Ventilation Quality Assessment of Dual Courtyard Configuration

Maintaining air quality within building spaces is a challenge under the increased pollutants resources. The optimization of the courtyard building layout toward natural ventilation has emerged as a critical factor in shaping air quality conditions. This study delves into the assessment of outdoor air quality infiltration within a dual courtyard typology characterized by varying proportions and orientations for both the connecting link and openings. To comprehensively analyse air quality within these integrated courtyards, a combination of on-site particle dispersion measurements and flow pattern simulations were employed. The outcomes of this investigation highlight the pronounced impact of atmospheric conditions, particularly wind direction, on the performance of courtyard configurations. The results revealed that dual courtyard provide an approach to address the geometric conflict between thermal comfort and air quality optimization. Even in circumstances with limited flow velocity, the direct connection between courtyards deepens the upward recirculating flow into the courtyard cavity at the activity level. The optimal air quality is attained by planning each courtyard’s upper-level intake and outflow openings, as it provides an evacuation flow out of court cavity and decline the PNC than outside surrounding conditions.


Introduction
Global warming has altered the physical properties of the environment, notably the air quality, threatening human health in inhabited public spaces.According to Zhang et al. (2023) the atmospheric particle matter (PM) exhibited an interaction factor dependent on the climatic circumstances [1].These suspended PM has been evaluated based on particle number concentration (PNC), which has shown a spatial dynamic variation in its amount and size in comparison to other contaminants [2].
The regulation factors of PNC dispersion within developed environment have determined at the built structure's resistance to dynamic flow and the metrological factors (wind speed, wind direction, and air temperature) [3], [4].The dynamic flow pattern dictate whether pollutants, heat, moisture, and other scalars are diluted or removed out of the built area [5].Furthermore, the flow fluctuation is affected by a pressure coefficient that changes along the constructed surface [5].Therefore, tracking the ventilation quality induced by built structure is essential to design decision framework, particularly in light of how the climate affects the driving natural force.In this study, the courtyard as passive architecture component is taken as the base of building typology to investigate its dispersion performance.As courtyard has a unique performance of buoyancy ventilation proved by literature.Based on the scanning, there are two approaches concerning courtyard ventilation.The first approach investigates the influence of courtyard ventilation on dispersion related to the pollutant source location.The second approach explores the possibilities for enhance its ventilation performance.
From first approach perspective, courtyard significantly protect its cavity against outside contaminants which reflect on the surrounding context.A study by Nosek et al. 2022 found that courtyard building with pitched roofs decrease traffic pollutants by 1.3 times at the canyon's centre due to the vertical flow penetration, while increasing them around the corners owing to recirculating flow [6].A study by da Silva et al., 2022 reported that the quantity of pollutants in the air and the pattern of urbanization significantly impact each other.According to the study, increasing the courtyard aspect ratio (AR) prevents pollutants from entering the court and reflecting in the accumulated concentration at the nearby street canyon.[7].On the other side when the source of pollutants was at the courtyard, decreases the AR is preferred to evacuate pollutants out of courtyard cavity.For instance, Hall et al. (1999) investigated the dispersion at courtyard depending on interior gas sources using a wind tunnel experiment.The findings indicated that courtyard hollow is totally encompassed by recirculating vortex when its Aspect Ratio (AR) is between (0.3 and 1.0).The recirculating flow stops developing or does not reach the courtyard bottom if the AR is lower or higher than this range [8].Based on the findings of Hall et al., Alvarez et al., 1998 created theoretical equations to study the link between flow and heat transported in the courtyard.The results revealed the contradiction of courtyard geometric optimization respect to both thermal and healthy requirements.As larger stratification induced by high courtyard AR >1.5; otherwise it was limited or non-existent at lower AR [9].Another study of airborne droplets at 1.6 m in the courtyard conducted by Leng et al.'s 2020 [10].The study's findings concluded that width (W) is the most critical factor among courtyard geometric parameters, which reduces the concentration up to 80% with W from 5.8 to 11.8 m [10].The previous scanning determined that increases the air flow increases the convection as well and accelerate contaminant evacuation.In contrast, lower wind speeds cause more pollution to build at court cavity.
The second approach is increasing ventilation flow performance along the courtyard.At this approach, evolving ventilation strategies at courtyard are recommended either through the use of external openings dispersed along building facades [11], or by exploiting pressure differences caused by duplicated courtyards by connecting them through other space volumes [12], [13].
From previous scanning, dual courtyard configuration is the form that developing ventilation speed and limited the outdoor transformation.Even though, dual courtyard approach has not explored regarding the particulate matter dispersion along both courtyards.Therefore, it is essential to explore how well courtyards ventilation impact on their activity level, particularly in public buildings, in order to reduce the danger of exposure for occupants and enhance the sustainability of design with regard to health issues.Furthermore, the design aspect of dual courtyard on ventilation performance must be taken into account throughout the building's design process.So, in this study, the flow quality is assessed based on site observations and numerical simulation to explore the flow interaction with various dual courtyard designs.The PNC is taken as the indicator to measure the flow quality throughout the case studies.The dual courtyard cases have different AR and orientation, opening and link proportions.

