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An examination of simple models for building influenced dispersion
Ammpheric
Emironmpnf
Vol.
18. No.
I. pp. 89-98.
ooo44981
1984
84 f3.W
+ 0.00
Pergamon Press Ltd.
P&cd in Great Bntam.
AN EXAMINATION OF SIMPLE MODELS INFLUENCED DISPERSION
FOR
BUILDING
J. E. FACKRELL Central Electricity Generating Board, Marchwood Engineering Laboratories, Marchwood, Southampton, SO4 4ZB (First received 9 MaY 1983 and received~or publjcarion 15 JulY 1983)
Abstract-This paper describes a comparison between fivesimple theoretical models for building influenced dispersion and extensive experimental data for ground level concentrations. The models are all based on the Gaussian plume formulation and account for building effects by specifying, in different ways, enhanced dispersion parameters related to the building shape. Wind tunnel concentration data, corresponding to fairly short time averagesat full scale ( - 10-30 min) has been mainly used for testing the models because of its good quality and repeatability. The data has been chosen for a wide range of building shapes, with stack heights up to 1.5 times the building height and includes studies of detailed nuclear power station sites, with full-scale hourly averaged data for one of these sites. The comparison shows that all the models give reasonable predictions of centreline ground level concentration for low level sources. For high level sources, the models with no allowance for stack height overpredict for normal orientations of the building with respect to the wind direction but are reasonable for diagonal orientations. Models which attempt to allow for variations in stack height tend to underpredict diagonal cases. The models predict plume spreads reasonably well, but cannot reproduce the non-Gaussian plume shapes or the behaviour with wind direction which are observed.
from low level release points and will be greatly affected by the presence of the building. This is the situation that must be considered with regard to nuclear power stations, for example, and there is some emphasis in this paper on nuclear power station shapes, since this is probably the main field of application of the models considered. If no allowance is made for the building effect then calculations of ground level concentration (glc) or dose could be seriously in error. Centreline concentrations could be very conservatively estimated by using a ground level point source at the building. However, even then, offaxis concentrations may well be under-predicted because of the greater lateral spreading experienced behind the building. Consequently, a number of simple models have been proposed over the years which claim to account for the building effect. Although differing in detail, these models are basically the same in their treatment of building influence. They are applied to the standard Gaussian plume formulation with whichever undisturbed flow spreading rate specification is being used (e.g. Pasquill-Gifford) and allow for building effects by specifying a large initial spread related to the building size. This is indeed one of the main features of piume entrainment into the highly turbulent wake of a building but there are other features (see, for example, Robins and Fackrell, 1980) which are totally ignored. Probably the most important of these is the generation of significant axial vortices in the wake for diagonal orientations of the building (i.e. the front face not normal to the flow). These vortices, formed by the rolling up of the shear layers from the leading roof top edges (as over a delta wing) can cause releases from
NOMENCLATURE A a, b, c C h, w, 1 Q u uh x, Y. 2
projectedarea of building transverse to flow (m2) constants concentration (pg m-‘) building height, width transverse to flow, and length in flow direction, respectively (m) source strength (pg s- *) convection velocity in Gaussian plume formula (ms-‘) measured velocity at building height in undisturbed flow (ms- ‘) co-ordinates along, transverse to wind and vertical, respectively (x normally measured from building rear unless stated differently) (m) lateral and vertical plume half widths (average distance between position of maximum concentration and position with half that value. (r = 0.85 d for a Gaussian profile) wind direction (degrees) lateral and vertical spread in undisturbed flow (m) enhanced spreads (m) initial enhanced spreads at building rear position (m).
1. INTRODU~IO~
The dispersion of releases of material near to buiidings is greatly influenced by the flow field around the buildings, which is determined by the shape of the buildings and the upstream flow conditions. If a stack is high enough compared to the building height then significant building influence on the dispersion can be avoided and this is usually arranged to he the case with large routine emissions, such as from conventional coal or oil-fired power stations. However, accidental releases or much smaller routine emissions can be made 89
90
J. E. FACKRE~L
roof tops and short stacks to be rapidly brought to ground level, leading to high glc’s near to the building. A number of examples of this behaviour will be seen in the experimental data given in this paper. It is clear from the detailed experimental data, that none of the simple models tested will be able to accurately predict the complex behaviour of the concentration field near to a building. For example, lateral profiles are often very skewed and non Gaussian for releases from low level point sources. These simple models will not apply until a plume has been well mixed throughout the wake so their use in this paper has been restricted to distances greater than about 3 building heights (h). Although some comparisons are made for plume spreads, the main emphasis in the paper is on the prediction of plume centreline glc’s, since these are the quantities most often required, and most often provided by the experimental data available for comparison. To keep the paper within reasonable limits, the comparison is restricted to release heights below 1.5 h, when the building influence can generally be expected to be important. Also, most of the theoretical models examined are only specified for low level releases and use the same formulation for ali source heights. The models, and hence the experimental data used for comparison, are also restricted to passive releases, i.e. non-buoyant and with negligible source momentum.
2. THE THEORETICAL
(i) Turner model (Turner, 1969). Here the spreads at the building are specified by oiO = n,;4.3
(1) G~ford model Gifford (1960) proposed a simple model for centreline glc which incorporates an allowance for initial plume dilution by a building of projected area A
Q (nop,+cA)U
The constant c was estimated to be between 0.5 and 2. The most conservative value (OS) has been generally used in the present work, although some tests with c = 1 have also been performed and in fact, the use of c= 1 provides better agreement with experiment in most cases. This model is clearly very simple to use but is only applicable to centreline glc. (2) Ifirtual source models The initial greater spreads produced by the building near-wake are allowed for in this model by assuming the plume comes from a virtual source at positions
o&, = h 2.15,
and the virtual source heights examined in this paper are at ground level for low level releases and at roof height, h, to model higher level releases. (ii) Barker Model (Barker, 1982). The spreads at the building are given by ’ = w/3 a;,=hl’3. OYO and the source height is z0 = h/3 (the same position is specified for both ground and roof level releases). This model has been used by the CEGB for nuclear applications. The specification of the lateral spread is restricted to w < 3h, otherwise b& = h. (31 F-C Model (Ferrara and Cagnetti. 1980) This model is representative of a group of models which linearly add some quantity proportional to the building dimensions to the lateral and vertical spreads to produce the increased wake dispersion
MODELS
A wide range of simple models has been proposed to account for building effects, differing mainly in the way in which the initial spreads are enhanced and in the numerical values of the constants used. (For examples, see Hosker, 1980; Barker, 1982). It would clearly be impractical to test all such models and only a representative sample has been attempted here. A brief description of each of the models is given below.
c=
upstream of the real source. The virtual source positions are chosen so that at the position of the rear face of the building, the vertical and lateral plume spreads are some specified fraction of the building height and width. Two different formulations have been studied.
