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The influence of wind flows on thermal comfort in the Daechung of a traditional Korean house
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Building and Environment 44 (2009) 18–26 www.elsevier.com/locate/buildenv
The influence of wind flows on thermal comfort in the Daechung of a traditional Korean house Youngryel Ryua,, Seogcheol Kimb, Dowon Leea a
Department of Environmental Planning, Graduate School of Environmental Studies, Seoul National University, Republic of Korea b Boolt Simulation Technology, Republic of Korea Received 19 September 2007; received in revised form 5 January 2008; accepted 15 January 2008
Abstract Daechung, a semi-open space with wooden floor located between the front and backyards of traditional Korean residences, is well known as a cool space in summer due to cross-ventilation, but it has not yet been scientifically explained thoroughly. The purpose of this paper is to characterize the wind flow measured at a Daechung to interpret the effects of the wind characteristics on thermal comfort. We measured 10-Hz turbulence data at the Daechung and partitioned the wind vector into two directions (i.e. backyard to Daechung and front yard to Daechung). Interestingly, the wind from the cool backyard flowing through the Daechung was of less frequency and shorter duration but had higher velocity compared to wind from the opposite direction, which can provide thermal comfort to the dwellers. We suggest that the wind characteristics were determined by various aspects of the house’s design, such as its location and the degree of enclosure in front and backyards. The results show that traditional Korean house made use of a natural ventilation system during the summer. The principles of this system could be helpful in constructing environmentally friendly and sustainable residences. r 2008 Elsevier Ltd. All rights reserved. Keywords: Wind flow; Natural ventilation; Thermal comfort; Traditional Korean home
1. Introduction Daechung is a unique space generally located in the middle of traditional Korean houses (Fig. 1). Korea experiences summer monsoon, so it is very hot and humid in the season (Fig. 2). To escape the heat, residents spend most of the daytime inside during summer, unless they work outside. In particular, people usually sit near the backdoors with their arms on the doorsill for exposure to the cool wind blown from the backyard. Generally, the Daechung is always open to the front yard, while backdoors are open to the backyard only in the summer daytime (Fig. 1), thereby allowing cross-ventilation to provide thermal comfort to dwellers. In addition, natural convection induces the cool air in the backyard to flow into Corresponding author at: Department of Environmental Science, Policy and Management, University of California at Berkeley, 137 Mulford Hall #3114, Berkeley, CA 94720-3114, USA. Tel.: +1 510 642 9048; fax: +1 510 643 5098. E-mail address: [email protected] (Y. Ryu).
0360-1323/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2008.01.007
the Daechung through the backdoors [1–4]. The crossventilation system of a Daechung is well known in the academic world, but it has not yet been characterized in depth. In this study, we analyzed naturally driven turbulence data measured at the Daechung in a traditional Korean house in order to explain the effects of turbulence characteristics on the thermal comfort of dwellers. Natural ventilation systems providing thermal comfort in traditional dwellings have been noted in various researches [1,4–7], but thorough analyses on turbulence measurement were rare. We characterized the turbulence at the Daechung by separating the v-axis wind component, the orthogonal vector to the back door of Daechung, into positive (from front yard to backyard) and negative (from backyard to front yard) parts. By analyzing these two wind parts independently, we successfully characterized the turbulence and found meaningful phenomena closely linked with thermal comfort. In this research, we conducted three steps: (1) characterization of turbulence measured in a Daechung, (2) analysis
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2. Material and methods 2.1. Site description The old house of Yunjeung (361160 5500 N, 127170 5200 E) is located in Gyochon-li, Nosung-myun, Nonsan City, Chungnam Province in Republic of Korea (Fig. 3(a)). It faces a rice paddy to the south and is backed by Mt. Nosung on the north (Fig. 3(b)). The backyard is enclosed by 1-m-high walls and the front yard is enclosed by the main buildings and the front gate (Fig. 3(c)). The front part of the Daechung is open to the front yard, and
Fig. 1. Daechung, the wooden-made main floor of traditional Korean residences (note the three backdoors).
2.2. Field measurements Field data were collected depending on the availability of monitoring equipment from September 2003 to August 2004: air temperature (September 2003–August 2004); wind components of u-, v-, and w-axis (August 2004); and wind velocity and direction (January 2004–August 2004). The most reliable summer data (August 5–August 8, 2004) were selected by testing data quality and analyzed for this study. Wind components (u-, v-, and w-axis) were measured by three sonic anemometers at a frequency of 10 Hz in the front yard and backyard at a height of 1.8 m (CSAT3, Campbell Sci. accuracy: 0.04 m s1), and adjacent to the middle backdoor of the Daechung at a height of 0.7 m (SATI/3Sx, Applied technologies Inc. accuracy: 0.03 m s1). To measure the ambient wind flow, an Automated Weather System (AWS) (HOBO wind velocity/direction and weather data station logger, Onset comp. accuracy: 74% of wind velocity and 751 of wind direction) was used. The AWS was set up 10 m in front of the house at a height of 1.5 m, and data were logged every 1 min. Air temperature (HOBO Temp, Onset comp. accuracy: 70.4 1C at 25 1C) was measured in the backyard,
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three rectangular back doors are open to the backyard (Fig. 1). The back doors are apart each other by 1.1 m and the dimension of each back door is 1.1 1.3 m2. The main gate is open in the daytime and closed at night throughout the year. The surface ratio of the main gate to the front wall of residential area is approximately one-eleventh. The 30-year averaged monthly precipitation and air temperature at Booyeo, the nearest National Weather System (NWS) site to Nonsan City, is presented in Fig. 2. The 30year averaged annual precipitation and air temperature are 1334 mm and 12 1C, respectively.
