Simplified representation of the radiant pattern in urban environment

G. Scudo, A. Rogora, V. Dessi
Politecnico di Milano, Dip. BEST
Via Durando 10, Milano

gianni. scudo@Dolimi. it

alessandro. roaora@Dolimi. it

valentina. dessi@Dolimi. it

Materials of built environment (in a broad sense: building materials for pavements and facades, shading devices, vegetation and water) have an important role in modifying microclimate and comfort conditions in urban space.

Materials surface temperature depends on energy balance which is given by solar and thermal radiation budget (short wave and wave radiation),plus convective (and conductive) flows.

In town context usually convective flows at pedestrian level are low, so the influence of materials is mainly due to radiant exchanges.

Fig 1 Surface temperature of typical “urban material” in summer afternoon in Milan

Also evaporation of water influence very much surface temperature both in vegetation and in pavement or ground ( in summer midday surface temperature of a tree crown can be a little lower than air temperature, let’s say 35 K in Milano, while grey concrete is around 50 and asphalt around 60°)

While the general effect on microclimate of building material in specific urban context and configurations have been largely inquired by microclimatologist (summer and winter heat island effect, albedo distribution, radiant fluxes in canyons… see Akabari et al., Oke, Santamouris…), the specific effect of single materials have been only recently inquired.

In example Asaeda inquired the role of heat capacity of different materials (asphalt, concrete, sand layer and soil) coming to the conclusion that low heat conductivity materials (asphalt) rise day temperature, while high heat conductivity ones ( concrete) raise night temperature: they are not therefore good performing materials for open space; at the contrary bare soil or highly water permeable materials have a much better microclimatic performance due to the low conductivity and the cooling effect of water evaporation.

This paper proposes to evaluate the variation of the radiant behaviour of an indiferenziated open space as it is perturbed from the energy point of view.

If we think of an unlimited and unique material paved open space, we’ll have omogeneus envirnmental conditions in all of its extension.

This condition can be considered as the reference case because the performance of the space depends only on the general conditions of the site (latitude, sky openess, season, radiation, etc.,) and the pavement materials.

A physical element in the open space, of any nature, alters the condition of it introducing local variation in which intensity and extension depend on the nature of the element.

From the geometric point of view the defined elements are: punctual, linear and spacial (deriving by the combination of linear elements).

We consider points those objects (plants, umbrellas, tents, fountains, etc.,) assimilable to mondimensional elements; their effect spreads to around changing the conditions of temperature and thermal balance in a decreasing and oriented way (anisotropic) for a defined spacial area.

The changing effect is different for each time period of the day.

A linear element is a vertical blade, with different layer and technology. The effect of the temperature and thermal balance variation can be described trought a variation curve analysed in the central point of the blade (infinite, i. e. without border effect), that change in the different time period of the day.

The presence of the blade defines three distinct environmental conditions: the ones perturbed before and after the blade, and the one in which the perturbation due to the shading element is not perceivable.

The variations depend on the change of the radiant field introduced by the different distribution of the solar radiation, on the capacity of accumulation and riemission of the radiant energy by the vertical and horizontal surfaces, on the wind and the irradiation conditions.

A continous facade of buildings is assimilable to a linear element.

A single linear element represents a road with a certain width whose one facade isn’t influenced by the presence of the other one in the opposite side.

Two facades of heightness of 24 m distant from each other 50 metres influence each other in a negligible way. The more close distances require some verifications and considerations.

This paper focus on the analysis of the radiant field variation due to the presence of the "blades” expressed in terms of D/H ratio.

For the spacial elements, it is intended "rooms”, realized by the different technologies (forest, pergola, canyon, corner, etc.,) in which the changing effect of the conditions of temperature and thermal balance is described, in the other way for the different time periods of the day, by the specific values of the environmental conditions (absolute values, about 33°C, or easier, parametrized values with respect to the air temperature, 4°C less of the air temperature).

This paper focuses the attention mainly on the linear elements. The influences depend on the different factors:


geografical

— climatical

— morphological

— technological

For simplicity we can consider only the summer season, for the latitude of 45° (Milan). The analysis have been done for the linear obstruction N-S and E-W oriented.

The first evaluation is related to the isolated blade, i. e. a road of infinite width (in which a facade doesn’t perturb the area close to the opposite facade).

If we analyse the MRT values in a creasing distance from the blade (for instance for each meter), we’ll find, for the five time periods of the day, specific values that can have plus or minus important differences from the MRT of a field not perturbed.

In this sample we show some points of two meters distant. The blade in the middle of the space means that there are sides facing to the south and the north.

Fig 4 The curve of the temperature trend in the E-W oriented street in summer in Milan

At noon the side facing to the south is completely reached by the solar radiation, and it means to have MRT values in between 33°C and 37°C.

As we approach to the vertical surface, the MRT increases; in fact the wall is reached by the solar radiation and riemittes in terms of thermal radiation (heat) in the infrared band.

In the side facing to the north there is an area always shaded. In this case, where there isn’t an important reirradiation, the MRT is about 19°C, without relevant variations.

Moving from the shadow to the sunlight the MRT increases, of about 13°C, reaching 32°C. Under the sun the MRT is almost costant except for a very light decrease as we go far from the wall.