0; = a,+wja a; = o, -+ h/b. In the F-C model u = b = 2.507. The model was actually specified using the dimensions of an equivalent flat plate to the building, a concept introduced by Halitsky (1977) to account for complex site layouts. However, in the present study, the same building dimensions have been used with this model as for the other models to enable a direct comparison to be made. Ferrara and Cagnetti (1980) showed that this model gives very similar results to that of Halitsky (1977) whilst being somewhat simpler to apply. (4) H--S tnvdei This is based on the wind tunnel measurements of Huber and Snyder (1976). the model being further tested by Huber (1977). It is a little more complicated than the other models tested, in that the wake region is divided into a near and far region. In the near region, 3h < x < 10/r. 61 = 0.7 (w/2)+ (x - 3h)/15 0; = 0.7h + (x - 3h)/ 15. A virtual source model is used for x > lOh, with the source located so that the spreads match the near wake model at 10h. If the spreads specified by the model are less than the undisturbed flow spreads then the latter are used. For source heights above h only the vertical
91
An examination of simple models for building influenced dispersion
spread is enhanced, the lateral spread using the undisturbed flow values. The actual source height is used in the model. This is the only model tested which specifically allows for stack height variation, although the Turner virtual source model has also been used with two source heights (0 and h) for comparison. 3. EXPERIMENTAL DATA In
choosing experimental data for comparison with the above models, data sets containing undisturbed flow, ground level release results for spreads and con~~tration have been used. The Gaussian plume model at the heart of all the above models can then be tuned to give the correct undisturbed flow behaviour by choosing the spreads to match the experimental ones and the convection velocity to give the right concentration values. The testing can then be solely of the building effect models and will not contain any additional uncertainty regarding the effectiveness of the particular Gaussian plume formulation. The majority of the data used were obtained in wind tunnels. This is the only data of sufficient detail and quality to enable meaningful testing of the models to be carried out. Full-scale data on building effects tends to be limited to a few tests which are so scattered that differences between the model results are small in comparison. Nonetheless we have included a comparison with the fuit-scale data of Start er at. (1980) since this is a fairly detailed study of the same building as that examined by Hatcher er nl. (1978) in a wind tunnel. This enables an interesting comparison between theoretical models, wind tunnel and full-scale results to be made, as well as illustrating the difficulties of using full-scale data. The experimental data has been chosen to cover a reasonably wide range of simple rectangular building shapes but a large part of the comparison uses results obtained on detailed models of nuclear power stations. There is a wide variety of both wind tunnel and full scale data available on building effects and it is not suggested that the particular data used are in any way exhaustive or the best available, but, as for the theoretical models, it is felt to be a sufficiently large representative sample to enable reasonably general conclusions to b-e drawn. Details of the various data sets used are given below. (1) Dean and Robins duta These experiments were performed in the Marchwood Engineering Laboratories wind tunnel using a 1.2 m neutral boundary layer with a rural type roughness. The undisturbed vertical spread from a ground level source was very close to the category D specification given by Clarke (1979) assuming a scale of 1:300.The lateral spread was given by eY = 0.5 x0.6* with uYand x in equivalent full scale metres. The ratio of the convection velocity used in the Gaussian plume model, U, to the measured velocity at the building height, Ij,, wasadjusted until good agreement with the undisturbed ground level source centreline concentration measurement was obtained; in this case with UjLr, = 0.8. All the buildings examined were of simple rectangular shape: three were of square planform with ratios of width to height of l/3, 1 and 3 and the rest were of square section transverse to the flow of various widths from 4 to 8. Releases were made from a roof top source or stack located in the centre of the roof, or from a ground level source immediately behind the building. The cases chosen for comparison with theory are summarized in Table 1. Some of this work is included in the M.Sc. thesis of Dean (1981), but the rest is, as yet, unpublished. (2) AGR nuclear power station model data This was obtained in the same boundary layer flow as that above. The prescriptions for the undisturbed eY and o2 are
Table 1. Dean and Robins’ data chosen for comparison with theories Buildings wxlxh (m full scale at 1:300) 9x9~27 27 x 27 x 27 81 x 81 x 27 13.5 x 27 x 27 54 x 27 x 27 135x27~27
Release heights
t?”
0, 27, 38 7,
0, 45 3,
42;:
40.5 1, ,I
il ,* .,
therefore the same as above, but since the building is taller U/U, is now 0.71. At full scale, the height of the reactor building is 70 m. It is about 110 m wide, with the smaller face about 40 m wide. Although there are a number of smaller buildings near the reactor building (principally the turbine hall) in making the comparison with theory the reactor ball is assumed to be dominant and its dimensions used in the theories. Concentration measurements were obtained for a complete range of wind directions at 45” intervals. A detailed description of this work is given by Fackrell and Robins (1981). (3) Hereher et al. data The previous data sets are for neutrally stable flow only, but for the study described by Hatcher er al. (1978) three different flow regimes were generated in the meteorological wind tunnel at Colorado State University; stable, neutral and unstable. The undisturbed flow parameters given in Table 2 were found to provide reasonable agreement with concentration values for ground level releases in the three flows. Releases from a 1:200 scale model of the EOCR reactor at Idaho Falls were studied in these flows. The reactor building is an approximately cubic shape joined to a lower, wider rectangular shape. For the present purposes, in the theoretical models, the building shape has been represented simply as a 23-m cube. This will aenemllv lead to higher nredicted concentrations than trying td represent the fuh shape. Ground level concentration values were measured for three release points, the three flows and at 45” intervals over the complete range of wind angles. Full details are given in Hatcher et a/. (1978). (4) Start et al. dota This is full scale field data for the same reactor site at Idaho Falls. Ground level concentration measurements were made over sampling arcs at 37,68,187,386and 794 m. Releases were made from ground level, roof top and stack as above, although the ground level position was not always the same. The releases were made in a variety of meteorological conditions although the emphasis was on light wind. stable conditions. Full detaifs of the experiments are given in Start et al. (1980).
Table 2. Hatcher et al. undisturbed flow parameters (Ail dimensions in equivalent full scale m) Flow category Unstable Neutral Stable
0 18 x”.s’ 0:13 x@ss 0.12 x0,7*
0.37 x0.85 0.26 x0.*5 0.18 x”.so
0.5 0.6 0.9
J.E. FACKRELL
92
(1)
Simple rectangular
interpretation is that the stack needs to be higher for a wide building to avoid downwash. Those models which allow for source height variation, Huber and Snyder and the Turner model with a roof height virtual source, show the correct trend with building width, although they overpredict the narrow building result and slightly underpredict the wide building result. The H-S model correctly predicts that the stack release leads to higher glc’s than the roof top reiease with the widest building. Figure 2 presents the Dean and Robins results for the square planform buildings. Results for normal orientations of the building tend to follow the trend with building width exhibited in the previous figure, but this time results are included for orientations with building faces diagonal to the wind, 0 = 45”. It can be seen that for the narrower buildings there can be substantial differences between the results produced by the two orientations. For the wider building this is not so apparent for the source heights examined, although it is stiI1 true for higher stack heights than shown. In the theoretical models the building width has been specified for the normal orientation only, i.e. l/3, 1, 3h, since using the actual projected area for An alternative
4. DETAILED COMPARISON
buildings
In Fig. 1 the various models are compared with the Dean and Robins results for rectangular buildings with a square section and various widths transverse to the flow. These results were all obtained with the front face of the building normal to the Bow. Some general points can be made from the comparison with theory. Looking at the gl source results first, we see that the predicted values from the different models tend to be fairly close together, within a factor 2. The concentration predictions are fairly close to the experimental points for the narrowest building and slightly high for the others. Generally, the Gifford model with c = ) is the highest and the FX model gives the lowest predictions. The F-C model gives closest overall agreement with experiment but slightly underpredicts the narrowest building result. The experimental results show that the roof and stack releases give much lower concentrations for the narrower buildings but become similar to the ground level results for the wide building.
I -
Cl /
L % fu 0 Ol,-
/ -
01 -
1
IO
0”
,
100
x/h
Fig. 1. Comparison of centreline ground level concentration for square section buildings (I = /I) with various widths, normal flow. (i) w = 5h, (ii) w = 2h, (iii) w = $h. Experimental data (Dean and Robins): +, GL source; x . roof source; 0, stack source (1.5h). Theoretical models: --, Barker; -, FC; --., Gifford with c = 3; I-.-., Gifford with c = 1; --.--., HS; ----, Turner. R and S indicate source at roof and stack height, respectively, in theoretical model.
