- Email: [email protected]
The influence of model scale on a wind-tunnel simulation of complex terrain
Journal of Wind Engineering and Industrial Aerodynamics, 12 ( 1 9 8 3 ) 1 2 5 - - 1 4 3
125
Elsevier Science Publishers B.V., A m s t e r d a m - - P r i n t e d in T h e N e t h e r l a n d s
THE INFLUENCE OF MODEL SCALE ON A WIND-TUNNEL SIMULATION OF COMPLEX T E R R A I N
D. N E A L
Vickers Dawson, Crayford, Kent (Gt. Britain) (Received N o v e m b e r 18, 1 9 8 1 ; a c c e p t e d O c t o b e r 14, 1 9 8 2 )
Summary T h e results o f w i n d - t u n n e l s i m u l a t i o n s involving 1 : 4 0 0 0 and 1 : 8 0 0 0 scale m o d e l s of G e b b i e s Pass (New Z e a l a n d ) are p r e s e n t e d and c o m p a r e d w i t h full-scale m e a s u r e m e n t s . T h e results f r o m t h e t w o m o d e l s are c o m p a r e d to investigate t h e possible effects o f w i n d - t u n n e l blockage and to test t h e validity o f s e g m e n t i n g a m o d e l for analysis. Speedu p / d o w n ratios are used to c o m p a r e t h e m e a n wind speeds over t h e t w o m o d e l s at a fullscale h e i g h t o f 12 m and result in high c o r r e l a t i o n s , in t h e range 0 . 9 0 - - 0 . 9 6 . Full-scale w i n d - s t r u c t u r e m e a s u r e m e n t s were m a d e to a h e i g h t o f 20 m at selected sites in t h e region. T h e full-scale and m o d e l results are c o m p a r e d in t e r m s o f velocity and turb u l e n c e - i n t e n s i t y profiles, energy s p e c t r a and length scales. In a d d i t i o n , velocity-profile d a t a collected by m e a n s of Tala kites are p r e s e n t e d and c o m p a r e d w i t h t h e results obt a i n e d in t h e w i n d - t u n n e l simulations. In all cases t h e r e is a high degree o f c o m p a t i b i l i t y , w h i c h suggests t h a t w i n d - t u n n e l m o d e l l i n g as for t h e G e b b i e s Pass c o m p l e x t e r r a i n region is a viable tool in t h e evaluation of p o t e n t i a l w i n d - e n e r g y sites.
1. Introduction One of the options available to investigators wishing to evaluate a potential wind-energy site involves physically modelling the area in a planetary boundary-layer wind tunnel. Although physical modelling offers advantages in terms of speed of data collection, ease of data handling and relatively low cost, there are other aspects that must be considered;these include: (1) t y p e of model surface finish; (2) selection of a suitable model scale; (3) blockage effects and their significance; (4) knowledge of approach-flow characteristics. This last factor becomes increasingly important if the wind regime does n o t meet the conditions required for a neutrally stable flow; however, some windtunnel facilities can model thermally stratified flow situations. Studies by Meroney et al. [ 1 ] , Neal [ 2 ] , Holmes et al. [3] and Chien et al. [4] produced data from wind-tunnel simulations which were shown to be in good agreement with full-scale measurements of the wind regimes over
0304~3908/83/$03.00
© 1 9 8 3 Elsevier Science P u b l i s h e r s B.V.
126
the actual terrains modelled. The study reported by Neal [2] involved simulation of the wind flow over a saddle region of complex terrain, Gebbies Pass, located in the South Island of New Zealand. A schematic diagram showing the major terrain features is given in Fig. 1; a full description of the saddle region and the approach terrain has been given by Neal [5].
Heights:-
Port Hills 600m 5ebbies Saddle 300m Mt Herbert Range 500m
Fig. 1. Terrain features of the Gebbies Pass region (viewed from the NE).
In the earlier work [5], Gebbies Pass was physically modelled at a scale o f 1:4000, resulting in a model 2.4 m wide and 3.6 m long which required subdividing to fit into the 1.2 m square cross-section of the wind tunnel; a photograph showing the three models produced from the region and their orientations is given in Fig. 2. The influence of the type of model surface finish in a complex terrain situation was considered by Neal et al. [6] who showed a terraced surface finish to be unsatisfactory for surface wind analysis. Other aspects o f this study involved the correlation of model data with full-scale measurements, and comparisons of several wind-structure parameters. The model results left some important questions unanswered, the major of these concerning: (1) the influence of wind-tunnel blockage (up to 7%) generated by the presence of the model itself; (2) the validity of subdividing the model and performing the analysis in sections; (3) the effects of removing significant portions of terrain from both sides of the area under analysis (this being the case for Model B). The influence of blockage will, of.course, vary from one terraia situation to another, and therefore the results of this aspect are applicable only to the Gebbies Pass situation, albeit a severe one.
