actiflow.com。

圖表來源：

圖1～3，6～9由Actiflow BV提供；圖4由Pinterest提供；圖5由ruimtevoorderivier.jouwweb.nl提 供； 表1～2由Actiflw BV提供。

（編輯/王亞鶯）

Wind Design in Smart Cities

1 Introduction: the Importance of Wind Assessment in Smart Cities

Cities become denser. We build more and more high-rise buildings, but these buildings typically induce high wind speeds on pedestrian level. As such, high buildings can reduce pedestrian wind comfort and the attractiveness of an entire neighborhood, but they can also lead to dangerous situations, especially for elderly or physically vulnerable people. Therefore, it is important to reduce high wind speeds in public spaces.

As cities become denser, the average temperature in the cities increases. Cities have difficulties to cool down, especially at night. The heat that is accumulated during daytime is trapped within the dense boundaries of the built environment. Significant research on this matter is performed at Eindhoven University of Technolog[1-2].

As the density of our cities increases, night temperatures are increasing. International research shows that increased temperatures in our cities leads to an increased mortality, mainly for people over 65 years of age, due to cardiovascular and respiratory problems. To mitigate this problem, wind can play a major role. Wind can help to ventilate streets, to dissipate heat, and as such, to cool down our cities.

2 Guidelines for Smart Cities to Design Wind in Public Spaces

In many places in the world, (local) governments and municipalities start realizing the important influence wind can have on the quality of life in our cities. However, we now see that wind assessments are mainly used to study problems when they already exist, and if assessments are used in the design phase, they are used to check the impact of a single building or project, when the design is almost fixed.

Wind is a large-scale phenomenon, and therefore it should be taken into account on a large scale in a city. This means that wind effects should not be studied or solved on a building level, but in an entire area or neighborhood(Fig. 1). Moreover, it is important to study the impact of a new development already in an early phase, when it is still possible to change building volumes, the landscape design or the orientation of buildings and streets. In a later phase, when the design of each single building is almost ready, a final check for pedestrian wind comfort can be performed and possible issues can then be mitigated with smaller measures like adding vegetation or small canopies or screens.

At Actiflow①, governments are advised as follows: 1) Assess the impact that wind has in your city, but also look at other aspects like shading, sound, heat islands, mobility, air pollution, etc.. 2) Determine the critical areas in the city, and consider how the challenges can be solved on a large scale(e.g. open up or densify the urban texture, introduce parks or other landscape elements, determine where highrise buildings can be allowed, determine the orientation of important streets and corridors, etc.. 3) Determine which quality standards you want to achieve, depending on the local function of the public space. 4) Translate the large-scale solutions and quality requirements into practical guidelines and codes for developers and architects: which building volumes are allowed, how to orientate the volumes, what is the maximum building height, where to place entrances or private outdoor spaces, and how to prove that a building design meets the requirements.

If good guidelines and rules are in place, developers and architects can easily check for each of their concept designs if they comply or not. By involving wind assessments during the design process, possible local issues can be identified in an early phase and the impact of a design modification can easily be assessed, e.g. the introduction of a canopy, a plinth or trees(Fig. 2). When the building design is fixed, a final check can be performed to show compliance with the norms set by the government.

3 Technologies of Wind Assessment

In general, it can be stated that there are 3 technologies to assess wind in the urban environment: 1) Real life measurements. 2) Wind tunnel experiments. 3) Computer simulations(CFD or“Computational Fluid Dynamics”).

3.1 Real Life Measurements

When assessing an existing situation or problem, real life measurements of wind speeds and wind directions can be performed. When measuring in real life, no simplifications or assumptions are needed in terms of building geometries or atmospheric conditions. However, in order to have statistically reliable data, it is necessary to perform measurements during a long period in time(typically several months), so that every possible wind condition has occurred during the measurement period. Another issue with real life measurements is that you can only collect data at a limited amount of measurement locations, which means that you don’t have a complete overview of all wind phenomena that occur in a certain area. A final challenge is that it is hard to relate a momentaneous measurement of the wind, to the undisturbed wind conditions known from the national meteorological institutes, which mostly have statistical wind data for the entire country.

Real life measurements can be useful to identify possible problems or challenges but are not recommended for a complete wind assessment in cities. Definitely when assessing a future situation(e.g. a new development in the city that is not yet built), real life measurements are not an option.

