dispersion from courtyards and other enclosed spaces

*Corresponding author. Tel.: 00 44 192 366 4713; Fax: 00 44192 366 4779

Atmospheric Environment 33 (1999) 1187—1203

Dispersion from courtyards and other enclosed spaces

D.J. Hall*, S. Walker, A.M. Spanton

Building Research Establishment Ltd., Garston, Watford, Herts WD2 7JR, UK

Received 19 August 1997; received in revised form 2 August 1998; accepted 4 August 1998

Abstract

The paper describes small scale wind tunnel experiments on the dispersion of contaminants discharged from thebottom of courtyards and other enclosed spaces. The experiments covered a range of courtyards with ratios of depth towidth from 5 (consistent with light wells and other very deep cavities) down to 0.1 (consistent with shallow enclosedsquares and piazzas that are frequently found in the urban environment). A variety of parameters within these shapeswere investigated, including the depth of the walls around the courtyard, the presence of openings in the walls, the effectsof wind direction, the presence of surface clutter and stratification of the air in the courtyard. In general courtyards arepoorly ventilating spaces and high residual concentrations of any discharged contaminant occur. However, thisbehaviour may also be used to advantage in using courtyards as a method of modifying the local climate. The effects ofthe parameters investigated on contaminant concentrations were complex and quite variable, which allows for thepossibility of designing courtyards to suit specific needs. ( 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Pollution; Dispersion; Courtyards; Enclosed spaces; Environmental control

Notation

A H.!9

/¼, aspect ratio of courtyardA

$area density, Proportion of ground surfacecovered by obstacles

C mean concentration of tracerd surface displacement, appearing in the usual

rough wall velocity profile equation [( ) )]D thickness of courtyard walls (see Fig. 3)F buoyancy flux, Eqs. (4) and (5)g acceleration due to gravityH height above groundH

.!9maximum height of courtyard, also used asreference height for windspeed

K dimensionless mean concentration, Eq. (2)q upward heat flux from surface or heat loss due

to evaporative coolingQ rate of discharge of tracer, Eq. (2)S stratification parameter, Eq. (7)º windspeedu longitudinal turbulence intensityº

3%&windspeed at height H

.!9, the height of the

courtyardº

*friction velocity

» volume flow rate of fluid in definition of buoy-ancy flux, F in Eq. (5)

¼ width of courtyard (courtyards were square inthe present experiments)

z0

aerodynamic roughness length, appearing inthe usual rough wall velocity profile equation,

º

º*"

1

iln A

z!d

zoB (1)

1352-2310/99/$— see front matter ( 1999 Elsevier Science Ltd. All rights reserved.PII: S 1 3 5 2 - 2 3 1 0 ( 9 8 ) 0 0 2 8 4 - 2

Fig. 1. Diagram of dispersion modelling wind tunnel.

o!

ambient fluid density*o fluid density difference from ambient

1. Introduction

Courtyards are a common architectural form. Theyare used for the generation of private spaces, improvingthe ingress of light and control of the local microclimateat almost all latitudes. In Northern climates they are usedas wind shelters and sun-traps and in Southern climatesfor shelter from the sun and the reduction of meantemperatures, for example by the evaporation of water. Ifthe definition of ‘courtyard’ is expanded somewhat totake in other enclosed spaces it can include deep lightwells and relatively shallow piazzas, largely enclosedsquares and the private spaces (usually gardens) formedby outward-facing buildings set in squares. On this basisthe enclosed square, courtyard or light well must be one ofthe most common features of the built environment.They are described here under the generic term of ‘court-yard’.

Despite this ubiquity, there seem to have been fewinvestigations of the ventilation of courtyards and en-closed spaces generally. An interrogation of the 9000 pluspapers and reports in the AIVC database yielded onlyabout a dozen references. There were some descriptionsof the use of courtyards for generating modified microcli-mates, for example Ahlemiddi (1991) and Al Azzawi(1991). There was also some work on the ventilation ofcourtyards. For example Bensalem and Sharples (1989)measured ventilation rates of courtyards and atria usingsmall orifice plate devices and Walker et al. (1993) and

Shao et al. (1993) investigated the use of CFD for deter-mining ventilation rates and used smoke for determiningthe clearance rates of contaminants in wind tunnel modeland full scale courtyards.

