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Some measurements of the surface pressure fluctuations on wind-tunnel models of a low-rise building
Journal o f Wind Engineering and Industrial Aerodynamics, 10 ( 1982 ) 361--372
361
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
S O M E M E A S U R E M E N T S OF THE S U R F A C E P R E S S U R E FLUCTUATIONS ON WIND-TUNNEL MODELS OF A LOW-RISE BUILDING
A.E. HOLDO* School o f Engineering, The Hatfield Polytechnic, Hatfield, Herts A L I O 9AB (Gt. Britain)
(Received February 25, 1982; accepted in revised form July 26, 1982)
Summary A knowledge of the pressure fluctuations on buildings exposed to strong winds is important for wind loading calculations. The statistical quantities of such fluctuations in terms of r.m.s, values and power spectra for models resembling the Aylesbury experimental building of the Building Research Establishment are presented and compared with full-scale results. The comparison suggests that provided the properties of the longitudinal velocity component are suitably simulated, then agreement between full-scale and model results in terms of r.m.s, values and power spectra can be achieved.
Introduction T h e m e a n pressure d i s t r i b u t i o n o v e r b l u f f b o d i e s i m m e r s e d in t u r b u l e n t flows, such as buildings in t h e a t m o s p h e r i c b o u n d a r y layer, has b e e n s h o w n t o be d e p e n d e n t o n shear and t u r b u l e n c e p a r a m e t e r s [ 1 - - 3 ] . Statistical i n f o r m a t i o n c o n c e r n i n g surface p r e s s u r e f l u c t u a t i o n s is generally m o r e i m p o r t a n t t h a n m e a n values w h e n c o n s i d e r i n g w i n d loading o n buildings and structures. It is t h e r e f o r e n e c e s s a r y t o ascertain w h e t h e r m o d e l s o f the a t m o s p h e r i c b o u n d a r y l a y e r in w i n d t u n n e l s w h i c h give a d e q u a t e m e a n pressure d i s t r i b u t i o n s o v e r m o d e l buildings will also give a c c e p t a b l e pressure d i s t r i b u t i o n s in t e r m s o f o t h e r statistical quantities. In this p a p e r , r.m.s, values and p o w e r s p e c t r a o f t h e pressure f l u c t u a t i o n s on w i n d - t u n n e l m o d e l s r e s e m b l i n g t h e A y l e s b u r y e x p e r i m e n t a l building o f the Building R e s e a r c h E s t a b l i s h m e n t are p r e s e n t e d [ 4 , 5 ] . T h e s e results w e r e o b t a i n e d using t h e I n d u s t r i a l A e r o d y n a m i c s wind t u n n e l at H a t f i e l d P o l y t e c h nic, w h e r e a s i m u l a t i o n o f the a t m o s p h e r i c b o u n d a r y l a y e r gave results in t e r m s o f m e a n pressure d i s t r i b u t i o n s close t o t h o s e m e a s u r e d b y t h e Building R e s e a r c h E s t a b l i s h m e n t o n t h e full-scale A y l e s b u r y building [ 4 ] . T h e p r e s e n t statistics f o r t h e f l u c t u a t i n g c o m p o n e n t s are also c o m p a r e d w i t h t h e p u b l i s h e d full-scale results o b t a i n e d at t h e A y l e s b u r y building [ 5 , 6 ] . * Present address: Division of Energy and Fluid Dynamics, S.I.N.T.E.F., 7034 Trondheim-NTH, Norway. 0304-3908/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company
362
Equipment and instrumentation Two models of the Aylesbury experimental building were used during the experiments. They were both 1/200th scale, but one model was solid whilst the other was hollow to allow measurements of internal pressure. The latter model had fewer pressure tappings because of the restricted space inside the model. However, the models complemented each other, as they facilitated double checks on a large number of pressure measurements. The wind tunnel used was of the open return type with a working section 1.2 m × 1.5 m, and the m a x i m u m continuous velocity through the working section was 25 m s -1. Modelling of the atmospheric b o u n d a r y layer was achieved by the use of a barrier--roughness combination [ 7,8]. The roughness elements were wooden blocks randomly positioned between a barrier 125 mm high and the model. The height of the blocks gradually decreased from 100 m m near the barrier to 10 mm just in front of the model, which was placed ~ 4 . 