3.2 Wind Tunnel Experiments

Instead of performing real life measurements, it is also possible to build a scale model of a part of a city and place the model in an atmospheric boundary layer wind tunnel. In such a wind tunnel, the real wind conditions can be simulated for many different wind directions, including the dynamic behavior of the wind. To understand the influence of individual buildings or vegetation, it is very easy to add or remove elements in the scale model and run the wind tunnel experiment again.

However, as with real life measurements, you can only collect wind data at the locations where the pressure probes are installed in the scale model, meaning that you don’t see or register what happens in the space between those locations. Another issue is caused by the small scale of the wind tunnel model. Local wind effects, for example on balconies of high-rise buildings or other private outdoor areas, are impossible to measure in the wind tunnel, simply because the measurement equipment is too large in relation to the building or location of interest.

3.3 CFD Computer Simulations

Instead of building a physical scale model, it is also an option to make a 3-dimensional computer model of a part of a city, and let the computer calculate the wind effects(Fig. 3). Specialized software is used to simulate the wind behavior in the entire city area under varying atmospheric conditions i.e. different wind speeds, wind directions, air temperatures, etc.. Also here, the influence of individual buildings or vegetation can be easily assessed by adding or removing elements from the computer model, and run a new simulation. However, the runtime of a computer simulation is typically longer then the runtime of a wind tunnel experiment.

The main advantage of CFD is that the software shows the local wind speeds and wind directions in the entire space in the city, which means that any remarkable or unexpected wind behavior can be identified. As the computer model is not scaled, all details in the flow around small balconies or other private places can be studied.

For the assessment of the mean wind velocities in cities, or for the design of wind in cities, CFD computer simulations are extensively validated and proven to be reliable, as long as the simulations are performed by an expert in the field of CFD. For the analysis of wind dynamics like gusts, CFD methods can still be improved and are under development.

4 Wind Assessment in The Netherlands

There are several ways to analyze wind conditions in cities. Some countries have their own national norms or methods, or they refer to scientific papers to deal with the subject. In other countries, there are no official rules in place.

The Netherlands is a country in Europe that has a long history with wind. In the 16th century, Dutch ships were sailing to the oversees areas to bring wealth to the country(Fig. 4). As of the 17th century, the typical and well-known Dutch windmills were built to gain land from the sea(Fig. 5). Since then, the old windmills are a national symbol for the Netherlands.

4.1 The Dutch Norm for Wind Assessment

It is no surprise that The Netherlands was one of the first countries in the world to introduce its own norm to analyze, quantify and classify the local pedestrian wind comfort. This is why we now have about 15 years of experience in wind assessments based on this NEN8100 norm, which is also referred to by several of our surrounding countries in Western and Northern Europe.

The Dutch NEN8100 norm describes 2 ways to assess the pedestrian wind comfort: wind tunnel experiments and CFD computer simulations. The results of the measurements or simulations are then combined with the local statistical wind data, in order to produce a map with local wind classes. The wind hindrance classes are defined by the probability of exceeding a local wind speed of 5 m/s. From a functional point of view, the wind classes describe whether a certain location is suitable for“walking through” “strolling” or“l(fā)ongterm sitting” (Fig. 6, Tab. 1).

According to the NEN8100 norm, also the risk for wind danger needs to be assessed. For this, a comparable picture can be made, but to determine the risk for wind danger, the NEN8100 norm looks at the probability of exceeding a local wind speed of 15 m/s, as shown in the Tab. 2. To classify the wind danger, there are only 2 categories: “l(fā)imited risk” or“dangerous”. Important to note is that the exceedance levels in the definition of wind danger are much lower than the exceedance levels in the definition of the wind comfort classes. The norm does not state which wind classes are acceptable or not, it just describes a method to classify the wind comfort and wind danger. It is up to the city governments to determine which wind classes are allowed in which parts of the city, or in which public spaces around a building.

4.2 Case Example of the Application of the Dutch Norm by Actiflow

Actiflow performs around 100 studies per year to advise governments, architects and developers about wind in the urban environment. Here below you can find a good example of the work performed by Actiflow. It involves a project for the city of Amsterdam, with the goal to create guidelines for real estate developers in the new Amstel III region.