The two main concerns in courtyard ventilation are,firstly, the direct rate of ventilation in terms of theremoval of unwanted contaminants and, secondly, thegeneration and retention of a microclimate. The require-ments are, of course, opposites, one requiring a maxi-mised rate of dispersion of any discharged contaminants,the other preferring a minimum. There appeared, how-ever, to have been no direct measurements of the ventila-tion of courtyards, or its equivalent, the dispersion ofdischarged contaminants from within them.

The present paper describes some direct measurementsof dispersion from courtyards, using trace gases in smallscale wind tunnel models. There are a large number ofvariables of interest and it has not been possible toinvestigate all of them thoroughly here. However thework presented here covers enough of these in sufficientdetail to reveal the essential character of courtyardventilation.

2. Details of experiments

The description of the experimental procedures havebeen kept relatively brief here. A more detailed discussioncan be found in Hall et al. (1995).

The experiments were carried out in the BRE disper-sion modelling wind tunnel at its Cardington laboratory.A diagram of the wind tunnel is shown in Fig. 1. Theworking section is 22 m long by 4.3 m wide and 1.5 m

1188 D. J. Hall et al. / Atmospheric Environment 33 (1999) 1187—1203

Fig. 2. Wind tunnel boundary layer — velocity and longitudinal turbulence profiles.

Fig. 3. Diagram of courtyard showing notation and terminology.

high. The forward part of the wind tunnel is used togenerate a thick rough wall boundary layer, using a mix-ture of Counihan’s(1969) turbulence generators plusa long fetch of rough surface. In the present experimentsthe boundary layer was grown over an array of 10 mmsquare by 20 mm high roughness elements on a 50 mmspacing, to produce a roughness length, z

0, for the

boundary layer of about 2 mm. A velocity and longitudi-nal turbulence intensity profile for the undisturbedboundary layer (measured with a pulsed wire anemo-meter) is shown in Fig. 2, it had a depth of about 1 m.Most of the experiments were carried out with a wind-speed of 1.5 ms~1 at 100 mm height, but some involvingstratified flows used windspeeds down to 0.3 ms~1 inorder to obtain the required levels of stratification.

The notation and terminology for the courtyards isgiven in Fig. 3, which shows a ‘folded out’ diagram of theinner walls. The ‘courtyards’ were constructed fromblocks of 100]100 mm plan section in a range of heightsfrom 10 to 100 mm. The blocks were close fitting whenstacked together, so that there was no significant leakagebetween them. A diagram of a typical courtyard layout inthe wind tunnel is shown in Fig. 4. The courtyards werealways of square planform, but varying depth, the ratio ofheight to width (H

.!9/¼, defined here as the courtyard

aspect ratio, A) ranging between 0.1 and 5. It was notpossible to achieve this range of aspect ratios witha single planform size between the constraints of remain-ing well within the wind tunnel boundary layer andhaving an acceptable minimum depth for practical pur-poses. Two courtyard plan dimensions were thereforeused, of 100mm]100 mm for the deeper courtyards, upto 500mm depth, and 200mm]200 mm for the shal-lower courtyards down to 20 mm depth. Overlapping

experiments with the same aspect ratio in both courtyardsizes, to be described later, showed no significant differ-ences in their behaviour. The thickness of the walls sur-rounding the courtyard, D, was normally 100 or 200 mm,

D. J. Hall et al. / Atmospheric Environment 33 (1999) 1187—1203 1189

Fig. 4. Diagram of a ‘Courtyard’ set up in the wind tunnel.

giving a value of D/W of 1.0. However, some experimentsinvestigated higher values of D/W.

Dispersion measurements were made using a dilutedmethane tracer (of 2.5% in air, with nearly neutral buo-yancy) discharged uniformly through the floor of thecourtyard at a fixed rate, as in Fig. 4. Typical tracerdischarge rates were low, about 0.1 lm~1, so that theefflux velocity from the courtyard floor did not directlyaffect the airflow in the courtyard. The dispersed tracerwas sampled through 1 mm bore tubing to a flameionisation detector system (FID) which was speciallydesigned for this type of work. It uses three FID’s run-ning through selector valves so that a large number ofsampling points can be used. A fourth FID constantlymonitors the background level of tracer so that this canbe subtracted from the measurement in progress. Sampleaveraging times were of two minutes duration, sufficientto give a stable mean concentration. Experiments arequality controlled by a number of cross checks, includinga mass balance measurement on a free plume for whichthe tracer emission rate and the integrated flux of concen-tration should agree, which checks out the whole system.Agreement was within 5.5% in the present experiments.Also the first measurement of each test uses the samesampling point for all three FID’s, which must agree.Overall accuracy of measurement of trace gas concentra-tion is about 10%. The system is more fully described inHall et al. (1995).