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of the factors which cause the turbulence characteristics, and (3) analysis of turbulence characteristics’ effects on thermal comfort.
19
-5
Fig. 2. Climate characteristics of the study site from 30-year averaged monthly precipitation and air temperature taken at Booyeo, site of the nearest National Weather System (NWS) to Nonsan City.
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Fig. 3. (color online) Study site, the old house of Yunjeung located in Gyochon-li, Nosung-myun, Nonsan City, Chungnam Province, South Korea: (a) location of Nonsan City, (b) topography around the house, (c) plain figure of the house (redrawn from Kim [21]), and (d) cross-sectional view and dimension (cm) of the house (reference for cross-section is indicated in (c) by the red arrow line).
front yard, and at the Daechung. Data were measured every 10 min at a height of 1.5 m, except at the Daechung, where it was measured at a height of 0.5 m. The experimental design is illustrated in Fig. 4.
2.3. v-component at the Daechung The v-component at the Daechung is the component parallel to the floor and perpendicular to the back door. In general, itpcomprised ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi at least 90% of the horizontal wind velocity ( u2 þ v2 ) and determined the horizontal wind direction at the backdoor of the Daechung. Therefore, we can regard this component as the dominant wind flow through the Daechung. A positive v-component means that the wind flows from the front yard to the backyard, while a negative v-component indicates the opposite direction. Analyzing turbulence characteristics including times, duration, velocity, skewness and kurtosis at the Daechung, we only dealt with the v-component measured by a sonic anemometer at the Daechung.
2.4. Skewness and kurtosis Skewness characterizes the degree of asymmetry of a distribution around its mean. If positive extremes dominate, the skewness is positive and if negative extremes dominate, the skewness is negative. If the probability density function is a Gaussian distribution, then the skewness is zero. Skewness can be written as n xj x 3 1X S¼ , (1) n j¼1 s where n is the number of data, s the standard deviation, xj the raw data, and x is the mean. Kurtosis is a measure of the extent to which observations cluster around a central point. A kurtosis is large if the tails of the probability density function are relatively large and small if the tails are relatively small. If the probability density function is Gaussian, the kurtosis value is three. So, kurtosis 43 indicates a peaked distribution and kurtosis under three shows a flat distribution. Kurtosis
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Fig. 4. Experimental design.
can be indicated by n xj x 4 1X . S¼ n j¼1 s
(2)
3. Results 3.1. Wind characteristics at the Daechung 3.1.1. Frequencies The numbers of negative and positive v-components from 9:00 to 17:00 during 4 days were analyzed from 10-Hz raw data (Fig. 5). The number of the positive v-component was approximately twice as much as that of the negative component throughout the 4 days. This indicates that the dominant wind blew from the front yard to the backyard.
Fig. 5. The numbers of negative and positive v-components from 09:00 to 17:00 (August 5–8, 2004) from raw 10-Hz data.
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3.1.2. Duration The duration of directional wind (i.e. positive or negative v-component) is shown in Fig. 6. A duration below 1 s contributed to around 60% of all durations in the positive and negative v-components. The duration from 0 to 10 s was higher for the negative v-component than for positive v-component, while the trend was reversed for durations over 10 s. This shows that most wind changed direction transiently, and the direction of continuous wind for longer periods of time was mainly southerly (i.e. from the front yard to the Daechung). 3.1.3. Velocity The velocities of positive and negative v-components ranged from 0.22 to 0.28 m s1 and from 0.42 to 0.48 m s1, respectively, indicating that wind velocity of the negative v-component is twice that of the positive v-component (Fig. 7(a)). Time series of 10-min-average wind velocity revealed that the negative v-component had both higher mean wind velocity and higher fluctuation (Fig. 7(b)). Standard deviations of positive and negative v-component velocities were 0.05 and 0.17 m s1, respectively. 3.1.4. Skewness and kurtosis The skewness and kurtosis of the v-component of wind are shown in Table 1. Throughout the 4 days (09:00–17:00), skewness was always negative, indicating that the mode of v-component velocity is greater than the median values. Kurtosis values were nearly higher than three, which means that the kurtosis of v-components was higher than is seen in a Gaussian distribution. 3.2. Wind velocity at the front yard and backyard The 30-min-average wind velocity at the front yard and backyard is presented in Fig. 8. The figure indicates that the wind at the front yard was very calm, o0.5 m s1.
Fig. 7. (a) The wind velocities of negative and positive v-components from 09:00 to 17:00 (August 5–8, 2004) from the raw data measured over 10 Hz, and (b) time series of 10-min average wind velocity of negative and positive v-components from 09:00 to 17:00 on August 7, 2004.
Table 1 Skewness and kurtosis of v-component wind at the Daechung from 09:00 to 17:00 (August 5–8)
Average Stdevc
August 5
August 6
August 7
August 8
Sa
Kb
S
K
S
K
S
K
1.29 0.74
5.98 4.63
1.06 0.39
3.90 1.57
1.29 0.56
5.34 2.87
1.23 0.49
5.23 1.92
Note: Skewness and kurtosis were calculated over a 10-min interval. a S ¼ skewness. b K ¼ kurtosis. c Stdev ¼ standard deviation.
Fig. 6. The durations of negative and positive v-components from 09:00 to 17:00 (August 5–8, 2004) from the raw data measured over 10 Hz.