127
A
\ \ \ \ \ \ \
Fig. 2. 1 : 4 0 0 0 scale m o d e l of G e b b i e s Pass.
C
"1
128
2. The model In response to the above questions, a 1:8000 scale model o f the same region was constructed for ~ . M ~ 1.2 m wkle by 1:8 m long, it was able to fit into the wind tunnel in one piece. simut tion. board 9 ram thick were obtained by le region and printin glued together uaing a P V A glue and r e ~ in a model witlt a terraced finish. A polyfiUer n o n ~ t r i n k material was ~ t o m in the ~ to produce the contoured model, shaping of the hills~and ~ being aided by photographs of the ~ o n . A series of tests w e r e have been given pl~vioualy by Neal [2]. The fmal water-based shelter.belts (typically 20 m high) and scrub areas {typically 3-~5m high). Wool was used to represent the shelt~-belt~ and an open form o f hessian for the scrub areas on the i:4000 scale model, and twine for the aheltar-belts and lint for the scrub areas on the 1:8000 ~ m o d e l A futI ~ r i p t i o n and justification for the use o f these materialshavebeen given by Neal [2] ; a photograph of the resulting 1:8000 scale model is given in Fig. 3,
The analysis of Model B, as shown in F~. 2, w a s o f particular interest, for two masons: (1) this model resulted in the saddle being positioned almost perpendicular to the general flow direction; (2} the resulting SSW alignment was found to be more representative of the full-scale weather patterns, which tended to be more southerly than southwesterly.
129 A cross-section through the saddle indicating the locations of the measurement points both for the models and for fun-scale data collection is given in Fig. 4. A dual¢hannel Thermo Systems hot-film anemometer was used to record wind velocity and turbulence. The hot-film cylindrical probe (Type 1210-20) has an effective length of 1.0 mm and a diameter of 0.051 mm. This type of sensor has the following inherent advantages over more traditional hot-wire systems: (1) better frequency response (when controlled electronically) than a hot wire of the same diameter, owing to the sensitive part of the hot-film sensor being distributed on the surface rather than including the entire cross-sectional area; (2) low heat conduction to the supports (end loss) for a given length-todiameter ratio, owing to the low thermal conductivity of the substrate material; a shorter sensing length can thus be used; (3) less susceptibility to fouling and greater ease of cleaning: a thin quartz coating on the surface resists accumulation of foreign material. model fu~Lscale daf~ dafo 40 &O 12 12 25 37
f{ow dlrechon
point 12
[point26 point 25 1 t -
~"~"
~
~
\ s o d d ~
po,nt31 point 37 PO,)t z*O
t
t
Fig. 4. Locations of m e a s u r e m e n t points on longitudinal cross-section through Gebbies Pass saddle.
The o u t p u t from the anemometer was passed to a Hewlett--Packard 2100A minicomputer for processing and extraction of various wind-structure parameters; full details of the data handling have been given by Pearse [7]. A traversing system supported on rails above the model was designed and commissioned for the Gebbies Pass study, the traverser being driven by stepperm o t o r and providing the following probe (sensor) positional accuracy: (1) longitudinal direction, + 2.0 mm; (2) vertical direction, + 0.1 mm; (3) lateral direction, + 1.0 mm; (4) angular (yaw about z-axis), +0.5 ° .
130 In conjunction with the hot-film anemometry, flow-visualisation techniques involving polystyrene beads and m i n i a t u ~ ~ were used tegether witha five. hole pressure probe to determine flow directions.
