Physical Modelling of Dispersion of Pollutant Emitted at the Portals of Road Tunnels

8/3/2019 Physical Modelling of Dispersion of Pollutant Emitted at the Portals of Road Tunnels

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2003 Firenze University Press244

Proceedings of PHYSMOD2003:International Workshop on Physical Modelling of Flow and Dispersion Phenomena

3-5 September 2003, Prato, Italy

PHYSICAL MODELLING OF DISPERSION OF POLLUTANT EMITTED AT THE PORTALS OFROAD TUNNELS

Daniele ContiniIstituto di Scienze dellAtmosfera e del Clima

Sezione di LecceConsiglio Nazionale delle RicercheStr. Prv. Lecce-Monteroni km 1,2

73100 Lecce, Italy

Lorenzo ProcinoCRIACIV

Boundary-Layer Wind TunnelPiazza Ciardi 2559100 Prato, Italy

Michela MassiniDipartimento di Energetica Sergio Stecco

Universit di FirenzeVia Santa Marta 3

50139, Firenze, Italy

Giampaolo ManfridaDipartimento di Energetica Sergio Stecco

Universit di FirenzeVia Santa Marta 3

50139, Firenze, Italy

ABSTRACTThe exit portals of road tunnels can be considered to anyextent as sources of pollution placed at ground level. Gaseouscontaminants, potentially dangerous (like, for example,nitrogen oxides, carbon monoxide and hydrocarbons) areemitted by vehicles passing through road tunnels and they arecollectively emitted at the exit portals. After release into theatmospheric boundary layer the contaminants will mix withambient air and dispersion will take place in the immediatesurrounding of the road tunnel itself generating zones withrelevant pollutant concentrations. This topic is importantespecially because sometimes road tunnels are placed in urbanenvironment or in proximity of areas of particular ecologicalinterest. Contaminants emitted by the road tunnels, basically atground level, will disperse in a high turbulence area in a way

that is different from the typical plumes emitted by elevatedsources like stacks and chimneys.

In this paper some experimental results obtained in theCRIACIV (Centro di Ricerca Interuniversitario diAerodinamica delle Costruzioni ed Ingegneria del Vento) windtunnel on a physical model of a road tunnel built under a hillare presented. In particular it will be described the modelcharacteristics and performances and the ground levelconcentration field for different wind conditions. The resultsobtained put in evidence the main aspects of the diffusionincluding recirculation of pollutant from one side of the roadtunnel into the other side and the transport above the hill oftracer released against the wind.

NOMENCLATURE

Symbol Description

C Tracer concentration

Uref Reference mean flow speed abovethe boundary-layer

U(z) Mean flow speed at height zu* Friction velocity

W Exit speed of the emissions at theportals of the model.

zref Boundary-layer height

zo Roughness length

Exponent of the exponential law forthe velocity profile

INTRODUCTION Nowadays pollution produced by exhaust of combustion

engines, used in both private and commercial vehicles, is one ofthe major sources of air contaminants in towns as well as insome extra-urban areas located near the major road networks.As matter of fact a source of gaseous contaminants, potentiallydangerous (like, for example, nitrogen oxides, carbonmonoxide and hydrocarbon), is represented by the emissions atthe exit portals of road tunnels. After release the contaminantswill mix with ambient air and dispersion will take place in theimmediate surroundings of the road tunnel itself generatingzones with relevant pollution concentrations. Even if the timeof stay inside the road tunnel is small it is important to take intoaccount the distribution of pollutant nearby the exit portals ofthe tunnel itself where inhabited areas can be located or areas of

particular environmental interest. The details of the plumedispersal are strongly related with the road tunnel ventilationcharacteristics and to the local climate and are thereforedominated by the interaction among the jet at the portal and the

ambient wind [Oettl et al, 2002]. Contaminants emitted by theroad tunnels, basically at ground level, will disperse in a waythat is different from the typical plumes emitted by elevatedsources like stacks and chimneys. This latter kind of source has

been widely studied with different typologies of mathematicaland experimental models but the configuration of sourceassociated with road tunnels emissions has been analysed to a


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2003 Firenze University Pres245

much lower extent and in specific cases in both full scale andlaboratory [Carr et al, 1996], [Ide et al, 1987], [Nadel et al,1996], [Lepade et al, 1996], [Contini et al, 2002]. Therefore itis necessary to develop reliable simulation codes that can beused in environmental impact assessment and in monitoring theexisting road tunnels. Validation of these codes could be

performed, at least partially, with the accurate development ofexperimental methodologies on small scale models tested incontrolled conditions by using the wind tunnels capabilities.

In this paper an experimental analysis has been conductedon a small scale model of a road tunnel placed under a hill in

order to investigate the principal characteristics of thedispersion phenomena related to road tunnel emissions. A first

part of the paper is a characterisation of the boundary-layer andof the model performances that are improved with respect to a

previous model [Contini et al, 2002]. In the second part of the paper an analysis of the concentration fields, especially atground level, has been carried out in order to investigate the

phenomena that are peculiar to this kind of dispersion. In particular it has been put in evidence the areas mainlyinterested in pollution dispersion and the effect of ambient windon the flow inside the pipes of the model. It has been put inevidence the recirculation effect that influence directly theemissions because part of the pollutant travelling in one of the

pipe of the road tunnel can be sucked into the other pipe and

travel under the hill.The results obtained show the complexity of the phenomena that have to be faced in modelling the dispersionfrom this kind of sources that are sometimes placed near urbanareas or near zones of particular environmental interest.

WIND TUNNEL SET-UP AND MODEL DESCRIPTIONA neutral boundary-layer has been artificially generated at

the CRIACIV (Centro di Ricerca Interuniversitario diAerodinamica delle Costruzioni ed Ingegneria del Vento) windtunnel facility. The wind tunnel facility is about 22.8 m long(test section length about 11m) with a test section 2.4m wideand 1.6m tall. The boundary-layer is generated by using largevortex generators (spires and quarter of ellipse shapedCounihan as shown in figure 1) placed at the beginning of the

flow development zone and a distribution of roughness placedin the floor of the wind tunnel of height variable between 10and 20 mm all over the wind tunnel floor. Largest roughnessare positioned at the beginning of the test section.

Fig. 1) Large vortex generators and surface roughness usedin the wind tunnel set-up for the generation of the neutralboundary layer.

The characteristics of the boundary-layer have beemeasured by using single hot-wire anemometer at differendistances from the position of the downwind portals of the roatunnel model (the source of the emissions) in absence of thmodel. In figure 2(a) the mean velocity profiles are reported foa reference flow speed, measured above the boundary-layeUref=2.8 m/s. The corresponding longitudinal turbulenc

profiles are reported in figure 2(b). The boundary-layer has alsbeen measured at different values of Uref obtaining very similaresults. The set-up used allows to generate a boundary-layeabout 0.7m tall (about 3.9 times the maximum height of th

model). The mean velocity profiles is characterised by a powe

law behaviour: ( )= refref z/zU)z(U with ranging from0.15 to 0.17 in the different positions. Friction velocity u* ha

been evaluated ranging from 5.2% to 5.4% of the referencflow speed above the boundary-layer and the values of zobtained by a two-parameters fit is ranging from 0.25 to 0mm.

