Richardson 1994

Journal of Wind Engineering and Industrial Aerodynamics, 51 (1994) 157-176 157 Elsevier

The Silsoe Structures Building: Comparison between full-scale and wind-tunnel data

G.M. R icha rdson a and D. Sur ry b

aProcess Engineering Division, AFRC Silsoe Research Institute, Wrest Park, Silsoe, Bedford MK45 4HS, UK bBoundary Layer Wind Tunnel Laboratory, University of Western Ontario, Faculty of Engineering Science, London, Ont., Canada N6A 5B9

(Received June 10, 1992; accepted May 26, 1993)

Summary

Full-scale wind profile and surface pressure measurements made at Silsoe are compared with a series of 1:100 scale experiments conducted in the University of Western Ontario's Boundary Layer wind-tunnel No. 1. The surface pressure comparison was limited to wind directions normal and parallel to the building ridge for a mid-building-length set of pressure tappings. The pressure coefficients from standard wind-tunnel tests were referenced to pressures measured above the model-scale boundary layer. The problems in correcting these to ridge height references are discussed. A time history experiment succeeded in over- coming these problems with the simultaneous recording of ridge-height velocity and a local reference static pressure just upwind of the 1:100 scale model. Recommendations for future model and full-scale experimentation are made.

Notation

CpMw

Cp,r CSR FB28 FSR

bui ld ing r idge he igh t re fe renced p ressure coefficient r idge he igh t Cp ca lcu la ted wi th addi t iona l p ressure m e a s u r e m e n t s ' g r ad i en t ' he igh t (above b o u n d a r y layer) re fe renced coefficient r idge he igh t Cp us ing hot wire ve loc i ty profile m e a s u r e m e n t s r idge he igh t Cp us ing a compar i son wi th full-scale da ta r idge he igh t Cp us ing a side m o u n t e d r idge he igh t p i to t Cei l ing s ta t ic re fe rence (see PR) F a r m bui ld ing n u m b e r 28 a t Silsoe: (A) cu rved (B) sha rp eaves F loor s ta t ic r e fe rence (using a sur face m o u n t e d t app ing 0.2 m up- s t ream)

Correspondence to: G.M. Richardson, Process Engineering Division, AFRC Silsoe Research Institute, Wrest Park, Silsoe, Bedford MK45 4HS, UK.

0167-6105/94/$07.00 O 1994 Elsevier Science B.V. All rights reserved. SSDI 0167-6105(93)E0035-W

158 G.M. Richardson and D. Surry/Silsoe Structures Building

h

hsm

H~

P

Pm Po

Psm

PR

q RMS Uz

Ur UR z

mean total pressure measured at ridge height above turntable centre (model removed) mean total pressure measured at ridge height 0.38 m from the centre of the model and 0.064 m to windward mean reference total head from ceiling mounted pitot 2.2 m to windward and 1.4 m above floor mean pitot-static pressure measured at ridge height above turntable centre (model removed) mean surface static pressure on the model mean surface static pressure at the centre of the turntable (model removed) mean pitot-static pressure measured at ridge height 0.38 m from the centre of the model mean ceiling mounted reference pitot-static pressure 2.2 m to windward and 1.4 m above floor ridge height dynamic pressure of the free-stream wind Root mean square mean longitudinal velocity at height z mean longitudinal velocity at ridge height mean longitudinal velocity at ceiling reference height of approx. 1.4 m height above wind-tunnel floor

Greek letters O mean full-scale incident wind angle (deg)

mean model-scale incident wind angle (deg)

1. I n t r o d u c t i o n

Recent comparisons between wind tunnel laboratories of surface pressure data for 1:100 scale models of the Aylesbury experimental building [1,2] have clearly i l lustrated the variabili ty of results. The instrumentation, tubing response, measurement technique and procedure need to be refined to minimise such differences. Fur ther demands are made when comparing with full-scale data, where a static reference pressure source that duplicates the full-scale equivalent pressure needs to be found within the wind-tunnel.

Unfortunately, the full-scale reference static pressure used in the Aylesbury experiment was not suited to model-scale comparison. The manhole reference pressure was affected by the wake of either the experimental building or the nearby housing estate for a number of wind directions [3,4]. In contrast the Silsoe Structures Building, sited at the Silsoe Research Institute, Wrest Park, Silsoe, UK, is situated on a flat and exposed open field site. It is orientated with the ridge aligned normal to the prevailing WSW wind direction and the site provides a clear fetch extending some 500 m or more over an arc from SW through N to E. The general terrain category [5] is open country with scattered

G.M. Richardson and D. Surry/Silsoe Structures Building 159

windbreaks. The Silsoe Structures Building reference pressure probe was mounted on a mast at the ridge height upstream of the building. It was thus unaffected by any wakes. Moreover, being an ongoing experiment, any questions arising from comparisons with model-scale can be investigated further at full-scale.

Wind-tunnel results from the Boundary Layer Wind Tunnel Laboratory (BLWTL), London, Ont., Canada, illustrate the kind of problems that exist in 1:100 scale modelling of wind loads on low-rise buildings. These model-scale experiments were principally made to compare mean pressure coefficients from the wind-tunnel with those obtained at full-scale, in order to assess the accu- racy of the modelling process. The wind-tunnel study was made in 1988 and results have already been published and presented at conferences for both standard pressure coefficient tests [6,7] and spectral analysis [8]. This paper presents further analysis of this comparative work with a view to reducing the variability of results between wind tunnel laboratories, by improving the experimental method.