/
IO
100
x/h
Fig. 2. Comparison of centreline ground level concentration for square planform buildings. (i) w=I=3h. (ii) w=l=h, (iii) w=I=h/3. Experimental data (Dean. 1981): 13, GL source, t?=O”: m, Roof source, 0’; 0. GLS, 45’; et Roof S., 45”; +, stack (1.4h), 0”; X, stack. 45”. Theoretical models as in Fig. I.
93
An examination of simple models for building influenced dispersion orientations would reduce the predicted concentrations, which is opposite to what is normally required, (except for the one case with the ground level source and the widest building). Conclusions regarding the low level source predictions are similar to before, although the models giving the lowest predictions now also slightly underestimate the wider building, normal wind results. The Barker model appears to give the lowest predictions which remain conservative. The comparison for roof top and stack releases is complicated by the wide difference in results for different wind directions. It is clear that if a model correctly predicts a normal orientation result then it will underpredict a diagonal orientation. For example, for the cube roof top release results, the diagonal orientation leads to glc’s similar to the ground level release and would be reasonably well predicted by a low level source model (such as the Barker model) but the normal orientation values are much lower and better predicted by the roof source models. None of the models tested allows for the extra downwash produced by diagonal orientations, so one must either choose a model which will cover the diagonal case and accept that it will be very conservative in some circumstances, or accept the complication of incorporating the effects of different wind directions into the models, possibly by using different parameters for normal and diagonal orientations. A few lateral plume half-widths measured at ground level were available for some of the building shapes examined above. These are shown in Fig. 3 together with the predicted values from the models (except Gifford’s model). The results from the different models lie close together for the narrowest building, but show diagonal
I 0
IO
20
30
increasing variation as the width increases. All are acceptably close to the experimental values, except the FC model, which overpredicts for the wider buildings. It is this greater lateral spread which tends to make the FC model concentration values lower than those of the other models. The slope of the predicted half-widths with downstream distance is generally slightly greater than that of the experimental values and the nearest experimental results at 1lh are mostly slightly underpredicted. Note that the restriction on maximum initial lateral spread is applied in the Barker model for the 5h wide building, effectively reducing the spread to that for a 3h wide building.
(2) AGR model The results for the AGR model are given in Fig. 4. Unlike the previous examples this was a fully detailed model of a station site, although the reactor hall does tend to be the dominant feature. In the theoretical models only the dimensions of the reactor building are specified; the two orientations at 6 = 0 and 90” being calculated (i.e. w = 1.6h and 0.6h, respectively). For the ground level source, the predicted results are generally reasonably close and slightly conservative with respect to the corresponding experimental results. However, for diagonal orientations, illustrated by the 45” experimental results, values near the building will be underpredicted, although use of the higher, w = 0.6h result to cover this case would give reasonable predictions by most of the models from * 3 to 4h outwards. The roof source results are reasonably well predicted except for the 45” case again, near to the building, when a low level source model would give better agreement.
I
I
40
0
I
IO
20
30
I
L
40
50
x/h Fig. 3. Comparison for lateral widths. (i) w= Sh, (ii) w=2h; (iii) w= h, (iv) w=)h. Experimental data (Dean and Robins): +, GL source, 0=0’; x, GLS, 45’; 0, Root source, 0’. Theoretical models as in Fig. 1.
J. E. FACKRELL
k.
a
Fig. 4. ACR model results for centreline ground level concentration. ii) GL source. w= 1.6h: (ii) GL source, w =0.6h: (iii) Roof source, w= 1.6fi; (iv) Roof source, w: =0.6/r. Experimental data (Fackrell and Robins, 1981);o. H = 0”:x, 45’; q (90”; +, 180’; 0,270’. Theoretical models as in F&. I.
x/h
Ground level lateral plume half widths and centreline vertical half widths were also measured for the AGR model and these are shown in Fig. 5. together with the predicted values. There is a tendency for the lateral plume half-width to be underpredicted in the near field by most of the models. except for the F-C model which for this building shape gives the most accurate predictions of lateral spread. For the vertical half-widths, the rate of increase of the predicted values with downstream distance is greater than for the experimental results, but the values are reasonably well predicted. Overall, both lateral and verticai half-width predictions from all the models are acceptable (within about a factor of 2) although individuat variations with wind direction will not be reproduced. as was true for concentration values. Note also that for cases with the ground level source upstream of the building, the experimental results showed that the plume could be displaced laterally by up to about a full building width from the downstream building centreline as it travelled around the side of the building. Also, plume profiles near to the building could be greatly skewed and distorted and not well reproduced by a Gaussian model. Together, these could be more important sources of error in predicting the concentration field near the building than any differences in plume spreads. (3) EOCR model The comparison with the results of Hatcher rf al. (1978) for the three flow stabilities examined, neutral, unstable and stable, is shown in Fig. 6. To be consistent
x/h
Fig. 5. AGR model results for lateral and vertical half widths. (i) lateral half width, w=0.6h; (iI) lateral half width, w= 1.6h; (iii) vertical half width, wr0.6h; (iv) vertical half width, w= 1.6h. Experimental data: o. GL source,0=00; l ,Roofsource,O;x,GLS,45’;i,RoofS,45”; Cl.GLS,W; m ,RoofS,W;+.GLS,lgO: 0. GLS, 270”. Theoretical models as in Fig. 1.
An examinarion of
simple models
for building
influenced
dispersion
Stock
(4
x/h
x/h
\
Stock \
\
“\
..
: + x
l...j e?
i ib)
i0 x/h
10
r/h
x/h
0
0
10
.x/h
Roof source 0
\
Stack i
x/h
x/h
Fig. 6. Comparison of centreline ground level concentration for EOCR wind tunnel data. (a) Unstable Row,(b) Neutral flow,(c) Stable flow. Experimental data (Hatcher et al., 1978): D ,B = 0”; +, 45”;X,90”; o, 135”; Cl. 180”; V, 225”; A, 270”; 0. 315”. Theoretical models as in Fig. I.
96
J. E. FACKRELL
with the AGR results, the wind angle, 0, is defined from the normal to the building face containing the Lowlevel source. This is different to the definition in the original paper. For the neutral and unstable results, the low level models’ predictions lie fairly close together and just above the top end of the experimental results. The stable predictions tend to be slightly lower relative to the experimenta results, especially far downstream, so that some wind directions are underpredicted, although by no more than about a factor of 2 in the worst case. In the roof and stack release comparison, the Turner roof and H-S model show the right behaviour for normal orientations of the building but underpredict some diagonal orientations. These are better predicted by a low level source model, as characterised in the figures by the Barker model results. (4) EOCR juwUscale The final set of comparisons is between theoretical models, wind tunnel results and full-scale results for the EOCR reactor (Fig. 7). In plotting the full-scale data the same symbols as in the Hatcher et al. data are used, the symbol for the nearest value to the actual wind direction being assigned. For the ground level source results, B is always taken from the normal to the face containing the source. The theoretical models are still apphed to a 23-m cube, as above, but now use the formulation of Clarke (1979) for the undisturbed flow spreads (categories A, D and F). Only the short term lateral spread has been enhanced in the models, the longer term wind meandering in the Clarke formulation remains the same. The full-scale results and the model predictions refer to hourly averaged data. Wind tunnel resultsare generally assumed to apply to shorter time averages than this, 10-30min. so that they would be expected to give higher concentrations. Generally, agreement between the theoreticai models. wind tunnef and full-scale results is reasonably good with the theoretical and wind tunnel results lying towards the top end of the range of full-scale data. Note, however. that the vertical scale in the figures has been greatly reduced to accommodate the scatter in the full-scale results. On this scale the differences in the theoretical models is fairly small and only representative samples are shown for the low level models. This illustrates the difTiculty of using full scale data to judge the models since the degree of scatter is much greater than differences between the models. This scatter is mainly from run to run rather than in the individua1 runs. It appears to be mainly caused because runs assigned to the same stability category (based on lapse rate) are in performed under differing flow confact ditions-different wind speed, direction and shear, slightly different temperature protiles and, in particular, quite large differences in lateral fluctuations of the wind, eg. Note that in the present work, categories EFG are plotted together, but the scatter in results for the individual categories would be similar to the combined one shown. The wind tunnel results and theoretical models tend to be conservative with regard
(a)
(bl
IO x/h
100
Fig. 7ia.b)
An examination of simple models for building influenced dispersion ’
91
others. Indeed their behaviour is fairly similar and there is usually less than a factor of two difference between the range of predictions for low level sources. This is obtained using the numerical values specified for the models. By suitably adjusting these, the differences could be even further reduced. It is perhaps not surprising that the models are similar, since they all incorporate the building effect as an enhanced spreading of a Gaussian plume, differing only in the details in which this is accomplished.