5 m downstream of the barrier. The results of the simulation in terms of the power spectrum of the longitudinal velocity c o m p o n e n t are compared with full-scale values in Fig. 1. The shear and turbulence-intensity profiles are shown in Fig. 2(a) and (b), respectively [4]. The flow characteristics were measured by means of pitot--static probes and standard hot-wire a n e m o m e t r y capable of accommodating both singleand cross-wire probes. Pressure transducers placed inside pressure scanners were used for the measurements of both fluctuating and mean pressures. The outputs from the hot-wire bridges were linearised so that a linear frequency response from 0 to 20 kHz was ensured. The plastic tubing from the pressure scanner to the pressure tappings on the model surface, as well as the response of the pressure scanner itself, severely restricted the frequency range of pressure transducer operation. Calibration of the tubes and pressure scanner was carried out, and the results from the surface pressure measurements were corrected using this calibration. The outputs from the anemometers and transducers were digitised and
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364
processed using either a D.E.C. PDP 8/e or PDP 11/03 on-line facility. T he latter was p r o g r am m ed to det er m i ne statistical quantities such as p o w e r spectra, cross-spectra and correlation functions. T he use of r.m.s, meters and analogue correlators allowed checks between the analogue and digital methods.
Results The r.m.s, values were averaged over 30 s and normalised to the freestream d y n amic pressure at a scaled height of 10 m. Since these values are all w i t h o u t sign, t h e y are shown by lines outwards from the models on the figures presented here. T h e r.m.s, levels {Figs. 3--6) are in general found to follow the same pattern as t he mean pressure levels e x c e p t in the region of wake flow. In this region the r.m.s, levels are very much lower t han in any o t h e r region of flow. It is interesting to n o t e t hat if the ratio of r.m.s, to mean pressure level based on values from refs. 4 or 5 is calculated, t hen it is f o u n d to be largely co n s tan t in all but t he region d o m i n a t e d by the wake. T he value of this ratio is on average 0.3; in the wake-dominated region, it is 0.1. These values o f the ratio o f r.m.s, to mean pressure level m ust be used with e x t r e m e caution if an a t t e m p t is made t o e x t r a c t i nf or m a t i on a b o u t r.m.s, levels from mean pressure results for configurations similar to those of the present tests, since slight deviations in terms of flow conditions or m o del g e o m e t r y m a y well p r o d u c e quite d i f f e r e n t results. Evidence o f the longitudinal velocity spectrum was found on all windward
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365
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Fig. 5. C o m p a r i s o n b e t w e e n m o d e l a n d full-scale r e s u l t s for w i n d w a r d s u r f a c e s : relative flow direction 90 ° .
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surfaces and in the region o f separated flow, b u t n o t in t h e region d o m i n a t e d by t h e w a k e flow. In the wake region the pressure p o w e r spectra display peaks which can be i n t e r p r e t e d o n l y as being due t o flow mechanisms in the wake. These peaks give S t r o u h a l n u m b e r estimates b e t w e e n 0.19 and 0.23, based o n the v e l o c i t y at t h e scaled 10 m height and the characteristic b o d y d i m e n s i o n n o r mal to the d i r e c t i o n o f flow. H o w e v e r , as seen f r o m Figs. 7 and 8, these peaks are v e r y b r o a d , and a c c u r a t e l y estimated i n t e r p r e t a t i o n s o f the flow in the wake are t h e r e f o r e v e r y difficult. C o m p a r i s o n o f m o d e l results with full-scale results T h e full-scale results used for this c o m p a r i s o n are those published in refs. 5 and 6. It should be n o t e d t h a t whilst the m o d e l results were o b t a i n e d for wind i n c i d e n t at 0 °, 45 °, 90 °, 180 ° and 270 °, the c o r r e s p o n d i n g full-scale results were o b t a i n e d at respectively 8 ° , 41 ° , 92 ° , 170 ° and 268 ° . Previous c o m p a r i s o n s in terms of m e a n pressures have suggested t h a t the modelling t e c h n i q u e s used were a d e q u a t e [ 4 ] .
