- Technical Article
- Open Access
Bioclimatic rehabilitation of an open market place by a computational fluid dynamics simulation assessment
Future Cities and Environment volume 1, Article number: 6 (2015)
These days urban design of open spaces is strongly related to bioclimatic techniques and practices. It is here presented the procedure of a bioclimatic study by the use of simulation tools. The area of an open market place is characterized of decreased human thermal comfort conditions during summer time. The employment of computational fluid dynamics has contributed in the understanding of what interventions should be made at the open space in order to succeed the defined thermal related targets. Table of the proposed rehabilitation explains what the interventions would contribute in the improvement of the local environment.
It is widely approved that densely urban developments in conjunction with the use of inappropriate external materials, the increased human related thermal energy emission and the lack of green areas, increase environmental temperature leading to significant environmental impacts and increased energy consumption (Santamouris et al. 2012a; Santamouris et al. 2012b; Fintikakis et al., 2011). Open spaces within urban developments are complicated due to thermal energy exchange between structures, shadowing and wind flow complication in comparison to general flow. Cooling materials (Kolokotsa et al. 2013) and other practices (water surfaces, green roofs) may be used in order to mitigate urban heat island effects (Santamouris 2014a; Mastrapostoli et al., 2014; Tang et al., 2014; Tsilini et al. in Press; Santamouris 2013; Pisello et al. 2013; Georgi and Dimitriou, 2010; Gartland, 2008; Gaitani et al., 2007; Akbari et al., 2001). The main problems that result from bad thermal conditions include decreased human thermal comfort, decreased air quality, increased heat illnesses and increased energy and water use (Stone, 2005; Baik et al., 2001).
Experimental measurements within urban developments must be carried out in order to identify the thermal situation. In the Greek territory there are intense thermal phenomena mainly during the summer period (Livada et al., 2002). These are observed in open urban areas all around country. Surface temperatures in relation to microclimatic conditions (wind, temperature, radiation) must be analyzed in order to better select rehabilitating strategy of open developments.
Simulation tools must be employed (Stavrakakis et al., 2011) in order to depict the present situation around the opencast area, usually during the warmest day of the hot period. Material identification and construction configuration must also be taken into account in the simulation process. A new configuration of materials and bioclimatic techniques is then proposed and simulated in order to show its influence to the thermal urban environment. Target of this procedure is to realize the microclimatic conditions improvement due to the rehabilitating bioclimatic techniques and practices. The selection of the measures to be proposed depends on the targets that will be defined for an improved thermal environment (e.g. thermal comfort) (Santamouris 2014b; Santamouris et al. 2011; Gulyas et al., 2006).
Due to the complicated urban environment in terms of materials, reflection, emission, wind flows around buildings, altitude differences etc. the simulation tool must be selected very carefully. It must be able to simulate three dimensional flows with solar radiation taken into account, simultaneously. This inevitably leads to the employment of computational fluid dynamics general codes (PHOENICS) but with increased demand of computational resources. Other tools may be useful for the assessment of individual parameters, such as surface materials and trees’ thermal influence (Matzarakis et al., 2006) but this would not assess the wind flow effect in geometrical detail.
It is here demonstrated the procedure of a bioclimatic study of an open urban space for a city of Northern Greece, Ptolemaida (Gaitani et al., 2014; Santamouris 2013; Skoulika et al., 2014; Gaitani et al., 2011). Experimental measurements, simulation tool verification and the simulation based assessment of the proposed architectural reformation are presented. The thermal targets of this study were defined by Center for Renewable Energy Sources and Saving (Center for Renewable Energy Sources and Saving www.cres.gr).
The effectiveness of the procedure being “measurement – simulation – bioclimatic proposal – simulation” depends on the following aspects:
knowledge of the area and experimental accuracy (materials, experimental instrumentation)
verified simulation against experimental measurements (simulation of the present situation)
qualified collaboration between the architectural bioclimatic design and simulation (viable architectural proposals that would improve thermal conditions e.g. green roof)
definition of the thermal related targets (thermal comfort, cooling degree hours, improved ventilation, surface temperatures, environmental temperature).
