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Typical building arrangements for urban air pollution modelling
Atmospheric Environment 33 (1999) 4057}4066
Typical building arrangements for urban air pollution modelling W. Theurer Ingenieurbu( ro Theurer, Ha( ndelweg 4, D-67346 Speyer, Germany
Abstract Typical building arrangements along urban roads and their parameters were determined for three cities in the south-western part of Germany. Some examples of the dispersion of vehicle emissions within idealized street canyons show how di!erent building parameters in#uence the concentration of air pollutants. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Building arrangements; Urban air pollution
1. Introduction Emissions caused by industry, tra$c and private households reduce the air quality in densely populated areas. The quantity of pollutants emitted in a certain area as well as transport and dispersion mechanisms and the background level determine the local concentration of air pollutants. Transport and dispersion depend on the local #ow between the buildings and obstacles. Flow direction and velocity are a result of interaction of the outer #ow and the type and arrangement of the buildings in their surroundings. Both numerical as well as physical models try to simulate the #ow and the concentrations in built-up areas. They need detailed information about the height and width of the buildings, their arrangement, the spacings between them, the width of the streets etc. These parameters need to be estimated for every single urban district or road or, even, building. To simplify this procedure, Theurer (1993) classi"ed typical building arrangements for wider urban areas and their aerodynamic parameters. This classi"cation allows to estimate the in#uence of building arrangements on the dispersion of releases from point sources near groundlevel up to about 1 km downstream the source. In urban or industrial areas with a systematic arrangement of the buildings in a certain orientation (e.g. building rows),
wind tunnel experiments indicated that dispersion depends much on the angle between the orientation of the buildings and the approach #ow direction. Compared to other areas with more homogeneous distributed buildings, a di!erent width of the dispersion plume occurred as well as di!erent positions of the maximum concentration in cross sections of the plume. There are other classi"cation schemes of urban building patterns (e.g. Roth and Haeubi, 1980; Busset et al., 1990) and buildings along urban roads (Glueck, 1976). Due to the fact that exhaust fumes by vehicles irreversingly dominate air pollution in urban areas, in this study focus is on the arrangements along the roads. The existing classi"cation of building arrangements has been modi"ed to deliver typical parameters. For instance, recent numerical investigations by e.g. Schaedler et al. (1994,1996) and wind tunnel experiments (e.g. Van den Hout and Baars, 1988) show a strong dependence of the pollutant concentration within street canyons on the dimensions of the canyon. For screening models such as PROKAS (Ingenieurbuero Lohmeyer, 1996), STREET (Pfeifer et al., 1995) or CAR (Eereurs et al., 1993) as well as to simplify wind tunnel investigations, the identi"cation of typical building arrangements along roads and the knowledge of their in#uence on the dispersion of air pollutants would be a helpful tool.
1352-2310/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 9 ) 0 0 1 4 7 - 8
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Fig. 1. Classi"cation scheme for urban building arrangements.
2. Classi5cation of the building arrangements 2.1. Proposed classixcation scheme Fig. 1 shows a classi"cation scheme for wider urban areas in German towns. The building arrangements are divided into single or double family houses (SFH), dense urban development, residential and commercial (DUD), building rows and blocks (R&B), block edge buildings (BEB), city centres (CC), commercial areas (CA) and industrial areas (IA). Green areas without larger trees (GA) and parks and forests (P&F) complete the classi"cation. In Fig. 2, the classi"cation scheme is applied to an urban area. First, the parameters of building arrangements along urban roads were quanti"ed along higher loaded main roads in three cities with 50.000 (3.5 km road length), 160.000 (4.5 km) and 300.000 inhabitants (11.4 km) in south-western Germany. There were 2 (in the town with 50.000 inh.) to 6 (in that with 300.000 inh.) lanes in the streets with tramway lanes in the larger towns. The tra$c
load had been approx. 14.000}25.000 vehicles/24 h in the smallest town, between 16.000}44.000 vehicles/24 h in the medium-sized one and 10.000}52.000 vehicles/16 h in the large one. To gain an insight into the change of typical building arrangements, the streets were observed from the city limit to the town centre. The length of the street sections, the spacings, more than 100 typical cross sections with the height of the buildings on both sides of the road and its width, the form of the roofs and additional remarks (obstacles within the street, etc.) were documented. Photographs do allow to control the documentation lists. Fig. 3 gives a frequency of the occurrence of the di!erent building arrangements in the road sections. Obviously, they depend on the size of the cities. In the smallest town, an open arrangement of single-family dwellings (SFH) could be found in about 10% of the 3.5 km distance. The building pattern decreases or disappears in the larger towns. The contrary is valid for the city centre (CC) pattern: about 6}8% in both larger towns, but only 2% in the small town. Industrial building arrangements
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Fig. 2. Application of the classi"cation scheme to an urban area.
building arrangements in that town besides commercial areas.
2.2. Investigated parameters
Fig. 3. Frequency of building arrangements.
(IA) were also missing along the investigated roads in the smaller town. Commercial areas (CA) were found in all three towns independent from their size with about 20%. For the medium-sized town, the large 35% contribution of green areas (GA) is remarkable. That is the case for the reduced dense urban development (DUD) and block edge building (BEB) patterns. Both of those building arrangements, often &historically grown', are less represented in the town founded in 1850. Building rows and blocks which were built after 1945 are the most frequent
The building arrangements di!er in typical geometrical parameters shown in Fig. 4. Geometrical parameters are, for example, the average size of an individual building, with its height H , width = and length ¸ . The basic area = ¸ of a building is named A , the considered 0 area is A . Theurer (1993) classi"ed building arrange2-2 ments among other by the dimensionless buildings packing ratio j " A /A . The width of the street 0 2-2 0 2-2 from wall to wall, = , including tramway and bicycle 1 lanes and sidewalks, and the distance from the line source (center of the vehicle lanes) to the building walls, *= , 1 have to be regarded. *= is assumed to = /2 for a sym1 1 metric arrangement of the vehicle lanes parallel to the centerline of the canyon independent of the number of lanes. Often, H and = are combined to a H/= -ratio of 1 1 the cross section of a street canyon. The length of a street section, ¸ , is measured from one crossing or junction to 1 the next, with a percentage of spacings between the buildings. Further, there might be even an in#uence of the roof type (saddle or #at roof), of the buildings in the surrounding and of the vegetation and obstacles within a street (trees, bushes, parking cars etc.) on the local concentration of pollutants.