R.m.s. pressure coefficients When t h e wind d i r e c t i o n is n o m i n a l l y 0 °, the r.m.s, pressure coefficients o n the w i n d w a r d surfaces o f t h e 1 / 2 0 0 t h scale m o d e l are v e r y similar to the fullscale results. No p a r t i c u l a r trends are f o u n d in the differences, which are on average o n l y 3%. It m a y be seen f r o m Figs. 3 and 4 t h a t c o m p a r i s o n s have b e e n m a d e o n l y for a limited n u m b e r o f positions, b u t these positions c o v e r m o s t of the w i n d w a r d wall. In the region o f separated flow on the short wall the d i f f e r e n c e is again 3% o n average, whilst in the wake region t h e d i f f e r e n c e bet w e e n the m o d e l and full-scale results is ~ 5%.
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Figures 5 and 6 show comparisons for the 90 ° case. The differences here are o f the same order as for the 0 ° case. Since differences in the direction of flow exist between these full-scale and m o d e l results it is somewhat unexpected that the results should be so similar. This illustrates the insensitivity of the r.m.s, levels to small changes in wind direction for this particular t y p e o f bluff body. Comparison for the 45 ° case, as seen from Fig, 7, shows the same overall agreement as for the two other flow directions. Pressure p o w e r spectra This comparison is limited to the nominally 0 ° case, owing to t h e lack of published full-scale data. The spectra on the windward model surfaces are quite similar to the full-scale spectra for those surfaces in terms o f spectral peak frequencies (Figs. 8--10). The main difference occurs in the range o f wavenumber from 0.01 to 0.02, as seen from Fig. 8. The full-scale spectrum displays a much larger amplitude at these frequencies. These disturbances could be due to bow vortices and vortex stretching in the vicinity of the surface. Thus, any misalignment between model and full scale could produce differences such as reported here. Since the flow directions are 0 ° and 8 ° respectively for the model and full-scale results, this is a possible cause. The pressure power spectra on the short wall (in separated flow) are also quite similar, as seen from Figs. 13 and 14. The model internal-pressure power spectrum (Fig. 15) for the case of evenly distributed permeability of all wall and roof surfaces coincides well with the full-scale intemal-pre~ure power spectrum. This is possibly a coincidence rather than a genuine match, as all peaks f o u n d in the extemal-pre'~ure power spectra are present in this model spectrum. The permeability of the full-scale Aylesbury building m a y well n o t correspond to t h a t modelled here, particularly as the difference in the r.m.s, levels is ~ 10%.
369 SMOOTHMODELSPECTRUM
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371 T h e largest d i f f e r e n c e b e t w e e n the m o d e l and t h e full-scale s p e c t r a is f o u n d on t h e lee side o f t h e building, as seen f r o m Figs. 11 and 12. T h e l o w - f r e q u e n c y p e a k o f t h e f r e e s t r e a m l o n g i t u d i n a l v e l o c i t y c o m p o n e n t is n o t clearly a p p a r e n t in t h e m o d e l s p e c t r u m , b u t it is seen in t h e full-scale A y l e s b u r y s p e c t r u m . A survey o f s o m e o t h e r full-scale s p e c t r a has suggested t h a t t h e a b o v e low-freq u e n c y p e a k is n o t generally p r e s e n t t o such a large e x t e n t in t h e lee-side pressure p o w e r s p e c t r a as