The Effects of Central Courtyard’s Geometric Form of the Hot and Dry Settlements in Kashan on the Airflow Passing by the Walls and Floor: Case Study-Kashan City

One way to achieve high energy efficiency in buildings and a reliable architectural system is to use the experiences of local architecture. In this connection, the central courtyard is an element that was used in the past to create comfort in the hot and dry climates of Kashan, Iran. A large part of the literature in the past has focused on this space and investigated factors such as height, length, and presence of water in the courtyard, which are directly related to the studied space. This study addresses the dynamic behavior of air in the courtyard under the effects of its geometric form and the airflow passing by its walls and floor to ventilate the space, aiming to increase efficiency. The Energy Plus analyzes this component, and Open Studio software and the research uses the descriptive-analytical method to investigate the findings. It is concluded that air circulation is best met by an airflow loop inside the courtyard in the central courtyard and it was ultimately revealed that the central courtyard priority per the geometric form is the oval with rectangular being the next. The circular, hexagonal and trapezoidal shapes of the yard have been shown to understand better the movement of the wind adjacent to the bottom of the yard (south-to-north) and the flow of the upper layer (north-to-south).

Climate has a pivotal role in the formation of architecture and settlements. One of the most important factors contributing to architectural designs is climate, which affects the individual buildings, the architectural elements of the interior space, the entire urban texture components, and human life as a whole [1]. Large hot and dry regions of Iran require specific solutions to make housing livable and desirable. Traditional Iranian architecture has long used various solutions to provide comfort. In old Iranian cities, spatial relations like central courtyards revealed various forms, functions, and meanings, thought to be major factors linking spatial architectural components. Central courtyards used natural elements of light, water, wind, and nature to provide better living conditions for the people [2]. Climate-based architecture in hot and dry and hot and humid regions is characterized by a central courtyard that serves as a major part of residential houses in Kashan. This special space is a passive solution to provide thermal comfort in the buildings. A central courtyard is a safe, private and comforting space that provides natural ventilation, daylight, and visual communication with nature. From an environmental perspective, a central courtyard serves as a component of the wind-based architecture which protects the residents from harsh climatic conditions by using passive energy supply systems in various regions. This element also uses evaporative cooling systems, passing ventilation, radiative cooling, and shading to cool down and reduce the cooling load of the building [3]. In this connection, structural components of the central courtyard, including the general form of the courtyard, courtyard proportions, and the formative structure of its walls, greatly contribute to the static cooling [4].

As stated, this research aims to investigate the effects of the geometric form of the central courtyard on the airflow in hot and dry settlements. The research, which uses a descriptive-analytical method, aims to quantitatively and qualitatively investigate the role of the central courtyard in modifying the climate of the intended region. Thus, in view of the structural nature of the research, other forms of the central courtyard are modelled and analyzed in the 3D-Ansys Workbench software, with all forms having an equal cross-sectional area and a height of 3 meters.

Climatic data used

To simulate the thermal performance of the model prepared under similar climatic conditions in the city of Kashan, Kashan’s climatic data files, formatted EPW, were used, which show climatic data on an hourly basis. This file is one of the several available climatic files in Iran, which has been uploaded on the site of developers of the Energy Plus software. Repository of free climate data for building performance simulation From the Creators of the EPW.

Thermal comfort limits for the residents of Kashan City

Consistent with the studies conducted on the residents of the city of Kashan and the temporal limits and thermal comfort ranges, it is determined that the said limits for the city of Kahan range from 27-218ºC in summers to 26-204ºC in winters. In contrast, the relative humidity ranges from 18-53% [5].

Wind situation in the city of Kashan

According to Kashan’s meteorological data, the average wind velocity at the Synoptic Station of Golbad in springs is clocked at 2.94 m/s, 3.97 m/s for the summers. As a result, the average wind velocity in the first six months of the year is estimated to be 3.455 m/s, with the desirable wind direction coming from the northeast.

