Experimental study on vertical void for improving natural ventilation in midrise double-loaded apartments

. Affordable apartments in tropical developing countries generally have double-loaded corridors to maximise the total floor areas. Building designs with double-loaded corridors often suffer from poor environmental conditions. Passive design using a vertical void can help improve the natural ventilation in the such building design. This study investigates the effectiveness of vertical voids in enhancing the wind speed in the building. An experimental building with a vertical void, open pilotis, penthouse at rooftop and wind fin on the ground floor to help direct air to the void was constructed in Tegal, Indonesia. Five cases were considered by controlling the window openings, louver openings on the roof and change in fin size at pilotis. Wind speed and direction were recorded at one-second intervals. High wind speed was experienced in the pilotis and void when the wind direction is from the north and the wind speed in the void improved due to the wind fin being placed on the south corridor of the building. The results show the average wind speed in the void is twice as high as compared to the corridors. The building design performed best with high wind speeds in the void and corridores when all windows were kept open.


Introduction
Affordable housing has become a major challenge for many countries of the Global South which are experiencing massive population growth. In case of Indonesia which is projected to have a population of 331 million of which 72% people will be living in urban areas by the year 2050 [1], one of the major challenges for the Government is to provide affordable housing to its growing and urbanising population. To cater to the needs of the people for housing, Government policies have led to the construction of public housing known as Rusun (essentially means low-cost apartment buildings) apartment buildings in all the major cities of Indonesia. Rusun is classified into Rusunami apartments which are developed by private companies for sale and Rusunawa developed by the central government for rental purposes (Three-year lease terms with one allowed extension).
It has been seen that design of these Rusun has double-loaded corridors aimed at maximising the usage of the area. In equatorial tropical regions like Indonesia, where both south-and north-facing facades get solar radiation all year long, solar radiation is unwelcome in tropical buildings. For this reason, the building's orientation concerning daylight is not prioritized. The design of the building is mainly concerned with accommodating the maximum number of units possible in the given land area. Poor environmental conditions, such as a lack of daylighting, thermal comfort, ventilation, and indoor air quality, frequently plague these designs [2]. The residents of these double-loaded flats who reside on the leeward side of the building may experience inadequate cross-ventilation in their residences. Particularly in low-cost affordable housing, the provision of natural ventilation can improve thermal comfort, energy efficiency, and resident health [3].
Continuous ventilation is regarded as one of the most essential comfort needs owing to the high humidity in tropical locations, which improves sweat evaporation efficiency and reduces discomfort from moisture on the skin and clothing. To increase the performance of crossventilation even on the leeward side of the structures, it is crucial to identify passive design techniques for double-loaded inexpensive flats.
The purpose of the current study is to parametrically evaluate the wind flow in the vertical void with changes in apertures and fin size in the structure. Three objectives of the study are: First, to understand and validate the wind flow patterns in the open pilotis and the void area of the double-loaded residential structures, considering the wind direction perpendicular to the building façade. Second, it is to determine how window E3S Web of Conferences 396, 02024 (2023) https://doi.org/10.1051/e3sconf/202339602024 IAQVEC2023 openings affect the variations in wind velocity in the vertical void. Thirdly, it is to check the impact of the fin on the wind flow in the pilotis and void region. From this study, we would be able to offer decisive evidence towards passive design architectural principles of a closed-vertical void which can be used into doubleloaded apartment complexes for future affordable housing.
The performance of natural ventilation is examined in this study using experimental measurements of wind speed and wind direction in response to changes in factors including (i) door and window openings, (ii) an open or closed penthouse, and (iii) wind fin size. The wind speed is then normalised using data from measurements of 2D ultrasonic anemometer placed at a distance of 6 m from the building.
The study will give a brief idea about improving natural ventilation with the help of passive design elements. Chapter 2 will explain the building design features which help increase natural ventilation, the measurement plan and the cases to test the effectiveness of the void have also been described. Chapter 3 compares the results from the measurements concerning the different sections of the building. Furthermore, a wind velocity ratio has been used to compare the cases. Finally, the study is concluded in Chapter 4 providing the future works and limitations of the study.

Building design, measurement setup and case design.
The present authors in their previous works have proposed an alternative design solution for such doubleloaded apartments for an effective ventilation system [4,5]. As shown in Fig.1a. The design consists of a closed vertical void between the two sides of the corridor. separating the building into two sections, open pilotis and wind fin at the bottom floor. The vertical void with a close-end roof will further provide a superior ventilation rate. The closed-vertical void dispersed wind pressure to the leeward side of the building and would induce natural ventilation inside the units. Additionally, the ground floor's vertical wind fin was built to channel wind upward and into the void. Fig. 1b. shows the areil view of the building and its surroundings there is a toilet of 3m height on the left and a water tank of 3m height on the right. Fig. 1c. shows the wind direction and wind speed recorded in the weather station placed at the the rooftop. Fig. 1d gives the outdoor air temperature and humidity during the measurement period.
In this study, we further focus on testing the proposed design in an actual experimental building to further advance the result to a real scenario. Fig. 1b. shows the experimental building. The experimental building in Tegal, Central Java, was created using data from earlier experiments conducted in Indonesia. Based on the local microclimate, its orientation was chosen to reduce heat gain and increase inside wind speed. First, as noted by Nugrahanti et al. [7], the overall plan of the building was created based on a typological analysis of existing middle-class residences, specifically Rusunami. Second, the building's concept for passive cooling is based on the findings of several pertinent studies, including those that looked at Dutch colonial buildings [8], middle-class apartments in Indonesia [9], vertical void space [5,6], and radiant floor cooling systems using phase change materials [10].