f o r t h e full-scale A y l e s b u r y building [ 9 - - 1 1 ] . It should, h o w e v e r , be n o t e d t h a t t h e r.m.s, levels o n t h e lee side are v e r y l o w ; t h u s a n y u n i n t e n d e d e x p o s u r e o f the m e a s u r e m e n t s y s t e m t o the f r e e s t r e a m , such as b y m e m b r a n e a c t i o n or t h r o u g h the l o n g lengths of flexible t u b i n g leading to the r e f e r e n c e p r e s s u r e t a p p i n g s , c o u l d cause t r a n s m i s s i o n o f low- (or high-)freq u e n c y signals and in this w a y d i s t o r t the pressure s p e c t r a at these c o m p a r a tively l o w r.m.s, levels. Such t r a n s m i s s i o n w o u l d , h o w e v e r , be u n n o t i c e d at the higher r.m.s, levels. Conclusions C o m p a r i s o n b e t w e e n full-scale and m o d e l results has suggested t h a t windt u n n e l s i m u l a t i o n o f buildings in t h e a t m o s p h e r i c b o u n d a r y l a y e r can p r o d u c e results in t e r m s o f t h e statistical q u a n t i t i e s o f the f l u c t u a t i n g surface pressures t h a t are v e r y close to t h e full-scale results. T h e r.m.s, pressure levels, as well as p e a k s in t h e p o w e r s p e c t r a , a p p e a r to be insensitive t o small changes in t h e d i r e c t i o n o f flow. T h e m o s t n o t i c e a b l e d i f f e r e n c e b e t w e e n full-scale and t h e p r e s e n t m o d e l results occurs in the w a k e region: this d i f f e r e n c e c o u l d be d u e t o t h e differing t y p e s o f i n s t r u m e n t a t i o n used in the m o d e l and full-scale studies. Acknowledgements W i n d - t u n n e l m o d e l s , o t h e r i t e m s o f e q u i p m e n t and initial f u n d s p r o v i d e d b y t h e Building R e s e a r c h E s t a b l i s h m e n t , and financial s u p p o r t given b y t h e Science R e s e a r c h Council, are gratefully a c k n o w l e d g e d . References 1 M. Jensen, The Model law for Phenomena in the Natural Wind, Part 2, Danish Technical Press, Copenhagen, 1965. 2 B.E. Lee, The effect of turbulence on the surface pressure field of a square prism, J. Flui~ Mech., 69 (1975) 263--282. 3 A. Laneville, I.S. Gartshore and G.V. Parkinson, An explanation of some effects of turbulence on bluff bodies, 4th Int. Conf. on Wind Effects on Buildings and Structures, in K.J. Eaton (ed.), Heathrow, 1975, Cambridge University Press, London, 1977, pp. 333341. 4 A.E. HoldS, Ph.D. Thesis, Hatfield Polytechnic, 1979. 5 K.J. Eaton and J.R. Mayne, The measurement of wind pressure on two-storey houses at Aylesbury, Build. Res. Establ., Curr. Pap. CP 70/74 (1974). 6 K.J. Eaton, J.R. Mayne and N.J. Cook, Wind loads on low-rise buildings--effects of roof geometry, Build. Res. Establ., Curr. Pap. CP 1/76 (1976).
372 7 J. Counihan, A method of simulating a neutral atmospheric boundary layer in a wind tunnel, Proc. Advisory Group Aerospace Res. Dev., Conf., Munich, 1970, No. 48. 8 N.J. Cook, Wind tunnel simulation of the atmospheric boundary layer by roughness~ barrier and mixing-device methods, J. Ind. Aerodyn., 3 (1978) 157--167. 9 C.M. Newberry, K.J. Eaton and J.R. Mayne, Wind loading on tall buildings: further results from Royex House, Build. Res. Establ., Curt. Pap. CP 29/73 (1973). 10 W.A. Dalgliesh, Comparison of model/full-scale wind pressure on a high-rise building, J. Ind. Aerodyn., 1 (1975) 55--66. 11 J.D. Holmes, Pressure fluctuations on a large building and alongwind structural response, J. Ind. Aerodyn., 1 (1975/76) 249--278.