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Fig. 4. Parameters to describe an urban building arrangement. Top left: parameters for wider urban areas, below right: parameters for urban roads.
2.2.1. Building height In Fig. 5, average building heights are assigned to the di!erent building arrangements. For all three towns, they lay close together. The lowest buildings can usually be found in SFH and CA arrangements, the highest buildings in city centres.
Fig. 5. Average building height for di!erent building arrangements and cities.
2.2.2. H/= -ratio and *= 1 1 The width = of the streets looked at ranged from 1 about 12 to 60 m } including the distances from the sidewalks to the walls of the buildings. For buildings on one side, = was set 2*= . Fig. 6 shows the frequency of 1 1 occurrence of the H/= -ratios along the street sections in 1 the three cities for both cases: buildings on one side and buildings on both sides. The sum of the frequency of occurrence for both cases becomes 1.0. There are about 35% buildings on one and 65% buildings on both sides of the streets. For the medium-sized city, buildings on one side seem the more
W. Theurer / Atmospheric Environment 33 (1999) 4057}4066
Fig. 6. Frequency of the H/= -ratios along the roads. 1
Fig. 7. H/= -ratios of street canyons for di!erent building 1 arrangements and cities.
typical case. From Fig. 6, a H/= -ratio of approx. 0.3 1 frequently occurs for streets with buildings only on one side (or negligible high buildings on the other), whereas with buildings on both sides, the H/= -ratio increases in 1 most cases to H/= "0.5 or higher. 1 Especially in city centres (see Fig. 7), the building heights along street canyons (with two lanes) reach 1.5 times the street width (H/= "1.5). In streets with 4 or 1 6 lanes as in the large city, the H/= -ratio will be halved 1 to about 0.7, if the building height was nearly kept constant. The smallest H/= -ratios (H/= "0.2}0.3) are 1 1 observed in areas with single family houses (SFH) or in commercial areas (CA). Industrial areas are sometimes surrounded by walls (which can be noticed in topographical maps). Sometimes, the buildings are placed at the border of the area. The "rst leads to low H/= -ratios, 1 the latter to relatively high ones, comparable with ratios in city centres.
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Fig. 8. Average street section length for di!erent building arrangements and cities.
2.2.3. Length of the road sections and percentage of the spacings In all the three cities, a similar typical length ¸ from 1 one crossing or junction to the next one could be observed for the di!erent building arrangements (Fig. 8). It ranges from 100 to 150 m for the city centre and the block edge building arrangements to 400}500 m in industrial areas. Streets in commercial areas are about 200}300 m long. In DUD and R&B arrangements, the street length is relatively constant with approx. 200 m. Glueck (1976) found for residential areas in "ve larger European cities with more than 800.000 inhabitants street length values from 89 m (in the inner urban districts) to 120 m (in the outer ones), which agrees to the city centre and block edge building arrangements. For one road section, the spacings between the buildings should be distributed approximately regular. Typical percentages of spacings in the building rows along streets are given in Fig. 9. They reach a maximum for single family houses with about 50%, and decrease continuously towards approx. 0% in the city centres. In commercial areas, a high percentage of spacings can be observed, also in industrial areas. If we classify the in#uence of the buildings on the dispersion as recognizable for a percentage of spacings between 60 and 20% and considerable for 20}0% (closed building rows), the arrangements SFH, DUD, CA and IA would mainly fall in the second category, whereas the rest (R&B, BEB, CC) falls in the third category. 2.2.4. Roof type, buildings in the surrounding and vegetation Typical roof types for the di!erent building patterns are:
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2.3. Typical parameters for diwerent building arrangements Table 1 summarizes the parameters of the building arrangements, which were discussed in Section 2. The values in brackets were taken from Theurer (1993) for comparison, bold typed values are recommendations for further calculations. The proposed classi"cation is simpler than looking at each individual building. Still, it might be restricted to German towns with a population of about 50.000}500.000 inhabitants, but it should be possible to adapt the procedure to other local conditions.
3. E4ects on the dispersion of pollutants Fig. 9. Percentage of spacings along the roads for di!erent building arrangements.
E SFH: houses mainly saddle roofs, if there is no other regulation, garages mainly #at roofs. E DUD: houses mainly saddle roofs, annexes as sheds or extensions, often #at roofs. E R&B: max. about 15% #at roofs, but one facility may completely have #at or saddle roofs. E BEB: max. about 10% #at roofs. E CC: 50}100% #at roofs, with roof tops. E CA: saddle roofs with small slopes or #at roofs. E IA: saddle roofs with small slopes, #at roofs or other roof types as shed roofs or desk roofs. The buildings packing ratio j (see Fig. 4) describes the 0 density of a building arrangement and should exceed a certain value for a ‘urban roughness’ situation (suggested: j '0.25, see Theurer, 1993). In addition to 0 Theurer (1993), for other locations packing ratios of E E E E E
R&B: 0.14}0.26 BEB: 0.3}0.4 CC: 0.4}0.65 CA: 0.23}0.27 IA: 0.41}0.58
were found. In our case, all urban streets were embedded into the built-up area. Streets with only two single building rows along their sides and open country around were not found. The percentage of street sections with trees in the middle or at least on one side of the road between sidewalks and tra$c lanes (alleys) was about 34% in the smaller town, 42% in the larger one and 36% in the city with 300.000 inhabitants. No obvious correlation could be made out with certain building arrangements, but streets with trees were normally wider than H/= "1.0. 1
To demonstrate the e!ect of di!erent building arrangements on local air pollution concentrations, some examples of tra$c induced air pollution along urban roads will be considered. Local concentrations c (x, y, z) (without any background concentration, although they might play an important role in real situations) can be calculated by a dilution factor c(x, y, z) ) u ) ¸ N CH(x, y, z)" q q c(x, y, z)" CH(x, y, z). (1) u )¸ Eq. (1) directly includes the approach #ow velocity u , the source strength per length, q, which depends on the number of vehicles per time (hour, day) and their emission factors, and a geometrical reference length ¸ as the typical building height. CH(x, y, z) is a function of the wind direction b, the vehicle induced turbulence v.i.t., the local building arrangement and the state of the atmosphere. The latter is often assumed to be neutral under urban conditions. CH(x, y, z)"f (b, v.i.t., building arrangement, state)
(2)
If we focus only on the building arrangement, CH(x, y, z) becomes a function of those parameters estimated in the section before CH(x, y, z)"f (H/= , *= , ¸ , spacings, roof types, 1 1 1 surroundings, vegetation and obstacles). (3) It seems to be impossible to make a simple superposition of the dilution factors or concentration values for di!erent building parameters because of the complex interaction between buildings and #ow. At least, the following examples will give a qualitative impression of the magnitude of that in#uence. Results from wind tunnel studies (Van den Hout and Baars, 1988; Muenchow, 1991; Meroney et al., 1994;
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Table 1 Recommended parameters for di!erent building arrangements Building arrange
H in m
H/= 1
SFH
10 (8}10) 10 11}17 (8}12) 13 15}17 (12}20) 15 13}17 (15}20) 15 18}21 ('20) 20 6}10 (5}15) 8 15}17 (15}25) 18
0.2
DUD
R&B
BEB
CC
CA
IA
¸ in m 1
}
Spacings
Roof type
Vegetation Surround. j 0 within the street
0.25}0.5
Approx. 1.0 s
0.34}0.42 (for all arr.)