Trace gas concentrations were measured at about 20sampling points (the number depending upon the depth

of the courtyard). One measurement was set in the centreof the courtyard floor, on a metal disc of 20 mm diameterand the remainder up the midlines of the inside walls andon the roof of the courtyard. A typical sampling layout isalso shown in Fig. 4. All concentration measurementswere non-dimensionalised in the usual form of a dimen-sionless concentration, K, defined here as

K"

Cº3%&¼2

Q(2)

Given a value of K for the experiment, the concentra-tions of contaminants at full scale can be determined byproviding appropriate values of the other variables.

In the experiments with stratified flow in the court-yards, either buoyant or heavier-than-air gases wereadded to the trace gas to generate unstable or stablestratifications. This is discussed in more detail in Section4.4, which deals with these cases.

Eq. (2) is that usually used to define the dimensionlessconcentration, K, based on dispersed contaminant con-centration, C, from a release at a discharge rate Q. How-ever, the other interest in the dispersion characteristics ofcourtyards noted in the introduction is related to thedispersion of heat and its effect on the air temperature inthe courtyard. In this latter context, Sanchez and Alvarez(1997) have noted that the analogy of Eq. (2) for thispurpose is

K "

oaC

1(¹!¹

!)º

3%&¼2

q. (3)


Fig. 5. Velocity and longitudinal turbulence intensity profiles through centre of courtyard. Courtyards of varying depth Set normally tothe wind. Wind direction 0°.

Thus values of K obtained from the dispersionof tracers (as in the present experiment) can be usedto predict the temperature rise, ¹!¹

!, due to a heat

flux q.

3. Velocity profiles through courtyards

Velocity profile measurements were made up thoughthe centre of the basic courtyards using a pulsed wire


Fig. 6. Sketches of the Three types of flow patterns in court-yards of different depth. The sketches are not to scale and theseparations have been exaggerated.

anemometer. This records both positive and negativevelocities and also produces a reliable measurement oflongitudinal turbulence intensity through the recirculat-ing flow in the courtyard where reversed flow occurs. Theresults of the measurements are shown in Fig. 5, forcourtyards set square to the wind. The profiles fellnaturally into three groups, which are plotted separately.These were, shallow courtyards with low aspect ratios (A"H

.!9/¼"0.1 and 0.2, the top plot), courtyards with

intermediate aspect ratios (0.3—1.0, the centre plot) anddeep courtyards with high aspect ratios (1—5, the bottomplot). Both mean velocity (the left hand profile) andlongitudinal turbulence intensity (the right hand profile)are shown. All heights are scaled relative to the courtyarddepth, H

.!9, and velocities and turbulence intensities are

scaled relative to the undisturbed windspeed, º3%&

, atheight H

.!9.

The two low aspect ratio courtyards, in the upper plot,showed mean velocity profiles with reduced velocitiesnear the ground but little or no reversed flow. The meanvelocity profiles for the intermediate depth courtyards, inthe centre plot, all collapsed on to the same curve, withnegative velocities near the ground becoming steadilymore positive up through the courtyard into the freestream. There was a sharp change in the profile gradientat the edge of the free shear layer over the top of thecourtyard, above which the velocity remained nearlyuniform. The deep courtyards, in the bottom plot,showed nearly zero or slightly negative mean velocitiesup through the courtyard until the region close to itsopening when the mean velocity became negative andthen sharply positive out through the top of the court-yard. Above the free shear layer the velocity was thenalmost uniform.

The turbulence intensities in Fig. 5 showed high levelsfor the shallow courtyards and levels that increased withheight for the intermediate and deep courtyards. In thelatter cases the maximum turbulence intensities occurredjust above the courtyard opening at the edge of the shearlayer. For each of the three courtyard groups, the turbu-lence intensity profiles showed similar forms but did notcollapse on to single curves as with the mean velocities.