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Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
S O M E M E A S U R E M E N T S OF THE S U R F A C E P R E S S U R E FLUCTUATIONS ON WIND-TUNNEL MODELS OF A LOW-RISE BUILDING
A.E. HOLDO* School o f Engineering, The Hatfield Polytechnic, Hatfield, Herts A L I O 9AB (Gt. Britain)
(Received February 25, 1982; accepted in revised form July 26, 1982)
Summary A knowledge of the pressure fluctuations on buildings exposed to strong winds is important for wind loading calculations. The statistical quantities of such fluctuations in terms of r.m.s, values and power spectra for models resembling the Aylesbury experimental building of the Building Research Establishment are presented and compared with full-scale results. The comparison suggests that provided the properties of the longitudinal velocity component are suitably simulated, then agreement between full-scale and model results in terms of r.m.s, values and power spectra can be achieved.
Introduction T h e m e a n pressure d i s t r i b u t i o n o v e r b l u f f b o d i e s i m m e r s e d in t u r b u l e n t flows, such as buildings in t h e a t m o s p h e r i c b o u n d a r y layer, has b e e n s h o w n t o be d e p e n d e n t o n shear and t u r b u l e n c e p a r a m e t e r s [ 1 - - 3 ] . Statistical i n f o r m a t i o n c o n c e r n i n g surface p r e s s u r e f l u c t u a t i o n s is generally m o r e i m p o r t a n t t h a n m e a n values w h e n c o n s i d e r i n g w i n d loading o n buildings and structures. It is t h e r e f o r e n e c e s s a r y t o ascertain w h e t h e r m o d e l s o f the a t m o s p h e r i c b o u n d a r y l a y e r in w i n d t u n n e l s w h i c h give a d e q u a t e m e a n pressure d i s t r i b u t i o n s o v e r m o d e l buildings will also give a c c e p t a b l e pressure d i s t r i b u t i o n s in t e r m s o f o t h e r statistical quantities. In this p a p e r , r.m.s, values and p o w e r s p e c t r a o f t h e pressure f l u c t u a t i o n s on w i n d - t u n n e l m o d e l s r e s e m b l i n g t h e A y l e s b u r y e x p e r i m e n t a l building o f the Building R e s e a r c h E s t a b l i s h m e n t are p r e s e n t e d [ 4 , 5 ] . T h e s e results w e r e o b t a i n e d using t h e I n d u s t r i a l A e r o d y n a m i c s wind t u n n e l at H a t f i e l d P o l y t e c h nic, w h e r e a s i m u l a t i o n o f the a t m o s p h e r i c b o u n d a r y l a y e r gave results in t e r m s o f m e a n pressure d i s t r i b u t i o n s close t o t h o s e m e a s u r e d b y t h e Building R e s e a r c h E s t a b l i s h m e n t o n t h e full-scale A y l e s b u r y building [ 4 ] . T h e p r e s e n t statistics f o r t h e f l u c t u a t i n g c o m p o n e n t s are also c o m p a r e d w i t h t h e p u b l i s h e d full-scale results o b t a i n e d at t h e A y l e s b u r y building [ 5 , 6 ] . * Present address: Division of Energy and Fluid Dynamics, S.I.N.T.E.F., 7034 Trondheim-NTH, Norway. 0304-3908/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company
362
Equipment and instrumentation Two models of the Aylesbury experimental building were used during the experiments. They were both 1/200th scale, but one model was solid whilst the other was hollow to allow measurements of internal pressure. The latter model had fewer pressure tappings because of the restricted space inside the model. However, the models complemented each other, as they facilitated double checks on a large number of pressure measurements. The wind tunnel used was of the open return type with a working section 1.2 m × 1.5 m, and the m a x i m u m continuous velocity through the working section was 25 m s -1. Modelling of the atmospheric b o u n d a r y layer was achieved by the use of a barrier--roughness combination [ 7,8]. The roughness elements were wooden blocks randomly positioned between a barrier 125 mm high and the model. The height of the blocks gradually decreased from 100 m m near the barrier to 10 mm just in front of the model, which was placed ~ 4 . 5 m downstream of the barrier. The results of the simulation in terms of the power spectrum of the longitudinal velocity c o m p o n e n t are compared with full-scale values in Fig. 1. The shear and turbulence-intensity profiles are shown in Fig. 2(a) and (b), respectively [4]. The flow characteristics were measured by means of pitot--static probes and standard hot-wire a n e m o m e t r y capable of accommodating both singleand cross-wire probes. Pressure transducers placed inside pressure scanners were used for the measurements of both fluctuating and mean pressures. The outputs from the hot-wire bridges were linearised so that a linear frequency response from 0 to 20 kHz was ensured. The plastic tubing from the pressure scanner to the pressure tappings on the model surface, as well as the response of the pressure scanner itself, severely restricted the frequency range of pressure transducer operation. Calibration of the tubes and pressure scanner was carried out, and the results from the surface pressure measurements were corrected using this calibration. The outputs from the anemometers and transducers were digitised and
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Fig. 2. (a) Comparison between model and full-scale shear profiles. (b) Comparison between model and full-scale intensity profiles.