The two-ways road tunnel is a 1:200 model (82 cm ilength) built with a longitudinal ventilation generated by twsmall DC axial fans placed in the middle of the model. Filtergrid and a distribution of obstacles are placed inside the modeat different distances from the fans in order to artificiallgenerate a mean velocity profile inside the road tunnel able tsimulate the real profiles present in full scale road tunnel as

consequence mainly of piston effect and eventual longitudinaventilation [Chen et al, 1998]. The tracer injection is madaround the centre of the model just before the axial fan anseparately for each pipe of the model. The total flow-rate of thtracer is controlled by an electronic mass-flow regulator and thsplit into the two pipes is obtained and checked with rotameteflow-meters. The tracer is therefore well mixed when it reachethe portals and concentration is quite homogeneous. Thereforthe model is able to reproduce an almost uniform, within a few

percent, distribution of pollutant at the portals that representative of full scale scenarios in which the vehicleemissions are well-mixed inside the road tunnel by the effecdue to the movements of the vehicles themselves.

The mean velocity W profile at the exit portal of the roa

tunnel model measured by using a single hot-wire anemometein the centre of the exit portal is reported in figure 3. The shapof the profile is in good agreement with the ones measured bChen et al [1998] as consequence of the piston effect. The mairesults obtained by Chen et al [1998] indicate that thdistribution of velocity due to the piston effects into the roatunnel is not strongly influenced by the details of the traffiinside the tunnel. The maximum speed is obtained nearby thvehicles themselves. However the piston effect is not confinenearby the vehicles because a significant speed is usuallmeasured well above the vehicles near the top of the roatunnel. The horizontal homogeneity of the exit flow at th

portals is around 6% relatively to the flow at the centre of thportal. Long term acquisition of W and of the concentration athe centre of the portal show that the model is stable an

measured variations are well within experimental errors.A description of the model and of its dimensions i

reported in figure 4.The model is placed in the wind tunnel under a hill wit

one pipe of the road tunnel releasing upwind of the hill againsthe wind and the other releasing downwind of the hill in thsame direction as the ambient wind.


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strength of the ambient wind it is necessary to change thevoltage at the axial fans inside the model in order to keep Wconstant according to the different values of Uref. The behaviourof W at different voltage and different Urefis reported in figure6. These kind of measurements allowed to evaluate the voltageto be applied in order to keep W=2.9 m/s in all conditions. Thisof course does not mean that the concentration at the two

portals is constant, because the effective concentration isinfluenced by the dilution with fresh and/or polluted air asconsequence of recirculation from one pipe into the other andto the pushing of air into the pipe releasing downwind of the

hill. Concentrations at the portals have therefore to be usedcarefully if a reference concentration for normalisation isneeded

CONCENTRATION FIELD RESULTSIn figure 8(a) longitudinal profiles measured along the road

tunnel pipe axis are reported for different values of Urefdownwind of the hill. The exit speed W was equal to 2.9 m/sfor both portals. In figure 8(b) the results of measurementsalong the axis of the pipe releasing against the wind upwind ofthe hill are reported for the same physical situations.Downwind of the hill it is present a sudden change in the

profiles due to the mixing of the jet emitted from the portal

with the flow coming from above the hill. This is more or lesspronounced as function of the wind velocity. The general areaof dispersion is quite small especially at high speed as aconsequence of the increased dilution. Upwind of the hill the

penetration of the pollutant against wind is quite small and thisgenerates an area of high concentration just nearby the exit

portal. The upwind jet seems to be folded back and part of thepollutant is transported downwind passing above the hill. Thetransport of pollutant above the hill is also confirmed by thevertical concentration profiles reported in figure 7 where it is

clear that at only 7 cm from the portals tracer is present up tabout 30 cm from the ground (1.7 times the height of the hill).

In figures 9 and 10 are reported the results of ainvestigation carried out with W=2.9 m/s and Uref=3 m/s to puin evidence quantitatively the recirculation effect from one sidof the model into the other side. In figure 9(a) longitudinaconcentration profiles are reported downwind of the hill, and ifigure 9(b) upwind of the hill, for the three different situations:

1) standard situation (both pipes with ventilation on antracer injection);

2) the injection in the pipe releasing downwind off buwith ventilation on;

3) the injection and ventilation in the pipe releasindownwind off.

In figure 10 similar results are reported for the three differensituation in which the changes in tracer injection and ventilatiohave been made on the pipe emitting against the wind upwinof the hill..

Figure 9(a) shows that at the portal releasing downwind is still present a certain concentration even if the traceinjection in this pipe is turned off. This means that part of thtracer emitted against the wind from the upwind portal actually re-circulated downwind inside the model itself. If th

ventilation into the pipe releasing downwind is turned off it still present a certain concentration although much lower thathe previous mentioned case. This means that the ambient winalone is actually able to push part of the tracer emitted againthe wind inside the model. Figure 9(a) shows that changes ithe working regime in one side of the tunnel influence the otheside because of the different efficiency of the recirculatio

phenomena.Figure 10 illustrates the situations when the changes ar

applied to the pipe releasing upwind putting in evidence similar effect of the recirculation but the pushing effect of thambient wind clearly disappears.

Figure 4) Schematic of the internal set-up of the model. All dimensions are in millimeters.

Tracer injection

Tracer injectionTracer injection

Hill


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490

510

530

550

570

590

610

1 2 3 4 5 6 7 8 9 10

Run number

Cattheportal(ppm)

Downwind portalUpwind portalDownwind portal averageUpwind portal average

Figure 5) Concentration at the centre of the portals indifferent runs with W=2.9 m/s and Uref=3 m/s. Tracer flow-rate was nominally constant.

Figure 11 shows normalised concentration maps upwind of thehill and downwind putting in evidence the area interested in the

dispersion of pollutant and it also illustrates the asymmetrygenerated by the presence of the two-ways road tunnel with therecirculation phenomena. Each map has been normalised withthe concentration at the releasing portals.

1.3

1.5

1.7

1.9

2.1

2.3

2.5

2.7

2.9

3.1

3.3

10 11 12 13 14 15 16 17 18 19 20 21 22 23

Axial Fans voltage (V)

W(

m/s)

Upwind portal; Uref=0Downwind portal; Uref=0Upwind portal; Uref= 5m/sDownwind portal; Uref=5 m/s

Figure 6) Exit velocity at the centre of the portals asfunction of the fan voltage for two different wind speed Urefin the wind tunnel.

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

45.0

0 100 200 300 400 500Concentration (ppm)

Z(cm) Upwind

Downwind

Measurements at 7 cm from the portals

Figure 7) Vertical concentration profiles measured at thetwo exit portals. W=2.9 m/s and Uref=3 m/s.


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0

100

200

300

400

500

600

0 25 50 75 100 125 150

Distance from the portal X (cm)

C(ppm)

2 m/s

3 m/s

4.5 m/s

6 m/s

Downwind of the hill (a)

0

100

200

300

400

500

600

700

-100 -75 -50 -25 0Distance from the portal X (cm)

C(ppm)

2 m/s

3m/s

4.5 m/s

6 m/s

Upwind of the hill(b)

Figure 8) Longitudinal concentration profiles measured at ground level along the model axis for different values of U ref. (adownwind of the hill and (b) upwind of the hill. W=2.9 m/s.