0.2 0.5}0.6
} 173}177
0.4 0.3}0.38
1.0 s Approx. 1.0 s
0.55 0.35}0.5
175 150}209
0.35 0.1}0.19
1.0 s 0.85}0.9 s
0.45 0.5}0.8
180 74}144
0.15 0.06}0.09
0.9 s 0.85}0.9 s
0.65
110 63}120
0.08 P0
0.9 s 0}0.5 s
0.7 or 1.5 0.15}0.3
90 182}316
0 0.4}0.5
0.5 s
0.25
250 390}535
0.4 0.2
1.0 f
0.15 or 0.7
450
0.2
1.0 f
} (0.1}0.2) } (0.2}0.3) 0.14}0.26 (0.2}0.3) 0.3}0.4 (0.25}0.35) 0.4}0.65 ((0.5) 0.23}0.27 (0.25}0.35) 0.41}0.58 (0.3}0.4)
} " no information available H/= "0.7 recommended for streets with 4 lanes, 1.5 for streets with 2 lanes H/= "0.15 recommended for streets between walls, 0.7 for streets between buildings s"saddle roof, f"#at roofs Saddle roof with small slopes or #at roofs Saddle roofs with small slopes, #at roofs or other roof types as shed roof or desk roofs
Kastner-Klein and Plate, 1995) will be presented as well as results from calculations with the numerical model MISKAM (Schaedler et al., 1994, 1996; Flassak et al., 1996). Unfortunately, from one investigation to another, input conditions as approach #ow, scale, dimensions and position of the source and the measuring points and volumes, ratio street length to source length etc. were partly di!erent. That leads to some uncertainties and scatter. To emphasize the in#uence of the individual parameters, dilution factors CH were made dimensionless by reference values CH , which are typical for an idealized symmetrical street canyon with a building height of about 15}20 m, a H/= of 1.0, with no spacings, build1 ings with #at roofs, and no vegetation or obstacles within the street canyon. Sources are placed parallel to the centerline of the canyon. Only the case with an approach #ow direction perpendicular to the canyon axis is inspected. The reference factor CH is estimated near ground level at the leeward wall of the upstream building, usually at half the street length. 3.1. H/= -ratio and *= 1 1 Wind tunnel and numerical investigations with at least two more H/= -ratios than 1.0 were inspected. For each 1
Fig. 10. CH/CH(H/= "1.0) versus H/= -ratio. 1 1
study, the concentrations CH for di!erent H/= -ratios 1 were made dimensionless with CH for H/= "1.0. The 1 results can be found in Fig. 10. The concentrations increase over-proportional with the H/= -ratio. Schaedler 1 et al. (1996) compared concentrations for di!erent H/= -ratios from some more wind tunnel investigations. 1
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Fig. 11. Recirculation patterns in street canyons for approach #ow direction b"03 (reference case) (from Hoydysh et al., 1974).
They found the same tendency, but more scatter, depending on the di!erent input conditions and the kind of representation. 3.2. Length of the street sections The dilution factor depends on the length of the street canyons. Vortices at the corners transport less loaded air into the streets (Hoydysh et al., 1974, Fig. 11). This ventilation e!ect fades with increasing street length, but the edge vortices are strong enough to inhibit a stable vortex perpendicular to the street canyon in the midsection which is typical for 2D situations. Because of that e!ect canyons with a street length ¸ smaller than ap1 prox. 200 m show an increase of the leeward concentration in the mid-section with increasing length (wind tunnel experiments by Plate and Kastner-Klein (1995); and calculations with MISKAM by Schaedler et al. (1994, 1996) and Flassak et al., (1996)). For longer street canyons, a stable vertical vortex in the mid-section is established and the dilution decreases to the limit for the 2D situation. For typical street lengths ¸ , CH values are presented in 1 Fig. 12. For each study, they were compared with the CH value occuring at the windward side in the midsection of a street with a H/= ratio of 1.0 and about 1 100 m length. Spacings between the buildings reduce the concentration level in a street. A "rst estimate is given in Lohmeyer (1994) for street canyons with a H/= ratio of 1.0 but 1 a street length of approx. 80 m. For 50% spacings one yields a CH/CH ratio of about 0.5. For the same spacing of 50%, but a longer street with H/= "0.45, Van den 1 Hout and Baars (1988) estimated also CH/CH "0.5 in the leeward wake of the buildings. 3.3. Roof type and buildings in the surrounding The roof type, the width of the buildings in #ow direction and other buildings in the surrounding cause small
Fig. 12. CH/CH(¸ "100 m) versus street length ¸ . 1 1
di!erences in the #ow above the canyon, generated by di!erent separation and recirculation zones (Meroney et al., 1994). Obviously, the dilution is strongly dependent on the position of the separation point and the size of the recirculation zone. Rafailidis and Schatzmann (1995) found for a H/= of 1 1.0 a reduction of the concentration in street canyons with saddle roofs, compared to those with #at roofs, to CH/CH "0.7 on the leeward side. For H/= "2, the 1 higher concentrations were observed at the windward side of the street canyon in the case of #at roofs. Other results indicate that not only the roof type but also the upstream building pattern and the width of the building determine the #ow and therefore the dilution factors. If we compare two situations &urban roughness' (with the street canyon embedded into an dense built-up urban area) and &open country' (with two rows of buildings only), the #ow regime can be characterized in the "rst case as a skimming #ow, whereas in the latter it is more similar to the #ow around separate buildings. That di!erence increases the concentration to about 1.4 for the
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Fig. 13. CH/CH and the in#uence of vegetation and obstacles, reference value CH "1.0 for H/= "1.0, leeward side of the street 1 canyon.
urban roughness, compared to the open-country situation (Plate and Kastner-Klein, 1995).