4.2.1 Background

Amstel III is an area of approximately 1 km2 at the South-east of Amsterdam. Currently the area is mainly occupied by offices. The average height of the current buildings is roughly 25 m, and the buildings typically have a square footprint.

The municipality of Amsterdam has the desire to transform this area into a lively urban district with mixed residential, office and recreational use. Between now and 2027 roughly 5,000 new houses will be realized, together with shops and new office spaces. Obviously, the municipality also envisions a comfortable outdoor space in this area. To realize the goals of the municipality, a significant amount of building volumes will be added to the area, leading to the introduction of high-rise buildings and a denser urban fabric(Fig. 7).

In the early planning phase, the municipality wanted to know if this increase in building volume would lead to problems regarding wind, sound and shading. Furthermore, the municipality wanted to create guidelines for individual real estate developments in the area. In cooperation with the Dutch company Cauberg Huygen, Actiflow simulated and analysed these physical quantities to inform and advise the municipality. Unique about this project was our early involvement, the large scale of the project and the combined simulation of multiple physical quantities. Exactly how Actiflow would recommend.

4.2.2 Results

As a starting point, a wind study is carried out according to the NEN8100. On the one hand, the results are shown in a cross section which is 1.75 m above ground level(Fig. 8, Tab. 1). In the picture it can be seen that there are several significant regions of wind class D and E, which have to be avoided in public areas. An important reason for these problematic areas is the orientation of the existing street plan. A large part of the streets is straight and SW-NE oriented. As the dominant wind direction in the Netherlands is SW, the wind from this direction can accumulate along the streets and cause problems at the corners of several buildings. A second reason is the addition of high-rise buildings. The wind that flows over the older(lower) buildings hits the fa?ade of the high-rise buildings, and then causes wind hindrance on pedestrian level through the downwash effect.

On the other hand, there are also enough regions with wind class A and B, which are perfectly suited for outdoor sitting and outdoor activities.

Wind danger needs to be avoided at all times. In the Fig. 9 and Tab. 2, it can be seen that there 2 zones with significant wind danger, which needs to be mitigated. Apart from that, there are also zones with limited risk for wind danger. In consultation with the municipality of Amsterdam, it was decided to also try to mitigate these zones. As the zones with(limited) wind danger are the same as the zones with wind comfort class D and E, local mitigations will improve the wind comfort and reduce the risk of wind danger.

4.2.3 Recommendations

Based on the results of the wind study, the following recommendations are formulated for the municipality: 1) Design the public space as much as possible in accordance with the governing wind classes; 2) Make use of natural elements like trees, hedges and other vegetation to break down the wind as much as possible.

These measures will improve the situation, but it is not expected that they will lead to an acceptable wind comfort in the entire area. Therefore, it is important to formulate guidelines for individual real estate projects in the area: As the dominant wind direction is SW, it is advisable to have rather low buildings in the SW part of Amstel III, and to slowly increase the maximum height of the buildings towards the NE. Critical buildings should be placed on a plinth or should have canopies that reduce the effect of the downwash on pedestrian level. Preferably, buildings should not be placed in a square grid, but rather in an irregular pattern, to avoid accumulation of wind from the SW.

Based on this study by Actiflow, the city of Amsterdam formulated guidelines and requirements for real estate developers in the Amstel III area. Each developer now has the obligation to prove in an early design phase that the design of their building(s) meets the requirements in terms of wind comfort.

Notes:

①Actiflow is a Dutch consultancy firm specialized in the fields of wind, indoor climate, aerodynamics and airflow design. For almost 15 years, Actiflow advises governments, architects, developers and engineering firms in the field of wind comfort and safety. Moreover, Actiflow offers shading studies, urban heat island assessments, indoor climate studies and smoke and heat removal studies. All these studies are based on complex airflow simulation technology. This technology is also used by Actiflow for the development of new products and systems. As such, Actiflow is involved in automotive, sports and wind turbine aerodynamics, in the development of HVAC systems, process equipment, and indoor skydive wind tunnels. More information about Actiflow can be found on the website www.actiflow.com.

Sources of Figures and Tables:

Fig. 1-3, 6-9 ? Actiflow BV; Fig. 4 ? Pinterest; Fig. 5 ? ruimtevoorderivier.jouwweb.nl; Tab. 1-2 ? Actiflow BV.