2. Experimental details

2.1. Silsoe Research Institute: Full-scale Wind profile measurements were made in October 1986 and September and

October 1988. Static pressure was measured at two anemometer positions via static pressure probes. The total wind pressure head was also measured at these anemometer positions using directional pitot-tube anemometers, each held into wind by a vane. The vertical profile up-stream of the building was measured progressively. One anemometer at a fixed height of 10 m was used as a reference whilst a second directional pitot tube anemometer was moved between heights of 0.32 m and 25 m. Two sensitive pressure transducers measured the difference between wind total and local static pressure to a resolu- tion of _ 0.2 N/m 2. The high frequency response of the directional pitot-tubes for wind speeds over 8 m/s reached 2.5 Hz (3dB down). The high frequency response of the wind pressure system varied between 2.5 Hz and 8 Hz (3dB down) for the longest tube lengths of 30 m and the shortest of approximately 8 m, respectively. Pneumatic filtering was incorporated into the tube systems by means of flow restrictors, to reduce the effects of resonance. Records of up to one hour's duration were recorded on FM magnetic tape and subsequently digitised for computer analysis. For spectral analysis the analogue record was filtered at 10 Hz and digitised at a rate of 20 Hz. Tethered kites were used to define the boundary layer velocity profile at heights beyond 25 m up to a height of 100 m.

The Silsoe Structures Building in its original 635 mm radius curved eaves state (FB28A) is shown in profile in Fig. 1, at model scale. The full-scale dimensions of ridge height, eaves height, length and span are 5.28, 3.60, 24.13 and 12.93 m, respectively. A full description of the Silsoe Structures Building, its instrumentation and research facilities has been presented elsewhere [9].


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The full-scale static reference pressure probe was mounted on a mast at the ridge height of the building, 20 m to windward of the western side-wall and in line with the southern gable end, for the prevailing winds. The dynamic pressure was measured by a differential pressure t ransducer connected via 30 m lengths of 6.3 mm bore flexible plastic tubing to the reference static probe and pitot tube, held into wind by a vane and mounted on the same mast at ridge height. The wind direction was measured by a potent iometer connected to the pitot-tube/windvane assembly. The response of the instrumentat ion was the same as for the profile measurements.

The pressures on the surface of the building over the mid-building-length section were sensed via 6.3 mm bore tubing (less than 8 m in length) to obtain spectral information to a frequency of at least 8 Hz. However, for the standard mean pressure coefficient data, longer tube lengths were used. Two types of surface pressure sensors were used. On the side walls, holes at the centres of 0.6 m square alloy plates, fitted flat to the crests of the profiled cladding, were used. On the roof, specially developed rain-proof probes [10] were used to overcome the problems of simple tappings becoming blocked with rain water. These probes sense the pressure field 125 mm above the cladding. In a pilot experiment, the probes were found to be unaffected by local pressure variations caused by a corrugated cladding profile of approximately 40 mm depth, trough to ridge, which is deeper than that on the Silsoe Structures Building (35 mm).

2.2. Boundary Layer Wind Tunnel: 1:100 scale model The boundary layer in the wind tunnel is developed over a distance of about

20 m as the air is sucked down the tunnel. The working section is approximately 2.5 m wide by 2.0 m high. Three large spires, near the inlet, were used to increase the boundary layer thickness and to increase the turbulence scales. For the "rough" profile, these were then followed by sets of one inch cubes to within a couple of metres of the model with randomly spaced 7 mm high


machine nuts over the remaining distance. For the "smooth" profile, the spires were followed by bare floor to the model position.

The standard procedure for determining the mean velocity and turbulent intensity profiles was followed. Two linearised single-wire anemometers were first calibrated against the reference pitot-static probe in the relatively undisturbed flow at the top of the wind-tunnel. The probes were then suspended from the horizontal beam of the traversing gear, above the centre of the turntable and model position, with the wires at 711 mm (28") vertical spacing. Thus, the velocity profile was defined between heights of 6.4 mm and the ceiling mounted reference height of 1.4 m by stepping the two probes up in tandem over the 711 mm distance. In addition a pressure profile was measured, for both the "rough" and "smooth" floor conditions, using standard pitot-static probes at 711 mm vertical spacing above the centre of the turntable.

The ceiling-mounted reference probe was approximately 140 mm to one side of the tunnel control pitot-static probe, 2.1 m to windward of the working section turntable-centre, and 1.1 m from the nearest side wall. Tube lengths of approximately 7 m were used to connect these probes to the pressure transducers. In order to check the model ridge-height dynamic pressure during the pressure coefficient tests, the dynamic pressure was measured using a floor mounted pitot-static probe to the side of the model at model ridge height. For FB28A and B this was 0.38 m out from the centre of the model and 0.064 m to windward.