.
(2) Low level releases
Stoble
(4 Fig. 7. Comparison of centreline ground level concentration for EOCR, full scale, wind tunnel and theoretical models. (a) Ground level source. (b) Roof source. (c) Stack source. Full scale experimental
data(StarteraL(1980)): D,e=W; +,45’; x (90”; 0,135”; 0,180”; V,225”;A, 270’; 0. 315”. Hatched areas indicate range of wind tunnel data for all t?. Theoretical models as in Fig. I.
to the full-scale results and the closest to the average values for neutral and unstable cases, being more conservative in the stable case. This is probably because neither the wind tunnel flow nor the model of Clarke (1979) adequately incorporate the observed large lateral meandering in light wind stable conditions. There is no clear difference between normal and diagonal results in the full scale data, as there was in the wind tunnel data, because the scatter is too great.
5. DISCUSSION
(1) Intercomparison of models In this paper the predictions of several simple models for building influenced dispersion have been compared with a fairly wide range of experimental data. It was hoped that this might allow the selection of a ‘best’ model but it is clear from the comparison that no one model is particularly outstanding relative to the
For low level sources, the GitTord model with c =) usually gave the highest predictions and the F-C model the lowest, with the others in between. The two virtual source models, Turner and Barker, were usually very close, differing most near the building, where the Turner model was more conservative. Generally the models gave reasonably good predictions for low level source results beyond 3-5 building heights (h), tending to be slightly conservative by less than a factor of 2, so that the lowest predictions, by the FC model, were most often closest to the experimental results. However, in some cases this model underpredicted concentration and overpredicted lateral spread for wide buildings. The Barker or Turner models gave the lowest predictions which remained conservative in most cases. None of these models as they stand will correctly predict behaviour with wind direction, particularly for diagonal orientations. Since, for example, the Hatcher data shows considerable variation with wind direction, the models which are to be conservative for all cases must considerably overpredict some wind directions (by a factor of 4 or 5). (3) Higher level releases (rooflop and above) The problem of wind direction variations is even more noticeable for higher level releases. The experimental data generally shows considerably higher concentrations for diagonal orientations of the building than for those with the front face normal to the flow. Models for higher level releases which predict normal orientations reasonably well, will underpredict diagonal ones. Using the models for low level releases can often predict diagonal orientations quite well but this will be very conservative for other high level release cases (by over an order of magnitude and for a considerable distance downstream, in some cases). As well as wind direction and stack height, the amount by which a stack release is influenced by a building can depend very much on roof area, i.e. the longitudinal extent of the building as well as the lateral extent. None of the models allows for this. Only short stacks of up to 1.5 building heights have been examined in this paper. Although building effect tends to decrease the effective source height of taller stacks, they generally behave as elevated releases and use of a low level model for stacks greater than 1.5h would normally be very pessimistic. To some extent the same conclusions apply to shorter stacks in very stable flows.
98
J. E. FACKRELL
(4) Plume spreads
This paper has mainly concentrated upon the prediction of centreline glc, but some comparisons were made for plume spreads. Generally, the predictions were reasonably close to the measured spreads (usually well within a factor of 2). The F-C model was found to overpredict lateral spread for wide buildings (w > Zh), but predicted quite well for narrower ones. The other models tended to slightly underpredict lateral spread near the building and there was a general tendency to slightly overpredict spreading rate. As for concentration, the models will not reproduce the complex variation of the spreads with wind direction, or with source height.
6.
however be difficult to incorporate all important building effects in a simple model. without having a great deal of a-priori empirical information for each case considered. To go further, requires a more complex model which directly takes into account, at least, the main features of the velocity field around a building.
Acknoaledgemenrs-~This work was carried out at Marchwood Engineering Laboratories and the paper is published by permission ofthe Central Electricity Generating Board. The work was sponsored under CEC Contract No. SR 012 UK, “Effects of Buildings on Low-Level Atmospheric Discharges”, as part of the Indirect Action Research Programme of the European Atomic Energy Community on the Safety of Thermal Water Reactors.
CONCLUSIONS
Very near to a building, a real piume, especially from a low level source, can be greatly distorted and displaced by the complicated flow field and by small local site features. These simple models should not be expected to predict the concentration field in this region. A wind tunnel study would seem best at the present time, especially for complex sites, and is probably also required if the source is not passive (i.e. with signifi~nt momentum or buoyancy). Further downstream (say S-iOh), many of the local effects will have become small and the main building effect can then be reasonably represented by a change in effective source height and enhanced initial spread, as in these models. The models will not predict variations with wind direction or source position, but for many practical applications, all that is required is to predict with minimum pessimism the worst wind direction result for a given source, the results for other wind directions then being more conservatively predicted. For such applications, one of the low-level release models can be used for all releases, in order to cover diagonal orientations for higher Ievel releases (at least up to 1Sh). Of those models tested, the virtual source models would seem marginally better from this point of view, although any of the other models could be made to perform similarly by changing the numerical values employed. If it is wished to do better than this using one of these simple modets, then at least some allowance must be made for source height and wind direction variations. The empirically based approach, as used for the W-S model, is attractive. but requires a wider range of input data than was used for that model. With this, it should be possible to incorporate the more obvious effects of wmd direction and source position into any of those models, at least for simple building shapes, (By, for example, distinguishing between normal and diagonal orientations of the building, with higher level releases). It would
REFERENCES
Barker C. D. (1982)A virtual source model for building wake dispersion in nuclear safety calculations. Central Electricity Generating Board Report No. TPRD~B/~72~N82. Clarke R. H. (1979) A model for short and medium range dispersion of.radionuchdes released to the atmosphere. National Radiological Protection Board Report No. NRPBR91. Dean M. (1981) M.Sc. thesis, University ofsydney, Australia. Fackrell J. E. and Robins A. G. (1981) Dispersion of releases at an AGR site. Central Electricity Generating Board Report No. RD/M~II72~R81. Ferrara V. and Cagnetti P. (1980) A simple model for estimating airborne concentrations downwind of buildings for discharges near ground level. Paper at the Commission of the European Communities’ Seminar on Radioactive Releases and their dispersion in theatmosphere following a hypothetical reactor accident. RI@. Denmark. Gifford F. A. (1960) Atmospheric dispersion calculations using the generalised Gaussian plume model. ivucl. Sal: 2, 56-59. Hahtsky J. (1977) Wake and dispersion model for the EBR II building complex. Armosplteric Encironment 11, 577-596. Hatcher R. V., Meroney R. N., Peterka J. A. and Kothari K. Dispersion in the wake of a model industrial complex. US Nuclear Regulatory Commission Report NUREG-0373. Hosker R. P. (1980) Dispersion in the vicinity of buildings. Proc. AMSiAPCA 2nd Joint ConJ on Application Poflution Mercorolugy, New Orleans.