364
processed using either a D.E.C. PDP 8/e or PDP 11/03 on-line facility. T he latter was p r o g r am m ed to det er m i ne statistical quantities such as p o w e r spectra, cross-spectra and correlation functions. T he use of r.m.s, meters and analogue correlators allowed checks between the analogue and digital methods.
Results The r.m.s, values were averaged over 30 s and normalised to the freestream d y n amic pressure at a scaled height of 10 m. Since these values are all w i t h o u t sign, t h e y are shown by lines outwards from the models on the figures presented here. T h e r.m.s, levels {Figs. 3--6) are in general found to follow the same pattern as t he mean pressure levels e x c e p t in the region of wake flow. In this region the r.m.s, levels are very much lower t han in any o t h e r region of flow. It is interesting to n o t e t hat if the ratio of r.m.s, to mean pressure level based on values from refs. 4 or 5 is calculated, t hen it is f o u n d to be largely co n s tan t in all but t he region d o m i n a t e d by the wake. T he value of this ratio is on average 0.3; in the wake-dominated region, it is 0.1. These values o f the ratio o f r.m.s, to mean pressure level m ust be used with e x t r e m e caution if an a t t e m p t is made t o e x t r a c t i nf or m a t i on a b o u t r.m.s, levels from mean pressure results for configurations similar to those of the present tests, since slight deviations in terms of flow conditions or m o del g e o m e t r y m a y well p r o d u c e quite d i f f e r e n t results. Evidence o f the longitudinal velocity spectrum was found on all windward
----~FULLS[ALE RESULTS
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Fig. 3. C o m p a r i s o n b e t w e e n m o d e l and f u l l - s c a l e r e s u l t s f o r w i n d w a r d s u r f a c e s : r e l a t i v e flow direction 0 ° .
365
E RESULTS
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LE RESULTS
Fig. 5. C o m p a r i s o n b e t w e e n m o d e l a n d full-scale r e s u l t s for w i n d w a r d s u r f a c e s : relative flow direction 90 ° .
366
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Fig. 6. Comparison between model and full-scale results for leeward surfaces: relative flow direction 270 ° .
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Wave number n[m -1) Fig. 8. Comparison b e t w e e n s m o o t h e d model and full-scale pressure power spectra: relative flow direction 0°; observation point close to centre of windward surface.
surfaces and in the region o f separated flow, b u t n o t in t h e region d o m i n a t e d by t h e w a k e flow. In the wake region the pressure p o w e r spectra display peaks which can be i n t e r p r e t e d o n l y as being due t o flow mechanisms in the wake. These peaks give S t r o u h a l n u m b e r estimates b e t w e e n 0.19 and 0.23, based o n the v e l o c i t y at t h e scaled 10 m height and the characteristic b o d y d i m e n s i o n n o r mal to the d i r e c t i o n o f flow. H o w e v e r , as seen f r o m Figs. 7 and 8, these peaks are v e r y b r o a d , and a c c u r a t e l y estimated i n t e r p r e t a t i o n s o f the flow in the wake are t h e r e f o r e v e r y difficult. C o m p a r i s o n o f m o d e l results with full-scale results T h e full-scale results used for this c o m p a r i s o n are those published in refs. 5 and 6. It should be n o t e d t h a t whilst the m o d e l results were o b t a i n e d for wind i n c i d e n t at 0 °, 45 °, 90 °, 180 ° and 270 °, the c o r r e s p o n d i n g full-scale results were o b t a i n e d at respectively 8 ° , 41 ° , 92 ° , 170 ° and 268 ° . Previous c o m p a r i s o n s in terms of m e a n pressures have suggested t h a t the modelling t e c h n i q u e s used were a d e q u a t e [ 4 ] .