Materials and Methods
This study assesses the quality of the ventilation flow distributed along dual courtyard in reference to the atmospheric variables.The evaluation phases are processed by measuring the on-site location of four different cases of dual courtyards.Each of these cases has a different proportion ((AR) aspect ratio, (LR) link ratio), opening orientation.The case study is selected at an educational building near a coastal city characterized by a hot-arid climate to trace the unsteady flow differences.Then, the measured duration is simulated to configure the flow performance along the selected cases.The methodology steps were illustrated in (Figure 1).

Case study
The case study is located at the E-just University's main campus, Borg El-Arab City, Alexandria governorate, Egypt.The location is adjacent to the north coast, so the atmospheric flow displays a fast variation related to the temperature differences between the land and the coast.The case study encompasses dual courtyards typology with different oriented openings and connections.The four structures that were chosen are (Bi-7), (Bi-8), (Bi-9), and (Bi-10).All of them are currently occupied as in (Figure 2).The specification for the integrated courtyards is displayed in

On-site Measurements
The buildings at the E-Just site are 223 m away from the city's transport network by a green belt.the courtyards within buildings are furnished with benches for student activities.The level of activity is determined at 1.5 m above the ground.At this level, environmental and PNC features have been assessed.The site campaign took place on June 25 and 26, which are also the end of spring and the beginning of the summer, when the flow condition undergoes a significant change.When scorching " khamsin " windstorms sweep Africa's northern shore while carrying sand and dust, Egypt occasionally witnesses this, according to (Egypt climate report).Due to climate change, this year's khamsin storms, generally between March and May, have lasted till June.The vast temperature contrasts between Africa's north coast and the rapidly rising temperatures in its deserts cause this hot-sandy wind to blow.The air quality was negatively disrupted by this phenomenon.So, the observed PNC during the campaign of measurements within the constructed environment is based on the atmospheric condition and the variation of the metrological factors.

Sampling site of PNC
The PNC observed during the campaign using the Ultrafine Condensation Particle Counter (UCPC) Model 3776 provides size-distribution measurements from 0.0025 to 1.0 µm.This alcohol-based device works by vaporising alcohol, which is then diffused into the sample stream.After the air sample is saturated, it condensed, and then the particles in the condensed sample are counted using an optical laser detector.Using the UCPC device, five samples have been explored along case studies.These five spots are distributed along both integrated courtyards, and one is taken at the outside as in (Figure3).The first point (P1), selected in front of the entrance, serves as a reference for the condition of the external air.The second point (P2) is located in the first courtyard, in the same line with the first point.Before the connecting link, the third point is on the first court (P3).The fourth point (P4) is placed in the second court after the link.The fifth point (P5) is obtained at the second courtyard's end expansion.Each sample has a 10-minute lifespan.Every dual courtyard situation requires an hour to measure the five spots.During two days, measurements are collected throughout occupation hours, from 9:00 am to 4:00 pm.As well as battery is used while device transporting, to reduce the gap of collecting samples.

