街谷配置對街谷風場之影響

Abstract

隨著全球都市化、熱島效應增強，都市熱環境成為一重要課題，都市風場亦成為廣泛討論的議題之一。人類頻繁活動之街道空間中，對流傳熱率受到都市冠層（urban canopy layer, UCL）內風速影響，密集的都市建築會對街谷內通風狀況產生負面影響，進而影響街谷的散熱效果，導致都市局部溫度增加。同時，街谷中1.5m高度之行人風場亦與行人之舒適性息息相關。由此可知都市冠層內風場之重要性。 都市冠層內風場受到街道結構影響，目前已有許多研究各別針對基礎風速、建築排列形式、街谷尺度或植栽配置等因子，對風場進行探討，然卻少有研究綜合考量不同街谷形式、植栽配置對風場產生之整體影響，且先前研究中多以氣流遇上形體所產生的機械湍流（mechanical turbulence）為切入角度，而沒有進一步考量街谷空間內溫差所形成之熱湍流（thermal turbulence），未能對都市風場做出較詳盡切實之模擬。故本研究欲更深入了解不同基礎風速、街谷高寬比以及行道樹排數對街谷風場的影響。本研究以計算流體力學（Computational Fluid Dynamics，CFD）數值模擬方法作為研究工具，針對不同基礎風速、街谷高寬比以及行道樹排數等共18個方案進行風場模擬。 結果顯示，(1)在基礎風速與行道樹排數相同的情況下，淺型街谷內風速高於深型街谷，而街谷高寬比與行道樹排數相同的情況下基礎風速越大街谷內風速亦越大，(2)隨著街谷吸收太陽輻射，熱湍流確實在街谷內形成，並且受到街谷高寬比、行道樹排數與基礎風速的影響，行道樹排數越多，街谷內熱湍流強度越大，而行道樹排數對熱湍流的影響在淺型街谷中較明顯，深型街谷中則較微弱。(3)不同基礎風速下，行道樹排數對街谷內風場強弱的影響不同。當基礎風速為4m/s時，栽植兩排樹木之方案其街谷內垂直剖面平均風速高於另外兩種栽植排數；基礎風速為12m/s時，無栽植樹木之方案其垂直剖面平均風速高於另外兩種栽植排數。(4)在不同街谷高寬比下，基礎風速以及行道樹排數對街谷內風場的影響度不同，在同樣植栽排數下，淺型街谷較深形街谷更容易受不同基礎風速之影響，而在同樣基礎風速下，植栽排數對熱湍流的影響在淺型街谷中較明顯。在淺型街谷下，基礎風速4m/s的方案中1.5m水平剖面之平均風速以栽植兩排行道樹時最高，栽植四排行道樹之方案次之，無栽植行道樹之方案最低；基礎風速12m/s的方案中1.5m水平剖面之平均風速以栽植兩排行道樹時最高，栽植四排行道樹之方案次之，無栽植行道樹之方案最低；在深型街谷下，基礎風速4m/s的方案中1.5m水平剖面之平均風速隨行道樹排數增加而增加；基礎風速12m/s的方案中1.5m水平剖面之平均風速隨行道樹排數增加而降低。

As urbanization and the urban heat island effect continue to increase, the urban wind farm have become significant considerations. In street environments where human activities are concentrated, the convection heat transfer rate is influenced by the wind speed within the urban canopy layer (UCL). Dense urban buildings negatively impact ventilation in street canyons, affecting cooling efficiency and bringing about localized temperature rises. The pedestrian wind field at a height of 1.5-2m within street canyons is closely linked to pedestrian comfort. As mentioned above, the wind field within the urban canopy layer is an important issue. The wind field within the UCL is influenced by street structure. While previous studies have examined factors like base wind speeds, street layout, street aspect ratio and vegetation configuration individually, a comprehensive analysis of their combined impact on the wind field is lacking. Additionally, existing studies have mainly focused on analyzing mechanical turbulence and have not further considered thermal turbulence within the urban canopy layer. Consequently, a more thorough simulation of urban airflow is needed. Thus, this study aims to investigate the influence of base wind speeds, street aspect ratios, and tree rows on street canyon wind fields using Computational Fluid Dynamics (CFD) simulations. A total of 18 cases were simulated. Results show that, (1)under the same base wind speed and the number of rows of trees, avenue canyons exhibit higher wind speeds compared to deep canyons. When the street canyon aspect ratio and the number of rows of trees are the same, wind speeds within street canyons increase with higher base wind speeds. (2)Thermal turbulence forms within the canyons due to solar radiation, and it is influenced by street aspect ratios, the number of rows of trees, and base wind speeds. The impact of vegetation on thermal turbulence is more pronounced in avenue canyons than in deep canyons, with a greater number of tree rows resulting in higher thermal turbulence intensity. (3)The influence of tree rows on the wind field within street canyons varies with different base wind speeds. In avenue canyons with a base wind speed of 4m/s, the case with two rows of trees exhibits the highest average wind speed, followed by the case with four rows of trees, while the case without trees has the lowest wind speed. In deep canyons with a base wind speed of 12m/s, the case without trees has the highest average wind speed, while the cases with two and four rows of trees have lower wind speeds. (4) Under different street canyon aspect ratios, the impact of base wind speeds and the number of rows of trees on the wind field within the avenue canyon differs. With the same number of tree rows, avenue canyons are more susceptible to the influence of different base wind speeds, while in the case of the same base wind speed, the influence of vegetation on thermal turbulence is more pronounced in avenue canyons. In avenue canyons, at a base wind speed of 4m/s, the case with two rows of trees exhibits the highest average wind speed at the 1.5m horizontal profile, followed by the case with four rows of trees, and the case without any trees has the lowest wind speed. At a base wind speed of 12m/s, the case with two rows of trees has the highest average wind speed at the 1.5m horizontal profile, followed by the case with four rows of trees, and the case without any trees has the lowest wind speed. In deep canyons, at a base wind speed of 4m/s, the average wind speed at the 1.5m horizontal profile increases with the number of rows of trees, while at a base wind speed of 12m/s, the average wind speed at the 1.5m horizontal profile decreases with an increasing number of rows of trees.