R.m.s. pressure coefficients When t h e wind d i r e c t i o n is n o m i n a l l y 0 °, the r.m.s, pressure coefficients o n the w i n d w a r d surfaces o f t h e 1 / 2 0 0 t h scale m o d e l are v e r y similar to the fullscale results. No p a r t i c u l a r trends are f o u n d in the differences, which are on average o n l y 3%. It m a y be seen f r o m Figs. 3 and 4 t h a t c o m p a r i s o n s have b e e n m a d e o n l y for a limited n u m b e r o f positions, b u t these positions c o v e r m o s t of the w i n d w a r d wall. In the region o f separated flow on the short wall the d i f f e r e n c e is again 3% o n average, whilst in the wake region t h e d i f f e r e n c e bet w e e n the m o d e l and full-scale results is ~ 5%.
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Figures 5 and 6 show comparisons for the 90 ° case. The differences here are o f the same order as for the 0 ° case. Since differences in the direction of flow exist between these full-scale and m o d e l results it is somewhat unexpected that the results should be so similar. This illustrates the insensitivity of the r.m.s, levels to small changes in wind direction for this particular t y p e o f bluff body. Comparison for the 45 ° case, as seen from Fig, 7, shows the same overall agreement as for the two other flow directions. Pressure p o w e r spectra This comparison is limited to the nominally 0 ° case, owing to t h e lack of published full-scale data. The spectra on the windward model surfaces are quite similar to the full-scale spectra for those surfaces in terms o f spectral peak frequencies (Figs. 8--10). The main difference occurs in the range o f wavenumber from 0.01 to 0.02, as seen from Fig. 8. The full-scale spectrum displays a much larger amplitude at these frequencies. These disturbances could be due to bow vortices and vortex stretching in the vicinity of the surface. Thus, any misalignment between model and full scale could produce differences such as reported here. Since the flow directions are 0 ° and 8 ° respectively for the model and full-scale results, this is a possible cause. The pressure power spectra on the short wall (in separated flow) are also quite similar, as seen from Figs. 13 and 14. The model internal-pressure power spectrum (Fig. 15) for the case of evenly distributed permeability of all wall and roof surfaces coincides well with the full-scale intemal-pre~ure power spectrum. This is possibly a coincidence rather than a genuine match, as all peaks f o u n d in the extemal-pre'~ure power spectra are present in this model spectrum. The permeability of the full-scale Aylesbury building m a y well n o t correspond to t h a t modelled here, particularly as the difference in the r.m.s, levels is ~ 10%.