0

100

200

300

400

500

600

0 25 50 75 100 125 150


C(ppm)

Standard

Downwind tracer off ventilation on

Downwind tracer and ventilation off

(a)

0

100

200

300

400

500

600

-50 -37.5 -25 -12.5 0


C(ppm)

Standard

Downwind tracer off ventilation on

Downwind tracer and ventilation off

(b)

Figure 9) Ground level concentration measured along the longitudinal axis of the model for different working conditions in thpipe releasing downwind of the hill. (a) Downwind of the hill. (b) Upwind of the hill.

0

100

200

300

400

500

600

0 25 50 75 100 125 150


C(ppm

)

Standard

Upwind tracer off ventilation on

Upwind tracer and ventilation off

(a)

0

100

200

300

400

500

600

-50 -37.5 -25 -12.5 0Distance from the portal X (cm)

C(ppm)

Standard

Upwind tracer off ventilation on

Upwind tracer and ventilation off

(b)

Figure 10) Ground level concentration measured along the longitudinal axis of the model for different working conditions inthe pipe releasing against the wind upwind of the hill. (a) Downwind of the hill. (b) Upwind of the hill.


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10 20 30 40 50 60 70 80

X (cm)

-30

-20

-10

0

10

20

30

Y(cm)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.400.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

-80 -70 -60 -50 -40 -30 -20 -10

X (cm)

-30

-20

-10

0

10

20

30

Y(cm)

0.00

0.05

0.10

0.15

0.200.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

(a)

(b)

Fig. 11) Normalized concentration maps. (a) downwind of the hill; (b) upwind of the hill. Uref=3 m/s and W=2.9 m/s.

CONCLUSIONS

The results obtained in the experiments described showthat the model built can reproduce with sufficient accuracy theknown details of the flow inside a road due to the piston effector the combined contribution of piston effect and internallongitudinal ventilation.

The wind external to the road tunnel influences the internal

flow, especially in the aligned configuration analysed in thispaper.Results show that a recirculation effect is present that

drives part of the pollutant released at one portal into the other pipe. This pollutant is therefore moved from one side of theroad tunnel into the other side passing under the hill. Asconsequence of the recirculation effect added to the influence


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of the external wind, the concentration at the portals placed atthe two sides of the road tunnel are different even if the exitspeed W at the portals of the model and the pollutant emissioninside the model are kept constant and equal in the two pipes ofthe road tunnel model.

The emissions released against the wind are rapidlystopped and folded back generating high level concentrationaround the exit portal of the road tunnel. Part of the pollutant issuccessively injected into the road tunnel itself and part istransported downwind passing above the hill and mixing withthe emissions released at the portal downwind.

REFERENCES

Carr E.L., Ireson R.G., Biltoft C., 1996, Analysis ofDispersion Characteristics and Induced Turbolent Flows NearRoadway Intersections from Mobile Sources, Presentation atthe 89th Annual Meeting & Exhibition, june 23-28, 96-WP86.03.

Chen T.Y., Lee Y.T., Hsu C.C., 1998, Investigations of piston-effect and jet fan-effect in model vehicle tunnels, Journal ofWind Engineering and Industrial Aerodynamics 73, pp. 99-110.

Contini D., Pasqualetti C., Massini M., Manfrida G., Corti A.,Bartoli G., Procino L., 2002, Studio della diffusione dicontaminanti gassosi emessi in gallerie stradali mediante un

modello fisico in scala ridotta, 7 Convegno Nazionale dIngegneria del vento - IN-VENTO-2002.

Ide Y., Ueyama S., Kobayashi K., 1987, "Wind tunnemodeling of gas diffusion from a road tunnel outlet", ThScience of the Total Environment, 59, 211-222.

Lepage M.F., Vanderheyden M.D., Davies A.E., Nadel C1996, Simulating Vehicle Emissions at the Exit of a VehiclTunnel, Presentation at the 89

thAnnual Meeting & Exhibitio

Nashville, Tennessee June 23-28.

Nadel C., Vanderheyden M., Lepage M., Davies A., Wan PGinzburg H., Schattanek G., 1996, "Physical modelling odispersion of a tunnel portal exhaust plume", Presentation at th89

thAnnual Meeting & Exhibition Nashville, Tennessee Jun

23-28.

Oettl D., Sturm P. J., Bacher M., Pretterhofer G., Almbauer RA., 2002, A simple model for the dispersion of pollutants froma road tunnel portal, Atm. Env. 36, pp. 2943-2953.

Panofsky H.A., Dutton J.A., 1984, "Atmospheric TurbulenceModels and Methods for Engineering Applications", JohWiley & sons.


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WIND TUNNEL MODELLING OF PEDESTRIAN WIND COMFORT AT A NEWBUILDING COMPLEX IN HAMBURG

B. LeitlMeteorological Institute at Hamburg UniversityBundesstrasse 55, 20146 Hamburg, Germany

M. SchatzmannMeteorological Institute at Hamburg UniversityBundesstrasse 55, 20146 Hamburg, Germany

MOTIVATIONThe process of urban planning and development requires a

number of environmental factors to be considered which arecritical with respect to living comfort and quality of live. Air

pollution, ventilation of built-up urban areas as well as aspectsof shading, thermal comfort and pedestrian wind comfort mayhave a significant impact on how people will accept forexample a new building complex. Also the "economical value"

of a building complex is affected by local environmentalfactors. Wind tunnel modelling can provide a very efficientway for predicting winds at pedestrian levels. Even within verycomplex urban structures the pedestrian wind comfort can beassessed safely using a combined model approach consisting ofsystematic flow visualization experiments and high resolutionflow measurements.

Within the scope of the structural design of a new buildingcomplex in the city center of Hamburg, a 'typical' pedestrianwind comfort study was carried out in the BLASIUS windtunnel at Hamburg University. Major aims of the project werethe identification of possible high wind areas at pedestrianlevels as well as an improvement of the aerodynamic design ofstructural details.

WIND TUNNEL BOUNDARY LAYER MODELLINGThe tests were carried out in the BLASIUS wind tunnel at

the Meteorological Institute of Hamburg University. Figure 1shows a sketch of the 16 m long conventional type boundarylayer wind tunnel. The facility consists of an air intake nozzlewith a contraction ratio of approx. 4:1, screens and modifiedStanden spires (Standen, 1972) at the entrance of the 11 m longtest section, a turn table and an adjustable double-ceiling alongthe entire test section. The cross section of the tunnel is 1.5 mwide and 1 m high. A radial-blow fan at the end of the testsection drives the tunnel and the wind speed can be preciselycontrolled between 0.2 m/s and about 12 m/s. Placing the wind

tunnel drive at the end of the wind tunnel test section avoids theflow disturbances generated by the fan affecting the flow in thetest section. The working section of the tunnel is equipped witha 3-axis stepper-motor controlled probe positioning systemwhich enables extensive sets of measurements to be carried outeffectively. Many different kinds of flow and dispersionmeasurement probes available in the laboratory can be

automatically positioned in the test section with an accuracy ofbetter than 0.1 mm.