The mean velocity profiles for the three courtyardgroups result from the different wind recirculation pat-terns within the courtyards in these cases. These aresketched (not to scale) in Fig. 6. In the shallow courtyardsthere was limited or no recirculation. The free shear layerover the courtyard spread back towards the surface andeither re-attached to the ground or came close to doingso. In the intermediate depth courtyards there wasa strong recirculating eddy in the courtyard cavity whichpenetrated to its base, thus there was a continuous velo-city gradient up through the courtyard, passing thoughzero (the centre of the recirculating eddy) at about halfthe courtyard height. In the deep courtyard there wasalso a recirculating eddy, but it remained approximately

circular in cross section and thus did not penetrate to thebottom of the courtyard, leaving a slow moving, un-steady airflow pattern close to the ground. Thus theregion of negative velocity and positive velocity gradientconsistent with the recirculation pattern was at the top ofthe courtyard opening in these cases.

4. Dispersion measurements in courtyards

4.1. The behaviour of closed empty courtyards of varyingaspect ratio

Fig. 7 shows vertical concentration profiles up thecentrelines of the walls of courtyards of varying aspectration set normally to the wind. The plot shows all thecourtyard aspect ratios used, with the shallower court-yards in the upper plots and the deeper courtyards in thelower plots. The three plots of each pair show the concen-trations respectively on the upwind wall, downwind walland the two side walls. Note that the concentration scalecovers five decades. All the plots for different walls and


Fig. 7. Vertical concentration profiles on courtyard walls. Courtyards of varying depth set normally to the wind. Wind Direction 0°.

aspect ratios showed similar behaviour, of a generallyreducing concentration with increasing height, thoughthe concentration gradients varied with wall position andaspect ratio. There was generally a steeper gradient ofconcentration at the upper levels of the courtyards anda very sharp gradient across the roof of the structure,except on the downwind side, where the concentrationremained relatively steady. Typically, concentrations inthe courtyards fell by an order of magnitude or morefrom the bottom to close to the top. The proportionatereduction was about the same irrespective of the aspectratio of the courtyard, so that the gradients reduced asthe aspect ratio increased.

All the data plots of this sort showed similar behaviourfor nearly all the courtyard parameters investigated. Itwas concluded from this that the vertical gradients ofconcentration were sufficiently well ordered for the con-centration at the base of the courtyard to be a goodindicator of the concentrations elsewhere within it. Thissingle measurement has therefore been used extensivelyin the results presented here.

Concentrations at the base of courtyards of varyingdepth, that is those in Fig. 7, are shown in Fig. 8, plottedagainst aspect ratio for courtyards normal to the wind.Two sets of data are shown, for the two scales of theexperiment. In the region of overlap of they are quitesimilar. The plot shows a curious behaviour, with a maxi-

mum in concentration at the base of the courtyard for anaspect ration of 0.3 and a minimum at an aspect ratio ofabout 1.5. For deeper courtyards, concentrations in-creased steadily to high values with increasing aspectratio. This behaviour is related to the recirculation pat-tern in the courtyard cavity, it is considered more fully inthe discussion in Section 5.

The effects of wind direction on the courtyard concen-trations are shown in Fig. 9, which gives the concentra-tion at the base as a function of wind direction for threeaspect ratios, 0.3, 1.0 and 3.0. These correspond to theaspect ratios giving concentrations at the maximum,minimum and some way up the rising levels for deepcourtyards in Fig. 8. A wind direction of 0° correspondsto a flow normal to the upwind face of the courtyard anddata are given around to 45°, the point of symmetry. Forthe lowest aspect ratio, of 0.3, the effects of wind directionwere relatively slight, with concentrations reducing byabout 20% around to 45° wind direction. For the aspectratio of 1.0, the concentration approximately doubled forwind directions around to 15°, beyond which it remainedroughly constant. However, for the deeper aspect ratiothe concentration remained roughly constant for winddirections around to 15°, beyond which the concentra-tion fell by about a factor of three to a minimum at 30°,before rising again round to 45°, where the concentrationwas about two thirds of its value at 0°.


Fig. 8. Effect of aspect ratio, H.!9

/W, on concentrations at the base of the courtyard. Wind direction 0°.

Fig. 9. Effect of wind direction on concentrations at the base of the courtyard.


Fig. 10. Effect of wall depth on concentrations at the base of the courtyard.