Hajian M, et al. [6] examined the spatial effect of the courtyard and its contribution to the formation of traditional Iranian houses in four examples of houses in the city of Kashan in their study, “The effect of the courtyard in the formation of the configuration of traditional Iranian houses in Kashan”. Hanif E [7] investigated the physical and spiritual qualities of Kashan homes during the Qajar era in work titled “The notion of occupancy and its influence on the courtyards of Iranian houses: a case study of Kashan houses in the Qajar period”. In an essay titled “The Effect of the Central Courtyard Pattern in Adjusting the Hot and Dry Climate Conditions of Kashan Houses”, Jafarian S, et al. [8] demonstrate that in addition to the spatial division and organization of the house's spaces, the presence of a central yard with the right proportions has significantly improved and controlled the climatic conditions in traditional houses of hot and dry climates, especially Kashan houses, and has led to the adjustment of the climatic conditions in winter and summer. In an article titled "Optimum design of the central courtyard in residential buildings against the 120-day wind of Zabul based on CFD analysis", Khaksafidi S, et al. [9] aims to find the best placement and appropriate geometrical shape of the buildings in the central courtyard type as well as ways to lessen wind speed, at least in a defined central courtyard area, and to lessen pollution emission in the central courtyard, which is amplified by the rotation. In a study by Mazaheri M, et al. [10] titled "Analysis of the role of the yard in the spatial structure of Iranian houses using the space syntax method", the spatial organization of various examples of Iranian homes is analyzed, and the common characteristics and elements of the homes are explained, which show the most characteristics of the courtyard in the architectural space of these homes. In an article titled “The role of the central courtyard pattern in adjusting the harsh conditions of the hot and dry climate of the Yazd region”, Zarei M, et al. [11] examined the impact of the central courtyard design of houses in Yazd and determined how the ratio of the central courtyard to the walls affected the region's harsh hot and dry climate. The literature on the central courtyard has specifically addressed its architectural features, with some studies dealing with the courtyards' thermal comfort and static cooling. One of these researches has analyzed structural parameters and courtyard design proportions in Mexico and Spain. In the fourth chapter of the book, “Responding Environments”, with the title of micro-climate, Bently Y, et al. [12] have proposed a way to reciprocally analyze the city and the climate, which stresses two climatic elements of radiation and wind. One of the major features of houses in this region is their interiority, inspired by the regional climate. This has caused the spaces to form on one or four sides of the central courtyard, with the courtyard being the most important space for the supply of light, ventilation, internal communication and other functions. The placement of spaces built around the courtyard, especially in large houses, and enclosed on four sides by rooms or walls, have rendered these spaces to be called central courtyards [13]. Mehdi SF, et al. [14] used the Ecotect software to study the interaction of the Abbasian house in Kashan city with solar energy. CFD-based simulation is another method to study the interaction between the climate and residential architecture. Kristianto M, et al. [15] used this technique to investigate the thermal comfort conditions in the internal environment of the traditional Minahasa houses. Nasrallahi F, et al. [16] investigated the effects of climatic elements in Iranian architecture on the central courtyards in four samples of Kashan’s houses in hot and dry regions. Nejad MB, et al. [17] also examined the central courtyard space in Kahan’s hot and dry climate.

Functioning of the central courtyard

Given the cases mentioned above, climatic issues, and the role of the courtyard, we aim to look deeper into the issue of the central courtyards in the houses. A courtyard is an open house space that constitutes the pivotal section in entrance spaces. It is, in fact, a room without a ceiling which the family members highly use. One can enjoy nature, trees, and wetness from the water pool in this space. This space is a place for association and communication with the guests as it provides comfort and a safe and silent haven. Other house spaces directly associated with the courtyard include the porches, adjacent rooms, corridors, and the stairway. Surrounding the courtyard are various rooms directly in contact with the light, water, plants, and fresh air. These surrounding spaces include three- and five-door rooms, the hall, wind tower, sash, balcony, squinch, and estrade [18].

People of the desert always seek to overcome severe climatic conditions and use various ways to alleviate their problems. The thinking behind an internal-oriented house has been the best response to the problems, and the central courtyard was the turning point of this solution. The residential architecture of central Iran's regions like Yazd, Isfahan, Kashan, and Shiraz are internally oriented, with various house spaces of these regions organized at a certain order in different building fronts. The way spaces are organized around the central courtyard, and the type of spaces constituting the house can be factors to configurate the residential units (Tables 1,2) [21].