@“Air
Huber A. H. (1977) Incorporating building/terrain wake effects on stack efIIuents. Joint Conf. on applications of air pollution meteorology, Salt Lake City. Huber A. H. and Snyder W. H. (1976) Building wake effects on short stack etftuents. 3rd Symp. on atmospheric turbulence, ditfusion and air quahty. Raleigh, NC. Robins A. G. and Fackrell J. E. (1980) Laboratory studies of dispersion near buildings. Paper at the Commission of the European Communities’ Seminar on radioactive releases and their dispersion in the atmosphere followmg a hypothetical reactor accident. Rise, Denmark. Start G. E., Hukari N. F., Sagendorf J. F.. Gate J. H. and Dickson C. R. (1980) EOCR building wake effects on di~usjon. US Nuclear Regulatory atmospheric Commission Report NUREG CR-1395 Turner D. B. (1969) Workbook of atmospheric dispersion estimates. US Dept. of Public Health Publ. No. 999AP-26.
Emironmpnf
Vol.
18. No.
I. pp. 89-98.
ooo44981
1984
84 f3.W
+ 0.00
Pergamon Press Ltd.
P&cd in Great Bntam.
AN EXAMINATION OF SIMPLE MODELS INFLUENCED DISPERSION
FOR
BUILDING
J. E. FACKRELL Central Electricity Generating Board, Marchwood Engineering Laboratories, Marchwood, Southampton, SO4 4ZB (First received 9 MaY 1983 and received~or publjcarion 15 JulY 1983)
Abstract-This paper describes a comparison between fivesimple theoretical models for building influenced dispersion and extensive experimental data for ground level concentrations. The models are all based on the Gaussian plume formulation and account for building effects by specifying, in different ways, enhanced dispersion parameters related to the building shape. Wind tunnel concentration data, corresponding to fairly short time averagesat full scale ( - 10-30 min) has been mainly used for testing the models because of its good quality and repeatability. The data has been chosen for a wide range of building shapes, with stack heights up to 1.5 times the building height and includes studies of detailed nuclear power station sites, with full-scale hourly averaged data for one of these sites. The comparison shows that all the models give reasonable predictions of centreline ground level concentration for low level sources. For high level sources, the models with no allowance for stack height overpredict for normal orientations of the building with respect to the wind direction but are reasonable for diagonal orientations. Models which attempt to allow for variations in stack height tend to underpredict diagonal cases. The models predict plume spreads reasonably well, but cannot reproduce the non-Gaussian plume shapes or the behaviour with wind direction which are observed.
from low level release points and will be greatly affected by the presence of the building. This is the situation that must be considered with regard to nuclear power stations, for example, and there is some emphasis in this paper on nuclear power station shapes, since this is probably the main field of application of the models considered. If no allowance is made for the building effect then calculations of ground level concentration (glc) or dose could be seriously in error. Centreline concentrations could be very conservatively estimated by using a ground level point source at the building. However, even then, offaxis concentrations may well be under-predicted because of the greater lateral spreading experienced behind the building. Consequently, a number of simple models have been proposed over the years which claim to account for the building effect. Although differing in detail, these models are basically the same in their treatment of building influence. They are applied to the standard Gaussian plume formulation with whichever undisturbed flow spreading rate specification is being used (e.g. Pasquill-Gifford) and allow for building effects by specifying a large initial spread related to the building size. This is indeed one of the main features of piume entrainment into the highly turbulent wake of a building but there are other features (see, for example, Robins and Fackrell, 1980) which are totally ignored. Probably the most important of these is the generation of significant axial vortices in the wake for diagonal orientations of the building (i.e. the front face not normal to the flow). These vortices, formed by the rolling up of the shear layers from the leading roof top edges (as over a delta wing) can cause releases from
NOMENCLATURE A a, b, c C h, w, 1 Q u uh x, Y. 2
projectedarea of building transverse to flow (m2) constants concentration (pg m-‘) building height, width transverse to flow, and length in flow direction, respectively (m) source strength (pg s- *) convection velocity in Gaussian plume formula (ms-‘) measured velocity at building height in undisturbed flow (ms- ‘) co-ordinates along, transverse to wind and vertical, respectively (x normally measured from building rear unless stated differently) (m) lateral and vertical plume half widths (average distance between position of maximum concentration and position with half that value. (r = 0.85 d for a Gaussian profile) wind direction (degrees) lateral and vertical spread in undisturbed flow (m) enhanced spreads (m) initial enhanced spreads at building rear position (m).
1. INTRODU~IO~
The dispersion of releases of material near to buiidings is greatly influenced by the flow field around the buildings, which is determined by the shape of the buildings and the upstream flow conditions. If a stack is high enough compared to the building height then significant building influence on the dispersion can be avoided and this is usually arranged to he the case with large routine emissions, such as from conventional coal or oil-fired power stations. However, accidental releases or much smaller routine emissions can be made 89
90
J. E. FACKRE~L
roof tops and short stacks to be rapidly brought to ground level, leading to high glc’s near to the building. A number of examples of this behaviour will be seen in the experimental data given in this paper. It is clear from the detailed experimental data, that none of the simple models tested will be able to accurately predict the complex behaviour of the concentration field near to a building. For example, lateral profiles are often very skewed and non Gaussian for releases from low level point sources. These simple models will not apply until a plume has been well mixed throughout the wake so their use in this paper has been restricted to distances greater than about 3 building heights (h). Although some comparisons are made for plume spreads, the main emphasis in the paper is on the prediction of plume centreline glc’s, since these are the quantities most often required, and most often provided by the experimental data available for comparison. To keep the paper within reasonable limits, the comparison is restricted to release heights below 1.5 h, when the building influence can generally be expected to be important. Also, most of the theoretical models examined are only specified for low level releases and use the same formulation for ali source heights. The models, and hence the experimental data used for comparison, are also restricted to passive releases, i.e. non-buoyant and with negligible source momentum.
2. THE THEORETICAL
(i) Turner model (Turner, 1969). Here the spreads at the building are specified by oiO = n,;4.3
(1) G~ford model Gifford (1960) proposed a simple model for centreline glc which incorporates an allowance for initial plume dilution by a building of projected area A
Q (nop,+cA)U
The constant c was estimated to be between 0.5 and 2. The most conservative value (OS) has been generally used in the present work, although some tests with c = 1 have also been performed and in fact, the use of c= 1 provides better agreement with experiment in most cases. This model is clearly very simple to use but is only applicable to centreline glc. (2) Ifirtual source models The initial greater spreads produced by the building near-wake are allowed for in this model by assuming the plume comes from a virtual source at positions
o&, = h 2.15,
and the virtual source heights examined in this paper are at ground level for low level releases and at roof height, h, to model higher level releases. (ii) Barker Model (Barker, 1982). The spreads at the building are given by ’ = w/3 a;,=hl’3. OYO and the source height is z0 = h/3 (the same position is specified for both ground and roof level releases). This model has been used by the CEGB for nuclear applications. The specification of the lateral spread is restricted to w < 3h, otherwise b& = h. (31 F-C Model (Ferrara and Cagnetti. 1980) This model is representative of a group of models which linearly add some quantity proportional to the building dimensions to the lateral and vertical spreads to produce the increased wake dispersion
MODELS
A wide range of simple models has been proposed to account for building effects, differing mainly in the way in which the initial spreads are enhanced and in the numerical values of the constants used. (For examples, see Hosker, 1980; Barker, 1982). It would clearly be impractical to test all such models and only a representative sample has been attempted here. A brief description of each of the models is given below.
c=
upstream of the real source. The virtual source positions are chosen so that at the position of the rear face of the building, the vertical and lateral plume spreads are some specified fraction of the building height and width. Two different formulations have been studied.
