خلاصة:
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 میهد 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.
Extended Abstract Objective and Background: The present study simultaneously analyzes urban skyline and natural dispersion of air in two urban blocks in Velenjak region of Tehran in order to assess skyline of urban blocks not only from visual aspect– what has been considered by most researchers so far_ but also from environmental perspective and as a matter of air ventilation. For this purpose, part of urban staggered texture in Velenjak region of Tehran is modeled in two patterns, and wind flow with a reference velocity of 4.5 m/s is applied above it: 1. The existing status; a situation in which vacant lands have been massed according to current laws and ultimately resulted in a heterogeneous skyline; 2. A pattern in which the residential density of sample 1 has been divided in 4 and 5 story buildings which results in creation of a homogenous skyline. The modeled area comprises 17 urban blocks and has an area of approximately 483,000 square meters. Methods: Wind simulation has been performed by Ansys Fluent Software and k-ε turbulence model, regardless of atmospheric thermal stratification. Comparison of the data obtained from numerical solution by Fluent Software with the results of reference wind tunnel indicates acceptable accuracy of the selected method. To access meteorological information of the area, the information of the nearest meteorological station to Velenjak region is used. The data of windfall of the region between 2007 and 2016 show that 93% of the times, wind speeds of less than 5 meters per second have been recorded. Therefore, the reference wind speed in this study is considered to be 4.5 m/s at a height of 10 m. Based on continuity and frequency of wind blow in any direction during the mentioned time period, an average coefficient of importance for each direction of wind blow is defined(Table 1). In this study, the criterion of exit velocity from wind stagnation state is considered a velocity of 1.7 m/s at a height of 10 m above the ground. The reference for assignment of this velocity is Beaufort’s table. Findings: According to Beaufort studies, wind up to 1 m/s of velocity at 1.75 m balance above the ground is considered to be stagnated. Using 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 contour of absolute wind speed at 10 m balance from the ground on two samples in four wind blow directions. The areas marked with light blue are areas of air stagnation. In these areas, rotational motion of air flow called a vortex or a sequence can be seen (Figure 7). The magnitude of the sequence area around buildings is directly related to the geometry and architecture of the building and how the urban texture is formed. Wind speeds in these areas are minimized and they are prone to accumulation of pollutants. To compare the samples data, a point grid with 100×100 points (Figure 8) is considered over the target area at a height of 10 m and the velocity value at each point is recorded. 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. As the importance of wind blow in each direction varies, therefore, a coefficient of importance according to Table 1 is applied to frequency percentage of the points. Comparison of air flow regime over two urban patterns with regular and irregular skylines indicates that in an area with a balanced skyline, at 29% of time-places we see air stagnation while in the first sample, at about 45% of time-places the average wind speed at 10 m balance above the ground is recorded as 1.7 m/s and thus we see air stagnation phenomenon. Conclusion: According to the results of this study, considering occupation area of 60% and current construction laws, given the wind characteristics of the study area and its urban texture pattern, the best response to the demanded density value with a constant occupancy level of 60% in terms of natural ventilation is 4 and 5 story buildings with a maximum height of 12 and 15 meters. This is while in the study area, due to the relatively high area of each urban plaque, 7 story buildings are licensed at a 60% occupancy level. Based on the results, it can be argued that a balanced skyline is more favorable both aesthetically and environmentally, and that creation of skyline buildings in residential areas is only recommended when the building has a symbolic value and plays a special role in the domain and skyline.
ملخص الجهاز:
تحلیل ترکیبی خط آسمان پلاکها و جریان طبیعی هوا در دو بلوک شهریِ تهران* (مورد پژوهش: منطقة ولنجک تهران) Integrated Analysis of the Skyline and Natural Airflow of Land Parcels in Two Urban Blocks of Tehran City (Case study: Velenjak Region of Tehran) سیده حمیده موسوی1، مرجان السادات نعمتی مهر2(نویسنده مسئول)، شهرام دلفانی3 محمدرضا حافظی4 تاریخ ارسال: 13/03/1397; تاریخ بازنگری: 15/10/1397; تاریخ پذیرش: 25/02/1398; تاریخ انتشار آنلاین: 01/10/1399 چکیده 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 بلوکهای 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.
(به تصوير صفحه مراجعه شود) Fig. 6 Determination of areas of flow-plan regime and side view (H is the height of the highest building) جهت شبیهسازی جریان هوا در این پژوهش از نرم افزار انسیس فلوئنت و مدل توربولانسی k-ε استفاده شده است.
(به تصوير صفحه مراجعه شود) Fig. 10 Contour of absolute wind speed in meters per second, at 10 m balance; wind direction: west همانطور که در شکل 11 نیز دیده میشود، به هنگام وزش باد از سمت شمال شرق، به مانند حالت قبل نواحی رکود ایجاد شده ناشی از حضور ساختمانها کمرنگ است و خیابانها در حکم کانالهایی برای جریان هوا ظاهر شدهاند.