The average wind velocities at the front yard and backyard from 09:00 to 17:00 during 4 days were 0.36 and 0.62 m s1, respectively; the wind velocity of backyard was, therefore, almost twice that of front yard.
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from the south and southwest around 1 m s1, while a nearly dead wind blew from the north at night. These diurnal patterns are consistent with valley wind in the daytime and mountain wind at night [9].
4. Discussion
Fig. 8. Time series of 30-min average wind velocity at the front yard and the backyard from August 5 to 8. Some data in former 2 days were missed due to the lack of power supply in the sonic anemometer.
Fig. 9. Time series of 30-min average air temperature in the front yard, at the Daechung and backyard from August 5 to 8.
3.3. Air temperature in the front yard, Daechung and backyard
Traditionally, the Daechung has been used as the place to escape the hot summer in Korea [1]. Actual field data indeed showed that the Daechung was the coolest site compared with the front yard and the backyard during the daytime (Fig. 9). The main reason for this can be attributed to the roof protecting from solar radiation and providing a shaded area. In another aspect, we focused on the effects of the turbulence characteristics at the Daechung on thermal comfort. As a first step, a thorough analysis of wind characteristics at a Daechung was conducted. The field data demonstrated that positive and negative v-components of wind at the Daechung were different in number, duration and velocity. The number of the positive component was nearly twice as much as that of the negative component (Fig. 5) and durations over 10 s mainly occurred in the positive component (Fig. 6). These phenomena can be ascribed to the southerly wind (from the rice paddy to the mountain) that consistently blew around 1 m s1 in the daytime (Fig. 10). Wind velocity of the negative component was almost twice that of the positive component (Fig. 7(a)) and showed a more fluctuating pattern compared to the positive component (Fig. 7(b)). Skewness and kurtosis analysis of the v-components support the characteristics of each component mentioned above. Skewness values consistently showed negative values in the daytime, indicating that the probability density function is biased to the right. This means that the number of the positive component consistently exceeds that of the negative component in the daytime (Fig. 5). Most of kurtosis values of 4-day data exceeded 3.00, indicating that (1) the probability density function has a pronounced
The time series of air temperature at the front yard, the Daechung and backyard are shown in Fig. 9. The standard deviations are 3.61, 1.76 and 3.47 1C, respectively. The temperature at the Daechung fluctuated the least, and maintained a relatively cool temperature in the daytime and a relatively warm temperature at night. Air temperature appeared to be 1–1.5 1C higher in the front yard than in the backyard during the daytime due to different land covers—bare soil in the front yard while full of grass and shrubs in the backyard—and the adjacency of the backyard to the forest in mountainous area. 3.4. Wind velocity/direction at 10 m in front of the house The diurnal patterns of wind velocity and direction at the local scale are given in Fig. 10. In the daytime, wind blew
Fig. 10. Time series of 10-min average wind velocity and direction at 10 m in front of the house from August 5 to 8. Wind velocity of 0 m s1 data were skulled.
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Fig. 11. Comparison of different time-scale average v-components at the Daechung. (a) 10-m average from August 5 to 8, (b) 1-min average during August 7, (c) 10-s average from 09:00 to 15:00, August 7 and (d) raw data measured over 10 Hz from 13:00 to 13:30, August 7. Shaded area indicates the same period from 13:00 to 13:30, August 7.
peaked distribution, and (2) the tails of the probability density function are large [10]. High kurtosis values explain that consistent wind blew dominantly from the front yard to the backyard (Fig. 5), and the wind velocity of the positive component was relatively constant around 0.3 m s1 (Fig. 7(b)). Second, the large tails in the probability density function mean that the v-component is subject to very extreme events, influenced by strong negative v-component winds of short number and duration (Fig. 11(d)). The factors that caused these turbulence characteristics can be attributed to the location and aspect of the home and the degree of enclosure in the front yard and backyard. Constructing residences on the south-facing foot of a
mountain exposes them to naturally induced wind flows [8]. In the hot summer, the tree canopies on the south face of the back mountain are heated more than the rice paddy because the incident angle of solar radiation on the mountain is nearly perpendicular, and the energy partitioning into latent heat flux is dominant in the rice paddy. So the wind blows from the rice paddy (south) to the mountain (north) to achieve mass conservation in the daytime. The naturally driven wind flow at the local scale passed through the entrance gate of the house and would influence the turbulence at the entrance of the Daechung. Next, the degree of enclosure in the front yard and the backyard can be considered as a factor impacting the turbulence at the Daechung. The wind
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velocity at front yard is quite calm and less fluctuated (Fig. 8), presumably due to the enclosing buildings (3–5 m height), and these characteristics are quite similar with the positive v-component (Fig. 7(b)). The backyard open to the environment with low walls (1-m height) showed higher wind velocity with more fluctuations (Fig. 8), which is similar with negative v-component. Thermal comfort is defined as the condition of mind which expresses satisfaction with the thermal environment [11], related to air temperature, humidity and wind speed [12]. We believe that the wind characteristics at the Daechung are very effective in providing thermal comfort to dwellers. Traditionally, dwellers would sit near backdoors with their arms on the doorsill to escape the heat in summer (note the position of matting and the height of the doorsill in Fig. 1. Numerical and laboratory studies have reported that the airflow through windows drops to and then flows along the floor [13,14]. This phenomenon explains