To compare information computed from time history measurements, it is necessary to provide wind tunnel sensors equivalent to the full-scale dynamic and static pressure sensors. A floor static reference (FSR) pressure sensor was positioned immediately below a ridge height cross-wire anemometer 200 mm to windward and in-line with the gable-end wall. The benefit of determining the ridge height dynamic pressure (q) from velocity measurements is the removal of errors due to static pressure fields in the wind-tunnel and an increased frequency response. The FSR consisted of a hole in the centre of a flat brass chamfered-edge disc, held to the floor by double-sided sticky tape. It had a mean bias Cp of 0.029 (and an RMS of 0.140) with respect to the ceiling static reference (CSR) in terms of q.

The chief benefit of this FSR over a pitot-static probe is its insensitivity to wind direction. Its location close to the floor has the advantage that it is positioned in a low velocity region and, hence, yields a minimal static pressure error due to dependence on q. However, this is potentially a disadvantage to windward of a model where the static pressure field, directly in front of the windward face, is more extensive near the ground than at ridge height. A further benefit occurs in this case, because the CSR requires a tube length significantly longer than that used for the tapping pressures. The use of a local static reference enables the effective matching of tube lengths and restrictors beyond the transducer when pressures both referenced to the ceiling static are differenced during analysis.


The 1:100 scale model was manufactured by the Building Research Establish- ment (BRE) near Watford, UK, and kindly lent to the author for this comparative study. The tappings were 1.0 mm internal diameter holes. The pressures sensed at the model surface were then transmitted via PVC tubing of 1.3 mm bore to a Scanivalve pneumatic selector unit. Each contained a brass tube restr ictor 360 mm from the tap and 250 mm away from the Scanivalve connec- tion, to provide pneumatic damping. A plan view of the roof, together with views of gable-end and side walls, is given in Fig. 2, showing the model- scale tap positions and numbering for the standard coefficient data. In the case of the subsequent time history experiment, the model-scale tap numbering was different as shown within the brackets in Fig. 1; the other numbers are the full-scale tap numbers for which short lengths of tubes were used when recording spectral data.

Statham 2.5 psi transducers were connected to each Scanivalve output. Despite using only about 1% of their range, at a tunnel control speed of 14 m/s, these transducers provide pressure coefficients accurate to within 0.04 to 0.08 of typical model ridge height q. The tap tubing characterist ics are such that there is a small variat ion in high frequency response between one tap and another due to the variat ion in restrictors. The pneumatic system, on average, has a slight resonance in the region of 70 Hz and otherwise is essentially flat to 90 Hz after which the response falls off.

3. R e s u l t s and d i s c u s s i o n

Comparison of pressure measurement data, assuming duplication of boundary layer characteristics, must address two fundamental questions relating to the locations of the reference pressure measurements. The first, "What factors are needed to correct the data sets in accounting for any height differences between the reference dynamic pressure measurements?". The second, "What factors are then needed to account for any static pressure differences between the data sets?".

The methods developed to enhance the comparison of the current data sets are appropriate for a much wider application. The objective is to both improve the comparison of the current data sets and also future data sets (from this and other wind tunnels) by avoiding practices that introduce variabili ty in the data. The key overall objective in the collaboration leading to this work was to assess the standard wind-tunnel 1:100 scale model testing method, where detailed pressure measurements are summarised in terms of maximum, min- imum, mean, and RMS pressure coefficients. The following discussion of these records is limited to mean pressure coefficients. However, data from a pilot 'time history' experiment, where spectral and thus unsteady pressure parameters are compared, are also addressed.

Three sub-sections are to follow. In 3.1. two model-scale boundary layers are compared to full-scale equivalent measurements. In 3.2. the method for accounting for static pressure differences is introduced. Four al ternative

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model-scale pressure coefficients are then compared, for the "rough" boundary layer that was chosen to simulate the full-scale Silsoe boundary layer. A comparison of CpL (the now preferred coefficient) is then made for each model for both the "rough" and "smooth" model-scale profiles compared to the full-scale measurements. Sub-section 3.3 contains the comparison of spectral data from the pilot 'time history' experiment.

3.1. Wind profile measurements The "rough" profile was chosen to model the full-scale velocity and local

turbulence intensity profiles which are compared in Figs. 3a and 3b. The comparison is good at the reference height of 5.28 m. However, below this height the model-scale velocities are higher. This is associated with the change in the surface roughness within two metres of the test section. The surface roughness (Zo) calculated from a line fitted to the points between 3 and 60 m full-scale equivalent height was 0.042 m. In contrast, the "smooth" profile (see Figs. 4a and 4b) had a local turbulence intensity of 13% at the reference height and a Zo of 0.001 m full-scale equivalent. This profile was used to assess the sensitivity of the model to the boundary layer simulation used. The most significant difference was the lower turbulence intensity which was approximately constant at 14% from ridge height to ground.

The velocity ratio ur/UR was 0.51 and 0.62 for the rough and smooth profiles, respectively. The square of this ratio is used when calculating ridge height Cp from the 1.4 m height referenced data (CpG).

3.2. Surface pressure coefficients Model-scale results are presented for three incident wind directions (¢):

the transverse direction ~b=270 ° (relating to the full-scale prevailing wind

G.M. Richardson and D. Surry / Silsoe Structures Building 165

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direction 8=0 °) and the longitudinal directions ~ = 180 and 360 ° (see Fig. 2). These mean pressure coefficients are tabulated in Table 1 in their measured form Cp~ for the mid-building-length taps (103 to 408 in Fig. 2) for ¢ = 270 °.