Environmental parameter measurements
In order to trace the metrological conditions while detecting air sample status, the air temperature (Ta), relative humidity (RH), and wind speed (WS) have been measured using a digital Anemometer IOP Publishing doi:10.1088/1742-6596/2754/1/0120155 (MS6252B).These parameters were measured at the same time of collecting air samples, as well as these measurements were taken to validate the simulated model.In addition, the hourly (Ta, RH, WS, and WD (wind direction)) are obtained from the weather station at the campus site to be used as input for computational software.

Computational simulation
Numerical simulation is implemented to predict how flow would behave within a complex building configuration.Reynolds-averaged Navier-Stokes (RANS) is the calculation based on computational fluid dynamic (CFD), which is widely used to visualise the dynamic flow performance [14], [15], [3].Envi-met software is a one of three-dimensional microclimate simulation that follows the RANS method of calculation [16], at which turbulence is calculated as a function of kinetic energy in the turbulence (k) and the turbulent dissipation (ꜫ) [17].Furthermore, the interaction of Surface temperature, plant effect, and turbulent flow with the outside constructed environment can be visualised by Envi-met [18].The buoyancy flow across created configuration patterns [19], [20], as well as inside courtyard volume [21] [22], is caused by temperature and pressure differences along the constructed surface.So, Envi-met software is used in this study to simulate the atmospheric conditions along the two observed days in order to configure the flow relation to the PNC within the dual courtyard cases.

Model area setting
The dimension of the case studies is obtained from the building documentation combined with a field survey.Envi-met software provides a resolution range between (0.5 -5).In order to achieve model optimization, the model boundaries have been justified based on prior investigations.In this study, for accurate prediction, the model area is implemented with (1, 1, 1) for (x, y, z) directions [23], [24].The lateral size has to be at least five times the highest building in the model zone [25], [26].According to the Envi-met setting, the vertical model limitation has been reported to be twice the height of the target building [27].Therefore, 12 nesting grid has been added to the model area with total grids of (237, 217, 60) for x, y, and z, respectively.The input variables of the Envi-met simulation are demonstrated in (Table .2).

Validation
The simulation results are calibrated based on the accepted limits of deviation illustrated by previous literature.Figure (4) revealed the agreement between simulated and measured results along the measured atmospheric factors.These parameters are (air temperature (Ta), relative humidity (RH), and wind speed (WS).Envi-met software can accurately predict Ta with lower deviation compared to the measured (Figure 4).The maximum deviation recorded at Ta is (2.7).This variation are within the accepted range which had reported from (2 to 4)•C by [28], [29].The simulated RH is also located within the accepted range (15.2%), as it is affected by water surface vapour and the vegetation within the location.The limitation of RH uncertainty has been reported to be around (≈15%) according to [27], [30].As well as, the WS prediction displayed a maximum deviation of around 0.33m/s, which is still within the accepted limits.The prior studies have determined the accepted deviation range between (0.7 to 0.2) m/s [21], [31].

Results of measurements
The air quality is assessed based on PNC dispersion through on-site measurements, and the metrological factors are measured at the same time of air sampling.The measurements are distributed as two points at both integrated courts and one point in front of the building entrance, which was taken as a reference point that demonstrated the quality of site location at each studied case.These points are (P1) represents the reference point, (P2, P3) represents the first courtyard, and (P4, P5) represents the second courtyard.