0; = a,+wja a; = o, -+ h/b. In the F-C model u = b = 2.507. The model was actually specified using the dimensions of an equivalent flat plate to the building, a concept introduced by Halitsky (1977) to account for complex site layouts. However, in the present study, the same building dimensions have been used with this model as for the other models to enable a direct comparison to be made. Ferrara and Cagnetti (1980) showed that this model gives very similar results to that of Halitsky (1977) whilst being somewhat simpler to apply. (4) H--S tnvdei This is based on the wind tunnel measurements of Huber and Snyder (1976). the model being further tested by Huber (1977). It is a little more complicated than the other models tested, in that the wake region is divided into a near and far region. In the near region, 3h < x < 10/r. 61 = 0.7 (w/2)+ (x - 3h)/15 0; = 0.7h + (x - 3h)/ 15. A virtual source model is used for x > lOh, with the source located so that the spreads match the near wake model at 10h. If the spreads specified by the model are less than the undisturbed flow spreads then the latter are used. For source heights above h only the vertical
91
An examination of simple models for building influenced dispersion
spread is enhanced, the lateral spread using the undisturbed flow values. The actual source height is used in the model. This is the only model tested which specifically allows for stack height variation, although the Turner virtual source model has also been used with two source heights (0 and h) for comparison. 3. EXPERIMENTAL DATA In
choosing experimental data for comparison with the above models, data sets containing undisturbed flow, ground level release results for spreads and con~~tration have been used. The Gaussian plume model at the heart of all the above models can then be tuned to give the correct undisturbed flow behaviour by choosing the spreads to match the experimental ones and the convection velocity to give the right concentration values. The testing can then be solely of the building effect models and will not contain any additional uncertainty regarding the effectiveness of the particular Gaussian plume formulation. The majority of the data used were obtained in wind tunnels. This is the only data of sufficient detail and quality to enable meaningful testing of the models to be carried out. Full-scale data on building effects tends to be limited to a few tests which are so scattered that differences between the model results are small in comparison. Nonetheless we have included a comparison with the fuit-scale data of Start er at. (1980) since this is a fairly detailed study of the same building as that examined by Hatcher er nl. (1978) in a wind tunnel. This enables an interesting comparison between theoretical models, wind tunnel and full-scale results to be made, as well as illustrating the difficulties of using full-scale data. The experimental data has been chosen to cover a reasonably wide range of simple rectangular building shapes but a large part of the comparison uses results obtained on detailed models of nuclear power stations. There is a wide variety of both wind tunnel and full scale data available on building effects and it is not suggested that the particular data used are in any way exhaustive or the best available, but, as for the theoretical models, it is felt to be a sufficiently large representative sample to enable reasonably general conclusions to b-e drawn. Details of the various data sets used are given below. (1) Dean and Robins duta These experiments were performed in the Marchwood Engineering Laboratories wind tunnel using a 1.2 m neutral boundary layer with a rural type roughness. The undisturbed vertical spread from a ground level source was very close to the category D specification given by Clarke (1979) assuming a scale of 1:300.The lateral spread was given by eY = 0.5 x0.6* with uYand x in equivalent full scale metres. The ratio of the convection velocity used in the Gaussian plume model, U, to the measured velocity at the building height, Ij,, wasadjusted until good agreement with the undisturbed ground level source centreline concentration measurement was obtained; in this case with UjLr, = 0.8. All the buildings examined were of simple rectangular shape: three were of square planform with ratios of width to height of l/3, 1 and 3 and the rest were of square section transverse to the flow of various widths from 4 to 8. Releases were made from a roof top source or stack located in the centre of the roof, or from a ground level source immediately behind the building. The cases chosen for comparison with theory are summarized in Table 1. Some of this work is included in the M.Sc. thesis of Dean (1981), but the rest is, as yet, unpublished. (2) AGR nuclear power station model data This was obtained in the same boundary layer flow as that above. The prescriptions for the undisturbed eY and o2 are
Table 1. Dean and Robins’ data chosen for comparison with theories Buildings wxlxh (m full scale at 1:300) 9x9~27 27 x 27 x 27 81 x 81 x 27 13.5 x 27 x 27 54 x 27 x 27 135x27~27
Release heights
t?”
0, 27, 38 7,
0, 45 3,
42;:
40.5 1, ,I
il ,* .,
therefore the same as above, but since the building is taller U/U, is now 0.71. At full scale, the height of the reactor building is 70 m. It is about 110 m wide, with the smaller face about 40 m wide. Although there are a number of smaller buildings near the reactor building (principally the turbine hall) in making the comparison with theory the reactor ball is assumed to be dominant and its dimensions used in the theories. Concentration measurements were obtained for a complete range of wind directions at 45” intervals. A detailed description of this work is given by Fackrell and Robins (1981). (3) Hereher et al. data The previous data sets are for neutrally stable flow only, but for the study described by Hatcher er al. (1978) three different flow regimes were generated in the meteorological wind tunnel at Colorado State University; stable, neutral and unstable. The undisturbed flow parameters given in Table 2 were found to provide reasonable agreement with concentration values for ground level releases in the three flows. Releases from a 1:200 scale model of the EOCR reactor at Idaho Falls were studied in these flows. The reactor building is an approximately cubic shape joined to a lower, wider rectangular shape. For the present purposes, in the theoretical models, the building shape has been represented simply as a 23-m cube. This will aenemllv lead to higher nredicted concentrations than trying td represent the fuh shape. Ground level concentration values were measured for three release points, the three flows and at 45” intervals over the complete range of wind angles. Full details are given in Hatcher et a/. (1978). (4) Start et al. dota This is full scale field data for the same reactor site at Idaho Falls. Ground level concentration measurements were made over sampling arcs at 37,68,187,386and 794 m. Releases were made from ground level, roof top and stack as above, although the ground level position was not always the same. The releases were made in a variety of meteorological conditions although the emphasis was on light wind. stable conditions. Full detaifs of the experiments are given in Start et al. (1980).
Table 2. Hatcher et al. undisturbed flow parameters (Ail dimensions in equivalent full scale m) Flow category Unstable Neutral Stable
0 18 x”.s’ 0:13 x@ss 0.12 x0,7*
0.37 x0.85 0.26 x0.*5 0.18 x”.so
0.5 0.6 0.9
J.E. FACKRELL
92
(1)
Simple rectangular
interpretation is that the stack needs to be higher for a wide building to avoid downwash. Those models which allow for source height variation, Huber and Snyder and the Turner model with a roof height virtual source, show the correct trend with building width, although they overpredict the narrow building result and slightly underpredict the wide building result. The H-S model correctly predicts that the stack release leads to higher glc’s than the roof top reiease with the widest building. Figure 2 presents the Dean and Robins results for the square planform buildings. Results for normal orientations of the building tend to follow the trend with building width exhibited in the previous figure, but this time results are included for orientations with building faces diagonal to the wind, 0 = 45”. It can be seen that for the narrower buildings there can be substantial differences between the results produced by the two orientations. For the wider building this is not so apparent for the source heights examined, although it is stiI1 true for higher stack heights than shown. In the theoretical models the building width has been specified for the normal orientation only, i.e. l/3, 1, 3h, since using the actual projected area for An alternative
4. DETAILED COMPARISON
buildings
In Fig. 1 the various models are compared with the Dean and Robins results for rectangular buildings with a square section and various widths transverse to the flow. These results were all obtained with the front face of the building normal to the Bow. Some general points can be made from the comparison with theory. Looking at the gl source results first, we see that the predicted values from the different models tend to be fairly close together, within a factor 2. The concentration predictions are fairly close to the experimental points for the narrowest building and slightly high for the others. Generally, the Gifford model with c = ) is the highest and the FX model gives the lowest predictions. The F-C model gives closest overall agreement with experiment but slightly underpredicts the narrowest building result. The experimental results show that the roof and stack releases give much lower concentrations for the narrower buildings but become similar to the ground level results for the wide building.
