Impact of dense masses on wind flow in urban ventilation, Case study: Babolsar city

Plans should improve the urban spaces quality in order to create access to a sustainable place for the residents’ comfort in the environment. For this reason, studies are essential on the micro-urban climate and in optimizing the urban structure for achieving this goal. Nowadays, the cities and population growth has disturbed nature and altered the original structure of the city. In recent years, the high-rise buildings idea has been raised in the cities of the world in order to prevent the horizontal growth of cities. One of the high-rise buildings negative effects is the change in urban wind flow. Our cities are now witnessing the growth and expansion of high-rise buildings and this issue occurs when there is no proper understanding of the subject and no laws and regulations are in place. The concerns existence and the need regulation for The correct application of the high-rise building phenomenon, conducting various research and studies in this field is necessary.In this study, the effects of elevation and changes in two urban blocks in the northern area of Babolsar were evaluated by Flow-3d software. In the warmest days of summer, the effect of altitude and mass arrangement on the blocks temperature and the wind flow velocity between them have been investigated. Increasing height, enclosure, and inappropriate orientation of the masses will not occur in the wind. This will increase the temperature between the masses and disappoint the inhabitants of comfort area. In order to improve the situation, it is necessary to change the physical conditions according to the wind direction and distribution.

The study area has two urban blocks with the total area of 111315 m2. The length of the area is 466 m and the width is 258 m. Comparing block A and B it is perceived that in block A density and setting of buildings has been changed relative to the nearby fabric while block B has maintained its traditional physical condition. Demand for construction is going to change morphology of block B and transform it into high-rise buildings. Minimum height of buildings is 4 m in block B, and the maximum height of buildings is 39 m in block A. The wind speed is measured using a hot wire anemometer st-3880 at 32 points in two blocks at a height of 1.75 (pedestrian level) and synoptic center meteorological information. The wind measurement data were obtained from 32 points carried out in two blocks site setting based on actual buildings/local neighborhood in northern part of Babolsar. These 32 points have been selected by their difference in height, enclosure, orientation and width between masses in this area. Their information has been surveyed in 5 times intervals of 2 hours a day in chart 3. Measurement is performed in normal street activity mode of neighborhood. For validation purposes, the wind velocity magnitude was done in-field on August 8, 2017. The research area has been computationally modeled in order to evaluate wind flow in Flow-3d (V11.2) software. The two blocks were studied as part of a small urban model, to simplify of modelling the experiments in CFD codes. The wind simulations data - at two blocks - were determined based on the time-series results, and subsequently compared with the measured wind data.

The validation for real urban areas is typically performed with data from infield measurements. This part of study was to provide experimental data for the validation of CFD simulations. Some special aspect of the flow between buildings setting were observed from the measurements. For further analysis of the flow aspect, CFD simulations could be used to attachment the present data and provided that these simulations are carefully validated. Velocity measured in in-field and computed in simulations are compared. Comparison between wind velocities extracted from 32 points in CFD simulation and infield wind measurements are shown. The mean correlation coefficient is 0.6563 that respectively represent a positive relation. Some low inconsistencies existed in certain points locations and this error rate is readily apparent due to unpredictable environmental factors.The velocity contours and streamlines were studied using the CFD method around the buildings. The results were presented for the annual wind velocity at a pedestrian height of 1.75 m from ground level. The basic results from the CFD simulations are presented for the proposed new building in Figures below. These figures show plan and section views of velocity streamlines for prevailing wind speeds varying from 0 m/s to 1.4 m/s. Evaluation of wind flow’s simulation:- Winds are driven by the prominence of masses in urban environment and are not randomly distributed.-The wind speed decreases by encountering the exterior of the existing masses (the form of the buildings) in the urban area.-The form of the masses increases and decreases the velocity and change the direction and streamlines of wind flow.-The wind speed increases in low density areas due to low prominence, simple form of the masses, low height and then low enclosure. But wind speed decreases by the new high-rise buildings in front of these masses.-When the buildings are rotated vertically and horizontally to the wind streamlines, the wind will be reduced behind the row buildings. In this case, the number of buildings will fall under the shadow of the wind, because the wind continuously hits the walls, and the wind moves around and above the buildings. These buildings are located on the street, so the air flow is also reduced in the street.-tall buildings have a lot of effects on the wind flow in the city. When the wind flows hit into high-rise buildings, there will be more flow around them. A pressure packet is created at the back of these buildings, which causes the air to flow downwards and on the ground.-tall buildings such as towers deflect a large part of the wind flow toward around.- When the width of the masses increases, the deviation rate of the velocities contours increases toward the surrounding. Therefore, the wind flow is more behind the buildings with high width than tower buildings with low width.-The acceleration of the wind movement is high near the edges and corners of the building.- Leeward is created in front of the building.- Wind turbulence occurs behind high-rise buildings.-The wind streamline rises in narrow spaces.- Creation rotational flow between buildings.-The inflow wind, which moves at 90 ° to the masses, causes collision and deviation of the wind and reduces the velocity.- The horizontal masses, along the wind direction, reduce the wind going up and down in the opposite of the vertical masses.

According to the results of the specified area simulation in Amir Mazandarani street, the type of the masses arrangement to the wind flow and other masses, height and low width between them changes the wind flow direction and velocity. In general, the enclosure increase between masses or barriers to wind, the velocity and wind direction distribution increase. The important point is that the shape and wind shades range of the masses change according to the enclosure and the masses orientation to the wind. Finally, the north-south streets have lower wind velocity than the east-west streets. the reason is that the north-south corridors are perpendicular to the wind and the masses, they have some continuity, and eventually the main air flow will flow over the building masses. Due to the low wind velocity is not able to climb above the buildings. At east-west streets, there is high wind velocity. The main reasons of the increasing velocity in these passages can be Same direction of wind flow and these passages, low width and canalization them. The high wind velocity in the range causing these conditions have inversely correlated with the amount of humidity between the masses, so that the optimum occurrence in this range is due to climatic conditions and high relative humidity.