For reference wind speed measurements a Pitot-tubeconnected to a differential pressure transducer is used. The

pressure transducer is calibrated against a fine pressure balance before and after each measurement campaign in order toeliminate any possible drift of the reference wind speed

measurements. For flow measurements a 2D Laser-Doppler-Anemometer (LDA) with a BSA burst processing unit from

Dantecwas used. The spatial resolution of the fiber-optic

probe applied is better than 1 mm, and two components of thelocal wind vector can be measured simultaneously with a datarate of at least several hundred Hertz. Although an LDAsystem does not need to be calibrated under normal operatingconditions, the accuracy of the system was checked using ascatter-disc calibrator.

In order to apply the wind tunnel results with confidence tofull scale conditions, the atmospheric boundary layer needs to

be replicated properly at model scale in the test section of thewind tunnel. The reference profile to be modelled in the windtunnel was derived from a field measurement station operated

by the Meteorological Institute at the Hamburg radiotransmission tower (see Pascheke et al, 2003) as well as fromthe guideline VDI 3783/12. For boundary layer modelling aspecific spires / floor roughness configuration was developed

by measuring the boundary layer flow in the test section,comparing the measured flow with the reference flow anditeratively improving the shape and the arrangement of spiresand floor roughness elements. Figure 2 shows the test sectionof the tunnel with modified Standen spires and floor roughnessfor modelling the essential wind and turbulence characteristicsat a model scale of 1:500. In Figure 3, the vertical mean wind

profile, turbulence intensity profile and the measured shearstress profile are plotted. After carefully adjusting the ceilingof the tunnel in order to minimize the longitudinal pressure

gradient in the test section, an almost ideal constant shear layercould be established in the test section of the tunnel. Inaddition, the spectral characteristics of the modelled turbulent

boundary layer were checked and found to be in goodagreement with full scale conditions for a model scale of 1:500.


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Figure 1: BLASIUS wind tunnel at the Meteorological Institute at Hamburg University.

Figure 2: Test section of the wind tunnel with optimized spires and floor roughness arrangement.


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Figure 4: Detailed physical model of the city district (red roof: new building complex, inner circle: core

model, outer circle: extended fetch).

Figure 5: Typical result of a sand erosion experiment for visualization of wind-critical zones (circles: areas

identified as critical with respect to pedestrian wind comfort).


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At the same point wind speed measurements were carried outwith and without noise protection screens located above themeasurement point. The effect of the noise screen increasing

pedestrian winds, can clearly be detected. Without theunfavourably designed noise protection mounted between the

building blocks the mean wind speed as well as the gustiness ofwinds at pedestrian levels was reduced significantly. In orderto improve the aerodynamic behaviour of the noise screens,several slightly modified noise screen structures were designed,tested in the model and suggested as replacements for theoriginal screens.

A final assessment of pedestrian wind comfort was givenbased on a number of wind comfort criteria which can be found

in literature. For instance in Baumller et al. (1993) a list ofthreshold values for winds speeds can be found. As listed inTable 2, critical wind speeds should be related to somemaximum allowable exceedance per year as well as to typicalzones of specific human activity. In the present study it wasfound that severe wind problems must be expected if the

building structure would be built as tested.

CONCLUSIONSPedestrian wind comfort in a complex urban area was

visualized and measured by means of physical modeling in a boundary layer wind tunnel. Building a detailed physicalmodel of the city district and carefully modeling a site-specific

boundary layer flow in the test section ensure a safe andreliable transfer of the model results to full scale conditions. Asand erosion technique was used to identify areas being

potentially critical with respect to pedestrian wind comfort. Ina second step of testing, a quantitative analysis of ground-levelwinds was carried out based on high resolution flowmeasurements using a 2D LDA system. Combining the resultsof flow measurements with the annual wind statistics of the siteenabled a reliable assessment of pedestrian winds in and around

the investigated structure to be given. In addition, a number ofaerodynamic corrections of the building complex wassuggested in order to assist the architects improving the overallvalue and environmental comfort in an around the newstructure.

Figure 6: Wind statistics and critical wind directions identified.


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Ui / Uref[ - ]

Uref= 5 m/s 8 m/s 12 m/s

Position A with noise protection screens

peak wind speed 1.97 9.86 15.78 23.67

minimum wind speed 0.04 0.19 0.3 0.45

mean wind speed 0.39 1.93 3.1 4.64

standard deviation of wind speed 0.35 1.76 2.82 4.22

gust wind speed 1.44 7.21 11.54 17.31

Position A without noise protection screens

peak wind speed 0.08 4.01 6.42 9.64

minimum wind speed 0.01 0.06 0.1 0.15

mean wind speed 0.24 1.2 1.92 2.89

standard deviation of wind speed 0.15 0.76 1.22 1.83

gust wind speed 0.7 3.49 5.59 8.39

Table 1: Exemplary results of wind speed measurements at 2m height above ground (full scale, wind

direction 160, gust defined as 10 seconds average)

gust wind speed allowable exceedancesin percent of time

assessment situation

< 6 m/s no problems related to wind comfort

> 6 m/s max. 5 % acceptable to pedestrian walk ways, shoppingareas, street cafes and play grounds

> 8 m/s max. 1 % acceptable to waiting / sitting areas

> 6 m/s> 15 m/s

max. 20 %max. 0.05 %

acceptable to areas quickly crossed by passers-by (relaxed criteria)

> 10 m/s max. 1 % acceptable to areas quickly crossed by passers-by (strong criteria)

> 13 m/s max. 1 % can be accepted at building corners

> 13 m/s > 1 % unpleasant / troublesome

WIND PROTECTION REQUIRED!

> 18 m/s potentially dangerousWIND PROTECTION REQUIRED!

Table 2: Assessment scheme adopted from Baumller et. al (1993).


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REFERENCES

Standen, N.M. (1972): A spire array for generating thickturbulent shear layers for natural wind simulation in windtunnels. Tech. Rept. LTR-LA-94, National AeronauticalEstablishment, Ottawa, Canada

Pascheke, F.; Leitl, B.; Schatzmann, M. (2003): Resultsfrom Recent Observations in an Urban Boundary Layer.Workshop on urban boundary layer parameterizations(Extended abstracts), COST Action 715, Office for official

publications of the European Communities, ISBN 92-894-4143-7.

VDI 3783/12 (2000): Physical modeling of flow anddispersion processes in the atmospheric boundary layer Application of wind tunnels. Part 12, Verein DeutscherIngenieure Clean Air Handbook, Vol. 1b

Baumller, J.; Hoffmann, U.; Reuter, U. (1993):Stdtebauliche Klimafibel: Hinweise fr die Bauleitung, Folge2, Hrsg. Witschaftsministerium Baden-Wrttemberg, 1993.