3.4. Vegetation and other obstacles Vegetation and trees are grown along urban streets for aesthetical reasons and to improve the air quality by absorbing dust and other particles. Van den Hout and Baars (1988) show for a H/= -ratio of 0.45 and an 1 approach #ow perpendicular to the street canyon that trees and obstacles such as parking cars within streets inhibit the ventilation. The magnitude of the increase is presented in Fig. 13. It has to be noticed that in their study, the value CH for H/= "0.45 and no vegetation 1 or obstacles is about 0.3 times the value CH for the same situation, but a H/= "1.0. 1 The few examples, moreover restricted to a #ow perpendicular to the street canyon, already show the di!erences in the magnitude of in#uence on the pollution concentrations by the building parameters. The dependence of the concentrations on the H/= 1 ratio and the number of spacings seems to be at least proportional, whereas the street length, especially for ¸ '200 m has less e!ect. Only trends of the 1 in#uence could be demonstrated for the roof type, buildings in the surrounding and obstacles in the street canyon.
4. Practical application This study is trying to o!er the basic parameters of typical building arrangements for wind tunnel or numerical investigations on the dispersion of air pollutants. Unfortunately, up to now it is not possible to present an explicit formula of the dilution factors in terms of the di!erent building parameters. The interaction between #ow and buildings is non-linear, and measured or calculated dilution data for the building arrangements are missing. A building arrangement might be modelled according to Table 1 and inspected for various approach #ow directions (e.g. 24 or 36) to get a representative data set for the dispersion of pollutants along streets in a surrounding, such as made in screening models as STREET (Pfeifer et al., 1995) or PROKAS (Ingenieurbuero Lohmeyer, 1996). Furthermore, the relevance of di!erent building parameters on the concentration values can be compared with themselves or with other parameters such as a variation of the emission or the meteorological conditions. For one single case could be shown that modeling a correct H/= ratio or number of spacings might be more 1 important than modeling the correct street length. But, as a recent study by Flassak et al. (1996) shows, the number of vehicles, the composition of a car #eet, a representative distribution of the wind direction and the wind statistics have the same or even more signi"cance on the pollution concentration than the building parameters.
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Acknowledgements The author wants to thank the reviewers and Dr. W. Baechlin for their critical but helpful remarks, and Mrs. C. Koller, M.A., for her support on preparing the manuscript.
References Busset, T., Garnier, A., Joye, D., Schuler, M., 1990. Une typologie exploratoire des batiments. Cahier IREC No. 88, EPFL Lausanne. Eereurs, H.C., Sliggers, C.J., van den Hout, K.D., 1993. The CAR model: The Dutch method to determine city street air quality. Atmospheric Environment 27 B(4), 389}399. Flassak, Th., Baechlin, W., Boesinger, R., Blazek, R., Schaedler, G., Lohmeyer, A., 1996. Ein#u{ der Eingangsparameter auf berechnete Immissionswert fuK r Kfz}Abgase: SensitivitaK tsanalyse. FZKA-PEF Bericht 150, Forschungszentrum Karlsruhe. Glueck, K., 1976. Ein Stadtmodell und der Umfang der LaK rmbelaK stigung durch den Stra{enverkehr. Schriftenreihe des Instituts fuK r Verkehrsplanung und Verkehrswesen, Heft 12, TU MuK nchen. Hoydysh, W.G., Gri$ths, R.A., Ogawa, Y., 1974. A scale model study of the dispersion of pollutants in street canyons. 67th Annual Meeting of the APCA, Paper 74}157, Denver, USA. Ingenieurbuero Lohmeyer, 1996. PROKAS } Berechnungsverfahren zur Bestimmung verkehrserzeugter Schadsto!belastungen. Informationsblatt. Kastner-Klein, P., Plate, E.J., 1995. Ausbreitung von Luftverunreinigungen. 8. ALS - Kolloquium, Stuttgart. Lohmeyer, A., 1994. Genauigkeit der Rechnungen. Seminar: Ausbreitung von Kfz-Emissionen, DuK sseldorf, Mai 1994, Schriftenreihe KRdL in VDI und DIN, Band 21, S. 103}118. Meroney, R.N., Rafailidis, S., Pavageau, M., 1994. Summary and discussion of the "ndings from the wind tunnel studies of April}June 1994. Meteorologisches Institut, UniversitaK t Hamburg.
Muenchow, Th., 1991. Untersuchung von AusbreitungsvorgaK ngen in einer turbulenten Windkanalgrenzschicht } Immissionsbelastung in Stra{enschluchten. Diplomarbeit. Meteorologisches Institut, UniversitaK t Hamburg. Pfeifer, T., Frank, W., Kost, W.J., Droescher, F., 1995. Lufthygienische Beurteilung der verkehrsbedingten Immissionen von Stra{en und Kreuzungen im Hinblick auf Paragraph 40 (2) Bundes-Immissionsschutzgesetz } Wissenschaftliche Dokumentation. TUG V Energie und Umwelt GmbH, Stuttgart. Plate, E., Kastner-Klein, P., 1995. Simulation der Ausbreitung von Kfz-Abgasen: Fachliche Impulse durch Windkanalmessungen. Experten } Workshop `Verkehr und Immissionena, FZKA, Karlsruhe, Juli 1995, Zusammenstellung der VortraK ge, S. 67}86. Rafailidis, S., Schatzmann, M., 1995. Concentration measurements with di!erent roof patterns in street canyons with aspect ratios B/H"1/2 and B/H"1. Meteorologisches Institut, UniversitaK t Hamburg. Roth, U., Haeubi, F., 1980. Wechselwirkungen zwischen der Siedlungsstruktur und WaK rmeversorgungssystemen. Schriftenreihe des Bundesministers fuK r Raumordnung, Bauwesen und StaK dtebau, Bonn. Schaedler, G., Baechlin, W., Lohmeyer, A., 1994. Vergleich und Bewertung derzeit verfuK gbarer mikroskaliger StroK mungsund Ausbreitungsmodelle. 10. Statuskolloquium des PEF vom 15. bis 17. MaK rz 1994 im Kernforschungszentrum Karlsruhe, KfK-PEF Bericht 117, Kernforschungszentrum Karlsruhe, S. 277}288. Schaedler, G., Baechlin, W., Lohmeyer, A., van Wees, T., 1996. Vergleich und Bewertung derzeit verfuK gbarer mikroskaliger StroK mungs- und Ausbreitungsmodelle. FZKA-PEF Bericht 138, Forschungszentrum Karlsruhe. Theurer, W., 1993. Ausbreitung bodennaher Emissionen in komplexen Bebauungen, Dissertation, IHW, UniversitaK t Karlsruhe. Van den Hout, K.R., Baars, H.P., 1988. Ontwikkeling van twee modellen voor de verspreiding van luchtverontreiniging door verkeer: het TNO-verkeer model and het CAR-model. TNO-Rapport Nr. R 88/192.