Building design
The existing experimental building is located in a tropical climate condition in the city of Tegal, Central Java, Indonesia. The building was designed based on passive design approaches to maintain thermal comfort for the residents. Among them, we are focusing on the impacts of passive design features in the building which help increase natural ventilation. As shown in Fig. 2a. there is a vertical void of 2.85 m width separating the two building sides. Fig 2b. shows the pilotis area which has a height of 4 m and is kept open for better natural ventilation. There is a wind fin of 12 m in length and 4 m in height in the pilotis area to direct the wind toward the vertical void. Fig 2c. shows the penthouse at the roof level of the building with dimensions of 3.05 m X 12.05 m and a height of 2.4 m.
Reinforced concrete was used to create the experimental building's main structure, while autoclaved aerated concrete (AAC) brick was used mostly for the outside walls. As shown in Fig. 3 the loft units ( ceiling height of 5.1 m) have a gross floor area (GFA) of 47.6 m2, whilst the standard units (ceiling height of 3 m) had a GFA of 43.2 m2. Both unit types include a full balcony with a 1.5 m width that faces the outside and serves as a shade structure. Both north and south-facing facades in the tropical tropics, like Indonesia, get solar radiation all year round. To further shield the experimental units from thermal impacts, high insulation materials (such as rock wool, which has a thermal conductivity of around 0.035 W/mK) were put on the west-and east-facing external walls. Semi-open and next to the vertical void space, the corridor in the experimental building was created. To reduce the interior air temperature through natural ventilation, it is crucial to maintain a lower air temperature in the neighboring corridor spaces. The windows and doors were specifically designed to maximize the ventilation performance in the units in addition to the ventilation strategies used in the building layout and configuration, including the use of operable insect screens, security grills, and small ventilation windows, among other things.

Measurement setup
A total of three 2D sonic anemometers, (YOUNG Model 86000 Ultrasonic Anemometer) were used for the measurements. 122 Hot wire anemometers were used to record the wind speed in the vertical void and the pilotis area of which 108 windgraphy (Wind speed sensors from KOA Corporation) and 14 Kanomax (Airflow Transducer model 6333 and sensor model 0976-03) sensors were used. Fig. 3a. shows the measurement setup for the study. The sensors were placed in three planes perpendicular to the building façade West (W), Middle (M) and East (E). A total of 8 planes starting from the north N1, N2, N3, Standard (Std.), Void, Loft, S1 and S2 were made parallel to the building façade. Sensors for wind speed and wind direction are placed on the intersection of these planes at various heights as shown in Fig. 3b. N1 being 6 m north of the building has two 2D ultrasonic anemometers placed at 1 m height from the ground at N1W and N1M. N2M has a 2D ultrasonic anemometer at 1 m with two hotwire anemometers at 1 m and 2 m respectively.
The pilotis have hot wire anemometers from Kanomax whereas the vertical void has windgraphy sensors from KOA corporates. The windgraphy sensors are placed in the corridors of the standard and loft units and the centre of the void. A total of 9 windgraphy sensors is placed at each intersection of StdW, StdM, StdE. Whereas 17 windgraphy sensors are placed in the intersection in the void. The first sensor is put at a height of 1m after that the distance between the two windgraphy sensors is 0.9 m. Kanomax sensors were placed only on the M plane at N2, N3, Std, Loft, S1 and S2.
Calibration was done for all 108 windgraphy sensors used for measurements with the help of a small wind tunnel at the Laboratory of Building Sciences, Ministry of Public Works and Housing. The 2D sonic anemometer and Kanomax sensors used were industry calibrated. Fig. 4. Shows the sensor placement in and around the building. the building (a) windgraphy sensors placed in the corridors and the void, (b) Kanomax sensor placed in the pilotis along the middle of the void (c) 2D sonic anemometers placed outside the building toward the windward side.

Case design
To compare the passive design elements and study their impacts a total of five cases were created by opening windows, opening penthouse windows and changing the fin size. The data were collected during the dry season when the days are sunny with minimal rainfall. Data collection for each case was done for five to six days in starting from 26 th June 2022 to 28 th July 2022. Table 1. explains the details of all the settings for the cases. The cases were designed in a way to figure out the impacts of the passive design elements used in the building design. Case 1 has all windows of units and penthouse open with a large size fin to direct air into the vertical void. Case 2 has closed windows to compare with Case 1 to know the impact of open windows in the units. Case 3 has the penthouse windows closed and compared to Case 1 will show the impact of a penthouse on the wind speed. Case 4 has a closed window, a penthouse and a large fin Case 5 has the same conditions as case 4 except the fin size is small to understand the impact of the fin size.