Documentation of the thermal bioclimatic problem in the open market place
To examine the weather conditions in the city of Ptolemaida meteorological data of the last 3 years were gathered, from the meteorological stations of the Environment Centre at the entrance of the city and the station of the Greek Public Power Corporation in Pentavrysos. Environmental Centre station located at the entrance of Ptolemaida may be considered as a suburban environment. Pentavrysos located at the north-east of Ptolemaida at a distance of about 10 km was considered as a rural environment (Fig. 1). The temporal coverage of the data covered the period between beginning of 2009 to November 2011. For both two stations hourly data were available. The meteorological parameter that was examined was the air temperature (°C).
Compared with the surrounding suburban and rural environment, urban climate varies in terms of solar radiation, characteristics of rainfall and air temperature. According to Oke (1973) almost every urban center in the world is warmer 1–4 °C than neighboring non-urban rural areas, and this enforces urban heat island effects. Also, Gilbert (1991) states that the air temperature on sunny days can be of up to 2.0 to 6.0 °C higher in urban compared to suburban locations.
The comparison of climate data during the three year period has shown a significant temperature difference between the suburban area (Environmental Centre) and the city center, which amounted to 6.1 °C, with a mean difference of 4.8 °C. The temperature difference is greater when the comparison is made against the rural area, about 10km from the city center (Pentavrysos), where the temperature difference amounts to 8.4 °C, with a mean difference of 7.1 °C. Thus the center acts as an urban heat island and open bioclimatic urban upgrading would be suggested.
Description of bioclimatic interventions in the open space of the market place
The 70% percent of the Greek power generation takes place in Northern Greece. Ptolemaida city is surrounded by power generation activities (lignite mining and combustion processes). In the commercial and social center of the city belongs the open market place that is heavy populated during the week. The urban summer time microclimate in this area is mainly affected by the presence of asphalt all over the ground surface of the open market. The southern part of the market along Pontou Street is neighbored by a multipurpose building with vegetation and pavements. Figure 2 shows the present configuration of materials.
The rehabilitation strategy of the area targets to conserve human activities and improve human thermal conditions in the open public market. Bioclimatic interventions could be divided in two main directions (see Fig. 2):
the open market place
the surrounding streets of the market place (Vas. Konstantinou – South of the multipurpose building, Foufa - North, Pontou - South, Dimokratias – East and Ethnikis Antistasis – West)
The above mentioned main bioclimatic directions were characterized by increasing water surfaces, vegetation, green roofs and by installing cool asphalt and flagstones (Fig. 3).
Table 1 presents the current situation of materials and surfaces in terms of the respective percentages in surface coverings in contrast with the proposed bioclimatic based configuration.
Description of the CFD software
Advanced CFD models can calculate with a high degree of accuracy microclimatic parameters at every grid point of the meshed space. However, the more complicated is the geometry of the urban open space the more resources of input data and calculation are needed. For the efficient simulation of the thermal energy condition in the areas of interest, the detailed three dimensional tool ANSYS CFX 13 has been used. ANSYS CFX is an advanced general code computational fluid dynamics model that solves the Navier Stokes differential equations and turbulence by the finite elements technique in the 3D space. It is a commercial software package that handles very detailed three dimensional geometry with the ability to solve heat transfer and fluid flow phenomena.
All simulations have been carried out in parallel by two processors (intel core™ i7-2600 CPU @ 3.40 GHz 3.00 GHz) with 16GB RAM.
The more detailed is the structural and 3D geometry of buildings, streets, pavements, urban equipment and vegetation the more representative and accurate simulation will be. The fluid domain (Fig. 4) encloses buildings, streets, pavements and trees as defined solids within a total dimension of 500 m * 400 m * 80 m (height) – this was at least four times the max height of structural domains in order to avoid during simulation flow reflection at boundaries and fluid returns. In the horizontal directions this was ensured due to longer boundaries.
The simulation domains have been meshed at solids’ surfaces and fluid domain volume. The mesh had an element of a min length dimension of 0.2 m, with dense tetra-, hexa-, octa- surface elements (building surfaces, vegetation, water elements, etc.). The final mesh constituted of 390,530 nodes and 2,129,384 elements (Fig. 5).