Particles transfer assessment.
In order to evaluate the dynamic transfer of particle concentration along case studies, the flow direction is analysed in accordance with the opening orientation.The hierarchy of concentration in all case studies determines that the airflow is the influential factor affecting particle dispersion (Table .3), (Figure 5).The particle dispersion is altered according to the permeability orientation of the first courtyard.Along four case studies, three relations are based on the wind direction and the opening orientation of the first courtyard.These relations are leeward, perpendicular, and against wind direction.These three relations control the distribution map of particle concentration along the studied cases.Firstly, the leeward relation was recorded at (Bi-7) at 1st day and (Bi-10) at 2nd measured day.For both (Bi-7) and (Bi-10), the outside reference point recorded the lower concentrations, while the higher accumulation was recorded at the endpoint of the 2nd courtyard in (Bi-7) and at the end of each 1st and 2nd courtyards at (Bi-10).The accumulation differences are due to the direct connection at (Bi-7), while at (Bi-10), the stair obstructs the transfer continuity.Secondly, the perpendicular phase was displayed at (Bi-8) and (Bi-10) during 1st day, as well as (Bi-7) and (Bi-9) during the 2nd day.At both (Bi-7) and (Bi-8), the reference outside point P1 demonstrated the lower PNC.At the same time, the higher values were displayed at the 2nd courtyard A7, due to a direct connection and at the 1st courtyard B8 due to the indirect link.Because of the building layout at (Bi-10), the PNC is around the same for all measured points.In case (Bi-9), the higher PNC was explored at reference point P1 while the lower was recorded at the 1st court B9.Finally, the opposite wind direction phase was recorded at (Bi-9) and (Bi-8) through the 1st and 2nd day, respectively.At the opposite flow condition, case (Bi-9) demonstrated almost the same PNC for all measured points; this can be due to the building layout effect.In case (Bi-8), the reference point has the most PNC, while the 2nd court A8 recorded the best air quality conditions with lower concentrations.

Atmospheric condition impact
The metrological conditions represent the surrounding environment that affects the dispersion of particles in the air.The relation between metrological factors and PNC is displayed in (Figure 5).Along the two measured days, the relative humidity RH displayed an inverse relationship with the PNC dispersion along all points.On the other side, the air temperature demonstrated a positive relationship with PNC.As wind speed is correlated to the flow pattern along the courtyard's geometrical configuration, studying the flow pattern is essential to configure the dynamic speed of airflow.

Results of simulation
The influence of wind direction and speed along the explored dual courts is simulated to configure the flow pattern performance of the integrated courtyards on ventilation quality.The case (Bi-7) at the leeward direction phase displayed a poor flow transferred between the integrated courts.This causes the flow at 2nd court A7 to perform as an isolated court, forming wide recirculating flow currents along its long axis.In comparison, the 1st court B7 has almost static flow at the lower level of the court.The second phase shown in the (Bi-7) case is perpendicular wind relation.At this status, a higher transferred flow current appears at the 2nd court A7, increasing the upward air exchanges.This behaviour influenced the PNC, which significantly declined than the first measured day.As well as the 1st courtyard B7 showed an increase in the flow speed current distribution.The perpendicular correlation of the flow has better performance in this case configuration.
The case (Bi-8) showed a perpendicular relation to the wind flow direction on the 1 st measured day.Even though there are flow transformations between the dual court layout at this phase, it was limited through the link and did not profoundly affect the flow currents at both courts' depth, where a recirculating vortex is formed around the end side of each court.The second measured day has shown an opposite wind direction relation.The flow performance at this phase is much better than before due to the opposite opening sides at the higher levels of 1 st courtyard B8 and the upper opening in 2 nd court A8.The flow currents deeply affect both courtyards due to wide vortex formation with higher speed.This relation positively influenced the PNC evacuation from both courtyards to the outside.The specification of (Bi-8) dual courtyard at opposite phase relation reduces the PNC even than outside conditions.
The case (Bi-9) witnessed an opposite flow relation on the first measured day.The dual courtyard specification has a contrary performance to what is shown in the case (Bi-8).At this phase, the influential flow is driven from the perpendicular corridor at the linked one between the 1st and 2nd courts (B9, C9).This pattern interrupted the flow current transformation between both courts.As a result, both courtyards displayed an isolated flow performance according to their aspect ratios (AR).An upward vortex at each court is formed along its long axis, increasing the flow velocity at lower levels of dual courts.As well as, the PNC appeared at steady conditions at both courts as the outside reference point.The second measured day displayed a perpendicular wind flow relation.This phase demonstrated a close behaviour to the previous phase as the perpendicular flow at the link still has an influential effect.The differences between the two phases represent at the shifting of the formed vortex to the upper corner of both courts, with lower flow currents at the lower courtyard level.At this phase, the 2nd court C9 displayed higher PNC accumulation than the 1st courtyard B9.