This argument is true from the morning to the early afternoon, in which the area facing to the south is reached by the sun and the one facing to the north at least for a D/H in between 0,22 and 0,44 is shaded.

In this time interval, that includes three of the five in a day, the curve tends to arise until the 3 p. m.. For instance, the highest value in the area facing the south at 9 a. m. is 30°C, at noon is 37°C and at 3p. m. is 42°C.

In the side facing to the north the difference in the shaded areas between the 9 a. m. and the 3 p. m. is about 7°C; infact is 15,5°C at 9 o’clock, 19 at 12 and 22 at 3 p. m.. There is the same difference under the sun, from the 29°C at 9 a. m., at 36°C at 3 p. m..

In the side facing to the south the trend during the day is quite the same, except the two moments in which the sun appears and disappears, at 6 a. m and 6 p. m.. In these points the change is evidence by a gap of temperature.

The side facing to the north has some time periods reached by the solar radiation, between 9 a. m and 3 p. m., while for the rest of the time, it is shaded. The trend is linear. At 6 p. m. both areas are shaded: the MRT values decrease a lot (about 18°C in the area facing to the south and 10°C in the one facing to the north), considering the accumulated radiation. The temperature continues to decrease till 4 a. m. very rapidly. From this point and the rest the decrease is more gradually.

If we pass from a high value of albedo (0,8) to a low one (0,2), the MRT trends remain the same, as the value decreases of about 10-20%.

the

45

0

p7 p6 p5 p4 p3 p2 pi pi p2 p3 p4 p5 p6 p7

When the blade is N-S oriented, i. e. has the sides exposed to the east and the west trend of the temperature is more linear.

Fig 6 The curve of the temperature trend in the N-S oriented street in summer in Milan

In the morning the radiation reaches the area and the blade east oriented, as the area west oriented is shaded. Viceversa, in the afternoon the west oriented area is the one by the solar radiation reached, as the one east oriented is shaded. For this reason the two points close to the opposite blade have a temperature difference of 12 °C.

At noon the difference decreases, till 5°C and they are contemporary under the sun.

After this moment, the trend changes, i. e. the values on the east side become lower, as those on the west side become higher.

At 6 p. m. the values of the west side are about 10°C higher then those on the east side.

To consider a blade with the two sides is like to have a road of which is considered all the points of the road section from those nearest to the facade to those at the center.

The similarity between one blade in an infinite space and one road is possible only if we consider a quite wide road, in order to avoid the interference between the two facades. When the fronts tend to approach the trend of the curve are the same, but we have also to consider the combinated effect of the two facades. In general we can say that the nearer are the facades (increasing the D/H ratio), the higher are the MRT temperature values (at the same sun/shade condition).

Fig 7 shadow path overlapped to the sections of NS oriented streets

This effect is due mainly to a couple of factors: the first is a less sky openess, i. e. svf low values, correspondes to a reduced radiant exchange with the sky vault, the second is the higher exchange between the wall and the man — energy fluxes receptor.

We consider the road with dimensional ratio D/H equal to 0,52, 0,26 and 0,12 for to understand what type of the roads can be tought as a road with facing buildig of hightness of 18 meters (6 floors) and width of 50, 26 and 16 meters respectivey. It means that on the E-W oriented roads at noon the road isn’t reached by the solar radiation.

str ada100 st гada 50 str ada 26

At noon the difference of temperature from a D/H to that of higher is about 1,5°C.

Fig 8 Temperature curves for different D/H ratio

Obviously the less is the distance between the facades, the less is the quantity of radiation on the street section. This difference tends to increase in the late afternoon, when it is more evident the heat accumulation effect that spreads hardly.

Fig 9 time 6 p. m. MRT difference between different streets

(1) Nikolopoulou M., Steemers K., Thermal comfort and psychological adaptation as a guide for designing urban spaces, Architecture, City, Environment, Cambridge, Proc. PLEA 2000, James &James, London

(2) Katzshner L., Bioclimatic Characterization of Urban Microclimates for the Usage of Open Staces, Proc. Architectural and Urban Ambient Environment, Nantes (Fr), 2002

(3) Scudo G., Rogora A., Dessi V., “Thermal comfort perception and evaluation in urban space” EPIC 2002 AIVC, Lyon, (Fr), 2002.

(4) Asaeda T. and Ca Thanh V., Heat storage of pavements and Its effect on the lower Atmosphere, Atmospheric Env., Vol. 3°, No 3, 1996.

(5) SOLENE++ Guide d’Utilisation, Laboratoire CERMA, Ecole d’Architecture de Nantes

(6) http://www. meteotest. ch

(7) Santamouris M., Doulos L., Comparative study o almost 70 different materials for streets and Pavements. M. sc. Final Report University of Athens, Department of physics, Athens, 2001

(8) Dessi V., Evaluation of microclimate and thermal comfort in open space, PLEA 2001, The 18th Conference on Passive and Low Energy Architecture, Florianopolis (BR), 2001

(9) Dessi V. “People’s behaviour in an open space as design indicator — Comparison between thermal comfort simulation and users’ behaviour in an open space”, Passive and Low Energy Architecture (PLEA) International Conference, Toulouse, (Fr), July 2002.