I -
Cl /
L % fu 0 Ol,-
/ -
01 -
1
IO
0”
,
100
x/h
Fig. 1. Comparison of centreline ground level concentration for square section buildings (I = /I) with various widths, normal flow. (i) w = 5h, (ii) w = 2h, (iii) w = $h. Experimental data (Dean and Robins): +, GL source; x . roof source; 0, stack source (1.5h). Theoretical models: --, Barker; -, FC; --., Gifford with c = 3; I-.-., Gifford with c = 1; --.--., HS; ----, Turner. R and S indicate source at roof and stack height, respectively, in theoretical model.
/
IO
100
x/h
Fig. 2. Comparison of centreline ground level concentration for square planform buildings. (i) w=I=3h. (ii) w=l=h, (iii) w=I=h/3. Experimental data (Dean. 1981): 13, GL source, t?=O”: m, Roof source, 0’; 0. GLS, 45’; et Roof S., 45”; +, stack (1.4h), 0”; X, stack. 45”. Theoretical models as in Fig. I.
93
An examination of simple models for building influenced dispersion orientations would reduce the predicted concentrations, which is opposite to what is normally required, (except for the one case with the ground level source and the widest building). Conclusions regarding the low level source predictions are similar to before, although the models giving the lowest predictions now also slightly underestimate the wider building, normal wind results. The Barker model appears to give the lowest predictions which remain conservative. The comparison for roof top and stack releases is complicated by the wide difference in results for different wind directions. It is clear that if a model correctly predicts a normal orientation result then it will underpredict a diagonal orientation. For example, for the cube roof top release results, the diagonal orientation leads to glc’s similar to the ground level release and would be reasonably well predicted by a low level source model (such as the Barker model) but the normal orientation values are much lower and better predicted by the roof source models. None of the models tested allows for the extra downwash produced by diagonal orientations, so one must either choose a model which will cover the diagonal case and accept that it will be very conservative in some circumstances, or accept the complication of incorporating the effects of different wind directions into the models, possibly by using different parameters for normal and diagonal orientations. A few lateral plume half-widths measured at ground level were available for some of the building shapes examined above. These are shown in Fig. 3 together with the predicted values from the models (except Gifford’s model). The results from the different models lie close together for the narrowest building, but show diagonal
I 0
IO
20
30
increasing variation as the width increases. All are acceptably close to the experimental values, except the FC model, which overpredicts for the wider buildings. It is this greater lateral spread which tends to make the FC model concentration values lower than those of the other models. The slope of the predicted half-widths with downstream distance is generally slightly greater than that of the experimental values and the nearest experimental results at 1lh are mostly slightly underpredicted. Note that the restriction on maximum initial lateral spread is applied in the Barker model for the 5h wide building, effectively reducing the spread to that for a 3h wide building.
(2) AGR model The results for the AGR model are given in Fig. 4. Unlike the previous examples this was a fully detailed model of a station site, although the reactor hall does tend to be the dominant feature. In the theoretical models only the dimensions of the reactor building are specified; the two orientations at 6 = 0 and 90” being calculated (i.e. w = 1.6h and 0.6h, respectively). For the ground level source, the predicted results are generally reasonably close and slightly conservative with respect to the corresponding experimental results. However, for diagonal orientations, illustrated by the 45” experimental results, values near the building will be underpredicted, although use of the higher, w = 0.6h result to cover this case would give reasonable predictions by most of the models from * 3 to 4h outwards. The roof source results are reasonably well predicted except for the 45” case again, near to the building, when a low level source model would give better agreement.
I
I
40
0
I
IO
20
30
I
L
40
50
x/h Fig. 3. Comparison for lateral widths. (i) w= Sh, (ii) w=2h; (iii) w= h, (iv) w=)h. Experimental data (Dean and Robins): +, GL source, 0=0’; x, GLS, 45’; 0, Root source, 0’. Theoretical models as in Fig. 1.
J. E. FACKRELL
k.
a
Fig. 4. ACR model results for centreline ground level concentration. ii) GL source. w= 1.6h: (ii) GL source, w =0.6h: (iii) Roof source, w= 1.6fi; (iv) Roof source, w: =0.6/r. Experimental data (Fackrell and Robins, 1981);o. H = 0”:x, 45’; q (90”; +, 180’; 0,270’. Theoretical models as in F&. I.
x/h
Ground level lateral plume half widths and centreline vertical half widths were also measured for the AGR model and these are shown in Fig. 5. together with the predicted values. There is a tendency for the lateral plume half-width to be underpredicted in the near field by most of the models. except for the F-C model which for this building shape gives the most accurate predictions of lateral spread. For the vertical half-widths, the rate of increase of the predicted values with downstream distance is greater than for the experimental results, but the values are reasonably well predicted. Overall, both lateral and verticai half-width predictions from all the models are acceptable (within about a factor of 2) although individuat variations with wind direction will not be reproduced. as was true for concentration values. Note also that for cases with the ground level source upstream of the building, the experimental results showed that the plume could be displaced laterally by up to about a full building width from the downstream building centreline as it travelled around the side of the building. Also, plume profiles near to the building could be greatly skewed and distorted and not well reproduced by a Gaussian model. Together, these could be more important sources of error in predicting the concentration field near the building than any differences in plume spreads. (3) EOCR model The comparison with the results of Hatcher rf al. (1978) for the three flow stabilities examined, neutral, unstable and stable, is shown in Fig. 6. To be consistent
x/h
Fig. 5. AGR model results for lateral and vertical half widths. (i) lateral half width, w=0.6h; (iI) lateral half width, w= 1.6h; (iii) vertical half width, wr0.6h; (iv) vertical half width, w= 1.6h. Experimental data: o. GL source,0=00; l ,Roofsource,O;x,GLS,45’;i,RoofS,45”; Cl.GLS,W; m ,RoofS,W;+.GLS,lgO: 0. GLS, 270”. Theoretical models as in Fig. 1.
An examinarion of
simple models
for building
influenced
dispersion
Stock
(4
x/h
x/h
\
Stock \
\
“\
..
: + x
l...j e?
i ib)
i0 x/h
10
r/h
x/h
0
0
10
.x/h
Roof source 0
\
Stack i
x/h
x/h
Fig. 6. Comparison of centreline ground level concentration for EOCR wind tunnel data. (a) Unstable Row,(b) Neutral flow,(c) Stable flow. Experimental data (Hatcher et al., 1978): D ,B = 0”; +, 45”;X,90”; o, 135”; Cl. 180”; V, 225”; A, 270”; 0. 315”. Theoretical models as in Fig. I.