369 SMOOTHMODELSPECTRUM
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371 T h e largest d i f f e r e n c e b e t w e e n the m o d e l and t h e full-scale s p e c t r a is f o u n d on t h e lee side o f t h e building, as seen f r o m Figs. 11 and 12. T h e l o w - f r e q u e n c y p e a k o f t h e f r e e s t r e a m l o n g i t u d i n a l v e l o c i t y c o m p o n e n t is n o t clearly a p p a r e n t in t h e m o d e l s p e c t r u m , b u t it is seen in t h e full-scale A y l e s b u r y s p e c t r u m . A survey o f s o m e o t h e r full-scale s p e c t r a has suggested t h a t t h e a b o v e low-freq u e n c y p e a k is n o t generally p r e s e n t t o such a large e x t e n t in t h e lee-side pressure p o w e r s p e c t r a as f o r t h e full-scale A y l e s b u r y building [ 9 - - 1 1 ] . It should, h o w e v e r , be n o t e d t h a t t h e r.m.s, levels o n t h e lee side are v e r y l o w ; t h u s a n y u n i n t e n d e d e x p o s u r e o f the m e a s u r e m e n t s y s t e m t o the f r e e s t r e a m , such as b y m e m b r a n e a c t i o n or t h r o u g h the l o n g lengths of flexible t u b i n g leading to the r e f e r e n c e p r e s s u r e t a p p i n g s , c o u l d cause t r a n s m i s s i o n o f low- (or high-)freq u e n c y signals and in this w a y d i s t o r t the pressure s p e c t r a at these c o m p a r a tively l o w r.m.s, levels. Such t r a n s m i s s i o n w o u l d , h o w e v e r , be u n n o t i c e d at the higher r.m.s, levels. Conclusions C o m p a r i s o n b e t w e e n full-scale and m o d e l results has suggested t h a t windt u n n e l s i m u l a t i o n o f buildings in t h e a t m o s p h e r i c b o u n d a r y l a y e r can p r o d u c e results in t e r m s o f t h e statistical q u a n t i t i e s o f the f l u c t u a t i n g surface pressures t h a t are v e r y close to t h e full-scale results. T h e r.m.s, pressure levels, as well as p e a k s in t h e p o w e r s p e c t r a , a p p e a r to be insensitive t o small changes in t h e d i r e c t i o n o f flow. T h e m o s t n o t i c e a b l e d i f f e r e n c e b e t w e e n full-scale and t h e p r e s e n t m o d e l results occurs in the w a k e region: this d i f f e r e n c e c o u l d be d u e t o t h e differing t y p e s o f i n s t r u m e n t a t i o n used in the m o d e l and full-scale studies. Acknowledgements W i n d - t u n n e l m o d e l s , o t h e r i t e m s o f e q u i p m e n t and initial f u n d s p r o v i d e d b y t h e Building R e s e a r c h E s t a b l i s h m e n t , and financial s u p p o r t given b y t h e Science R e s e a r c h Council, are gratefully a c k n o w l e d g e d . References 1 M. Jensen, The Model law for Phenomena in the Natural Wind, Part 2, Danish Technical Press, Copenhagen, 1965. 2 B.E. Lee, The effect of turbulence on the surface pressure field of a square prism, J. Flui~ Mech., 69 (1975) 263--282. 3 A. Laneville, I.S. Gartshore and G.V. Parkinson, An explanation of some effects of turbulence on bluff bodies, 4th Int. Conf. on Wind Effects on Buildings and Structures, in K.J. Eaton (ed.), Heathrow, 1975, Cambridge University Press, London, 1977, pp. 333341. 4 A.E. HoldS, Ph.D. Thesis, Hatfield Polytechnic, 1979. 5 K.J. Eaton and J.R. Mayne, The measurement of wind pressure on two-storey houses at Aylesbury, Build. Res. Establ., Curr. Pap. CP 70/74 (1974). 6 K.J. Eaton, J.R. Mayne and N.J. Cook, Wind loads on low-rise buildings--effects of roof geometry, Build. Res. Establ., Curr. Pap. CP 1/76 (1976).
372 7 J. Counihan, A method of simulating a neutral atmospheric boundary layer in a wind tunnel, Proc. Advisory Group Aerospace Res. Dev., Conf., Munich, 1970, No. 48. 8 N.J. Cook, Wind tunnel simulation of the atmospheric boundary layer by roughness~ barrier and mixing-device methods, J. Ind. Aerodyn., 3 (1978) 157--167. 9 C.M. Newberry, K.J. Eaton and J.R. Mayne, Wind loading on tall buildings: further results from Royex House, Build. Res. Establ., Curt. Pap. CP 29/73 (1973). 10 W.A. Dalgliesh, Comparison of model/full-scale wind pressure on a high-rise building, J. Ind. Aerodyn., 1 (1975) 55--66. 11 J.D. Holmes, Pressure fluctuations on a large building and alongwind structural response, J. Ind. Aerodyn., 1 (1975/76) 249--278.