Form of the central courtyard

Several studies have sought to provide optimal sizes of central courtyards through circular, polygonal, square, and rectangular forms in different climates and geographical latitudes. As the literature suggests, changing of central courtyard’s proportions with a circular form had a significant impact on the level of shading and exposure to direct sunlight, while low-height samples were found to outperform the deep samples. On the other hand, simulations suggested that the buildings with deep central courtyards required less energy consumption to meet their cooling needs [22]. Another research modelled the square-shaped central courtyards of different proportions in four different regions and investigated the effects of their forms on shading and direct sunlight. It was concluded that the optimal height of the central courtyards, aimed at meeting the desired function in summers and winters, was three floors in the hot and humid climates, two floors in the dry and mild climates, and one floor in the cold climate [23].

Ventilation in the central courtyard

Natural ventilation is highly dependent on building openings. The use of mechanical systems has made this issue be ignored; however, natural ventilation can help increase the quality of the inside airflow and save energy, which can eventually serve as a major strategy to achieve green building as intended. The literature also suggests that besides wind towers, atrium, central courtyard, domed ceiling, and windbreak walls can, if regarded to be part of the building design, have significant impacts on the quality of the inside air. It was also found that the central courtyard, which serves as a transfer zone, can provide peace and comfort by modifying the micro-climate in the building and increasing airflow velocity inside the building. This causes the central courtyard to be considered an effective strategy in the hot and humid climate. In these climates and summers, it is highly required to direct the wind to the interior side of the space to modify the warmth and humidity and improve thermal comfort. This is also an energy-saving strategy that is much too effective [22].

Investigating the effects of the geometry of the central courtyard on the airflow passing by the walls and the floor

The courtyard’s geometric form can greatly affect the air circulation of the inside in the courtyard, its intensity, and the velocity of the wind in the adjacency of the surfaces of the building, the courtyard, and its floor. Central courtyards of the local houses in Iran’s hot and dry climates have a rectangular and elongated form aimed at taking advantage of the wind. In addition to our local forms, there are essentially new architectural forms in other cultures which can also be examined by 2-D modeling in the FLUENT Software, the temperature and wind speed contours are calculated for each model. The 2-D structure mesh and the K-epsilon turbulence model were used to solve the problem in the FLUENT. The wind flow in all states is left-to-right (north-to-south). The average wind speed is 3.455 m/s, and its temperature is 31.88 centigrade, equal to the monthly temperature of Kashan City. To validate the problem-solving procedure in the FLUENT the CFD simulation article for steam in the pool has been used. The studied forms consist of the following: Central courtyard with a rectangular section, central courtyard with a circular section, central courtyard with an oval section, and central courtyard with a hexagonal section.

Effects of the geometric form of the central courtyard on the natural ventilation

The table illustrates that the geometric forms of the central courtyards are oval, rectangular, circular, hexagonal, and trapezoidal. The longitudinal section of the rectangular central courtyard suggests that the air well circulates there. There is an airflow loop inside the courtyard which almost affects all the floor and the surfaces facing and back on to the wind. Under this situation, the wind shadow with a velocity of 0-0.6 m/s inside the courtyard covers a small area in the middle of the courtyard, almost the roof level. Wind contours inside the courtyard suggest a large part of the courtyard, i.e., surfaces facing and back on to the wind, the courtyard floor to half the height of courtyard, is affected by the wind velocity of around 1.3-2.8 m/s. In the circular form, the airflow loop and its intensity are lower than those of the rectangular form, and the wind shadow inside the courtyard is larger. Transverse contours in adjacency and middle of the courtyard floor are asymmetrically distributed while the airflow in the center and right side of the courtyard has south to north orientation, but the right side has an inverse orientation. In this model, the courtyard is affected, but the intensity of the contours is reduced, and the wind velocity ranges from 1.3 to 2.7 m/s. The section is rectangularly elongated in the oval form, while the adjacent surfaces are circularly involved. Here, the airflow is well made, and the contours in the adjacency of the courtyard have a symmetrical distribution. The airflow in the middle of the courtyard is greater than that in the edges, which covers up to the middle height of the building mass. The greater intensity of the flow and the affected area are properties of this model, where the wind velocity amounts to around 1.3 to 2.8 m/s. The hexagonal form has a relatively poor airflow, the maximum of which occurs in the middle of the courtyard adjacent to the floor. Here, the airflow has an asymmetrical distribution. The surfaces facing or back on to the wind are affected by the flow, while other surfaces (four other surfaces) are not placed in the flow course. Here, the velocity ranges from 1.3 to 2.6 m/s. Like the hexagonal form, the trapezoidal form has a relatively poor airflow which mostly occurs in the middle of the courtyard adjacent to the floor. Only the surfaces facing or back on to the wind are affected by the flow, which reduces the intensity of the contours. Air velocity here reaches 1.3 to 2.6 m/s (Figure 1).