3. Fuli~cale data collection A mobile survey technique was used to provide mean wind speed data over the region at a height of 10 m. Mean wind speeds were collected over fiveminute intervals using Rimco three-cup fast-response anemometers, Datarecording start times were synchronised with those at a reference tower at a site upstream of the region modelled, thus allowing the use of normalised velocities for comparisons; a full description of the technique and the justification for its use have been given b y Neal [ 2 ] . Tala kites were used to collect wind-velocity data to a height of 300 m. Forces exerted on the aerodynamically lifting kite, of sled design, produce tension on the flying line which is connected to a calibrated spring to give velocities directly. Recordings of the length of the flying line (nonstretch kelvar) and the angle of inclination of the kite were required in order to apply the manufacturer's formula to obtain the actual altitude of the kite; the accuracy of this formula has been verified by Shieh and Frost [8]. Orthogonally arranged propeller anemometers, aerodynamically similar to the Gill propeller anemometer, were supported on a lattice t o w e r at various intervals to a height of 20 m and the o u t p u t collected on seven-track magnetic tape for analysis using a Burroughs 6700 computer. Details of the anemometry, data collection and software have been given by Flay [9]
4. Approach-flow characteristics The model approach-flow conditions for both the 1:4000 and 1:8000 scale models were established by simulation of the following parameters: (1) velocity profile; (2) local turbulence-intensity profile; (3) roughness length; (4) longitudinal c o m p o n e n t of energy spectrum; (5) boundary-layer depth. The modelling of these parameters was based on data for flat rural terrain presented by ESDU [10] and, where possible, on full-scale measurements made on the p r o t o t y p e approach terrain. In obtaining the high degree o f compatibility in the parameters discussed it was found that the boundarylayer height was significantly higher than the initial specification o f Z~ = 400 m (50 ram). In the 1:4000 simulation the choice of a boundary-layer h e i g h t o f 400 m was based on available literature and the earlier work o f M e r o n e y et al. [ 1 ] . Subsequent to the 1:4000 study, local Meteorological Office measurements suggested that owing to the nature of the southwesterly weather patterns
131 this parameter could be as high as 1000 m. Therefore, owing to this uncertainty, it was considered reasonable to accept the modelled boundary height in the approach flow of Z~ --- 640 m in a t r a d e ~ f f in order to hold the other parameters to the required values. 5. Results The results are presented in two sections. The first shows the influence of blockage and model sectioning, by means of data correlations. The second considers Model B in more detail using data correlations between the model and full-scale data, together with wind-structure comparisons between the model and full-scale measurements. 5.1. R e s u l t s f o r M o d e l s A a n d C
If the wind-tunnel blockage of 6--7% for the 1:4000 scale model had been a significant factor it would have resulted in compression of the flow streamlines and a subsequent increase in the velocity close to the model surface, together with possible changes in flow direction. These effects would be superimposed on the velocity and direction changes induced by the topography itself. Flow direction tests using miniature flags and the pressure probe showed that there were in fact no significant variations in the flow directions between the two models, thus eliminating this aspect of the blockage influence. The speed-up/down ratio for each analysis point relative to the approachflow velocity was calculated for the heights Z p = 12, 20 and 40 m, and used to assess the possible influence of blockage on the wind-velocity regime. These data were used to produce a sample correlation coefficient which, based on 23 data sets, ranged from 0.812 at Z p = 12 m to 0.833 at Z p = 40 m. The percentage differences between the 1:4000 and 1:8000 data at Z p = 12, 20 and 40 m were calculated and an arbitrary differential of 15% set to account for any small modelling discrepancies and to allow for experimental errors. It was apparent that five of the data points were significantly above this value, with 18--40% differences. With these exceptions all data fell well within the 15% difference level, and in the majority of cases the difference was in the region of 5%. A detailed comparison of these points provided clear explanations for the larger discrepancies, which were in all cases due to modelling errors. Although care was taken in the construction of both models it will be appreciated that at a scale of 1:8000 very small model errors can represent large full-scale discrepancies. The feature most prone to this was the formation of hilltops, where there is often an absence of contours for guidance. Indeed, four of the five points concerned were situated on isolated hilltops. It should also be noted that in several cases where the percentage difference was 20% at Z p = 12 m, this dropped to ~10% at Z p = 40 m, thus illustrating the localised influence that small model discrepancies can produce. With the extreme data removed, the sample correlation coefficient r for
132
Zp = 12 m improved to 0.958, as shown on the scatter diagram in Fig. 5, which also includes the linear regresaion curves. The scatter d ~ in Fig. 6 represents the Zp = 40 m data, for which r has the value o f 0.918. The high correlations obtained between the two models suggest the following: (a) the blockage introduced by the 1:4000 scale model (up to 7%) does not significantly influence the flow directions or magnitudes; (b) the results obtained by the analysis o f Models A and C separately are representative of the area as a whole. 22 2-0 1-B
1.6 1"4. o ~ o
1"2 1.0 0.8 0.6
, ~/.~/
n:18
0-~ 0.2 0
0.2 0./., 0"6 0-8 1-0 1.2 1./+ 1.6 1"8 2"0 2.2 O
Regression tines 0
~6/~ooo = 1°s6 "~6/8ooo-o.o79
Corretafion coefficient r= 0-95B Fig. 5. Scatter diagra.-~ for Models A and C at Zp = 12 m.