Typical building arrangements for urban air pollution modelling W. Theurer Ingenieurbu( ro Theurer, Ha( ndelweg 4, D-67346 Speyer, Germany
Abstract Typical building arrangements along urban roads and their parameters were determined for three cities in the south-western part of Germany. Some examples of the dispersion of vehicle emissions within idealized street canyons show how di!erent building parameters in#uence the concentration of air pollutants. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Building arrangements; Urban air pollution
1. Introduction Emissions caused by industry, tra$c and private households reduce the air quality in densely populated areas. The quantity of pollutants emitted in a certain area as well as transport and dispersion mechanisms and the background level determine the local concentration of air pollutants. Transport and dispersion depend on the local #ow between the buildings and obstacles. Flow direction and velocity are a result of interaction of the outer #ow and the type and arrangement of the buildings in their surroundings. Both numerical as well as physical models try to simulate the #ow and the concentrations in built-up areas. They need detailed information about the height and width of the buildings, their arrangement, the spacings between them, the width of the streets etc. These parameters need to be estimated for every single urban district or road or, even, building. To simplify this procedure, Theurer (1993) classi"ed typical building arrangements for wider urban areas and their aerodynamic parameters. This classi"cation allows to estimate the in#uence of building arrangements on the dispersion of releases from point sources near groundlevel up to about 1 km downstream the source. In urban or industrial areas with a systematic arrangement of the buildings in a certain orientation (e.g. building rows),
wind tunnel experiments indicated that dispersion depends much on the angle between the orientation of the buildings and the approach #ow direction. Compared to other areas with more homogeneous distributed buildings, a di!erent width of the dispersion plume occurred as well as di!erent positions of the maximum concentration in cross sections of the plume. There are other classi"cation schemes of urban building patterns (e.g. Roth and Haeubi, 1980; Busset et al., 1990) and buildings along urban roads (Glueck, 1976). Due to the fact that exhaust fumes by vehicles irreversingly dominate air pollution in urban areas, in this study focus is on the arrangements along the roads. The existing classi"cation of building arrangements has been modi"ed to deliver typical parameters. For instance, recent numerical investigations by e.g. Schaedler et al. (1994,1996) and wind tunnel experiments (e.g. Van den Hout and Baars, 1988) show a strong dependence of the pollutant concentration within street canyons on the dimensions of the canyon. For screening models such as PROKAS (Ingenieurbuero Lohmeyer, 1996), STREET (Pfeifer et al., 1995) or CAR (Eereurs et al., 1993) as well as to simplify wind tunnel investigations, the identi"cation of typical building arrangements along roads and the knowledge of their in#uence on the dispersion of air pollutants would be a helpful tool.
1352-2310/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 2 - 2 3 1 0 ( 9 9 ) 0 0 1 4 7 - 8
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Fig. 1. Classi"cation scheme for urban building arrangements.
2. Classi5cation of the building arrangements 2.1. Proposed classixcation scheme Fig. 1 shows a classi"cation scheme for wider urban areas in German towns. The building arrangements are divided into single or double family houses (SFH), dense urban development, residential and commercial (DUD), building rows and blocks (R&B), block edge buildings (BEB), city centres (CC), commercial areas (CA) and industrial areas (IA). Green areas without larger trees (GA) and parks and forests (P&F) complete the classi"cation. In Fig. 2, the classi"cation scheme is applied to an urban area. First, the parameters of building arrangements along urban roads were quanti"ed along higher loaded main roads in three cities with 50.000 (3.5 km road length), 160.000 (4.5 km) and 300.000 inhabitants (11.4 km) in south-western Germany. There were 2 (in the town with 50.000 inh.) to 6 (in that with 300.000 inh.) lanes in the streets with tramway lanes in the larger towns. The tra$c
load had been approx. 14.000}25.000 vehicles/24 h in the smallest town, between 16.000}44.000 vehicles/24 h in the medium-sized one and 10.000}52.000 vehicles/16 h in the large one. To gain an insight into the change of typical building arrangements, the streets were observed from the city limit to the town centre. The length of the street sections, the spacings, more than 100 typical cross sections with the height of the buildings on both sides of the road and its width, the form of the roofs and additional remarks (obstacles within the street, etc.) were documented. Photographs do allow to control the documentation lists. Fig. 3 gives a frequency of the occurrence of the di!erent building arrangements in the road sections. Obviously, they depend on the size of the cities. In the smallest town, an open arrangement of single-family dwellings (SFH) could be found in about 10% of the 3.5 km distance. The building pattern decreases or disappears in the larger towns. The contrary is valid for the city centre (CC) pattern: about 6}8% in both larger towns, but only 2% in the small town. Industrial building arrangements
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Fig. 2. Application of the classi"cation scheme to an urban area.
building arrangements in that town besides commercial areas.
2.2. Investigated parameters
Fig. 3. Frequency of building arrangements.