why dwellers feel the most thermal comfort sitting at that site. Further, the unique characteristics of the negative v-component—less numbers and short duration but strong velocity—can provide thermal comfort to dwellers. Considering that the air temperature in the backyard is around 1 1C lower than in the front yard in the daytime (Fig. 9), the intermittent strong cool wind from the backyard could cool the dwellers. In addition, the Venturi effect at the backdoors can contribute to strengthening the negative v-component. Many studies have also reported that window opening can effectively regulate indoor airflow strength by the Venturi effect [15–18]. Our findings support the importance of turbulence on thermal sensation [12,19,20]. We additionally suggest the importance of time-averaged wind velocity for considering the effect of turbulence on thermal comfort. We measured 10-Hz turbulence data and averaged them over 10 s, 1 and 10 min (Fig. 11). Note the change in minimum wind velocity depending on the scale of average time during the daytime. During the shaded period (13:00–13:30, Aug. 7), average minimum wind velocities for 10 and 1 min, and 10 s and raw 10-Hz data are 0.05, 0.76, 1.8 and 2.4 m s1, respectively, indicating that the wind velocity is highly sensitive to the averaging time. With respect to thermal comfort, we should consider how long a human can sense wind in any given environment. If 1 s is the threshold to sense wind, for example, averages longer than 1 s can underestimate the effect of turbulence on thermal comfort. These characteristics cannot be considered in the previous indices of thermal comfort (e.g., percentage dissatisfied [19], draught index (DR) [11]). These indices use wind speed as an input variable, because it is related to cooling effect of the air flow on the skin. Two issues should be stressed out. First, wind speed is always higher than wind velocity due to the fact that the former is always positive and the latter has both signs (i.e., plus and minus. Fig. 11). As described earlier, wind direction is an important component for thermal comfort in Daechung, so wind velocity needs to be considered rather than wind speed. Second, there is no
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explicit reference of average time interval for analysis. An objectively oriented average time scale should be carefully selected to effectively explain thermal comfort. 5. Summary and conclusions Our main findings are as follows: (1) less frequent, short duration but strong wind blew from the cool backyard to the Daechung, (2) the home received naturally driven local wind and (3) the degree of enclosure in the front yard and backyard caused different wind characteristics between two yards, and consequently affected v-components at the Daechung. From these findings, we conclude that the intermittent strong cool wind blown from the backyard provided the thermal comfort to the dwellers at Daechung, and a natural ventilation system for thermal comfort was used in a traditional Korean home during the summer. The principles on which this system relies could be helpful in constructing environment-friendly, sustainable residences. Further, we pointed out the sensitivity of wind velocity measures to the time over which they are averaged, indicating that the selection of that time in a given environment should be carefully considered for understanding the effect of turbulence on thermal comfort. Acknowledgments We thank Insu Koh, Seung Kim and Youngil Kim for their assistance of our field experiments. We greatly appreciate the members of ‘Research for Traditional Ecology’ about their continued participation and discussion on this paper. Especially, we are grateful to Drs. Jin Il Yun, Dong-Hwan Sung, and Sinkyu Kang for their constructive advices and comments on our ideas, experimental skills, data processes, and manuscript. Special thanks should go to the family of ‘the old house of Yunjeung’ for providing us with lots of accommodation during field works. This research was supported by NCIRF and NICEM at the Seoul National University, and funded by a Grant no. 03-03-211-39 from the Institute of Korean Studies, Seoul National University, Republic of Korea. References [1] Lee K, Han D, Lim H. Passive design principles and techniques for folk houses in Cheju Island and Ullung % Island of Korea. Energy and Buildings 1996;23(3):207–16. [2] Chung D, Choi Y, Kim S, Bae S, Lee K. A study on the passive cooling design principles and techniques in traditional folk house (I). Proceedings of the Architectural Institute of Korea 1994;14(2):259–64 [in Korean]. [3] Shin J, An B, Lim H, Yun J, Lee K. A study on the passive cooling design principles and techniques in traditional folk house (II). Proceedings of the Architectural Institute of Korea 1994;14(2): 365–70 [in Korean]. [4] Kim D. The natural environment control system of Korean traditional architecture: comparison with Korean contemporary architecture. Building and Environment 2006;41(12):1905–12.
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[5] Cardinale N, Micucci M, Ruggiero F. Analysis of energy saving using natural ventilation in a traditional Italian building. Energy and Buildings 2003;35(2):153–9. [6] Borong L, Gang T, Peng W, Ling S, Yingxin Z, Guangkui Z. Study on the thermal performance of the Chinese traditional vernacular dwellings in Summer. Energy and Buildings 2004;36(1):73–9. [7] Feriadi H, Wong NH. Thermal comfort for naturally ventilated houses in Indonesia. Energy and Buildings 2004;36(7):614–26. [8] Ryu Y. Horizontal wind flows over Daechung as affected by spatial differentiation of thermal and turbulent environments within a traditional Korean dwelling, Master thesis, Graduate School of Environmental Studies, Seoul National University, 2005. [9] Defant F. Local winds. In: Malone TF, editor. Compendium of meteorology. American Meteorological Society; 1951. p. 655–72. [10] Jacobs AFG, van de Wiel BJH, Holtslag AAM. Daily course of skewness and kurtosis within and above a crop canopy. Agricultural and Forest Meteorology 2001;110(2):71–84. [11] ISO 7730. Moderate thermal environments determination of the PMV and PPD indices and specification of the conditions for thermal comfort. Geneva: International Organisation for Standardisation; 1994. [12] Olesen BW, Parsons KC. Introduction to thermal comfort standards and to the proposed new version of EN ISO 7730. Energy and Buildings 2002;34(6):537–48.