Surfaces with materials (concrete, glass, pavement, water, trees) and properties (emission and reflection coefficients) have been defined. The top and horizontal boundaries were defiened as simple openings that let air flow free to pressure gradients generated in the domain. In the north boundary was imposed the northern wind direction that was observed during the simulation period. Turbulence was simulated by the Shear Stress Transport model with K-Turbulence KE and O-Turbulence frequency (Stavrakakis et al., 2012). Thermal energy was simulated by the discritised model in surface to surface and medium to surface modes. This takes into account opposite surfaces energy exchange that is very important in the heat balance of the open spaces in case of replacing conventional surfaces with cool materials. Solar radiation has been taken into account in slope and deviation through the top boundary. This way surface temperatures were calculated by the CFD simulation in relation to the input material thermal characteristics from Table 1. Vegetation has been considered as physical barrier to wind flow with shadowing without evapotranspiration effects. Water elements were defined as free surfaces (zero friction) with a constant temperature of 15°C without evaporation effects. Boundary conditions were gathered by the experimental data (air temperature, wind speed and direction, radiation, surface temperatures) that were taken during the summer period.
Simulation time step was set at 5 sec and convergence criteria at 10−4 RMS of residuals for steady state or transient calculation.
Real experimental data of the thermal conditions in the area of interest and at surrounding locations obtained the analytical verification of the CFD model and consequently the accurate simulation of both the current situation and the proposed interventions.
A number of experimental procedures were organized in the study area. The field surveys involved microclimatic monitoring by fixed measurements and a set of portable equipment. Note that, this was a measurement campaign that did not include a fully equipped monitoring station such the stations in the entrance and away from the city.
The fixed data included continuous measurements and specifically the air temperature (T), the relative humidity (RH), the wind speed (WS) and the wind direction (WD) at Environmtnal Centre site. The portable station recorded at 1.8m height the air temperature, the relative humidity, the wind speed and wind direction and solar radiation on the horizontal. The technical characteristics of the measuring instrumentation are given in Table 2 and the measuring location (ML) in Fig. 2. This height was selected as representative of the conditions prevailing at pedestrian‟s level and additionally measurements are not affected by activities at pedestrian level (walking, cars’ motion).
In order to prove the ANSYS CFD model validity for the open area simulation, it was verified against experimental data that have been taken during summer 2011. From the period of experiment the warmest day was selected in terms of the completeness of the microclimatic data (i.e. air temperature, air velocity, surface temperature). It was chosen September the 1st, 2011, which recorded the highest temperatures for the period of measurement. The data used for validation of the model is the measurement data within the study area, i.e. the air temperature, the temperature of material surfaces, surface temperature of streets, sidewalks and facades of 1.8m height and the wind speed at the same height.
It was compared the simulation results against the measured values of the surface temperatures, the ambient temperature and the wind speed. Used climatic data from that period were obtained from the Environmental Centre meteorological station at the entrance of the city and climatic data were simulated in the intervention area and in places where measurement was made. Since as mentioned previously, the surface temperature measurements made during midday, the comparisons were made for the same period.
The concept of model validity was that if meteorological input from Ptolemaida’s station are applied in ANSYS CFD then this could efficiently calculate the thermal behavior of within the urban complex. From Table 3 seemed that a satisfactory convergence between experimental and simulation results. This was due to low wind velocities during hot summer days with apnea. Therefore, the achieved accuracy was quite high for the model that was developed for the study area. The above comparison substantiates the high reliability of the model for the assessment of both the current situation and bioclimatic upgrade in the study area.
The bioclimatic concept of this study was defined according to what could be succeeded in terms of the thermal environment improvement. The climatic targets of the proposed interventions were defined by the Centre for Renewable Energy Sources and Saving, under the framework of “Bioclimatic Reformations of Open Public Spaces”, OPERATIONAL PROGRAMME ENVIRONMENT AND SUSTAINABLE DEVELOPMENT 2007–2013, AXIS 1 "Protection of Atmospheric Environment & Urban Transport - Addressing Climate Change - Renewable Energy". The microclimatic parameters that should be improved were the following:
Mean maximum summer temperature during noon of the warmest day. The use of the term mean maximum depicts the hottest thermal conditions during noon.
Conditioning hours during the typical day
Mean surface temperature during noon of the warmest day
Mean human thermal comfort index
Wind field during the typical summer day
Materials with their properties of the area (solar reflectance and thermal emission) to existing and proposed configuration are presented in Table 4.
Simulation results and discussion
The simulations were carried out for the present situation and for the proposed rehabilitated configuration. The same meteorological data from the station in Ptolemaida were applied for the simulation before (i.e. Fig. 2) and after (i.e. Fig. 3). Then, the results were compared in order to obtain the microclimatic improvement. In order to clarify matters, the input data of meteorological measurements were different between the verification of the model and the simulation procedure (i.e. selection of the warmest day) due to the necessity of having actual microclimatic measurements in the verification procedure.