Discussion
The ventilation quality at the dual courtyard is explored within the activity level to trace the influence of case specifications and the best zones of activity distributions.The dual courts provide a different performance of flow than single court flow.As illustrated by prior studies, the flow pattern at single courtyard is strongly dependent on geometric proportions, particularly (AR and W) [8], [9], [10].In addition, the flow velocity within a single courtyard at activity levels between (1m to 2m) height has recorded limited values according to [27], [32].The flow rate that increased due to linked courtyards is in line with what was reported by [12].Furthermore, this study estimated the flow quality according to the site measurements to comprehensively analyses the effect of the predicted flow direction interaction with different dual courtyard configurations.The study explored two connection statuses, direct and indirect link influence on the ventilation performance and its quality to alter suspended particle dispersion.The study revealed that the indirect link at case (Bi-9) has a limited impact on ventilation and health conditions at both courtyards.Furthermore, direct integration is preferred due to increasing the flow velocity at the lower activity level, even with limited wind speed conditions.The previous flow velocity is reported to decline within isolated courtyard even within the higher speed atmospheric flow [27].Moreover, the opening orientation at the courtyard can derive higher flow velocity as the skimming flow over the courtyard limits the exchanged flow, as proved by [8].
In general, a significant correlation between the wind direction and the performance of dual courtyard configuration must be considered during the building design phase.Moreover, the direct connection layout, such as in (Bi-7) and (Bi-8) cases, has a better influence on the ventilation quality and current distribution than the indirect connection as in the case (Bi-9).

Conclusion
The study explores the dual courtyard typology's influence on preserving a healthy space for activity at educational buildings.PNC dispersion is assessed as an air quality parameter, and the ventilation flow pattern as the dynamic atmospheric force that interfaces with case configuration.The assessment is followed by site measurements and numerical simulation to achieve the target.The results revealed the influential factors adjusting the particle dispersions at each dual courtyard's configurations.The main factors that have to be considered while designing courtyard configurations: -considering the prevailing wind direction in characterizing dual courtyard parameters through design process is highly impacted on the air quality and ventilation performance.
-the opening at the higher level of the courtyard increases the flow through connection, as the sky opening of the courtyard has a limited effect at high AR due to the skimming flow over the courtyard.
-the feature implemented at courtyard has to be away from the connection link of dual courts to sustain the flow speed and continuity.
Eman Mohamed Balah: conception and design, analysis and interpretation of the data, writing original draft, Hassan Shokry: analysis and interpretation of the data, Aya Hagishima: supervision, revising the article, and Hatem Mahmoud: project administration and revising paper critically for important intellectual content.

Figure 2 .
Figure 2. Case-study location (a), city and surrounding topography (b), main campus buildings(c), selected four courtyards building and their layout (d), the blue arrow refers to the entrance side (e).

Figure 3 .
Figure 3. Measured point location along the studied cases.

Figure 4 .
Figure 4. Comparison between the measured and simulated temperature (Ta), relative humidity (RH), and wind speed parameters.

Figure ( 6
) illustrates different sections taken at the axis of measured points (P1, P2) representing sec.(1), (P3, P4, P5) represents sec.(2) another one is add sec.(3) for case (Bi-7) where the last point P5 of the 2 nd courtyard does not exist at the same line of P3 and P4.The flow direction significantly influences the flow pattern at the dual courtyard layout.Each of the studied cases has different flow behaviour due to the changed flow direction.

Table 1 .
Case study specifications

Table 2 .
Input variables for Envi-met simulation.

Table 3 .
PNC transfer assessment