The effects of wall thickness on courtyard concentra-tion are shown in Fig. 10, which covers wall thicknessratios, D/¼ from the basic value of the experiments, 1.0,up to 5.0. Results are given for two aspect ratios, 0.3 and1.0. In both cases increasing the wall thickness uniformlyincreased concentrations in the courtyard. For the loweraspect ratio, of 0.3, concentrations increased by about30% up to the greatest wall thickness. The larger aspectratio, of 1.0, showed a much greater increase in concen-tration, by about a factor of five, over this range of wallthickness.

Fig. 11 shows vertical concentration profiles up thecourtyard wall centrelines for the results of Fig. 9 and 10,for varying wind direction and wall thickness. As with thevertical concentration profiles of Fig. 7, for varying as-pect ratio, there was mainly a well ordered concentrationgradient, with concentration reducing steadily with in-creasing height.

4.2. Courtyards with openings

A common feature of courtyards and enclosed spacesis the presence of openings in the walls. Two types ofopening, shown in Fig. 12, have been investigated here.The first was an archway or tunnel entrance at theground, which was of square cross section and size ¼/2.The second was an opening in a corner up the whole ofone wall, of width ¼/4. The first type of opening isa common form of entrance to courtyards. The second

type of opening was intended to represent an access streetto an otherwise enclosed square. There are often, ofcourse, several of these, but in the first instance a singleopening of this type was investigated. A single courtyardaspect ratio, of 1.0, was used.

Fig. 13 shows concentrations at the base of the court-yard for the two types of opening and a range of winddirections round to 180°, the point of symmetry. A winddirection of 0° corresponds to the opening facing into thewind. Data for a closed courtyard, with no openings, areshown for comparison. The concentrations show a com-plex behaviour with varying wind direction. With anarchway opening, concentrations were generally belowthose of the closed courtyard by a factor of two to three,except in the 90° and 180° wind directions, when theywere greater. With a corner opening facing into the windthere was a considerable increase in concentration in thecourtyard, by a factor of three or more. However, asthe wind direction passed 90° (with the opening along thewind), concentrations fell well below those for a closedcourtyard, again by about a factor of three. Then, as thewind direction approached 180° (with the opening facingdownwind), concentrations rose again to a level abovethat for the closed courtyard.

4.3. The effects of ground clutter

It is common to find substantial amounts of groundclutter in courtyards and other enclosed spaces, both


Fig. 11. Vertical concentration profiles for courtyards with varying wind direction and wall thickness.

Fig. 12. The two types of courtyard with openings investigated. Both of aspect ratio 1.0.

solid obstacles and those porous to airflows like foliage.The effects of this were investigated using the two types ofmodel ‘clutter’ shown in Fig. 14. These were surfacemounted obstacles in the form of cylinders of about twice

height to diameter, solid or porous. The porous obstacleswere of gauze with a 2 mm mesh and an open area ofabout 50%, mainly intended to simulate trees andbushes. The obstacles were placed with an even spacing


Fig. 13. Concentrations at the base of courtyards with openings, showing the effects of wind direction.

covering a range of area densities (the proportion of thesurface area covered by obstacles) from 12% to 48%. Thecentral space of the courtyard floor, where the singlesampling point lay, was left clear, as in the example planlayout in Fig. 14. Measurements were made for twocourtyard aspect ratios, 0.3 and 1.0, constructed at thelarger experimental size of 200 mm square. Thus theclutter occupied 25% of the height of the deeper court-yard and 83% of the height of the lower courtyard.

The effect of surface obstacles on concentrations on thefloor of the courtyard is shown in Fig. 15, for both solidand porous obstacles, as a function of the area density ofthe obstacles. For both courtyard aspect ratios, an in-creasing area density of obstacles generally produced anincrease in concentrations at the ground. The behaviourwas similar for both solid and porous obstacles, butporous obstacles usually produced the greatest propor-tionate increase in concentration. The greatest differencewas in the effects of the obstacles on the two courtyarddepths. The lower courtyard, for which the obstaclesoccupied nearly the whole depth, showed only a limitedincrease in concentration at its base over the range ofoccupational densities of around 50%. However, for thedeeper courtyard, for which the obstacles occupied 25%of the total depth, the concentration at the courtyardbase increased by about a factor of 8 over the range ofarea densities, the most marked change occurring forarea densities beyond 20%.

4.4. The effects of surface stratification

It is readily possible for the surfaces of courtyards tobecome cooler or warmer than the surrounding air and,as a result, to generate stable or unstable stratification inthe air near the ground. This may occur naturally, or bydesign if deliberate attempts are made at climate modifi-cation in the courtyard. One common example of this isusing the evaporation of water for cooling.