Results
A parametric study was conducted to understand the natural ventilation flow in the vertical void of the building. The wind speed measurements were normalized to wind velocity ratio (WVR) by Eq. 1. (1) where VM is the measured velocity magnitude and UO is the velocity magnitude recorded at the 2D sonic anemometer at a height of 1 m and distance of 6 m from the building. ܹܸܴ തതതതതതത represents the average value of wind velocity ratio.

WVR = VM / UO
For this study, we have considered the data which has wind speed above 1 m/s and a wind angle perpendicular to the façade has been considered allowing for 30° change in direction on each side. This was decided from the previous study by the authors in which it was proved that a wind angle of 30° from the perpendicular to the windward side can provide a similar level of wind pressure in the void [5]. The building is at an angel of 22.5° to the North, hence a wind angle between 352.5° and 52.5° (60°) was considered for the study. Rest of the data was discarded for this study.        തതതതതതത in the vertical void increased by 0.1 due to the penthouse opening this directly means the flow of wind from the pilotis to the outside was improved cause of the penthouse opening. Hence penthouse should be opened when the flow is to be taken outside the building and can be kept closed in case the flow is to be directed toward the units.  Fig. 9 shows the comparison of ܹܸܴ തതതതതതത in the middle plain for Case 4 and Case 5. In Case 4 the fin size was large whereas Case 5 has a smaller fin size rest of the parameters are the same in both cases with closed windows in the units and penthouse. The ܹܸܴ തതതതതതത on the windward side of the pilotis for Case 5 was the highest at N2 (2.85) slowly decreasing towards N3 (1.6) and standard (1.6) which was higher than Case 4 by 0.4 in all the cases. The ܹܸܴ തതതതതതത on the leeward side of the pilotis was again higher in Case 5 as the smaller wind fin allowed higher volumes of air to pass through. The ܹܸܴ തതതതതതത on the leeward sides for Case 5 are 1.2 at the loft, reducing to 0.9 at S1 and S2 whereas Case 4 has an average ܹܸܴ തതതതതതത of 0.3 which was almost 0.8 times less than Case 5. Case 5 recorded a higher ܹܸܴ തതതതതതത in the pilotis area. In the vertical void Case 5 and Case 4 show similar trends after the first floor starts. In the case of standard corridors, both cases show similar trends and values of ܹܸܴ തതതതതതത are similar for all three corridors between 0.1 to 0.3. The loft corridors show a similar trend for the second loft floor with the ܹܸܴ തതതതതതത between 0.1 to 0.2. In the case of the first-floor loft corridor the ܹܸܴ തതതതതതത was higher in Case 5. This explains that the smaller fin size allows better flow of air in the pilotis as well as allows a better direction of flow into the void. In the case of a large wind fin, the incoming air volume gets completely stopped by the larger fin and was forced to move in the void area thereby reducing the wind speed by backflow after hitting the large fin.

Conclusion
The study intended to do an experimental measurement to facilitate natural ventilation in the vertical void with the help of a wind fin at pilotis level. The passive design elements to increase natural ventilation were parametrically assessed in the building. A total of 5 cases were studied by opening and closing the windows, doors and changing the fin size. The minimum wind speed for analysis was assumed at 1m/s and wind direction at 352.5° and 52.5°. The following are the key conclusions that can be drawn: x The ܹܸܴ തതതതതതത in the void when the doors and windows are open is approximately 0.15 higher than when the doors and windows are closed.
x The large fin size directs more air into the vertical void but higher ܹܸܴ are recorded in Case 5 with a small fin size compared to Case 4 where the fin size is bigger though the rest of the conditions are kept the same. x The impact of the penthouse opening can be seen in the vertical void area with a minor increase in the ܹܸܴ. x The ܹܸܴ in the corridors is much less than that in the vertical void, Overall the corridors act as an interruption to the wind flow. x The first-floor loft receives the highest ܹܸܴ as the wind fin at pilotis level directs all the wind towards leeward units when the wind is perpendicular to the windward façade.
x The ܹܸܴ at the pilotis level is almost 1.5 times the outside natural ventilation due to the venturi effect. x The leeward side pilotis area is not impacted by the opening and closing of windows.
These findings can be further advanced by adding more environmental factors for measurements like air temperature, humidity, etc. There is a need to further study the wind pattern and thermal comfort inside the units too.
There is a need to further discuss the location of the corridor in the building design. There is a need to study the airflow patterns in the case of an external corridor which can further enhance the wind environment and can add more privacy to the residents. Overall the results can help ascertain whether the vertical void and wind fin help improve the natural ventilation in affordable housing.