P m - - P R

Cp~ Hx--PR" (1)

The four cases for which the data are presented are split evenly between the curved eaves (FB28A) and the sharp eaves (FB28B) 1:100 scale model versions of the Silsoe Structures Building. For both models, data are presented for the rough and smooth profiles.

For the combined longitudinal wind directions, the average Cpo for the mid-building-length tap set are presented in Table 2 for each of four cases, together with the full-scale average C a value for 0 = 90 °. The longitudinal wind direction provides an opportunity to correct any discrepancy in the reference static pressure between different data sets and models. It is the geometric similarity between the FB28A and B models for this wind direction and the distance of the taps 103 to 408 from the gable-end separation of the surface flow that makes this possible. It is reasonable to assume that the flow will be reattached by the mid-length of the building and that the characteristics of this flow will be relatively unaffected by the differences between the two model- scale profiles. Thus, all four sets of model-scale data, whose averages are presented in Table 2, can be calibrated against the full-scale datum to correct for any zero static pressure inconsistencies between data sets. The resulting correction factors to be applied to Cp~ are given in Table 3 for the four cases concerned. The major change in static pressure occurs not between profile changes but between occasions in the wind-tunnel; the FB28A measurements


Table 1

A compar i son of Cp,~ for mid-bui ld ing l eng th taps for t r ansve r se wind 6 = 2 7 0 ' inc lud ing full-scale da ta 0 = 90 °

Tap No. Curved eaves Sha rp eaves FB28A FB28B Rough Smooth Rough Smooth Full-scale Full-scale

103 0.072 0.139 0.108 0.165 0.35 0.36 104 0.095 0.161 0.119 0.185 0.43 0.42 105 0.093 0.160 0.126 0.201 0.44 0.50 106 --0.254 -0 .476 --0.256 -0 .468 - 1 . 1 3 --1.35 107 --0.239 -0 .414 -0 .235 -0 .444 0.83 - 1.31 108 --0.143 --0.245 --0.169 --0.385 --0.68 - 1.25

203 --0.109 -0 .201 --0.137 --0.321 - 0 . 5 8 1.07 204 -0 .115 -0 .191 -0 .095 --0.255 --0.54 - 0 . 8 8 205 --0.110 --0.193 -0 .082 --0.182 --0.54 --0.64 206 --0.119 --0.214 --0.082 --0.165 --0.58 --0.55 207 --0.151 --0.254 --0.099 --0.185 --0.71 - 0 . 5 7 208 -0 .166 --0.293 --0.105 --0.214 --0.84 - 0 . 6 6

303 -0 .188 -0 .340 -0 .134 -0 ,249 -1 .02 - 0 . 7 6 304 -0 .239 -0 .394 -0 .152 -0 .299 - 1 . 0 6 - 0 . 7 8 305 -0 .226 -0 .390 -0 .145 -0 .255 - 0 . 9 0 - 0 . 6 9 306 -0 .172 -0 .280 -0 .103 -0 .184 - 0 . 7 0 - 0 . 5 7 307 -0 .110 -0 ,178 -0 .069 -0 ,134 - 0 . 5 3 - 0 . 4 8 308 -0 .081 -0 .141 -0 .042 -0 .105 -0 .47 - 0 . 3 8

403 -0 .063 -0 .108 -0 .033 -0 .081 - 0 , 3 2 - 0 . 3 0 404 -0 .061 -0 .094 -0 .022 -0 .071 - 0 . 2 9 - 0 . 2 7 405 -0 .051 -0 .083 -0 .022 -0 .060 - 0 . 2 5 - 0 . 2 4 406 -0 .046 -0 .075 -0 .018 -0 .048 - 0 . 2 2 -0 ,21 407 -0 .043 -0 .067 -0 .014 -0 .043 -0 .21 - 0 . 2 0 408 - 0.034 - 0.054 - 0.001 - 0.033 - 0.13 - 0.16

Table 2

A compar i son of the overa l l average Cpo for the mid-bui ld ing- length taps wi th l ong i tud ina l winds

FB28A FB28B Full-scale

Rough Smooth Rough Smooth FB28A (Cp) - - 0.0290 - 0.0444 0.0015 -- 0.0209 - 0.0861

o c c u r r i n g a f u l l m o n t h b e f o r e F B 2 8 B . T h i s i s n o t o n l y a p p a r e n t f r o m t h e s t a t i c

p r e s s u r e c o r r e c t i o n f a c t o r s b u t a l s o f r o m t h e m e a s u r e m e n t o f t h e s i d e m o u n t e d

r i d g e - h e i g h t p i t o t - s t a t i c p r e s s u r e (Psm) t a p 1406 a l s o p r e s e n t e d i n T a b l e 3. I n

f u t u r e , t h e e f f e c t o f s u c h i n c o n s i s t e n c i e s c a n b e a v o i d e d b y r e p e a t i n g a l l

r e f e r e n c e m e a s u r e m e n t s w h e n r e p e a t i n g e x p e r i m e n t s .