It must be appreciated that it is not temperature diffe-rences per se that maintain the stratification, but the con-tinuous transfer of heat, either into or out of the ground orby evaporative cooling, to the surrounding air, withoutwhich any stratification would collapse rapidly. The criticalparameter for scaling the stratification is the dimensionlessbuoyancy flux parameter, the usual form of which is,

F

º3¸. (4)

Here, º is a windspeed and ¸ is a characteristic lengthscale of the flow, taken here as ¼. F is a flux of buoyancy,which is expressed either as the introduction of fluid ofdifferent density into the flow (as in a hot plume, forexample) or as an equivalent transfer of heat. Thus in thefirst definition,

F"g»

n*oo

, (5)


Fig. 14. Types and layout of obstacles used to simulate surfaceclutter.

where » is a volume rate of flow of a gas of densitydifference)*o from the surrounding fluid, of density o. g isacceleration due to gravity. It can also be shown thatF can be defined in terms of a heat flux to the ground, Q,in the form,

F"8.96Q, (6)

where Q is then in units of MW.This form of scaling is commonly used to model buo-

yant discharges in small scale wind tunnel models.A fuller consideration can be found in Hall et al. (1995).

The larger the value of the buoyancy flux parameter,Eq. (3), the greater will be any effects of stratification. Itcan be seen that its value is very sensitive to windspeed,diminishing rapidly as the windspeed increases. Thusstrong stratification is mainly a phenomenon associatedwith low windspeed. With their generally low internalwindspeeds, courtyards should have a significant poten-tial for generating stratification.

In the present case the buoyancy flux can be either theupward heat flux from the ground (either positive forunstable stratification or negative for stable) or an equiva-lent heat flux from the evaporation of water where this isused deliberately for climate modification. Since the sur-face heat flux is normally measured in Watts m~2, a suit-able dimensionless form of the buoyancy flux parameter,called here the stratification parameter, S, for stratifica-tion in courtyards would be,

S " 8.96]10~6q¼

º33%&

, (7)

where q is the surface heat flux in W m~2 (positive foroutward heat flux and unstable stratification and nega-tive for inward heat flux and stable stratification), ¼ isthe side length of the courtyard as previously and º

3%&is

the windspeed as previously.For normal atmospheric surface heat transfer, in the

UK values of q range between about #250 Wm~2 and!30 W m~2 (Clarke (1979), Fig. 4). If q is generated bythe evaporation of water near the ground, the evapora-tion of 1 g s~1 of water per m2 of ground surface areagives rise to a value of q of 2257 Wm~2, so that theevaporation of quite small amounts of water will gene-rate the equivalent of significant stabilising heat fluxes.

Particular values of the stratification parameter, S, willproduce specific flow and dispersion patterns. Fig. 16shows the values of S produced by different combinationsof windspeed, courtyard size and surface heat flux. It isgiven as a plot of the values of S obtained for the productof courtyard size, ¼, and surface heat flux, q (in W m~2),described on the plot as a ‘dimensional heat flux’, againstwindspeed. The plot is valid for both negative and posi-tive values of q, except of course that the sign of S isreversed when q is negative.

The wind tunnel experiments simulated the heat trans-fer by directly injecting dense or buoyant gas (argon orhelium) through the porous base of the courtyard, alongwith the trace gas. This produces a flux of buoyancy,which is equivalent to a surface heat transfer or ground-based evaporative cooling as discussed above. Furtherdetails of this principle can be found in Hall et al. (1995).As was noted above, the value of S, depends upon thewindspeed as well as the buoyancy flux and courtyardscale. In order to obtain the larger values of S whilekeeping the buoyant gas discharge within acceptablelimits, it was necessary to reduce the wind tunnel wind-speeds, down to 0.35 m s~1 in the most extreme cases.This is a normal procedure in this type of wind tunnelexperiment. Due consideration is given to maintainingadequate Reynolds numbers in this type of experiment,though it is often necessary to strike a balance with theneed to obtain the required buoyancy fluxes. Hall et al.(1995) consider this in more detail.

Fig. 17 shows vertical profiles of concentration up thecentres of the courtyard walls for a range of values of the


Fig. 15. Concentrations at the base of courtyards with surface clutter.