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Proceedings of PHYSMOD2003International Workshop on Physical Modelling of Flow and Dispersion Phenomen

3-5 September 2003, Prato, Ital

WIND TUNNEL EXPERIMENTS WITHIN THE SCOPE OF THEOKLAHOMA CITY TRACER EXPERIMENTS

B. LeitlMeteorological Institute at Hamburg UniversityBundesstrasse 55, 20146 Hamburg, Germany

F. PaschekeMeteorological Institute at Hamburg UniversityBundesstrasse 55, 20146 Hamburg, Germany

M. SchatzmannMeteorological Institute at Hamburg UniversityBundesstrasse 55, 20146 Hamburg, Germany

P. Kastner-KleinSchool of Meteorology, University of Oklahoma

100 E. Boyd, Norman, OK 73019, USA

ABSTRACTThe papers describes the basic concept of systematic wind

tunnel experiments carried out in support of an extensive field

measurement campaign, studying instantaneous dispersion in acomplex urban area. Wind tunnel results are used for anefficient and reliable planning of the measurements as well asin support of the interpretation of the results from a limitedamount of field data. The basic goals of accompanying windtunnel modelling are to increase the efficiency of timeconsuming and expensive field measurements as well as to gainfurther information on instantaneous flow and dispersion

phenomena in complex urban areas.

INTRODUCTIONAlthough the emissions from urban air pollution sources

have been significantly reduced in most industrializedcountries, urban air quality problems are still a matter of

concern. While dispersion from distributed urban pollutionsources can be modelled numerically in many cases withreasonable accuracy, the short-term behaviour of instantaneousemissions from ground level point sources within the urbancanopy is not yet completely understood. For the improvementof numerical codes and model validation, field data as well asresults of systematic laboratory experiments are needed. Inorder to generate a validation data set for dispersion modellingin complex urban terrain, an extensive set of tracer gasexperiments will be carried in Oklahoma City (USA) in thesummer of 2003. Within the scope of the 'Joint Urban 2003'experiments, releases from different point sources will besimulated using a passive tracer and the resulting plume will bemeasured by means of a variety of tracer gas sampling systems.

In addition, an extensive set of meteorological measurementswill be carried out simultaneously in order to quantify theboundary conditions during the tracer release periods.

MOTIVATIONThe emergence of increasingly powerful computer

stimulated the development of complex micro-scale flow antransport model. Numerical modelling plays an increasingimportant role in many practical environmental applicationsuch as urban air quality prediction or modelling accidentareleases of hazardous gases. The models are now commonlapplied to predict pollutant dispersion in complex structureurban canopy layers and use of such models is made, foinstance in the licensing of new industrial plants and in safetanalysis studies.

The model used vary in their level of complexity, ranginfrom simple obstacle accommodating or so-called diagnostimodels to prognostic model which employ not only masconservation but also the equations for momentum and energyWhile a number of models are already commercially availableothers are still treated as research tools. Independent of th

type of model they all contain a substantial amount of empiricinput, for example in the turbulent closure scheme applied.

The increasing use of numerical models is accompanied ba growing awareness that most of the model have never beetested in a procedure of careful and rigorous evaluation focomplex urban type dispersion problems. Nevertheless, thesmodels are used as a basis for making decisions with profouneconomic and social consequences.

Within the scope of the Oklahoma City field campaign, high-quality validation data set is generated, which should bsuitable for testing the 'fitness for purpose' of numerical modelin a unified and comprehensive way.

Validation data for development and testing of numericamodels are not just any experimental data. Validation da

must fulfil a solid set of requirements in order to be callecomplete, representative, sufficiently accurate and completeldocumented (Leitl/Schatzmann, 1999, Leitl 2000). If thesrequirements are not met, the freedom in the setup of numericamodel runs is likely to distort the results of a model test. It well known that a wide variety of numerical results can bgenerated within the limits of reasonable assumptions regardin


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input data. Consequently, a solid conclusion concerning thecapabilities of a model cannot be achieved as long as the degreeof freedom in setting up model test runs is minimized.

Field data are often designated as the one and only sourceof validation data. However, it must be considered that fieldresults as well as other experimental data have their specific

limitations. Results from field campaigns usually representunique situations with a complex set of boundary conditions.Appropriate recording of all essential boundary conditions isoften impractical because of the limitations in instrumentation.Changing boundary conditions like the constantly changingweather due to the diurnal circle can cause a large variation

Figure 1: The Large Boundary Layer Wind Tunnel "WOTAN" at Hamburg University.

Figure 2: Oklahoma City Model mounted in the test section of the wind tunnel upwind view withspires/roughness in front of the model.


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Figure 3: Oklahoma City model in the test section of the wind tunnel (downwind view, floor roughness

in front of the model, traverse system).

Figure 4: Exemplary result of Laser light sheet visualizations(horizontal light sheet, wind flow from the upper left corner

to the lower right corner of the image).


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even if the results are time averaged. As could be shown inlaboratory experiments and long-term field data sets, theremaining scatter in typical half-hour averages of measuredflow and dispersion quantities can vary by one order ofmagnitude. Consequently, the limited representativeness offield data limits their usefulness for comparison with the steadystate results delivered by most of the numerical dispersionmodels.

Another source of validation data is made availablethrough laboratory experiments in boundary layer wind tunnels.Although the wind tunnel modelling is limited in most cases to

neutral stratification and the results of physical modellingincorporate some simplification and abstraction from physicalreality, results of systematic wind tunnel experiments havesome major advantages over field data. Different levels ofcomplexity can be generated, ranging from single obstaclesituations to complex urban configurations containing allgeometric details. The boundary conditions can be preciselycontrolled and kept constant over a long period of time. Thus,it is possible to simulate the same steady-state situation ascalculated by most of the micro-scale flow and dispersionmodels. The accuracy of laboratory grade instrumentation andthe spatial and temporal resolution of up-to-date wind tunnelinstrumentation enable measurements to be carried out with atleast the same full-scale resolution as in field experiments.

Another important advantage of laboratory data is of course,that most of the physical boundary conditions of a test can becontrolled at will and that all of them can be measured withhigh accuracy.

To overcome a number of limitations from both validationdata sources it is wise to combine field and laboratory data.Supporting field measurement campaigns by correspondingwind tunnel experiments can add significant value to field data

because gaps of information inherent in any field data set can be filled by laboratory data. In addition, systematic windtunnel testing can help to plan extensive field campaigns moresafely. If a physical model of a complex urban structure isavailable, results from wind tunnel simulations can be used forexample to define what source locations will deliver the mostvaluable results depending on wind direction and release

condition. Wind tunnel measurements carried out in advancecan also used for planning the layout of instrumentation and thetiming of releases because all instantaneous behaviour of thedispersion process due to turbulence is modelled directly.

OUTLINE OF THE WIND TUNNEL EXPERIMENTSThe experiments are to be carried out in the new Large

Boundary Layer Wind Tunnel 'WOTAN' at HamburgUniversity (Figure 1). The 25 m long facility provides an 18 mlong test section equipped with two turn tables and anadjustable ceiling. The cross section of the tunnel is 4 m wideand 2.75 to 3.25 m high (variable ceiling). For probe

positioning and automated measurements, the tunnel has a

computer controlled traverse system with a positioningaccuracy of better than 0.1mm on all three axes for all types ofprobes used in the tunnel. An extensive custom made softwarepackage has been developed for automated and semi-automatedmeasurements, probe calibration and positioning, online datavisualization, data reduction and data validation.

In order to achieve a high accuracy of the wind tunnel data,all measurement systems as well as the precision mass flowcontrollers for emission sources are calibrated againstindependent certified standards available in the laboratory.