[13] Heiselberg P, Svidt K, Nielsen PV. Characteristics of airflow from open windows. Building and Environment 2001;36(7):859–69. [14] Gan G. Effective depth of fresh air distribution in rooms with single-sided natural ventilation. Energy and Buildings 2000;31(1): 65–73. [15] Olgyay V. Design with climate—bio-climatic approach to architecture regionalism. New Jersey: Princeton University Press; 1973. [16] Raja IA, Nicol JF, McCartney KJ, Humphreys MA. Thermal comfort: use of control in naturally ventilated buildings. Energy and Buildings 2001;33(3):235–44. [17] Prianto E, Depecker P. Characteristics of airflow as the effect of balcony, opening design and internal division on indoor velocity: a case study of traditional dwelling in urban living quarter in tropical humid region. Energy and Buildings 2002;34(4):401–9. [18] Prianto E, Depecker P. Optimization of architectural design elements in tropical humid region with thermal comfort approach. Energy and Buildings 2003;35(3):273–80. [19] Fanger PO, Melikov AK, Hanzawa H, Ring J. Air turbulence and sensation of draft. Energy and Buildings 1988;12(1):21–39. [20] Koskela H, Heikkinen J, Niemela R, Hautalampi T. Turbulence correction for thermal comfort calculation. Building and Environment 2001;36(2):247–55. [21] Kim B. Rediscovery of Korean architecture, vol. 2. Seoul: Ideal Architecture Publishers; 1999 [in Korean].
Building and Environment 44 (2009) 18–26 www.elsevier.com/locate/buildenv
The influence of wind flows on thermal comfort in the Daechung of a traditional Korean house Youngryel Ryua,, Seogcheol Kimb, Dowon Leea a
Department of Environmental Planning, Graduate School of Environmental Studies, Seoul National University, Republic of Korea b Boolt Simulation Technology, Republic of Korea Received 19 September 2007; received in revised form 5 January 2008; accepted 15 January 2008
Abstract Daechung, a semi-open space with wooden floor located between the front and backyards of traditional Korean residences, is well known as a cool space in summer due to cross-ventilation, but it has not yet been scientifically explained thoroughly. The purpose of this paper is to characterize the wind flow measured at a Daechung to interpret the effects of the wind characteristics on thermal comfort. We measured 10-Hz turbulence data at the Daechung and partitioned the wind vector into two directions (i.e. backyard to Daechung and front yard to Daechung). Interestingly, the wind from the cool backyard flowing through the Daechung was of less frequency and shorter duration but had higher velocity compared to wind from the opposite direction, which can provide thermal comfort to the dwellers. We suggest that the wind characteristics were determined by various aspects of the house’s design, such as its location and the degree of enclosure in front and backyards. The results show that traditional Korean house made use of a natural ventilation system during the summer. The principles of this system could be helpful in constructing environmentally friendly and sustainable residences. r 2008 Elsevier Ltd. All rights reserved. Keywords: Wind flow; Natural ventilation; Thermal comfort; Traditional Korean home
1. Introduction Daechung is a unique space generally located in the middle of traditional Korean houses (Fig. 1). Korea experiences summer monsoon, so it is very hot and humid in the season (Fig. 2). To escape the heat, residents spend most of the daytime inside during summer, unless they work outside. In particular, people usually sit near the backdoors with their arms on the doorsill for exposure to the cool wind blown from the backyard. Generally, the Daechung is always open to the front yard, while backdoors are open to the backyard only in the summer daytime (Fig. 1), thereby allowing cross-ventilation to provide thermal comfort to dwellers. In addition, natural convection induces the cool air in the backyard to flow into Corresponding author at: Department of Environmental Science, Policy and Management, University of California at Berkeley, 137 Mulford Hall #3114, Berkeley, CA 94720-3114, USA. Tel.: +1 510 642 9048; fax: +1 510 643 5098. E-mail address: [email protected] (Y. Ryu).
0360-1323/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2008.01.007
the Daechung through the backdoors [1–4]. The crossventilation system of a Daechung is well known in the academic world, but it has not yet been characterized in depth. In this study, we analyzed naturally driven turbulence data measured at the Daechung in a traditional Korean house in order to explain the effects of turbulence characteristics on the thermal comfort of dwellers. Natural ventilation systems providing thermal comfort in traditional dwellings have been noted in various researches [1,4–7], but thorough analyses on turbulence measurement were rare. We characterized the turbulence at the Daechung by separating the v-axis wind component, the orthogonal vector to the back door of Daechung, into positive (from front yard to backyard) and negative (from backyard to front yard) parts. By analyzing these two wind parts independently, we successfully characterized the turbulence and found meaningful phenomena closely linked with thermal comfort. In this research, we conducted three steps: (1) characterization of turbulence measured in a Daechung, (2) analysis
ARTICLE IN PRESS Y. Ryu et al. / Building and Environment 44 (2009) 18–26
2. Material and methods 2.1. Site description The old house of Yunjeung (361160 5500 N, 127170 5200 E) is located in Gyochon-li, Nosung-myun, Nonsan City, Chungnam Province in Republic of Korea (Fig. 3(a)). It faces a rice paddy to the south and is backed by Mt. Nosung on the north (Fig. 3(b)). The backyard is enclosed by 1-m-high walls and the front yard is enclosed by the main buildings and the front gate (Fig. 3(c)). The front part of the Daechung is open to the front yard, and
Fig. 1. Daechung, the wooden-made main floor of traditional Korean residences (note the three backdoors).
