تحلیل ترکیبی خط آسمان پلاک ها و جریان طبیعی هوا در دو بلوک شهری تهران (مورد پژوهش: منطقه ولنجک تهران)

پژوهش ذیل به تحلیل همزمان خط آسمان شهری و جابجایی طبیعی هوا در دو بلوک شهری در منطقه ولنجک تهران پرداخته است تا خط آسمان بلوک های شهری را نه تنها به لحاظ بصری – آنچه تا کنون مد نظر اکثر پژوهشگران این حوزه بوده بلکه از منظر محیطی و برقراری جریان هوا بررسی کند.  بدین منظور بخشی از بافت شطرنجی شهری در منطقه ولنجک تهران، در دو الگو شبیه سازی شده و باد با سرعتِ مرجع m/s 4.5، بر فراز آن اعمال شده است. شبیه سازی باد توسط نرم افزار انسیس فلوئنت و مدل توربولانسی k-ε، بدون در نظر گرفتن لایه بندی حرارتی اتمسفر انجام پذیرفته است. در این راستا، رژیم جریان هوا بر 1) مدل وضع موجود که اراضی خالی آن مطابق با قوانین جاری جاگذاری و به دنبال آن خط آسمانی نامتوازنی ایجادگردیده و 2) مدلی جایگزین با ساختمان های 4 و 5 طبقه در سطح اشغال 60 % که منتج به خط آسمان یکدست شده است،  مورد بررسی قرار گرفته است. نتایج نشان می ‌هد در الگوی جایگزین، در 71 % مکان زمان ها، سرعت باد به اندازه ایست که شاهد تهویه طبیعی هوا هستیم؛ این در حالیست که این شاخص در وضعیت خط آسمان نامتوازن، 55 % است. لذا با توجه به جهت گیری زمینه شهری منطقه مورد مطالعه و ویژگی های باد منطقه، در شرایطی که سطح اشغال 60 % است (الگوی متداول کنونی) الگوی شماره دو، می تواند به لحاظ تهویه و زیبایی شناختی، الگویی بهینه معرفی گردد.

Integrated Analysis of the Skyline and Natural Airflow of Land Parcels in Two Urban Blocks of Tehran City(Case study: Velenjak Region of Tehran)

Extended Abstract Objective and Background: The present study analyzes the urban skyline and natural ventilation of land parcels in two urban blocks in Velenjak region of Tehran simultaneously. This study tries to assess the efficiency of the urban blocks not only from the visual aspects– what has been considered by most researchers so far but also from an environmental perspective. For this purpose, a part of the urban texture in Velenjak region of Tehran is modeled in two patterns, and the airflow with a reference velocity of 4.5 m/s is applied to the models. The models are prepared in two patterns:1. The current status; a situation in which vacant lands have been massed according to existing data and ultimately resulted in a heterogeneous skyline; 2. A pattern in which 4 and 5 story buildings are added to the site, resulting in a homogenous skyline. The modeled area comprises 17 urban blocks and has an area of approximately 483,000 square meters. Wind simulation has been performed by Ansys Fluent Software and k-ε turbulence model, regardless of the atmospheric thermal stratification. Comparing the data obtained from the numerical solutions by Fluent Software to the reference wind tunnel results indicates acceptable accuracy of the selected method. Methods: The information of the nearest meteorological station to Velenjak region is used to assess the data in this area. The data achieved between 2007 and 2016 show that the wind speed is less than 5 meters per second 93% of the time. Therefore, the reference wind speed in this study is considered to be 4.5 m/s at the height of 10 m. Based on the continuity and the frequency of wind in any direction during the mentioned time, an average for the coefficient of importance is defined for each wind direction (Table 1). In this study, a velocity of 1.7 m/s at the height of 10 m above the ground is considered the criterion for wind stagnation state. The reference for the assignment of this velocity is Beaufort’s table. Findings: According to equation 1, a velocity of 1 m/s at 1.75 m balance is equal to a velocity of 1.7 m/s at 10 m balance (α= 0.33). Figures 9 to 12 show the absolute wind speed contours at 10 m balance above the ground on two samples for four wind directions. The areas marked with light blue color are areas of air stagnation. In these areas, the rotational airflow motion, also known as a vortex or a sequence, can be seen (Figure 7). The magnitude of the sequence area around the buildings is directly related to the building’s geometry and architecture and the urban fabric formation. Wind speeds in these areas are minimized, and they are prone to the accumulation of pollutants. A point grid with 100×100 points (Figure 8) is considered over the target area at the height of 10 m to compare the sample data and the velocity value at each point. Outdoor points with velocities greater than 1.7 m/s are separated from points with velocities less than 1.7 m/s, and their frequency percentages are calculated. Since the importance of wind blow in each direction varies, a coefficient of importance is applied to the frequency percentage of the points, shown in Table 1. Comparing airflow in two urban patterns with regular and irregular skylines indicates that air stagnation is seen at 29% of time/place in an area with a balanced skyline. However, in the first sample, at about 45% of time/place, the average wind speed at 10 m balance above the ground is recorded as 1.7 m/s, and thus we experience air stagnation phenomenon. Conclusion: According to the results, considering buildings to cover 60% of the land parcel according to the construction regulations, and given the wind characteristics of the study area and its urban texture pattern, natural ventilation is provided better in 4 and 5 story buildings with a maximum height of 12 and 15 meters for buildings that cover 60% of the land parcel. This is while 7 story buildings are also allowed to cover 60% of the land parcel due to the relatively highly constructed urban blocks. Based on the results, it can be argued that a balanced skyline is more favorable both aesthetically and environmentally, and that creation of tall buildings in residential areas is only recommended when the building has a symbolic value and plays a special role in the region and affects the skyline.