In the first step of the project, the atmospheric boundarylayer flow across Oklahoma City was replicated at a scale of1:300 in the test section of the boundary layer wind tunnel. Asa reference the results from local wind measurements inOklahoma City as well as from the University of Oklahoma(School of Meteorology) were applied. The boundary layerflow approaching the Central Business District was classified

as type B/C according to ANSI/ASCE 7-95. The large crosssection of the tunnel enables a large city area to be modelled.For a model area 1.8 km x 1.8 km all significant buildings andobstacles have been replicated in the model enabling a fetch ofabout 1 km to be considered in the wind tunnel model for eachwind direction. For the given model scale, the typicalgeometrical resolution of velocity measurements is about 0.6mm in the wind tunnel which corresponds to about 18 cm atfull scale conditions. For concentration measurements, thesampling area is about 0.2 mm in diameter in the wind tunnel,which corresponds to about 5 cm in diameter in full scale.

For boundary layer modelling, a conventionalspires/roughness set up is used. In an iterative process theshape and arrangement of the spires and the floor roughness

were gradually optimised until agreement with the requiredfull-scale conditions was reached. The proper scale of the windtunnel boundary layer was documented by high-resolution flowmeasurements. The mean wind profiles as well as integrallength scales and spectra of turbulence were found to be ingood agreement with full-scale conditions. In addition it can beshown, that a careful adjustment of the boundary layer enableseven large scale wind fluctuations up to a time scale of about 1hour are replicated by the model boundary layer flow. Thecomplete documentation of the modelled boundary layer flowconsists of:

documentation of the spatial and temporal homogeneityand stability of the approaching flow in the test section

representative vertical and lateral mean flow profiles (all

three components of the wind vector) representative vertical and lateral profile of turbulence

intensities (all three components of the wind vector)

representative vertical and lateral profiles of turbulentmomentum fluxes of the approaching flow

representative integral length scales for different heightsabove ground

representative power spectral density for all threecomponents of the wind vector

complete temporal and angular analysis of the winddirection fluctuations inherent in the modelled boundarylayer flow.

Once the geometric scale of the model boundary layer was

defined, a detailed aerodynamic model of the city district of thearea to be covered in both field measurements and numericalsimulations was constructed. Figure 2 and Figure 3 shows themodel mounted in the test section of the wind tunnel. Theextended fetch around the core model allows an assessment ofthe effects on calculated results due to the limited size of thearea modelled for example in a numerical model. Suitable


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point source were located in the model at all locations possiblyused for tracer gas releases. The point source design enablessmoke releases to be used for flow visualization experiments.

A first measurement campaign in the wind tunnel wasconducted concerning mainly with flow visualizationexperiments. The results from Laser light sheet experimentsare assisting the set up of the field experiments. The overall

behaviour of the plumes formed at different release locationswas documented by taking still images as well as recordingvideo sequences. The latter was of special interest to the fieldmeasurement groups because the highly instantaneous

behaviour of the plumes as well as the big scatter of theemissions due to turbulent fluctuations will lead to a significantscatter in field data even for similar and stationary boundaryconditions. Figure 4 shows a exemplary result of the Laserlight sheet experiments

In a further step, the instantaneous behaviour of the tracer plumes resulting from different sources was measured fordifferent wind directions. In order to quantify the concentrationfluctuations to be expected during full scale experiments longtime series of concentration fluctuations were recorded atseveral measurement points all across the different plumes. Anoff-line analysis of the time series enabled all statistical

parameters as well as the intermittency and the mean time ofthe presence of the plume at different measurement locations to

be documented. The results show that the resulting plumes areclearly affected by the high-rise building structures and the axisof the plume was not found to be parallel with the mean winddirection. Measurement points with a high intermittency factorcould be found all across the model area. From this, a bigscatter in the field data thus is expected.

Further experiments are planned to simulate the JoinUrban 2003 field experiments in order to prove the ability othe wind tunnel model to replicate all essential features of thdispersion processes. The main part of the project will be aextensive set of high-resolution flow and dispersiomeasurements. The intension is to measure flow anconcentration patterns in several horizontal and verticmeasurement planes throughout the entire core model. Thresulting flow and dispersion maps will enable a qualitative anquantitative comparison of global measurements witcorresponding results from numerical modelling.

ACKNOWLEDGMENTSThe support by staff of the Department of Geosciences a

the University of Oklahoma is gratefully acknowledged.

REFERENCES

Leitl, B.; Schatzmann, M. (1999): Generation of HigResolution Reference Data for the Validation of Micro-Scalmodels. published in: Recent Developments in Measuremenand Assessment of Air Pollution, VDI-Komission Reinhaltunder Luft, Report 1443, ISBN 3-18-091443-2, pp. 647-656

Leitl, B. (2000): Validation Data for Microscale DispersioModelling. EUROTRAC Newsletter, 22. pp. 28-32


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DEVELOPMENT OF VERTICAL TURBULENT FLUXES OVER AN IDEALISED URBANROUGHNESS

Schultz M.Div. of Technical Meteorology

Meteorological Institute, University of Hamburg

Dr. Leitl B.Div. of Technical Meteorology


Schatzmann M.Div. of Technical Meteorology


ABSTRACTSystematic measurements above a regularly arranged array

of cubes with h = 25 mm height were performed in the wind

tunnel BLASIUS of Hamburg University.In a first series of measurements the spacing of the cubeswas changed from 0.5h over 1h to 2h. The experiments werecarried out without using vortex generators in the wind tunnel.It was intended to study solely the influence of the bottomroughness on the flow development. During the secondmeasurement campaign the cube distance was 2h all the time,

but the length of the array varied from 38 rows over 45 rowsand 65 rows up to 85 rows. Again no spires were used. Theexperimental programme was rounded up by a third series ofmeasurements in which the cubes were approached by a

boundary layer generated in the common way by means ofvortex generators and a ground roughness. Cube spacing herewas 2h, the number of rows was 38.

Subsequently the results will be presented and compared

with each other.

NOMENCLATURE

d0 : = displacement heighth : = height of roughness elements

: = von Karman constantu, v, w : = components of wind speed along x,y, and

z axis, respectively.u* : = friction velocityx, y, z : = coordinates of an rectangular Cartesian

system with x-axis defined in direction ofmean wind.

z0 : = roughness length

INTRODUCTIONIn the last decades more and more studies in the field as

well as in the wind tunnel deal with the examination of the

urban boundary layer. But in contrast to the rural boundarylayer that is comparatively well understood, urban boundarylayer concepts are not yet settled. A detailed review of results

from field measurements carried out within and above theurban canopy layer can be found in the paper by Roth (2000).According to him, the boundary layer over an urban structurecan be subdivided into two layers, the inertial sublayer and theroughness sublayer. Inside the inertial sublayer (hereafterdenoted by IS), the turbulent fluxes are assumed to be constantwith height, and under neutral conditions the usual logarithmiclaw applies:

=

0

0* lnz

dzuu

(1)

The roughness sublayer (hereafter denoted by RS) lies beneath the IS. Inside the RS turbulent fluxes and all other boundary layer properties are assumed to be influenced byindividual roughness elements. Therefore, the turbulent fluxesare neither spatially homogeneous nor is the logarithmic lawapplicable. The height of the RS is still subject of debate but itis in the range of 2 to 5 times the average building height(Raupach 1991, Roth 2000). Rotach (1999) states that the RSover a very rough surface can occupy a substantial portion ofthe surface layer.