2.2. Field measurements Field data were collected depending on the availability of monitoring equipment from September 2003 to August 2004: air temperature (September 2003–August 2004); wind components of u-, v-, and w-axis (August 2004); and wind velocity and direction (January 2004–August 2004). The most reliable summer data (August 5–August 8, 2004) were selected by testing data quality and analyzed for this study. Wind components (u-, v-, and w-axis) were measured by three sonic anemometers at a frequency of 10 Hz in the front yard and backyard at a height of 1.8 m (CSAT3, Campbell Sci. accuracy: 0.04 m s1), and adjacent to the middle backdoor of the Daechung at a height of 0.7 m (SATI/3Sx, Applied technologies Inc. accuracy: 0.03 m s1). To measure the ambient wind flow, an Automated Weather System (AWS) (HOBO wind velocity/direction and weather data station logger, Onset comp. accuracy: 74% of wind velocity and 751 of wind direction) was used. The AWS was set up 10 m in front of the house at a height of 1.5 m, and data were logged every 1 min. Air temperature (HOBO Temp, Onset comp. accuracy: 70.4 1C at 25 1C) was measured in the backyard,
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three rectangular back doors are open to the backyard (Fig. 1). The back doors are apart each other by 1.1 m and the dimension of each back door is 1.1 1.3 m2. The main gate is open in the daytime and closed at night throughout the year. The surface ratio of the main gate to the front wall of residential area is approximately one-eleventh. The 30-year averaged monthly precipitation and air temperature at Booyeo, the nearest National Weather System (NWS) site to Nonsan City, is presented in Fig. 2. The 30year averaged annual precipitation and air temperature are 1334 mm and 12 1C, respectively.
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Jan
Feb
Mar
Apr
May
Jun Jul Month
Aug
Sep
Oct
Nov
Dec
Temperature (°C)
of the factors which cause the turbulence characteristics, and (3) analysis of turbulence characteristics’ effects on thermal comfort.
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-5
Fig. 2. Climate characteristics of the study site from 30-year averaged monthly precipitation and air temperature taken at Booyeo, site of the nearest National Weather System (NWS) to Nonsan City.
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Fig. 3. (color online) Study site, the old house of Yunjeung located in Gyochon-li, Nosung-myun, Nonsan City, Chungnam Province, South Korea: (a) location of Nonsan City, (b) topography around the house, (c) plain figure of the house (redrawn from Kim [21]), and (d) cross-sectional view and dimension (cm) of the house (reference for cross-section is indicated in (c) by the red arrow line).
front yard, and at the Daechung. Data were measured every 10 min at a height of 1.5 m, except at the Daechung, where it was measured at a height of 0.5 m. The experimental design is illustrated in Fig. 4.
2.3. v-component at the Daechung The v-component at the Daechung is the component parallel to the floor and perpendicular to the back door. In general, itpcomprised ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi at least 90% of the horizontal wind velocity ( u2 þ v2 ) and determined the horizontal wind direction at the backdoor of the Daechung. Therefore, we can regard this component as the dominant wind flow through the Daechung. A positive v-component means that the wind flows from the front yard to the backyard, while a negative v-component indicates the opposite direction. Analyzing turbulence characteristics including times, duration, velocity, skewness and kurtosis at the Daechung, we only dealt with the v-component measured by a sonic anemometer at the Daechung.
2.4. Skewness and kurtosis Skewness characterizes the degree of asymmetry of a distribution around its mean. If positive extremes dominate, the skewness is positive and if negative extremes dominate, the skewness is negative. If the probability density function is a Gaussian distribution, then the skewness is zero. Skewness can be written as n xj x 3 1X S¼ , (1) n j¼1 s where n is the number of data, s the standard deviation, xj the raw data, and x is the mean. Kurtosis is a measure of the extent to which observations cluster around a central point. A kurtosis is large if the tails of the probability density function are relatively large and small if the tails are relatively small. If the probability density function is Gaussian, the kurtosis value is three. So, kurtosis 43 indicates a peaked distribution and kurtosis under three shows a flat distribution. Kurtosis
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Fig. 4. Experimental design.
can be indicated by n xj x 4 1X . S¼ n j¼1 s
(2)
3. Results 3.1. Wind characteristics at the Daechung 3.1.1. Frequencies The numbers of negative and positive v-components from 9:00 to 17:00 during 4 days were analyzed from 10-Hz raw data (Fig. 5). The number of the positive v-component was approximately twice as much as that of the negative component throughout the 4 days. This indicates that the dominant wind blew from the front yard to the backyard.
Fig. 5. The numbers of negative and positive v-components from 09:00 to 17:00 (August 5–8, 2004) from raw 10-Hz data.
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3.1.2. Duration The duration of directional wind (i.e. positive or negative v-component) is shown in Fig. 6. A duration below 1 s contributed to around 60% of all durations in the positive and negative v-components. The duration from 0 to 10 s was higher for the negative v-component than for positive v-component, while the trend was reversed for durations over 10 s. This shows that most wind changed direction transiently, and the direction of continuous wind for longer periods of time was mainly southerly (i.e. from the front yard to the Daechung). 3.1.3. Velocity The velocities of positive and negative v-components ranged from 0.22 to 0.28 m s1 and from 0.42 to 0.48 m s1, respectively, indicating that wind velocity of the negative v-component is twice that of the positive v-component (Fig. 7(a)). Time series of 10-min-average wind velocity revealed that the negative v-component had both higher mean wind velocity and higher fluctuation (Fig. 7(b)). Standard deviations of positive and negative v-component velocities were 0.05 and 0.17 m s1, respectively. 3.1.4. Skewness and kurtosis The skewness and kurtosis of the v-component of wind are shown in Table 1. Throughout the 4 days (09:00–17:00), skewness was always negative, indicating that the mode of v-component velocity is greater than the median values. Kurtosis values were nearly higher than three, which means that the kurtosis of v-components was higher than is seen in a Gaussian distribution. 3.2. Wind velocity at the front yard and backyard The 30-min-average wind velocity at the front yard and backyard is presented in Fig. 8. The figure indicates that the wind at the front yard was very calm, o0.5 m s1.