Fig. 16. Values of the stratification parameter, S, produced by different combinations of courtyard size, windspeed and surface heat flux.The plot is valid for both positive and negative heat fluxes.

stratification parameter, S. Results are shown for twocourtyard aspect ratios, 0.3 and 1.0 and normal winddirections (0°). With one exception, the vertical concen-tration gradients show no particular differences overthose for other experiments presented here, being fairly

well ordered and showing monotonic gradients. Thesingle exception is the downwind wall for the lowercourtyard aspect ratio, of 0.3, where the vertical gradientwas much more pronounced and concentrations fell tozero before the top of the courtyard. It appears that the


Fig. 17. Vertical concentration profiles in courtyards with stratified flows.

stratification (both stable and unstable) had some effecton the air circulation pattern within the courtyard, allow-ing external air to penetrate more deeply down the down-wind face of the courtyard than was normally the case.

Fig. 18 shows concentrations at the base of the court-yard as a function of the stratification factor, S, for thetwo aspect ratios. A value of S of zero is neutrally strati-fied, as has been all the data presented so far. The rangesof S for stable and unstable stratification are based onestimates related to the different maximum surface heatfluxes which occur in the atmosphere. Maximum nega-tive heat fluxes, from solar radiation, are about an orderof magnitude greater than positive heat fluxes, which arelargely due to conduction at the surface unless the evapo-ration of water is involved.

The courtyards for both aspect ratios showed similarbehaviour with stratification. With increasingly positivevalues of S (unstable stratification) concentrations at thebase of the courtyard fell and with increasingly negativevalues of S (stable stratification) concentrations rose, atleast initially. Small amounts of stratification causedquite large changes in concentration at the ground. Withunstable stratification concentrations fell steadily as thelevel of stratification increased. With stable stratification,however, concentrations increased to a high maximum atvalues of S around !0.002, before falling rapidly forfurther decreases in S. The reason for this behaviour is

not clear. The courtyards for both aspect ratios showedthis general behaviour, the only difference between thembeing that the effects of stratification were more markedfor the deeper courtyard.

From these limited experiments it appears that stratifi-cation is an important feature in dispersion from court-yards. The range of concentrations at the base of thecourtyards over the range of stratification covered herewas about 10 : 1 for the shallow courtyard, of aspect ratio0.3, and about 30 : 1 for the deeper courtyard, of aspectratio 1.0.

5. Discussion

The first point that must be made in the light of thepresent work is that courtyards are generally very poorlyventilating places. The values of the dimensionlessconcentration, K, measured here at the base of thecourtyards ranged from a low of about 5 to a highapproaching 300. Typical values were around 50—200. Incomparison with the relatively large number of disper-sion measurements of contaminant concentrationaround the outside of rectangular buildings (see, forexample, the reviews of Hosker (1980) and Hall et al.(1996a)), if there is complete entrainment of the con-taminant in the building wake, values of K immediately


Fig. 18. Concentrations at the base of courtyards with stratified flows.

downwind in the wake would be of order unity. With thedefinition of K used here, based on the width of thecourtyard rather that the overall width of the building,a typical value of K ought to be about 0.5. During thecourse of the experiment, in view of the very high valuesof K being obtained in the courtyards, we made sucha measurement and obtained a value of K of 0.6, close towhat would be expected.

From the point of view of the dispersion of con-taminants, this seems a serious matter and it is clearlydesirable to avoid the discharge or entrainment of con-taminants into courtyards. For example, it would appearthat a single motor vehicle exhaust discharging ina courtyard (large courtyards are commonly used as carparks) would produce contaminant levels on a par withabout one hundred vehicle exhausts discharging just out-side the building.

As far as the opposite interest is concerned, courtyardsdo indeed seem to be very good places in which togenerate a microclimate and their use for this purposeappears to be technically well founded. In view of theprofound effects of even modest levels of stratification,the use of evaporating water to provide both cooling andto stably stratify the atmosphere in courtyards to assistretaining the generated microclimate seems to havea sound physical basis. The use of distributed surfaceobstacles, whether foliage or solid, also seems to bea useful method of assisting retention of microclimates.