After a roughness change an internal boundary layerdevelops that is influenced by the new roughness. This internal

boundary layer is growing with fetch. Inside the internalboundary an equilibrium layer is forming consisting of the RSand IS. Above the internal boundary layer lies a transition

region, which is covered by the outer boundary layer. Chengand Castro (2002) found in their study that a growingequilibrium layer behind a step change in roughness first has tomodify the RS before a new equilibrium IS can develop.

Task of this study was to investigate up to which height itis possible to model a boundary layer that is representative for


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the underlying roughness. Second task was to investigate thecharacteristics of this boundary layer for different types of cubearrays, e.g. the heights of the RS and IS etc..

EXPERIMENTAL SET-UPAll measurements were carried out in the small boundary

layer wind tunnel Blasius of the Meteorological Institute.This wind tunnel has a test section, which is 1.5 m wide, 1mhigh. The usable length of the flow establishment and testsection is 17.5 m. The wind tunnel is equipped with an

adjustable double ceiling (see fig. 1). During the twomeasurement campaigns an array of regularly arranged sharpedged wooden cubes with cube height of h = 25 mm was put inthe test section of the wind tunnel. Above the cube array

profiles were measured with a LDA fiber probe of Dantec.The probe had a focal length of 50 mm and a measuringvolume of dx = 0.121 mm, dy = 0.122 mm and dz = 1.151 mm.The measured wind components were u and w for all profiles.

16 mintakehoneycombs

double ceilingadjustable

blowerfl owdirection

screens boundary layer development section(7.5ml ong)

DCmotor

variable speed

test section1.5mwide,1mhigh,4mlongturntable

Figure 1. Schematic view of the small wind tunnel of theMeteorological Institute of Hamburg University.

The first measurement campaign was performed fromDecember 2002 to January 2003. To represent an idealizedurban roughness a regularly arranged array of cubes with size h= 25 mm was positioned in the test section of the wind tunnel.During this campaign the array of cubes consisted of 38 rows.After 32 rows a small array in the centre of the tunnel with size3 x 3 cubes was painted black in order to minimize reflectionsduring the LDA measurements. The last three rows were

placeholders to guarantee undisturbed measurements inside theblack array. Three different configurations were measured witha a spacing between the cubes varying from 0.5h, over 1h to 2h.Figure 2 shows the set-up for the 0.5h-configuration.

For all three configurations the following measurementswere carried out:

1. A profile was measured upstream of the cube arrayto determine the characteristics of the approachflow.

2. Along the centre line of the whole cube array profiles were measured between 30 and 390 mm

height with a vertical resolution of 40 mm and ahorizontal distance of 50 mm. This should give afair representation of the development of the flowabove the cube array with increasing fetch.

3. Above the black array the density of measurement points was increased to 21 spatially distributed

profiles each with 29 Points between 26 mm an390 mm height.

Figure 2. Set-up for the configuration 0.5h

Figure 3. Set-up for the cube distance 2h and the extendedarray of 40 rows.

During all measurements no spires were used in the wintunnel. The flow development was solely governed by throughness elements. The Reynolds number criterion for full


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rough flows given in Snyder and Castro (1998) was wellfulfilled in all experiments.

In the second campaign, only experiments with the cubespacing 2h were carried out, again without spires as in the firstcampaign. Task in this part of the campaign was to extend thenumber of cube rows behind the black array from 3 to 10 inorder to surely exclude possible upstream effects from changein roughness at the end of the array. Subsequently, the fetchwas extended by adding 20 rows and then 40 rows in front ofthe (black) intensive measurement array (figure 3). Again

profiles above the centreline were measured, but compared tothe first campaign the vertical resolution was reduced to 10 mmon the expense of the horizontal distance that varied now

between 150 and 300 mm. Above the black cube array onlyfour spatially distributed profiles were measured, but with thesame resolution and position as in the first campaign.

Finally, in a third series of experiments the short array ofcubes consisting of 32 rows in front of the black field and acube spacing of 2 h was used again, but now the approach flowwas fully turbulent. Spires were installed at the entrance of theflow establishment section and staggered arranged cubes withdistance 6h served as roughness elements. Again Profiles above

the centreline were measured between the heights of 30 and390 mm with a vertical resolution of 10 mm. The horizontaldistance between these profiles was 300 mm. In the blackintensive measurement field 4 profiles at the same positions asin the first part were measured.

RESULTS AND DISCUSSIONOnly results from the first measurement campaign are

given here. The data from the second and third series ofexperiments are presently being processed. They will be

presented at the conference. For further details see Schultz(2003).

A proper equilibrium boundary layer should becharacterised by fully developed mean velocity and turbulence

intensity profiles. Since after a roughness step the height up towhich equilibrium is to be expected should grow with fetch, itwas worthwhile to document the development of flow

properties with downstream distance for selected heights abovethe elements. Figs. 5a, b and c show that at the example ofturbulence intensities measured along the centreline of the testsection for the three different cube arrays.

As can be seen in Fig. 5a, for the cubes spaced by 0.5h aconstant turbulence intensity is found in 70 mm above groundat a distance of X= 1000 mm whereas at height levels z = 110mm and z= 150 mm the turbulence intensities are stillincreasing. For the less dense roughness arrays (1h and 2h) thefetch needed to achieve equilibrium 70 mm above groundincreases to 1200mm and 2000 mm, respectively (Figs 5b and

c). Again, at higher elevations equilibrium was not yetobtained.

Figure 4. Set-up for the experiments with a fully turbulentapproach flow. Cube spacing was 2 h, spires were used inthe flow establishment section.

Results from turbulent flux measurements above the (blackpainted) intensive measurement section are presented in Fig. 6.Shown are numerous vertical uw profiles taken at differenthorizontal positions relative to the cubes. Adopting the

procedure applied in Cheng and Castro (2002), an averagemean profile (white points) was calculated which was then usedto determine the roughness sublayer and the inertial sublayer.Cheng and Castro defined the upper limit of the RS as theheight at which all flux profiles are converging. The verticalextent of the IS is defined as the region within which thevertical variation of the spatially averaged profile is below 5%.Other possible ways to determine the IS like fitting thelogarithmic law to the spatially averaged profile are lessstringent but give similar results (see Schultz 2003). The greydashed lines indicate the scatter.

For all three cube arrays the RS extends up to a height of38 to 40 mm which corresponds to 1.5 h. The upper edge of theIS for the cube arrays with spacing 0.5h and 1h reaches 60 mm,

whereas for the low density array (spacing 2h) the IS height isonly 48 mm.


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a)

X [ mm ]

urms/u

0 500 1000 1500 20000

0.05

0.1

0.15

0.2

0.25

Z = 70 mm

Z = 11 0 mm

Z = 15 0 mm

height

0.5harray ofcubes

b)

X [ mm ]

urms/u

0 500 1000 1500 2000 2500

0.01

0.03

0.05

0.07

0.09

0.11

0.13

0.15

0.17

0.19

0.21

0.23

0.25

Z = 7 0 mm

Z = 1 10 m m

Z = 1 50 m m

1h arrayof cubes

height

c)

X [ mm ]

urms/u

0 1000 2000 3000

0.01

0.03

0.05

0.07

0.09

0.11

0.13

0.15

0.17

0.19

0.21

0.23

0.25

Z = 7 0 mm

Z = 1 1 0 mm

Z = 1 5 0 mm

2h arrayof cubes

height

Figure 5a)-c). Development of turbulence intensities atlevels 70mm, 110mm and 150 mm above ground for

roughness arrays with cube spacing of 0.5h (a), 1h (b) an2h(c). The error bars indicate the experimental scatter.