Fig. 7. (a) The wind velocities of negative and positive v-components from 09:00 to 17:00 (August 5–8, 2004) from the raw data measured over 10 Hz, and (b) time series of 10-min average wind velocity of negative and positive v-components from 09:00 to 17:00 on August 7, 2004.
Table 1 Skewness and kurtosis of v-component wind at the Daechung from 09:00 to 17:00 (August 5–8)
Average Stdevc
August 5
August 6
August 7
August 8
Sa
Kb
S
K
S
K
S
K
1.29 0.74
5.98 4.63
1.06 0.39
3.90 1.57
1.29 0.56
5.34 2.87
1.23 0.49
5.23 1.92
Note: Skewness and kurtosis were calculated over a 10-min interval. a S ¼ skewness. b K ¼ kurtosis. c Stdev ¼ standard deviation.
Fig. 6. The durations of negative and positive v-components from 09:00 to 17:00 (August 5–8, 2004) from the raw data measured over 10 Hz.
The average wind velocities at the front yard and backyard from 09:00 to 17:00 during 4 days were 0.36 and 0.62 m s1, respectively; the wind velocity of backyard was, therefore, almost twice that of front yard.
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from the south and southwest around 1 m s1, while a nearly dead wind blew from the north at night. These diurnal patterns are consistent with valley wind in the daytime and mountain wind at night [9].
4. Discussion
Fig. 8. Time series of 30-min average wind velocity at the front yard and the backyard from August 5 to 8. Some data in former 2 days were missed due to the lack of power supply in the sonic anemometer.
Fig. 9. Time series of 30-min average air temperature in the front yard, at the Daechung and backyard from August 5 to 8.
3.3. Air temperature in the front yard, Daechung and backyard
Traditionally, the Daechung has been used as the place to escape the hot summer in Korea [1]. Actual field data indeed showed that the Daechung was the coolest site compared with the front yard and the backyard during the daytime (Fig. 9). The main reason for this can be attributed to the roof protecting from solar radiation and providing a shaded area. In another aspect, we focused on the effects of the turbulence characteristics at the Daechung on thermal comfort. As a first step, a thorough analysis of wind characteristics at a Daechung was conducted. The field data demonstrated that positive and negative v-components of wind at the Daechung were different in number, duration and velocity. The number of the positive component was nearly twice as much as that of the negative component (Fig. 5) and durations over 10 s mainly occurred in the positive component (Fig. 6). These phenomena can be ascribed to the southerly wind (from the rice paddy to the mountain) that consistently blew around 1 m s1 in the daytime (Fig. 10). Wind velocity of the negative component was almost twice that of the positive component (Fig. 7(a)) and showed a more fluctuating pattern compared to the positive component (Fig. 7(b)). Skewness and kurtosis analysis of the v-components support the characteristics of each component mentioned above. Skewness values consistently showed negative values in the daytime, indicating that the probability density function is biased to the right. This means that the number of the positive component consistently exceeds that of the negative component in the daytime (Fig. 5). Most of kurtosis values of 4-day data exceeded 3.00, indicating that (1) the probability density function has a pronounced
The time series of air temperature at the front yard, the Daechung and backyard are shown in Fig. 9. The standard deviations are 3.61, 1.76 and 3.47 1C, respectively. The temperature at the Daechung fluctuated the least, and maintained a relatively cool temperature in the daytime and a relatively warm temperature at night. Air temperature appeared to be 1–1.5 1C higher in the front yard than in the backyard during the daytime due to different land covers—bare soil in the front yard while full of grass and shrubs in the backyard—and the adjacency of the backyard to the forest in mountainous area. 3.4. Wind velocity/direction at 10 m in front of the house The diurnal patterns of wind velocity and direction at the local scale are given in Fig. 10. In the daytime, wind blew
Fig. 10. Time series of 10-min average wind velocity and direction at 10 m in front of the house from August 5 to 8. Wind velocity of 0 m s1 data were skulled.
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Fig. 11. Comparison of different time-scale average v-components at the Daechung. (a) 10-m average from August 5 to 8, (b) 1-min average during August 7, (c) 10-s average from 09:00 to 15:00, August 7 and (d) raw data measured over 10 Hz from 13:00 to 13:30, August 7. Shaded area indicates the same period from 13:00 to 13:30, August 7.