G.M. Richardson and D. Surry / Silsoe Structures Building

Table 3

Static pressures measured in June and July experiments

167

Curved eaves Sharp eaves

Rough Smooth Rough Smooth

Correction factor for calculation + 0.009 of CpL (Add) Average value Tap 1406 -0.017 C~odA=O ° to 350 ° Correction factor for calculating 4.272 Cp~ (Multiply)

+ 0.011 -- 0.022 - 0.012

- 0.020 + 0.009 + 0.002

2.599 4.272 2.599

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I 0 0

Fig. 5. Within-run variability of Cpo for longitudinal winds: FB28A rough profile.

The mid-bui ld ing- length long i tud ina l Cpo da t a also prov ide an o p p o r t u n i t y to assess the va r i ab i l i t y of the da t a be tween taps. The Cpo da t a for t aps wi th a c o m m o n first in teger , wi th in a p a r t i c u l a r exper iment , have been compu ted f rom sur face p ressure m e a s u r e m e n t s us ing a common pressure t r ansduce r . The C~o da t a for taps wi th the las t two in tegers in common, for a p a r t i c u l a r model, h a v e been made s imul taneous ly . A n o t h e r fac tor a f fec t ing the s t a n d a r d Cpo da t a is t h a t all sur face p ressure m eas u rem en t s , for a p a r t i c u l a r wind direct ion, are subsequen t ly divided by an ' above b o u n d a r y l ayer he igh t ' dynamic -p res su re r e f e r e n c e - m e a s u r e m e n t made dur ing a p reced ing t ime in terva l . F igures 5 and 6


EAVES 0.01

0.0

-0 .01

- 0 . 0 2

Cp 1-0 03

-0.0

-0.0~

-0.0~

-0 .0 '

RIDGE EAVE~

• .... A . .,~. ~ ': "'"' & / : i : = : ~ _ . ~ / .,._ .~ A

• !

1

o I o : .... ,,, ~.~" "~-,~,~,

z

i

360 Degrees

I ~ ' - FB28A R o u g h

[ - [ ~ - FB2flA S m o o t h

[ . . A - . FB28B Rough

i ; [ " ~ " FB28B S m o o t h

25 50 75 100

% SPAN

Fig. 6. Between experiment (FB28A/B) and profile variability of CpG.

show the variability of roof tap data for this relatively uniform flow field. Figure 5 illustrates the within-run variability and Fig. 6 indicates the effect of changing the velocity profile in two experiments. The variation due to the changed velocity profile would be expected to be removed when using an upstream model-ridge-height reference. However, the unexplained static pressure variation between the two models is too large to be ignored and needs to be removed if a sensible comparison of the model and full-scale data is required.

To convert transverse wind direction C~ data presented in Table 1 to ridge height C~ the standard procedure is to use the mean-velocity vertical-profile to obtain the velocity ratio and, hence, the pressure ratio between the heights of 1.4 m and 0.053 m within the wind-tunnel. Alternatively, the side-probe total head measurement can be used as a direct source of pressure ratio and hence a multiplication factor. The Cp resulting from these multiplication factors are Cp. w and Cp.r, respectively.

Cp. w = CpG ( U R / u f l , (2)

Cp,, = CpG ( H a - PR)/( hsm-- PR) . (3)

For comparison with full-scale measurements, Po, the static pressure measured at the centre of the turntable surface before any model was introduced, was thought to be the best possible reference static pressure. Thus, using additional non-standard pressure measurements, made separately from the

G.M. Richardson and D. Surry/ Silsoe Structures Building 169

Cpo tests, another form of Cp was calculated.

Pro--P0 Cp,= h -po

This was calculated from

Cp,=[Cp~ (po--PR) ] HR--PR ~ ) J (h--p) - (P0 - P ) '

where

P - P 0 P o - PR 0.009, 0.016,

HR-- PR HR- PR

(4)

(5)

h - p - - - - - 0 . 2 4 3 . (6) H R - - PR

If the correction factors (Table 3), determined from the calibration with full-scale data for 0 = 90 °, are included within the calculation of Cp~, we obtain

CpL = (Cpo + Add) Mult. (7)

These four alternative Cp are assessed for reliability in Figs. 7 and 8 and Table 4. The large difference between Cp. and Cp. W for both the curved and sharp eaves models is due to the combined influence of the static pressure field around the model and the effect of turbulence intensity on the side probe measurement. The significantly reduced dynamic pressure results in a larger multiplication factor compared to that derived from the hot-wire vertical profile data. However, this does not account for the variation in base leeward- roof pressure for Cps r between the FB28A and B models ( a feature not present at

0.C

-O.f

-I,0

EAVES RIDGE EAVES

ir : '~ :~ ,

.g

. . . .

.."

- 1 . 5 0 25 50 75 100

% SPAN

:i| u~]~'~! ~ ih~

CURVED EAVES

Cp sr

] - ~ - . Cp HW

]..A-. cp B

Fig. 7. A comparison of four forms of Cp: Curved eaves roof (¢ = 270 °).


C~:

0.0

-0.5

-1.0

-1.5

EAVES RIDGE EAVES ! ! . ~]

# g'

c~

25 75 100 I

5O

% SPAN

SHARP EAVES " O - - Cp sr

- t ~ " Cp HW

• .A.. Cp B

Cp L

Fig. 8. A comparison of four forms of Cp: Sharp eaves roof (~b =270 °).