COMPARISON OF SPATIALLY AVERAGED PROFILEFOR DIFFERENT CUBE DISTANCES

Fig. 7 shows spatially averaged profiles for the three cubarrays for other flow characteristics, i.e. the horizontacomponent of mean velocity u (Fig. 7 a) and the verticcomponent of mean velocity w (Fig. 7 b). Fig. 7 c replicate

Fig.6 but shows only mean turbulent fluxes - '' wu as a functioof roughness spacing and normalized height z/h.The profiles of the mean wind speed u for the cube array

0.5h and 1h have the same shape, but the profile of 1h islightly shifted to lower wind speeds. The gradient for thestwo profiles is the same. The profile of the cube distance 2h haa different gradient in wind speed up to the level z= 7h. Abovthis level all three profiles lie within the scatter band. Thindicates, that free stream conditions have been reached at thlevel.

The shape of the three w-profiles for the 0.5h, 1h and 2arrays are similar with the tendency of higher values for lesdensely packed cube arrays. However, the measured values arnear to detection limit of the LDA instrument and lie more oless within the scatter band.

The turbulent flux profiles (Fig 7 c) for 1h and 2h fanearly on top of each other whereas those for roughnesspacing 0.5 h are smaller, above all near to the roughneselements. It appears that approach flow conditions are founabove z = 6 h.

SUMMARIZING AND CONCLUSIONFor the investigated cube arrays a fully develope

equilibrium layer was found up to a level of 70 mm. Thinertial sublayer was found to be more or less independent othe cube spacing. The cube arrays 0.5h and 1h had a versimilar influence on the mean velocity components. Thesrather densely packed roughnesses behaved more like a san

roughness whereas the cube array with 2h spacing showefeatures of individual obstacles. With respect to turbulencthere were more similarities between the cube arrays 1h and 2hBoth arrays generate more turbulence through the individuaobstacles than was found in the experiments with the arra0.5h. Also, the turbulence generated by cube array 2h reacheto higher elevations than the others.

However, before final conclusions can be made, thfindings should be consolidated by experiments with one or twadditional roughness arrays.


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a)

- u'w'/ u ref

Z

[mm

]

0 0.001 0.002 0.003 0.004 0.005 0.006 0.0070

10

20

30

40

50

60

70

80

90

100

Roughness sublayer

Inertialsublayer

cubespacing 0.5h

b)

- u'w' / u ref

Z[mm

]

0 0.001 0.002 0.003 0.004 0.005 0.006 0.0070

10

20

30

40

50

60

70

80

90

100

Roughness sublayer

Inertialsublayer

cubespacing 1h

c)

- u'w' / u ref

Z[mm

]

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007

0

10

20

30

40

50

60

70

80

90

100

Roughness sublayer

Inertialsublayer

cubespacing 2h

Figure 6a)-c). Normalized turbulent flux profiles abovecube arrays with cube spacing 0.5h (a), 1h (b) and 2h (c).For further explanations see text.

a)

u / u r ef

Z/

h

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

1

2

3

4

5

6

7

8

9

0.5h

1h

2h

cube spacing

b)

w/ uref

Z/h

-0.05 -0.04 -0.03 -0.02 -0.01 0 0.01 0.02 0.03 0.04 0.050

1

2

3

4

5

6

7

8

9

0.5h

1h2h

cubespacing

c)

u'w' / u ref

Z/h

0 0.0025 0.005 0.00750

1

2

3

4

5

6

7

8

9

0.5h

1h

2h

cubespacing

Figure 7a)-c). Comparisons of spatially averaged Profilesover cube arrays with cube spacing 0.5h (a), 1h (b) and 2h(c). Error bars indicate the scatter band.


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REFERENCES

Cheng,H. , Castro,I. 2002: Near wall flow developmentafter a step change in surface roughness, Boundary LayerMeteorology 105, 411-432

Cheng,H. , Castro,I. 2002: Near wall flow over urban-like roughness,Boundary Layer Meteorology 104, 229-259

Feigenwinter,C. , Vogt,R. , Parlow,E. 1999: VerticalStructure of Selected Turbulence Characteristics above anUrban Canopy, Theoretical and Applied Climatology 62, 51-63

Hgstrm,U. , Bergstrm,H. , Alexandersson, H. 1982:Turbulence characteristics in a near neutrally stratified urbanatmosphere,Boundary Layer Meteorology 23, 449-472

Kastner-Klein,P 2001: Overview of near-surfaceturbulence parametrisations, COST Action 715 workshop onurban boundary layer parametrisations, extended abstracts,Zurich, Switzerland, 31-40

Macdonald R.W. 2000: Modelling the mean velocityprofile in the urban canopy layer, Boundary Layer Meteorology97, 25-45

Macdonald,R.W. , Carter,S. , Slawson, P.R. 2002:Physical modeling of urban roughness using arrays of regularroughness elements, Water, Air and Soil Pollution: Focus 2,541-554

Macdonald,R.W. , Griffiths,R.F. , Hall,D.J. 1998: Acomparison of results from scaled field and wind tunnelmodeling of dispersion in arrays of obstacles, AtmosphericEnvironment, 32, 3845-3862

Oikawa,S. , Meng,Y. 1995: Turbulence characteristicsand organized motion in a suburban roughness layer, BoundaryLayer Meteorology 74, 289-312

Raupach,M.R. , Thom,A.S. , Edwards,I. 1979: A windtunnel study of turbulent flow close to regularly arrayed roughsurfaces,Boundary Layer Meteorology, 18, 373-397

Rotach, M.W. 1993a: Turbulence close to a rough urbansurface part I: Reynolds stress, Boundary Layer Meteorology,65, 1-28

Rotach, M.W. 1993b: Turbulence close to a rough urbansurface part II: Variances and Gradients, Boundary LayerMeteorology, 66, 75-92

Rotach, M.W. 1995: Profiles of turbulence statistics inand above an urban street canyon, Atmospheric Environment,29, 1473-1486

Rotach, M.W. 1999: On the influence of the urbanroughness sublayer on turbulence and dispersion, AtmosphericEnvironment, 33, 4001-4008

Roth,M. 2000: Review of atmospheric turbulence overcities, Quart. J. Roy. Meteorol. Soc., 146, 941-990

Roth,M. ,Oke,T.R. 1993: Turbulent transfer relationshipsover an urban surface. I: Spectral characteristics, Quart. J. Roy.Meteorol. Soc., 119, 1071-1104

Schultz, M. 2003: Development of turbulent fluxes over anidealized urban roughness. Diploma thesis at the

Meteorological Institute at the University of Hamburg.Snyder, H.S., and Castro, I.P. (1998) Surface roughness in

laboratory modelling - how rough is rough ? Proc. 4 th UKConference on Wind Engineering, pp. 83-88.

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