(IA) were also missing along the investigated roads in the smaller town. Commercial areas (CA) were found in all three towns independent from their size with about 20%. For the medium-sized town, the large 35% contribution of green areas (GA) is remarkable. That is the case for the reduced dense urban development (DUD) and block edge building (BEB) patterns. Both of those building arrangements, often &historically grown', are less represented in the town founded in 1850. Building rows and blocks which were built after 1945 are the most frequent
The building arrangements di!er in typical geometrical parameters shown in Fig. 4. Geometrical parameters are, for example, the average size of an individual building, with its height H , width = and length ¸ . The basic area = ¸ of a building is named A , the considered 0 area is A . Theurer (1993) classi"ed building arrange2-2 ments among other by the dimensionless buildings packing ratio j " A /A . The width of the street 0 2-2 0 2-2 from wall to wall, = , including tramway and bicycle 1 lanes and sidewalks, and the distance from the line source (center of the vehicle lanes) to the building walls, *= , 1 have to be regarded. *= is assumed to = /2 for a sym1 1 metric arrangement of the vehicle lanes parallel to the centerline of the canyon independent of the number of lanes. Often, H and = are combined to a H/= -ratio of 1 1 the cross section of a street canyon. The length of a street section, ¸ , is measured from one crossing or junction to 1 the next, with a percentage of spacings between the buildings. Further, there might be even an in#uence of the roof type (saddle or #at roof), of the buildings in the surrounding and of the vegetation and obstacles within a street (trees, bushes, parking cars etc.) on the local concentration of pollutants.
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Fig. 4. Parameters to describe an urban building arrangement. Top left: parameters for wider urban areas, below right: parameters for urban roads.
2.2.1. Building height In Fig. 5, average building heights are assigned to the di!erent building arrangements. For all three towns, they lay close together. The lowest buildings can usually be found in SFH and CA arrangements, the highest buildings in city centres.
Fig. 5. Average building height for di!erent building arrangements and cities.
2.2.2. H/= -ratio and *= 1 1 The width = of the streets looked at ranged from 1 about 12 to 60 m } including the distances from the sidewalks to the walls of the buildings. For buildings on one side, = was set 2*= . Fig. 6 shows the frequency of 1 1 occurrence of the H/= -ratios along the street sections in 1 the three cities for both cases: buildings on one side and buildings on both sides. The sum of the frequency of occurrence for both cases becomes 1.0. There are about 35% buildings on one and 65% buildings on both sides of the streets. For the medium-sized city, buildings on one side seem the more
W. Theurer / Atmospheric Environment 33 (1999) 4057}4066
Fig. 6. Frequency of the H/= -ratios along the roads. 1
Fig. 7. H/= -ratios of street canyons for di!erent building 1 arrangements and cities.
typical case. From Fig. 6, a H/= -ratio of approx. 0.3 1 frequently occurs for streets with buildings only on one side (or negligible high buildings on the other), whereas with buildings on both sides, the H/= -ratio increases in 1 most cases to H/= "0.5 or higher. 1 Especially in city centres (see Fig. 7), the building heights along street canyons (with two lanes) reach 1.5 times the street width (H/= "1.5). In streets with 4 or 1 6 lanes as in the large city, the H/= -ratio will be halved 1 to about 0.7, if the building height was nearly kept constant. The smallest H/= -ratios (H/= "0.2}0.3) are 1 1 observed in areas with single family houses (SFH) or in commercial areas (CA). Industrial areas are sometimes surrounded by walls (which can be noticed in topographical maps). Sometimes, the buildings are placed at the border of the area. The "rst leads to low H/= -ratios, 1 the latter to relatively high ones, comparable with ratios in city centres.
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Fig. 8. Average street section length for di!erent building arrangements and cities.
2.2.3. Length of the road sections and percentage of the spacings In all the three cities, a similar typical length ¸ from 1 one crossing or junction to the next one could be observed for the di!erent building arrangements (Fig. 8). It ranges from 100 to 150 m for the city centre and the block edge building arrangements to 400}500 m in industrial areas. Streets in commercial areas are about 200}300 m long. In DUD and R&B arrangements, the street length is relatively constant with approx. 200 m. Glueck (1976) found for residential areas in "ve larger European cities with more than 800.000 inhabitants street length values from 89 m (in the inner urban districts) to 120 m (in the outer ones), which agrees to the city centre and block edge building arrangements. For one road section, the spacings between the buildings should be distributed approximately regular. Typical percentages of spacings in the building rows along streets are given in Fig. 9. They reach a maximum for single family houses with about 50%, and decrease continuously towards approx. 0% in the city centres. In commercial areas, a high percentage of spacings can be observed, also in industrial areas. If we classify the in#uence of the buildings on the dispersion as recognizable for a percentage of spacings between 60 and 20% and considerable for 20}0% (closed building rows), the arrangements SFH, DUD, CA and IA would mainly fall in the second category, whereas the rest (R&B, BEB, CC) falls in the third category. 2.2.4. Roof type, buildings in the surrounding and vegetation Typical roof types for the di!erent building patterns are:
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2.3. Typical parameters for diwerent building arrangements Table 1 summarizes the parameters of the building arrangements, which were discussed in Section 2. The values in brackets were taken from Theurer (1993) for comparison, bold typed values are recommendations for further calculations. The proposed classi"cation is simpler than looking at each individual building. Still, it might be restricted to German towns with a population of about 50.000}500.000 inhabitants, but it should be possible to adapt the procedure to other local conditions.
3. E4ects on the dispersion of pollutants Fig. 9. Percentage of spacings along the roads for di!erent building arrangements.