Table 4

Mid-building length side-wall tap pressures ~b = 270 ° for different Cp

Tap No. Curved eaves: Rough Sharp eaves: Rough

Cp.. CpH~ G, cpL c~.. c ~ G, GL

103 0.36 0.28 0.24 0.35 0.54 0.42 0.39 0.37 104 0.47 0.37 0.34 0.44 0.60 0.47 0.44 0.42 105 0.46 0.36 0.33 0.43 0.63 0.49 0.47 0.45 408 -0.17 -0.13 -0.21 -0.11 -0.01 -0.00 -0.07 -0.10

full-scale). This va r i a t ion is also present in Cp. W. It is clear from these comparisons tha t a fac tor va ry ing between the FB28A and B experiments needs to be added to Cpo if these base pressures are ever to coincide. Cp, relies on one measurement for each of the profiles. In future, all measurements should be repeated for each model to avoid this. However, CpL has different factors for FB28A and FB28B models which when applied resul t in a much more consis- tent compars ion of the side-wall and roof da ta for this mid-building-length tap set. This is far from ideal since it relies on there being full-scale data for the longi tudinal wind direct ion cal ibrat ion.

A compar is ion of CpL with full-scale da ta for both FB28A and B is given in Figs. 9 and 10 for roof-taps and in Table 5 for side-wall taps. Included in Fig. 9 and Table 5 is a set of points resul t ing from the t ime his tory experiment where


CPL

o.o EAVES RIDGE EAVES

-0.5

-1.0

-1.5

s~..B.-.:. '

~.:

~..'? "el ~. . '"~,: ,

A.-" /

25 50 75 100 % SPAN

'i

CURVED EAVES

Silsoe F/S

-~]- BLWTL Rough

• .A.. BLWTL Smooth

BLWTL T/Hist

Fig. 9. A comparison of CpL and "Time History" data with full scale: Curved eaves roof.

Cp[~

0.0

-0.5

-l.O

EAVES RIDGE EAVES

/ / '

,' . ®

Cf®"

-1"50 25

Fig. 10. A comparison of C~_

~ I ' ~ - ' - ~ ,

50 75 % SPAN

with full-scale data: Sharp eaves roof.

SHARP EAVES

Silsoe F/S "E~]" BLWTL R o u g h

• .~. . BLWTL S m o o t h

100

r idge he ight dynamic pressure could be computed from cross-wire da ta simul- t aneous ly recorded with tap pressures and a local re fe rence s ta t ic pressure used in the ca lcu la t ion of these Cp. (This da ta has not been cor rec ted accord ing to the longi tudina l wind d i rec t ion ca l ib ra t ion procedure . )

In the rough profile, bo th models have an average windward wall CpL of 0.41, a leeward roof slope base CpL of --0.15 and a mid-leeward-wall CpL of


Table 5

Mid-building length side-wall tap pressures C~, ~ = 270

Tap No. Curved eaves Sharp eaves

Rough Smooth Full-scale Time history Rough Smooth Full-scale

103 0.35 0.39 0.35 - 0.37 0.40 0.36 104 0.44 0.45 0.43 0.40 0.42 0.45 0.42 105 0.43 0.45 0.44 0.38 0.45 0.49 0.50 408 -0.11 -0.11 -0.13 -0.24 0.10 -0.12 -0.16

approx imate ly -0 .10 . In the smooth profile, both models exper ience an increased average windward wall Cp~: more p ronounced wi th the sharp eaves model where Cp~ is 0.45 on average. The leeward-roof base CpL and mid-leeward- wall CpL are v i r tua l ly unchanged from the rough profile.

In the full-scale the bui lding exper iences an average windward wall Cp of 0.41 for the curved eaves increas ing to 0.43 for the sharp eaves building. The leeward-roof base Cp is in bo th cases approximate ly - 0 . 2 0 and the mid-leeward- wall Cp changes from -0 .13 to - 0 . 16 be tween FB28A and FB28B.

The full-scale da ta seen in Figs. 9 and 10 show very similar maximum values of Cp first on the windward roof slope before the flow acce lera tes over the ridge and then on the bot tom of the leeward roof slope. This same pa t t e rn occurs in the model-scale data, a l though the t endency is for the s tandard mean pressure coefficient wind tunnel da ta to underes t imate the uplif t over the roof, espe- cial ly where the flow is separa ted at the windward eaves of the sharp eaves building. The average uplifts der ived from model-scale measurements are 87% and 79% of the full-scale values for the curved and sharp eave vers ions of the Silsoe S t ruc tu res Building, respect ively.

3.3. Removing acoustic effects from spectral and urtsteady pressure parameters A cons idera t ion for reduc ing differences in measurements be tween wind-

tunnels is the removal of acoust ic effects resu l t ing from the dimensions of the tunne l itself, which can select ively amplify f requencies in a s imilar fashion to 'o rgan pipe' r esonance of pressure tubing. This removal can be done by using a re fe rence or back ing pressure t aken in close proximi ty to the model, prefer- ably outs ide the inf luence of the s ta t ic pressure field of e i ther the model or the side-wall boundary layer. This s ta t ic pressure is then e i ther in s t an taneous ly dif ferenced wi th each of the o the r recorded pressures or sub t rac ted dur ing subsequent analysis. It is impor t an t for all the tube lengths to be closely ma tched for a genuine dif ferencing across the ent i re bandwidth of interest .