Mean maximum air temperature at noon of the warmest day
Simulation of the average (i.e. the mean value from a specific number of grid points) maximum summer period temperature has been carried out in the open area during noon of the warmest day. The warmest day was obtained from the period of the three year 2009–2011 being the 16th of July 2011. Meteorological data (Table 5) from that day at noon were used in the simulation, in steady state mode, for the present and the rehabilitated situations. At each case the same meteorological data from Environmental Centre station were applied as input in ANSYS CFD with the respective materials and interventions of each configuration. This way modeling predicted the thermal situation in the urban complex by the use of the city’s meteorological data.
It was considered that in the open urban space of the street market and in the surrounding roads of the market in Ptolemaida, the average maximum temperatures were appeared during the selected warmest day. Moreover, simulation has shown that the maximum temperatures within roads and the street market appeared at the same times. The area of interest was divided in the surface of the open market place and in five surrounding streets (Fig. 2). For each road and the market’s surface the respective air temperatures were calculated at noon of the warmest day. Then, the resulted average maximum air temperature was compared for the case before and after rehabilitation.
The simulated air temperature for each individual space (streets and open market) was obtained from at least 500 grid points of the mesh at 1.8m height. Table 6 shows the predicted air temperatures before and after rehabilitation with the surface areas on the 16th of July 2011. The calculated total air temperature before and after case was 37.61 °C and 35.42°C, respectively. So, the air temperature improvement if bioclimatic measures are taken would be 2.19°C.
Conditioning hours during the typical day
Conditioning hours of 26 °C base at 1.80 m height have been calculated for the typical day. From meteorological data analysis during 2009–2011 from Environmental Centre station in Ptolemaida it was obtained that the warmest month was July 2011, with a mean air temperature of 23.9 °C. On the 25th of July 2011 it was observed the closer mean daily air temperature to that value being 23.6 °C. So, the 25th of July 2011 was selected as the typical summer day.
Transient simulations have been carried out with data from the meteorological station in Ptolemaida at Environmental Centre (Table 7) for the selected typical day before and after bioclimatic reformation. Conditioning hours have been calculated with a degree base of 26 °C. In Table 8 it is presented the calculated hourly air temperatures before and after reformation between 10:00 and 20:00 hours. The air temperature from sunset to 10:00 o’clock was lower than 26 °C and thus, it were not taken into account. The mean hourly air temperature for each road and the open market place was calculated from at least 500 grid points.
The predicted conditioning hours of 26 °C base due to the calculated mean air temperatures during 7:00 to 20:00 hours for the typical summer day, were 41.9 for the present case and equal to 26.7 for the proposed bioclimatic case (Table 9).
Mean surface temperature at noon of the warmest day
In order to improve thermal microclimate in the area of the open market new cool materials must be used that may reduce surface temperatures of buildings, streets and sidewalks. The proposed materials have relatively high reflectivity of solar radiation and increased emission rate. The structural surfaces should be reduced and replaced by water surfaces, soil and vegetation. Green roofs in the open area or where people are accommodated would also contribute in the reduction of material thermal storage.
CFD simulations have been carried out for the present case and for the proposed bioclimatic one. The results of the two simulations have compared concerning the warmest day of the 16th of July 2012 (Figs. 8–9).
It was assumed that the mean surface temperature peaks during the warmest day and then, it was calculated for all the individual roads and the market place separately. Therefore, it was assumed that the calculated surface temperatures were maximums for each individual road or open space. The mean surface temperature during noon of the 16th of July for all surfaces reached 43.73°C in contrast with the proposed bioclimatic configuration (Fig. 3) that reached 36.62°C (Table 10). The total predicted temperature difference was 7.11°C. The significant material surface temperature reduction was due to shadowing from vegetation, water surfaces, green roofs, cool asphalt and cool flagstones of pavements and sidewalks.