Figure 1: Geometric form of the central courtyard on the passing airflow.

The courtyard’s geometric form can greatly affect the air circulation of the inside in the courtyard, its intensity, and the velocity of the wind in the adjacency of the surfaces of the building, the courtyard, and its floor. Central courtyards of the local houses in Iran’s hot and dry climates have a rectangular and elongated form aimed at taking advantage of the wind. The table illustrates that the best geometric forms of the central courtyards are oval, rectangular, circular, hexagonal, and trapezoidal. To better understand the movement of the wind (south to north) in the adjacency of the courtyard floor and the upper layer flow (north to south), the transverse section of the oval courtyard is illustrated as an example in the following figure. The dominant flow orientation is from left to right (north to south), which has more intensity at a greater height of the courtyard. The more the flow approaches the height of the walls surrounding the courtyard, the more its intensity decreases, with the maximum airflow of the right to left (south to north) occurring in the adjacency of the courtyard floor and in conclude (Figure 2).

Figure 2: Transverse section of the oval courtyard; examination of the layers of the flow passing onto each other.

Utilizing regional architectural expertise will result in a building that is more energy efficient and an effective system. Due to the benefits of shade, water, and vegetation within, as well as depending on the kind of walls, height, and proportions, the central courtyards can lower the air temperature on the hot days of Iran's hot and dry environment On the other side, in the early spring and late fall, air currents created in the yard can be employed as natural ventilation for cooling. The present study aimed to investigate the geometric form of the central courtyard in residential houses in the arid climate of Kashan, taking into account the aerial data of this city and the length of the courtyard in the direction of the prevailing wind, using Energy Plus and FLUENT software. Although the simulations in this research are not based on a specific foundation and are limited to the arid climate and the rectangular courtyards with the same wind speed and temperature for all the models, and other factors such as the temporal vaporization from the walls, their materials, etc. have not been considered for the calculations, it was ultimately revealed that the central courtyard priority per the geometric form is the oval with rectangular being the next. In addition, according to the tables, the circular, hexagonal and trapezoidal shapes of the yard have been shown to understand better the movement of the wind adjacent to the bottom of the yard (south-to-north) and the flow of the upper layer (north-to-south). The cross-section of the oval yard is shown as an example in the figure. The prevailing flow is left-to-right (north-to-south) with a higher intensity in the heights, and the closer it gets to the surrounding walls, the lower its intensity. The maximum right-to-left flow (south-to-north) occurs near the floor. Finally, since previous studies have considered the analysis of the courtyard’s role in modifying the climatic conditions using the climate-related software and optimal design of the courtyard, we, in the present research, have investigated the hydrodynamic behavior of the wind in the central courtyard for reduction of the temperature by taking the geometric shape as a variable and the central courtyard proportions. It is suggested to investigate the role of central courtyards in other climates in future studies and evaluate the behavior of the central courtyard in the reduction of the temperature alongside other effective factors on the flow of airflow passage near the walls and the floor.

M. Karbasforoushha performed the literature review, model design, analyzed and interpreted the data, prepared the manuscript text, and manuscript edition. F. Habib and H. Zabihi prepared the manuscript text, manuscript edition, and compiled the data and manuscript preparation.

We would like to express our special thanks of gratitude to all those who have taken part in the study but have not been among the authors of the article. Also, everyone who has contributed to the writing of the article, methods and provided general support, we are truly thankful to them. Lastly, it is upon us to acknowledge the Sciences' Library of Tehran Branch. Departement of architecture, Tehran-west Branch, Islamic Azad University which provided support and funding for this project.

The authors declare no potential conflict of interest regarding the publication of this work. In addition, the ethical issues including plagiarism, informed consent, misconduct, data fabrication and, or falsification, double publication and, or submission, and redundancy have been completely witnessed by the authors.

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