peaked distribution, and (2) the tails of the probability density function are large [10]. High kurtosis values explain that consistent wind blew dominantly from the front yard to the backyard (Fig. 5), and the wind velocity of the positive component was relatively constant around 0.3 m s1 (Fig. 7(b)). Second, the large tails in the probability density function mean that the v-component is subject to very extreme events, influenced by strong negative v-component winds of short number and duration (Fig. 11(d)). The factors that caused these turbulence characteristics can be attributed to the location and aspect of the home and the degree of enclosure in the front yard and backyard. Constructing residences on the south-facing foot of a
mountain exposes them to naturally induced wind flows [8]. In the hot summer, the tree canopies on the south face of the back mountain are heated more than the rice paddy because the incident angle of solar radiation on the mountain is nearly perpendicular, and the energy partitioning into latent heat flux is dominant in the rice paddy. So the wind blows from the rice paddy (south) to the mountain (north) to achieve mass conservation in the daytime. The naturally driven wind flow at the local scale passed through the entrance gate of the house and would influence the turbulence at the entrance of the Daechung. Next, the degree of enclosure in the front yard and the backyard can be considered as a factor impacting the turbulence at the Daechung. The wind
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velocity at front yard is quite calm and less fluctuated (Fig. 8), presumably due to the enclosing buildings (3–5 m height), and these characteristics are quite similar with the positive v-component (Fig. 7(b)). The backyard open to the environment with low walls (1-m height) showed higher wind velocity with more fluctuations (Fig. 8), which is similar with negative v-component. Thermal comfort is defined as the condition of mind which expresses satisfaction with the thermal environment [11], related to air temperature, humidity and wind speed [12]. We believe that the wind characteristics at the Daechung are very effective in providing thermal comfort to dwellers. Traditionally, dwellers would sit near backdoors with their arms on the doorsill to escape the heat in summer (note the position of matting and the height of the doorsill in Fig. 1. Numerical and laboratory studies have reported that the airflow through windows drops to and then flows along the floor [13,14]. This phenomenon explains why dwellers feel the most thermal comfort sitting at that site. Further, the unique characteristics of the negative v-component—less numbers and short duration but strong velocity—can provide thermal comfort to dwellers. Considering that the air temperature in the backyard is around 1 1C lower than in the front yard in the daytime (Fig. 9), the intermittent strong cool wind from the backyard could cool the dwellers. In addition, the Venturi effect at the backdoors can contribute to strengthening the negative v-component. Many studies have also reported that window opening can effectively regulate indoor airflow strength by the Venturi effect [15–18]. Our findings support the importance of turbulence on thermal sensation [12,19,20]. We additionally suggest the importance of time-averaged wind velocity for considering the effect of turbulence on thermal comfort. We measured 10-Hz turbulence data and averaged them over 10 s, 1 and 10 min (Fig. 11). Note the change in minimum wind velocity depending on the scale of average time during the daytime. During the shaded period (13:00–13:30, Aug. 7), average minimum wind velocities for 10 and 1 min, and 10 s and raw 10-Hz data are 0.05, 0.76, 1.8 and 2.4 m s1, respectively, indicating that the wind velocity is highly sensitive to the averaging time. With respect to thermal comfort, we should consider how long a human can sense wind in any given environment. If 1 s is the threshold to sense wind, for example, averages longer than 1 s can underestimate the effect of turbulence on thermal comfort. These characteristics cannot be considered in the previous indices of thermal comfort (e.g., percentage dissatisfied [19], draught index (DR) [11]). These indices use wind speed as an input variable, because it is related to cooling effect of the air flow on the skin. Two issues should be stressed out. First, wind speed is always higher than wind velocity due to the fact that the former is always positive and the latter has both signs (i.e., plus and minus. Fig. 11). As described earlier, wind direction is an important component for thermal comfort in Daechung, so wind velocity needs to be considered rather than wind speed. Second, there is no
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explicit reference of average time interval for analysis. An objectively oriented average time scale should be carefully selected to effectively explain thermal comfort. 5. Summary and conclusions Our main findings are as follows: (1) less frequent, short duration but strong wind blew from the cool backyard to the Daechung, (2) the home received naturally driven local wind and (3) the degree of enclosure in the front yard and backyard caused different wind characteristics between two yards, and consequently affected v-components at the Daechung. From these findings, we conclude that the intermittent strong cool wind blown from the backyard provided the thermal comfort to the dwellers at Daechung, and a natural ventilation system for thermal comfort was used in a traditional Korean home during the summer. The principles on which this system relies could be helpful in constructing environment-friendly, sustainable residences. Further, we pointed out the sensitivity of wind velocity measures to the time over which they are averaged, indicating that the selection of that time in a given environment should be carefully considered for understanding the effect of turbulence on thermal comfort. Acknowledgments We thank Insu Koh, Seung Kim and Youngil Kim for their assistance of our field experiments. We greatly appreciate the members of ‘Research for Traditional Ecology’ about their continued participation and discussion on this paper. Especially, we are grateful to Drs. Jin Il Yun, Dong-Hwan Sung, and Sinkyu Kang for their constructive advices and comments on our ideas, experimental skills, data processes, and manuscript. Special thanks should go to the family of ‘the old house of Yunjeung’ for providing us with lots of accommodation during field works. This research was supported by NCIRF and NICEM at the Seoul National University, and funded by a Grant no. 03-03-211-39 from the Institute of Korean Studies, Seoul National University, Republic of Korea. References [1] Lee K, Han D, Lim H. Passive design principles and techniques for folk houses in Cheju Island and Ullung % Island of Korea. Energy and Buildings 1996;23(3):207–16. [2] Chung D, Choi Y, Kim S, Bae S, Lee K. A study on the passive cooling design principles and techniques in traditional folk house (I). Proceedings of the Architectural Institute of Korea 1994;14(2):259–64 [in Korean]. [3] Shin J, An B, Lim H, Yun J, Lee K. A study on the passive cooling design principles and techniques in traditional folk house (II). Proceedings of the Architectural Institute of Korea 1994;14(2): 365–70 [in Korean]. [4] Kim D. The natural environment control system of Korean traditional architecture: comparison with Korean contemporary architecture. Building and Environment 2006;41(12):1905–12.
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