Results from the pilot t ime h is tory exper iment using the a l t e rna t ive FSR yielded a set of da ta much closer to the full-scale equivalents . The roof da ta can be seen in Fig. 9 and the side-wall da ta in Table 5. It is possible t h a t the

G.M. Richardson and D. Surry / Silsoe Structures Building 173

100.0

10.0

1.0

p* 0.1

0.01 . . . . .

0.001

RUN 0 3 5 / 0 3 5 M

Tap 19 (FSR)

I

0.0001 0.1 1.0 10 .0 1 0 0 . 0

Frequency (Hz)

, , , , j

Fig. 11. The effect of changing from the remote (CSR) to local (FSR) static reference on Tap 19 power spectrum.

increase in magnitude of the leeward-roof base pressure is in part due to traversing gear blockage and a local acceleration of the flow. This was mini- mised by mounting the cross-wire probe from rods extended to windward and below the traversing boom by approximately 0.4 and 0.5 m, respectively. No time history data were recorded for a longitudinal wind direction, which would have provided a valuable assessment of this method when compared to CpL directly.

The effect of changing the reference static pressure from the 1.4 m high, 2.1 m to windward position, to FSR 200 mm to windward can be observed in the power spectra for leeward roof tap 19 in Fig. 11. A comparison with the FSR model-scale data with the full-scale data can be seen in Fig. 12, where the model-scale frequency is reduced by a factor of 100 on both axes. This comparison is extremely good from the lowest frequencies to approximately 0.3 Hz above which the model-scale shows increased power. This increased power, above 0.3 Hz (30 Hz model-scale), is probably due to the weak separation of the model-scale flow at the ridge of the model. This separation, seen in a flow visualisation study, does not occur at full-scale. However, the power in this frequency range above 30 Hz is very low and the sensitivity of the transducer, the resonance of the pneumatic system and possible aliasing effects could all be contributing to this difference. The spike at approximately 1.2 Hz is due to the fan blade crossing frequency (117 Hz at model-scale). In Fig. 11 the spike at 60 Hz is due to the mains power supply frequency.

The benefit of the local static reference is twofold. In addition to removing the increased power in the frequency range 1 to 10 Hz in Fig. 11, it provides

174 G.M. Richardson and D. Surry / Silsoe Structures Building

1 0 0 . 0

10.0

1.0 t~

v

0.1

v t/~ 0 .0!

0.001

0 . 0 0 0 1

RUN 0 6 5 / 0 3 5 M i A

! ! i ,, Tap lo F/Scaio !

. . . . Tap 19 (FSR) i

• • i r

. . . . . __ ! . _ _ . " ' , j . , : [

o.ooJ o.ol o.1 1.o Frequency (Hz)

Fig. 12. A comparison of the (FSR) model-scale spectrum for Tap 19 with full scale.

a better mean reference static pressure for comparison with full-scale. This can be appreciated from the comparison of power in frequencies below 1 Hz in Fig. 11. The mean bias of the FSR happens to cancel out the reference static bias of the experiment, assuming the full-scale reference static pressure provides an absolutely repeatable reference source.

4. C o n c l u d i n g r e m a r k s

A number of problems highlighted in this study are relevant to the difficulty of comparing results from different wind tunnels. In this paper the problems have been illustrated with data from one particular wind-tunnel, using the sharp and curved eave 1:100 scale models of the Silsoe Structures Building. The kind of difference that can result in mean pressure coefficients, simply due to the method used for obtaining the ridge height dynamic pressure, has been illustrated (Figs. 7 and 8). The need for repeating all pressure measurements in experiments separated by a significant time interval was indicated by the comparison of Cp, (using a hole in the centre of the turntable as the reference static pressure). This was then confirmed by using full-scale longitudinal wind data to remove unwanted static pressure inconsistencies that occurred at model-scale. The Cp~ data from this exercise fitted the full- scale data, in particular Cp at the bottom of the leeward-roof was unaffected by the change in eaves detail. Moreover, logical comparison of model-scale data resulted when investigating the effect of roughness in the fetch upon CpL (Fig. 9 and 10).

G.M. Richardson and D. Surry/ Silsoe Structures Building 175

Finally, results from a time history record underlined the advantages of recording all measurements simultaneously and using floor mounted local static pressure sensor. Fundamental-mode pressure fluctuations associated with the acoustic characteristics of the wind tunnel cylinder were removed with the use of this local sensor. Thus, not only is the requirement of all pressure measurements being repeated between experiments ensured, but an advantage gained due to their simultaneous record and the opportunity for subsequent changes during analysis.

Ideally a common methodology for experimentation needs to be adopted during wind-tunnel studies. Measurement of ridge height velocity or dynamic pressure and a reference static pressure close to the model must be made simultaneously with the surface pressure measurements. The positioning of all reference measurements should carefully avoid any pressure field effects due to the model, wind-tunnel side-walls, and surface roughness elements. The only reason for departing from this would be to repeat a feature of a full-scale experiment [11] that for some practical reason could not be avoided. Neverthe- less at model-scale there is probably no such restriction and for the sake of comparison with other laboratories both the pressure field affected full-scale reference positions and unaffected references should be used. Thus, differences attributed to the affect of a particular simulation on the model's pressure field can be avoided when comparing one set of model-scale data with another.