96
J. E. FACKRELL
with the AGR results, the wind angle, 0, is defined from the normal to the building face containing the Lowlevel source. This is different to the definition in the original paper. For the neutral and unstable results, the low level models’ predictions lie fairly close together and just above the top end of the experimental results. The stable predictions tend to be slightly lower relative to the experimenta results, especially far downstream, so that some wind directions are underpredicted, although by no more than about a factor of 2 in the worst case. In the roof and stack release comparison, the Turner roof and H-S model show the right behaviour for normal orientations of the building but underpredict some diagonal orientations. These are better predicted by a low level source model, as characterised in the figures by the Barker model results. (4) EOCR juwUscale The final set of comparisons is between theoretical models, wind tunnel results and full-scale results for the EOCR reactor (Fig. 7). In plotting the full-scale data the same symbols as in the Hatcher et al. data are used, the symbol for the nearest value to the actual wind direction being assigned. For the ground level source results, B is always taken from the normal to the face containing the source. The theoretical models are still apphed to a 23-m cube, as above, but now use the formulation of Clarke (1979) for the undisturbed flow spreads (categories A, D and F). Only the short term lateral spread has been enhanced in the models, the longer term wind meandering in the Clarke formulation remains the same. The full-scale results and the model predictions refer to hourly averaged data. Wind tunnel resultsare generally assumed to apply to shorter time averages than this, 10-30min. so that they would be expected to give higher concentrations. Generally, agreement between the theoreticai models. wind tunnef and full-scale results is reasonably good with the theoretical and wind tunnel results lying towards the top end of the range of full-scale data. Note, however. that the vertical scale in the figures has been greatly reduced to accommodate the scatter in the full-scale results. On this scale the differences in the theoretical models is fairly small and only representative samples are shown for the low level models. This illustrates the difTiculty of using full scale data to judge the models since the degree of scatter is much greater than differences between the models. This scatter is mainly from run to run rather than in the individua1 runs. It appears to be mainly caused because runs assigned to the same stability category (based on lapse rate) are in performed under differing flow confact ditions-different wind speed, direction and shear, slightly different temperature protiles and, in particular, quite large differences in lateral fluctuations of the wind, eg. Note that in the present work, categories EFG are plotted together, but the scatter in results for the individual categories would be similar to the combined one shown. The wind tunnel results and theoretical models tend to be conservative with regard
(a)
(bl
IO x/h
100
Fig. 7ia.b)
An examination of simple models for building influenced dispersion ’
91
others. Indeed their behaviour is fairly similar and there is usually less than a factor of two difference between the range of predictions for low level sources. This is obtained using the numerical values specified for the models. By suitably adjusting these, the differences could be even further reduced. It is perhaps not surprising that the models are similar, since they all incorporate the building effect as an enhanced spreading of a Gaussian plume, differing only in the details in which this is accomplished.
.
(2) Low level releases
Stoble
(4 Fig. 7. Comparison of centreline ground level concentration for EOCR, full scale, wind tunnel and theoretical models. (a) Ground level source. (b) Roof source. (c) Stack source. Full scale experimental
data(StarteraL(1980)): D,e=W; +,45’; x (90”; 0,135”; 0,180”; V,225”;A, 270’; 0. 315”. Hatched areas indicate range of wind tunnel data for all t?. Theoretical models as in Fig. I.
to the full-scale results and the closest to the average values for neutral and unstable cases, being more conservative in the stable case. This is probably because neither the wind tunnel flow nor the model of Clarke (1979) adequately incorporate the observed large lateral meandering in light wind stable conditions. There is no clear difference between normal and diagonal results in the full scale data, as there was in the wind tunnel data, because the scatter is too great.
5. DISCUSSION
(1) Intercomparison of models In this paper the predictions of several simple models for building influenced dispersion have been compared with a fairly wide range of experimental data. It was hoped that this might allow the selection of a ‘best’ model but it is clear from the comparison that no one model is particularly outstanding relative to the
For low level sources, the GitTord model with c =) usually gave the highest predictions and the F-C model the lowest, with the others in between. The two virtual source models, Turner and Barker, were usually very close, differing most near the building, where the Turner model was more conservative. Generally the models gave reasonably good predictions for low level source results beyond 3-5 building heights (h), tending to be slightly conservative by less than a factor of 2, so that the lowest predictions, by the FC model, were most often closest to the experimental results. However, in some cases this model underpredicted concentration and overpredicted lateral spread for wide buildings. The Barker or Turner models gave the lowest predictions which remained conservative in most cases. None of these models as they stand will correctly predict behaviour with wind direction, particularly for diagonal orientations. Since, for example, the Hatcher data shows considerable variation with wind direction, the models which are to be conservative for all cases must considerably overpredict some wind directions (by a factor of 4 or 5). (3) Higher level releases (rooflop and above) The problem of wind direction variations is even more noticeable for higher level releases. The experimental data generally shows considerably higher concentrations for diagonal orientations of the building than for those with the front face normal to the flow. Models for higher level releases which predict normal orientations reasonably well, will underpredict diagonal ones. Using the models for low level releases can often predict diagonal orientations quite well but this will be very conservative for other high level release cases (by over an order of magnitude and for a considerable distance downstream, in some cases). As well as wind direction and stack height, the amount by which a stack release is influenced by a building can depend very much on roof area, i.e. the longitudinal extent of the building as well as the lateral extent. None of the models allows for this. Only short stacks of up to 1.5 building heights have been examined in this paper. Although building effect tends to decrease the effective source height of taller stacks, they generally behave as elevated releases and use of a low level model for stacks greater than 1.5h would normally be very pessimistic. To some extent the same conclusions apply to shorter stacks in very stable flows.
98
J. E. FACKRELL
(4) Plume spreads
This paper has mainly concentrated upon the prediction of centreline glc, but some comparisons were made for plume spreads. Generally, the predictions were reasonably close to the measured spreads (usually well within a factor of 2). The F-C model was found to overpredict lateral spread for wide buildings (w > Zh), but predicted quite well for narrower ones. The other models tended to slightly underpredict lateral spread near the building and there was a general tendency to slightly overpredict spreading rate. As for concentration, the models will not reproduce the complex variation of the spreads with wind direction, or with source height.
6.
however be difficult to incorporate all important building effects in a simple model. without having a great deal of a-priori empirical information for each case considered. To go further, requires a more complex model which directly takes into account, at least, the main features of the velocity field around a building.
Acknoaledgemenrs-~This work was carried out at Marchwood Engineering Laboratories and the paper is published by permission ofthe Central Electricity Generating Board. The work was sponsored under CEC Contract No. SR 012 UK, “Effects of Buildings on Low-Level Atmospheric Discharges”, as part of the Indirect Action Research Programme of the European Atomic Energy Community on the Safety of Thermal Water Reactors.
CONCLUSIONS
Very near to a building, a real piume, especially from a low level source, can be greatly distorted and displaced by the complicated flow field and by small local site features. These simple models should not be expected to predict the concentration field in this region. A wind tunnel study would seem best at the present time, especially for complex sites, and is probably also required if the source is not passive (i.e. with signifi~nt momentum or buoyancy). Further downstream (say S-iOh), many of the local effects will have become small and the main building effect can then be reasonably represented by a change in effective source height and enhanced initial spread, as in these models. The models will not predict variations with wind direction or source position, but for many practical applications, all that is required is to predict with minimum pessimism the worst wind direction result for a given source, the results for other wind directions then being more conservatively predicted. For such applications, one of the low-level release models can be used for all releases, in order to cover diagonal orientations for higher Ievel releases (at least up to 1Sh). Of those models tested, the virtual source models would seem marginally better from this point of view, although any of the other models could be made to perform similarly by changing the numerical values employed. If it is wished to do better than this using one of these simple modets, then at least some allowance must be made for source height and wind direction variations. The empirically based approach, as used for the W-S model, is attractive. but requires a wider range of input data than was used for that model. With this, it should be possible to incorporate the more obvious effects of wmd direction and source position into any of those models, at least for simple building shapes, (By, for example, distinguishing between normal and diagonal orientations of the building, with higher level releases). It would
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