Mean thermal comfort index during the typical day
The most important effect of the proposed bioclimatic intervention (Harlan et al., 2006) would be the improvement to human living conditions. This has been assessed by the calculation of thermal comfort indices across the area under consideration. The thermal comfort index should take into account climatic factors like global solar radiation on the horizontal and thermal radiation, environmental temperature, air velocity and humidity. The ΤSP (Thermal Sensation Perception) index (Monteiro et al. 2009) was selected being as an appropriate one for external spaces. This index has been considered and validated for the assessment of several thermal comfort studies of open spaces against real experimental data. The equation of its calculation was the following:
Tα is the environmental temperature, (°C)
Τmrt the mean radiant temperature, (°C)
RH the relative humidity (%), and
V the air velocity, (m/sec)
When TSP index lies between −0.5 to +0.5 then thermal comfort is obtained, between 0.5 to +1.5 environment is assumed warm, 1.5 to +2.5 very warm and higher than 2.5 excessively warm. For intervals of −0.5 to −1.5 environment is considered cool, −1.5 to −2.5 cold and below −2.5 excessively cold.
From the transient simulation have been gathered the hourly air temperature, mean radiant temperature and wind velocity for the typical summer day between 10:00 to 18:00 hours. Note that, the mean radiant temperature was estimated by considering shadowing effects at each road and the market place, global solar radiation on the horizontal from meteorological station and material surface temperatures. Relative humidity has been obtained from the meteorological station of Environmental Centre in the 25th of July 2011.
In Table 11 is shown the calculated values of microclimatic data and TSP index at each road and in the market place before and after rehabilitation. All parameters where calculated by the mean values of at least 500 mesh points at each defined location. In Table 11 it was calculated the mean spatial thermal comfort index TSP at each individual space during 10:00 to 18:00 o’clock.
From Table 12 it is perceived that the human thermal comfort before and after rehabilitation belongs mainly in the warm interval (0.5 to +1.5). This was expected due to the thermal analysis of surface and air temperatures where significantly high temperatures were observed during summer time. However, the present mean spatial TSP index of 0.88 is improved to a value of 0.64.
Wind field during the typical summer day
Transient simulation of the wind field during the typical summer day with meteorological input of the area has been carried out. The typical wind field between 10:00–18:00 hours has been simulated in the area under consideration. From Figs. 10–11 it was concluded that at 1.80 m height the wind velocity and turbulence would not change significantly due to the proposed bioclimatic interventions. Velocity vectors in the streets and in the market place do not differ significantly at two cases with low velocities below 1.5 m/s at all spaces. Generally, human comfort would not be affected at all open locations due to the low wind velocities before and after rehabilitation.
Computational fluid dynamics simulation predicted the thermal conditions at the present case and the bioclimatically reformatted one. In Table 13 is given the percentages of improvement for each targeted parameter if the proposed rehabilitation would take place in future. From the table is obvious that relatively low air temperature improvement may lead to significant thermal comfort improvement. This is because thermal comfort is rather independent from air temperature but mostly correlated to radiant temperature. Note that, the wind field of velocities was approximately the same for the case before and after rehabilitation and therefore, did not influence significantly the human thermal comfort conditions. It would not be possible to obtain significant thermal comfort condition improvement by just targeting to air temperature improvement.
Approximately 100% replacement of ground surface conventional materials by cool ones, an increase of about 15% in roofed areas, doubled trees and 10% increase of green and water surfaces caused an improvement in surface temperature of 16.30%. Reflection coefficients of cooled materials were higher than the conventional materials but emission coefficients were approximately the same. Therefore, it is rather obvious the significance of roofs, green areas and trees within the open urban complex in relation to the improvement of air temperature. Relatively high thermal comfort improvement is “easier” to be succeeded if surface thermal exchange is manipulated by any of the abovementioned ways. The exact influence of a bioclimatic intervention to microclimatic parameters must be studied in relation to urban complexity and climatic zone characteristics.
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The authors greatly acknowledge the support of the Mayor of Eordaia Mrs Paraskevi Vrizidou during all simulation stages.
ANSYS-CFD simulations were carried out in the framework of student instruction and demonstration of the Department of Environmental Engineering, Democritus University of Thrace in Greece.
The authors declare that they have no competing interests.
SZ was the overall scientific responsible that directed the present study AD contributed at material and civil engineering guidance VE contributed in microclimatic analysis and simulation SL constructed the meshed 3D CFD domain SD contributed in 3D drawings and proposal interventionAMT contributed in 3D drawings and proposal intervention VS consulted in the bioclimatic implementation ALP contributed in data collection and implementation ED contributed in data collection. All authors read and approved the final manuscript.