اثرات خنک کنندگی دریاچه های شهری بر ساختمان های اطراف، نمونه مطالعاتی: دریاچه چیتگر، تهران (مقاله علمی وزارت علوم)
درجه علمی: نشریه علمی (وزارت علوم)
آرشیو
چکیده
توسعه شهرنشینی چالش های زیست محیطی بزرگی از جمله انتشارات گازهای مضر را به همراه داشته است. این امر با افزایش دما و تراکم شهری به اوج خود رسیده است. یکی از مهم ترین چالش ها جزیره گرمایی شهری (UHI) است که در مناطق شهری پرجمعیت به طور برجسته تر ظاهر می شود. در میان اجزای تشکیل دهنده مورفولوژی شهری، پهنه های آبی مانند رودخانه ها و دریاچه ها تأثیر قابل توجهی بر خنک کنندگی شهری دارند. به دلیل ویژگی های خنک کنندگی، پهنه های آب مانند یک فضای باز شهری بیشتر از دیگر عناصر، بر جزایر گرمایی شهری از طریق فرآیندهای تبخیر و انتقال حرارت تأثیر می گذارند. تهران در سال های اخیر به دلیل افزایش جمعیت با مشکلات گرمایشی شدیدی مواجه بوده است. احداث دریاچه چیتگر در تهران فرصتی قابل توجه برای بررسی اثرات خنک کنندگی آب و بهینه سازی ساختارهای شهری فراهم کرده است. در این تحقیق با نرم افزار انوی مت 4.4.5 بر اساس مطالعات انجام شده، 16 مدل ترکیبی برای مقایسه سناریوهای شهری بر اساس فرم و هندسه شهری با عوامل اقلیمی و شاخص آسایش حرارتی PET (دمای معادل فیزیولوژیک) طراحی شد. از نتایج به دست آمده مشخص شد که با احداث دریاچه چیتگر دمای محیط کاهش و رطوبت افزایش یافته است. شاخص آسایش حرارتی PET نشان داد که پس از احداث دریاچه چیتگر، آسایش حرارتی از شرایط بهتری برخوردار است. نتایج به دست آمده از سناریوی فرم شهری بهینه نشان داد که هر چه نسبت ارتفاع به عرض دره های شهری بیشتر باشد، شاخص دما و آسایش حرارتی کاهش، اما رطوبت افزایش می یابد. همچنین محل قرارگیری ساختمان ها در جهت شمال شرقی به جنوب غربی کمترین درجه حرارت و آسایش حرارتی PET را دارد اما از رطوبت بیشتری برخوردار است.The Cooling Effects of Urban Lakes on the Surrounding buildings ( Case Study: District 22 of Chitgar Lake, Tehran)
HighlightsUrban designs that allow the incorporation of natural elements lead to improved environmental quality.Deepening urban valleys facilitates greater wind flow from lake surfaces, enhancing cooling effects.Reducing building surface coverage enhances airflow and improves thermal comfort.Increasing building height and decreasing the surface coverage factor (SC) reduces temperature levels. IntroductionUrban areas are responsible for two-thirds of the world’s primary energy consumption, which contributes significantly to urban heat islands (UHI) (Keirstead et al., 2012). UHIs are a major environmental issue, characterized by higher urban temperatures due to concentrated human activities. This study examines the cooling effects of Chitgar Lake on its surrounding urban area, aiming to mitigate future heating issues in Tehran by exploring how urban lakes can influence UHI. Specifically, it analyzes the role of urban geometric configurations in mitigating heat around lakes, which serve as urban open spaces.Theoretical FrameworkUrban planners utilize indicators like Floor Area Ratio (FAR), Surface Coverage (SC), and Building Height (H) to regulate urban forms. FAR relates to building volume, while SC denotes the proportion of built surfaces, indicating building permeability. Figure 1 illustrates the calculation of FAR and SC.Figure 1: Illustration of Urban Planning Indicators - FAR and SC.Among urban morphology elements, water bodies (e.g., rivers and lakes) significantly impact urban heat islands (Sarralde et al., 2015). The cooling effect of urban water bodies results from two main mechanisms: (a) evaporation, and (b) latent heat absorption, both of which lower surrounding temperatures compared to areas without water.MethodologyThis research compares two time periods—before and after the construction of Chitgar Lake—to assess the lake’s cooling effects on its surroundings. Two critical thermal conditions were selected from each period: one representing the environment after the lake’s construction and another before it. Meteorological data from these periods were analyzed, and simulations were conducted using the ENVI-met modeling software. The Physiological Equivalent Temperature (PET) was subsequently calculated with Biomet software (Wang et al., 2021).Figure 2: Research Process Diagram (a) and Sample Model Descriptions (b).Results and DiscussionComparison of Conditions Pre- and Post-Lake Construction: Simulation data reveal a general temperature decrease in the area following Chitgar Lake’s construction. Model A, representing the current urban fabric, acts as a heat absorber. Although it generates a warmer atmosphere, the shadowed areas remain cooler, providing enhanced thermal comfort. In Model A, humidity intensifies during pre-dawn hours, gradually decreasing as sunlight increases.Impact of Height-to-Width Ratio: Taller buildings deepen urban valleys (H/W > 1), casting longer shadows that lower temperatures and improve thermal comfort. However, these deep valleys tend to retain moisture due to limited sunlight and airflow. In contrast, shorter valleys (H/W < 1) permit more sunlight and airflow, leading to reduced humidity but higher temperatures, resulting in less favorable thermal comfort.Comparison of Urban Form Layout: Analyzing the SB-1 and SB-2 models showed that increasing building height and reducing surface coverage (SC) lead to temperature reductions due to higher FAR (increased density). Larger cross-sectional areas hinder wind flow, retaining humidity in street valleys. The urban form model with the best thermal comfort index was CC-2, characterized by high humidity and optimal thermal conditions. Conversely, SB-1, with a high occupancy coefficient and high permeability, exhibited the most critical thermal state, acting as a heat absorber with elevated temperatures.ConclusionThis study demonstrates that urban open spaces with natural cooling elements, such as water bodies, along with wind flow and shading, significantly improve thermal comfort during hot summer days. Models show that urban spaces allowing natural factors to penetrate are of higher quality. As building height increases and urban valleys deepen, extended shadows help lower temperatures and create areas with balanced thermal comfort. Streets oriented from northeast to southwest, and to a lesser degree east-west, experience the lowest temperatures while maintaining high humidity, which improves thermal comfort through lake-enhanced wind flow.Additionally, denser urban street valleys with low permeability hinder wind flow, contributing to inadequate thermal comfort. This study found that modifying urban form, building orientation, and the height-to-width ratio can enhance urban valley permeability, reducing thermal pollution effects and lowering cooling energy demands.
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