E SFH: houses mainly saddle roofs, if there is no other regulation, garages mainly #at roofs. E DUD: houses mainly saddle roofs, annexes as sheds or extensions, often #at roofs. E R&B: max. about 15% #at roofs, but one facility may completely have #at or saddle roofs. E BEB: max. about 10% #at roofs. E CC: 50}100% #at roofs, with roof tops. E CA: saddle roofs with small slopes or #at roofs. E IA: saddle roofs with small slopes, #at roofs or other roof types as shed roofs or desk roofs. The buildings packing ratio j (see Fig. 4) describes the 0 density of a building arrangement and should exceed a certain value for a ‘urban roughness’ situation (suggested: j '0.25, see Theurer, 1993). In addition to 0 Theurer (1993), for other locations packing ratios of E E E E E
R&B: 0.14}0.26 BEB: 0.3}0.4 CC: 0.4}0.65 CA: 0.23}0.27 IA: 0.41}0.58
were found. In our case, all urban streets were embedded into the built-up area. Streets with only two single building rows along their sides and open country around were not found. The percentage of street sections with trees in the middle or at least on one side of the road between sidewalks and tra$c lanes (alleys) was about 34% in the smaller town, 42% in the larger one and 36% in the city with 300.000 inhabitants. No obvious correlation could be made out with certain building arrangements, but streets with trees were normally wider than H/= "1.0. 1
To demonstrate the e!ect of di!erent building arrangements on local air pollution concentrations, some examples of tra$c induced air pollution along urban roads will be considered. Local concentrations c (x, y, z) (without any background concentration, although they might play an important role in real situations) can be calculated by a dilution factor c(x, y, z) ) u ) ¸ N CH(x, y, z)" q q c(x, y, z)" CH(x, y, z). (1) u )¸ Eq. (1) directly includes the approach #ow velocity u , the source strength per length, q, which depends on the number of vehicles per time (hour, day) and their emission factors, and a geometrical reference length ¸ as the typical building height. CH(x, y, z) is a function of the wind direction b, the vehicle induced turbulence v.i.t., the local building arrangement and the state of the atmosphere. The latter is often assumed to be neutral under urban conditions. CH(x, y, z)"f (b, v.i.t., building arrangement, state)
(2)
If we focus only on the building arrangement, CH(x, y, z) becomes a function of those parameters estimated in the section before CH(x, y, z)"f (H/= , *= , ¸ , spacings, roof types, 1 1 1 surroundings, vegetation and obstacles). (3) It seems to be impossible to make a simple superposition of the dilution factors or concentration values for di!erent building parameters because of the complex interaction between buildings and #ow. At least, the following examples will give a qualitative impression of the magnitude of that in#uence. Results from wind tunnel studies (Van den Hout and Baars, 1988; Muenchow, 1991; Meroney et al., 1994;
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Table 1 Recommended parameters for di!erent building arrangements Building arrange
H in m
H/= 1
SFH
10 (8}10) 10 11}17 (8}12) 13 15}17 (12}20) 15 13}17 (15}20) 15 18}21 ('20) 20 6}10 (5}15) 8 15}17 (15}25) 18
0.2
DUD
R&B
BEB
CC
CA
IA
¸ in m 1
}
Spacings
Roof type
Vegetation Surround. j 0 within the street
0.25}0.5
Approx. 1.0 s
0.34}0.42 (for all arr.)
0.2 0.5}0.6
} 173}177
0.4 0.3}0.38
1.0 s Approx. 1.0 s
0.55 0.35}0.5
175 150}209
0.35 0.1}0.19
1.0 s 0.85}0.9 s
0.45 0.5}0.8
180 74}144
0.15 0.06}0.09
0.9 s 0.85}0.9 s
0.65
110 63}120
0.08 P0
0.9 s 0}0.5 s
0.7 or 1.5 0.15}0.3
90 182}316
0 0.4}0.5
0.5 s
0.25
250 390}535
0.4 0.2
1.0 f
0.15 or 0.7
450
0.2
1.0 f
} (0.1}0.2) } (0.2}0.3) 0.14}0.26 (0.2}0.3) 0.3}0.4 (0.25}0.35) 0.4}0.65 ((0.5) 0.23}0.27 (0.25}0.35) 0.41}0.58 (0.3}0.4)
} " no information available H/= "0.7 recommended for streets with 4 lanes, 1.5 for streets with 2 lanes H/= "0.15 recommended for streets between walls, 0.7 for streets between buildings s"saddle roof, f"#at roofs Saddle roof with small slopes or #at roofs Saddle roofs with small slopes, #at roofs or other roof types as shed roof or desk roofs
Kastner-Klein and Plate, 1995) will be presented as well as results from calculations with the numerical model MISKAM (Schaedler et al., 1994, 1996; Flassak et al., 1996). Unfortunately, from one investigation to another, input conditions as approach #ow, scale, dimensions and position of the source and the measuring points and volumes, ratio street length to source length etc. were partly di!erent. That leads to some uncertainties and scatter. To emphasize the in#uence of the individual parameters, dilution factors CH were made dimensionless by reference values CH , which are typical for an idealized symmetrical street canyon with a building height of about 15}20 m, a H/= of 1.0, with no spacings, build1 ings with #at roofs, and no vegetation or obstacles within the street canyon. Sources are placed parallel to the centerline of the canyon. Only the case with an approach #ow direction perpendicular to the canyon axis is inspected. The reference factor CH is estimated near ground level at the leeward wall of the upstream building, usually at half the street length. 3.1. H/= -ratio and *= 1 1 Wind tunnel and numerical investigations with at least two more H/= -ratios than 1.0 were inspected. For each 1
Fig. 10. CH/CH(H/= "1.0) versus H/= -ratio. 1 1
study, the concentrations CH for di!erent H/= -ratios 1 were made dimensionless with CH for H/= "1.0. The 1 results can be found in Fig. 10. The concentrations increase over-proportional with the H/= -ratio. Schaedler 1 et al. (1996) compared concentrations for di!erent H/= -ratios from some more wind tunnel investigations. 1
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Fig. 11. Recirculation patterns in street canyons for approach #ow direction b"03 (reference case) (from Hoydysh et al., 1974).
They found the same tendency, but more scatter, depending on the di!erent input conditions and the kind of representation. 3.2. Length of the street sections The dilution factor depends on the length of the street canyons. Vortices at the corners transport less loaded air into the streets (Hoydysh et al., 1974, Fig. 11). This ventilation e!ect fades with increasing street length, but the edge vortices are strong enough to inhibit a stable vortex perpendicular to the street canyon in the midsection which is typical for 2D situations. Because of that e!ect canyons with a street length ¸ smaller than ap1 prox. 200 m show an increase of the leeward concentration in the mid-section with increasing length (wind tunnel experiments by Plate and Kastner-Klein (1995); and calculations with MISKAM by Schaedler et al. (1994, 1996) and Flassak et al., (1996)). For longer street canyons, a stable vertical vortex in the mid-section is established and the dilution decreases to the limit for the 2D situation. For typical street lengths ¸ , CH values are presented in 1 Fig. 12. For each study, they were compared with the CH value occuring at the windward side in the midsection of a street with a H/= ratio of 1.0 and about 1 100 m length. Spacings between the buildings reduce the concentration level in a street. A "rst estimate is given in Lohmeyer (1994) for street canyons with a H/= ratio of 1.0 but 1 a street length of approx. 80 m. For 50% spacings one yields a CH/CH ratio of about 0.5. For the same spacing of 50%, but a longer street with H/= "0.45, Van den 1 Hout and Baars (1988) estimated also CH/CH "0.5 in the leeward wake of the buildings. 3.3. Roof type and buildings in the surrounding The roof type, the width of the buildings in #ow direction and other buildings in the surrounding cause small
Fig. 12. CH/CH(¸ "100 m) versus street length ¸ . 1 1
di!erences in the #ow above the canyon, generated by di!erent separation and recirculation zones (Meroney et al., 1994). Obviously, the dilution is strongly dependent on the position of the separation point and the size of the recirculation zone. Rafailidis and Schatzmann (1995) found for a H/= of 1 1.0 a reduction of the concentration in street canyons with saddle roofs, compared to those with #at roofs, to CH/CH "0.7 on the leeward side. For H/= "2, the 1 higher concentrations were observed at the windward side of the street canyon in the case of #at roofs. Other results indicate that not only the roof type but also the upstream building pattern and the width of the building determine the #ow and therefore the dilution factors. If we compare two situations &urban roughness' (with the street canyon embedded into an dense built-up urban area) and &open country' (with two rows of buildings only), the #ow regime can be characterized in the "rst case as a skimming #ow, whereas in the latter it is more similar to the #ow around separate buildings. That di!erence increases the concentration to about 1.4 for the
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Fig. 13. CH/CH and the in#uence of vegetation and obstacles, reference value CH "1.0 for H/= "1.0, leeward side of the street 1 canyon.