It also appears from the experiments here that rates ofdispersion from courtyards are sensitive to most of thevariables investigated, in some cases showing order ofmagnitude variations. This is useful from the architect’sand designer’s point of view, as the judicious choiceof design parameters can be used to modify courtyardventilation rates either upwards or downwards as de-sired. Though most of the design parameters of interesthave been investigated here at least briefly, thereare some other items which would also merit attention.These are the effects of courtyard plan shapes and theeffects of variable wall heights around courtyards.Both are common features of real courtyards and thereis good reason to suppose that rates of dispersion willalso be sensitive to them. It is also likely that the sur-rounding structures will affect courtyard ventilation bymodifying flow patterns at the top of the courtyard. Theeffects of increasing wall thickness investigated here isprobably a good indicator of the behaviour that might beexpected.

The character of dispersion from Courtyards is inti-mately bound up with the behaviour of the wind-drivenrecirculating vortex in the courtyard space, some ofwhich has been described in Section 3 and shownsketched approximately in Fig. 6. Modifications to thecourtyard shape and character which diminish or inten-sify this vortex equally result in matching increases anddecreases respectively in the rates of dispersion. Thus the


apparently curious results of Fig. 8, which show theeffects of aspect ratio on dispersion, can be explained inthese terms. In general, the recirculating vortex prefers tobe circular in cross section and in this state it appears tohave the greatest intensity. If the space in which it iscontained is far from this form then its intensity is dimin-ished in some way. Thus in Fig. 8, the minimum concen-tration in the courtyard occurred when its aspect ratiowas approximately unity and the recirculation could becircular in cross section and fill the courtyard. As thecourtyard depth increased beyond this, the recirculationremained of circular cross section, but remained at thetop the courtyard, leaving a low velocity region with lowrates of dispersion at the base of the courtyard. When thecourtyard depth reduced below unity aspect ratio, therecirculating vortex was deformed out of its preferredform and the velocities and resultant rates of dispersiondiminished. As a result, concentrations in the base of thecourtyard continued to rise as the aspect ratio fell untilthe courtyard was shallow enough for the separatedshear layer over its opening to re-attach back at theground within the courtyard. This occurred with aspectratios below 0.3 and the resultant increased rate of dis-persion explains the maximum in the concentration atthis point.

This phenomenon has also been observed by the firstauthor in other work on the flows in closed containers,which have quite similar aerodynamic properties tocourtyards. In an investigation of the water retentionproperties of cylindrical rain gauges of varying depth(Hall et al. 1993), a measurement of the windspeed re-quired to remove droplets and snowflakes from the gaugeproduced a very similar plot to that of Fig. 8 for essential-ly the same reasons. That is, the highest velocities inthe base of the gauge (and the most rapid removal ofits content) were associated with aspect ratios close tounity.

In a similar way, the presence of openings sometimesincreased and sometimes decreased concentrations in thecourtyards, though it might be expected in principle thatany additional form of ventilation of the courtyard spaceought to reduce concentrations. This is again related tothe effects of the additional cross flows in the courtyardssometimes diminishing and sometimes re-inforcing therecirculating vortex.

6. Conclusions

Measurements of dispersion from courtyards haveshown that these are quite poorly ventilating spaces withresultant high concentrations of contaminants dis-charged at the base of the courtyard. Values of thedimensionless concentration, K, at the base of the court-yards ranged between 5 and 300, with typical valuesbetween 50 and 200. These values are orders of magni-

tude higher than levels around the exteriors of buildingsfrom locally discharged contaminants, which are typi-cally of order unity.

This is disadvantageous for removing contaminants,but advantageous in generating desired microclimateswithin courtyards, for example stabilisation and coolingof the local atmosphere using evaporated water.

A wide range of courtyard parameters have been inves-tigated, aspect ratio, wind direction, wall thickness, thepresence of openings and surface clutter and the genera-tion of stratification by heating or cooling the base of thecourtyard. All had significant effects on concentrationswithin the courtyards, often altering them by orders ofmagnitude. An implication of this is that there is goodpotential for designing courtyards for high or low rates ofventilation, as desired, by the judicious choice of therelevant design parameters.

Acknowledgements

The work described here was part of the POLIS Pro-ject (on integrated urban planning) in the EC JOULE 95research programme under contract No. JOR3-CT95-0024. It was co-funded by the European Commission(DG XII/F) and by the Construction Directorate of theUK Department of Environment, Transport and theRegions.

The authors are indebted to their colleagues onthe POLIS project, and in particular to Professor Ser-vando Alvarez of the University of Seville, for manyinteresting discussions on this topic and for introducingthem to the concept of using courtyards to generatemicroclimates.

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