5. R e c o m m e n d a t i o n s for future work

The pitot-static probe is most commonly used for measuring the reference static or backing pressure. Unfortunately, this is not reliable when measuring close to the ground in turbulent boundary layers. There is a need to investigate the potential for a static reference probe possibly modelled on the full-scale probe [12] used in the Silsoe experiments. This vertical cylindrical probe is based upon a design which was adopted by the United States National Bureau of Standards. The static pressure is sensed via eight 3 mm holes drilled through the circumference of a 38 mm outer diameter tube. The holes are shrouded by an outer cylinder of 45 mm internal diameter. The position of the shroud can be varied to give a zero mean static pressure when the probe is mounted at the ridge height of the building of interest. At full-scale this is calibrated against a hole in the ground. A model-scale probe could similarly be calibrated against a hole in the centre of the wind-tunnel working section before the model is positioned there.

Further work is required both at full-scale and model-scale to investigate and compare static pressure fields around the Silsoe Structures Building. An improvement in the model-scale profile needs to be attempted due to the instability of the simulation following the change in surface roughness approximately one metre upstream of the model. An improvement in model-scale frequency response tubing characteristics to beyond 100 Hz is essential in determining significant flow differences between full-scale and 1:100 model scale data.


Acknowledgements

G e o f f R i c h a r d s o n w o u l d l i ke to t h a n k c o l l e a g u e s a t S i l s o e R e s e a r c h I n s t i t u t e w h o g a v e h i m e x c e l l e n t s u p p o r t w h i l s t in C a n a d a a n d h a v e p a t i e n t l y a w a i t e d t h e w r i t i n g o f t h i s p a p e r . H e a l so a c k n o w l e d g e s t h e f u n d i n g p r o v i d e d by t h e M i n i s t r y o f A g r i c u l t u r e , F i s h e r i e s a n d F o o d . F i n a n c i a l a s s i s t a n c e w a s a l so r e c e i v e d f rom t h e N a t u r a l S c i e n c e s a n d E n g i n e e r i n g R e s e a r c h C o u n c i l o f C a n a d a t h r o u g h o p e r a t i n g g r a n t s m a d e a v a i l a b l e to D. S u r r y a n d A.G. D a v e n p o r t .

References

[1] B.L. Sill, N.J. Cook and P.A. Blackmore, IAWE Aylesbury comparative experiment: Preliminary results of wind tunnel comparisons, J. Wind Eng. Ind. Aerodyn., 32 (1989) 285-302.

[2] B.L. Sill, N.J. Cook and C. Fang, The Aylesbury comparative experiment: A final report, Proc. 8th Int. Conf. on Wind Engineering, London, Ont., 1991.

[3] M.E. Greenway and C.J. Wood, Wind-tunnel pressure measurements on the Aylesbury low-rise housing estate, Part I: Simulation design and mean pressures, University of Oxford, Department of Engineering Science, Rep. QUEL 1213/77, 1977.

[4] K.J. Eaton and J.R. Mayne, The measurement of wind pressures on two-storey houses at Aylesbury, BRE Current Paper CP70/74, July 1974.

[5] British Standards Institution, Basic data for the design of buildings, Chapter V Loading: Part 2 Wind Loads, British Standard Code of Practice CP3: 1970, London BSI, 44 pp.

[6] G.M. Richardson and D. Surry, Comparisons of wind-tunnel and full-scale surface pressure measurements on low-rise pitched-roof buildings, in: C. Kramer and H.J. Gerhardt (eds.), Part 2: Building Aerodynamics, Proc. 8th Colloquium on Industrial Aerodynamics, Aachen, 1989, pp. 37~48.

[7] G.M. Richardson, A.P. Robertson, R.P. Hoxey and D. Surry, Full-scale and model investigations of pressures on an industrial /agricultural building, J. Wind Eng. Ind. Aerodyn., 36 (1990) 105~1062.

[8] G.M. Richardson and D. Surry, The Silsoe Structures Building: A comparison of pressure coefficients and spectra at model and full-scale, Proc. 8th Int. Conf. on Wind Engineering, London, Ont., 1991.

[9] A.P. Robertson and A.G. Glass, The Silsoe Structures Building: Its design, instrumentation and research facilities, Divisional Note DN 1482, AFRC Engineering, Silsoe, 1988, 59 pp.

[10] P. Moran and R.P. Hoxey, A probe for sensing static pressure in two-dimensional flow, J. Phys. E., 12 (1979) 752 753.

[11] A.M. Goliger, R.V. Mitford, J.L.Waldeck and G.L. Beattie, Jan Smuts wind tunnel comparative study: Mean and root-mean-square pressures, Internal Report IR 90/01, Engineering Structures and Information Technology Programme, CSIR, Pretoria, 1990.

[12] R.P. Hoxey and D.A. Wells, A method of calibrating a static pressure sensing head under natural wind conditions, Departmental Note DN/G/826/04014, Nat. Inst. agric. Eng., Silsoe, November 1977.

Richardson 1994

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