urban roughness, compared to the open-country situation (Plate and Kastner-Klein, 1995).
3.4. Vegetation and other obstacles Vegetation and trees are grown along urban streets for aesthetical reasons and to improve the air quality by absorbing dust and other particles. Van den Hout and Baars (1988) show for a H/= -ratio of 0.45 and an 1 approach #ow perpendicular to the street canyon that trees and obstacles such as parking cars within streets inhibit the ventilation. The magnitude of the increase is presented in Fig. 13. It has to be noticed that in their study, the value CH for H/= "0.45 and no vegetation 1 or obstacles is about 0.3 times the value CH for the same situation, but a H/= "1.0. 1 The few examples, moreover restricted to a #ow perpendicular to the street canyon, already show the di!erences in the magnitude of in#uence on the pollution concentrations by the building parameters. The dependence of the concentrations on the H/= 1 ratio and the number of spacings seems to be at least proportional, whereas the street length, especially for ¸ '200 m has less e!ect. Only trends of the 1 in#uence could be demonstrated for the roof type, buildings in the surrounding and obstacles in the street canyon.
4. Practical application This study is trying to o!er the basic parameters of typical building arrangements for wind tunnel or numerical investigations on the dispersion of air pollutants. Unfortunately, up to now it is not possible to present an explicit formula of the dilution factors in terms of the di!erent building parameters. The interaction between #ow and buildings is non-linear, and measured or calculated dilution data for the building arrangements are missing. A building arrangement might be modelled according to Table 1 and inspected for various approach #ow directions (e.g. 24 or 36) to get a representative data set for the dispersion of pollutants along streets in a surrounding, such as made in screening models as STREET (Pfeifer et al., 1995) or PROKAS (Ingenieurbuero Lohmeyer, 1996). Furthermore, the relevance of di!erent building parameters on the concentration values can be compared with themselves or with other parameters such as a variation of the emission or the meteorological conditions. For one single case could be shown that modeling a correct H/= ratio or number of spacings might be more 1 important than modeling the correct street length. But, as a recent study by Flassak et al. (1996) shows, the number of vehicles, the composition of a car #eet, a representative distribution of the wind direction and the wind statistics have the same or even more signi"cance on the pollution concentration than the building parameters.
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Acknowledgements The author wants to thank the reviewers and Dr. W. Baechlin for their critical but helpful remarks, and Mrs. C. Koller, M.A., for her support on preparing the manuscript.
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Muenchow, Th., 1991. Untersuchung von AusbreitungsvorgaK ngen in einer turbulenten Windkanalgrenzschicht } Immissionsbelastung in Stra{enschluchten. Diplomarbeit. Meteorologisches Institut, UniversitaK t Hamburg. Pfeifer, T., Frank, W., Kost, W.J., Droescher, F., 1995. Lufthygienische Beurteilung der verkehrsbedingten Immissionen von Stra{en und Kreuzungen im Hinblick auf Paragraph 40 (2) Bundes-Immissionsschutzgesetz } Wissenschaftliche Dokumentation. TUG V Energie und Umwelt GmbH, Stuttgart. Plate, E., Kastner-Klein, P., 1995. Simulation der Ausbreitung von Kfz-Abgasen: Fachliche Impulse durch Windkanalmessungen. Experten } Workshop `Verkehr und Immissionena, FZKA, Karlsruhe, Juli 1995, Zusammenstellung der VortraK ge, S. 67}86. Rafailidis, S., Schatzmann, M., 1995. Concentration measurements with di!erent roof patterns in street canyons with aspect ratios B/H"1/2 and B/H"1. Meteorologisches Institut, UniversitaK t Hamburg. Roth, U., Haeubi, F., 1980. Wechselwirkungen zwischen der Siedlungsstruktur und WaK rmeversorgungssystemen. Schriftenreihe des Bundesministers fuK r Raumordnung, Bauwesen und StaK dtebau, Bonn. Schaedler, G., Baechlin, W., Lohmeyer, A., 1994. Vergleich und Bewertung derzeit verfuK gbarer mikroskaliger StroK mungsund Ausbreitungsmodelle. 10. Statuskolloquium des PEF vom 15. bis 17. MaK rz 1994 im Kernforschungszentrum Karlsruhe, KfK-PEF Bericht 117, Kernforschungszentrum Karlsruhe, S. 277}288. Schaedler, G., Baechlin, W., Lohmeyer, A., van Wees, T., 1996. Vergleich und Bewertung derzeit verfuK gbarer mikroskaliger StroK mungs- und Ausbreitungsmodelle. FZKA-PEF Bericht 138, Forschungszentrum Karlsruhe. Theurer, W., 1993. Ausbreitung bodennaher Emissionen in komplexen Bebauungen, Dissertation, IHW, UniversitaK t Karlsruhe. Van den Hout, K.R., Baars, H.P., 1988. Ontwikkeling van twee modellen voor de verspreiding van luchtverontreiniging door verkeer: het TNO-verkeer model and het CAR-model. TNO-Rapport Nr. R 88/192.