5.2. Results for Model B Since a more detailed analysis was ~ e d o u t for the Model B results, the analysis points used are presented on the map in ~ g . 7. The procedure o f correlating the s ~ - u p / d o w n ratios was alsoapptied to
133 22 20 1.8 1'6 1.z,
§
1"2
Oo
10 0-8
//
0.6
n=
18
0.4 02 I
0
I
I
I
i
0"2 04., 0"6 08
I
I
I
i
I
,,
l
1'0 1"2 1"/., 1"6 1"8 2.0 2'2
°/, U&
000
Re~]ression Lines
O
/,ooo o°/
& 8000
Corre[otion coefficient
g
bU
-0'028
000
+ 0"197
r = 0.918
Fig. 6. Scatter diagram for Models A and C at Zp = 40 m. the Model B data: based on 15 data sets, values varying from r = 0.912 at Zp = 12 m to r = 0.925 at Zp = 40 m were obtained, as shown by the scatter diagrams in Figs. 8 and 9. The 1:4000 scale model results were shown by Neal [5] to correlate very well with full-scale data, producing correlation coefficients in the range 0.73-0.90. It follows that there exists an implied high correlation between the 1:8000 scale mo de l and the full-scale data.
5.2.1. Velocity and local turbulence-intensity profiles Nondimensional velocity and local turbulence-intensity profiles are presented in Figs. 10 and 11 f or points 40 and 31, which are situated on the flat terrain approaching the saddle region. In Fig. 10, for point 40, full-scale local turbulence-intensity data collected to a height o f 20 m are included and show excellent agreement with the model data. The model velocity data have been
134
:¢ modelled but not photogra~ed Fig. 7. Measurement points for analysis of Model B.
135
c-4
o
~
o
o
d:,
.'9 ÷
~_
÷
o o
o
I.~ o--
I:::~ II~
i:::) II:)
II c_
d~
tl
II
~t o
o
c-4
N
I
•
.
cM
t'~
.
o
.
c_ o kJ
.
o
o
o
Q4 N o
d~ o
o
~
,9
<:~ 0
n
II
II
,-
0
O
N
m
r~
E N
o ~
o
12~ Ii2~
~,
ap o
o,
"9 o
~y o
t
I
,~
N
I
I
I
I
I
I
0 000~/911
I
o
~ |
/ - -
r - -
I
J
I \~I~
0
o
~6
136
-5 O
"~
@
O
O
•
O
O
~..
CO
O
O
"-0
,4D
0
0
i
0
o
o
-4"
c'~
CD
0
Cr~
0
O
0
@
0
@
0
c I
o
o
o
0
0
0
'?
""
~
~
I
I
o
o
c~
(w)z 4q6!aq
o ~
i ~
o
o
N
i ~
8
M
N
~p
5
e~
E
0
•
0
• .
0
IPl
?1,="
~
0
4.J
~
o
_o _o
o
C
o
0
-~
_o _o
o
d
~
6
o o
c 0
0
d~,
~
t-
o
o
o
Z
Z
o
--
~
r,,.
r,,
0 o
•
o o•
[]
"7"
o I
I
o
o~
(wlZ
446!a4
0
137
normalised by the velocity at the gradient height. Figure 12 presents similar information for point 12 and shows clearly the uniform nature of the velocity profile at this point and the significant reduction in the local turbulence values. Again there is excellent agreement with full-scale local turbulenceintensity data to a height of 20 m. The results for point 26 are presented in Fig. 13: although there is good agreement in terms of slope, there is a small variation in magnitude between the two models. In order to provide some comparisons with full-scale measurements the Tala velocity data were used in normalised form. The velocity at 160 m height was used for normalisation, since this was the highest c o m m o n denominator for all measurement points. The resulting profiles are presented in Figs. 14-16 for points 40, 12 and 25, together with the profiles for both model scales. In all cases there is general agreement with the Tala profiles in terms of slope, which is most encouraging from the modelling point of view. a
160
•
•
o
sccl[e
symb0[ 14.0
120
UZ
o
1:4000
0 ~
•
1:8000
cru
[]
I:/+000
UZ
m
1:8000
100 N
.E
Ell
80
•
0
.c
60
40
13
•
0
Q
20
O
manful[- scale [l
0 0 I
I
I
X
0
I
I
.I
0-1 0-2 0"3 0.4 05 I
I
• o
I
I
I
I
I
l
I
|.
~ F-Iz
0.6 0-7 0-8 0.9 10_--I
I
t
1
I
0-04 0.06 008 0.10 0-12 0.14 0.16 0.18 0'20 0,22 0"24 O-U
0z
Fig. 12. Nondimensiona] velocity and local turbulence-intensity profiles for point 12.
138
b'
o
o
o
r-,IW
4
I
o
"--
,5
o
I
I
,.6
I
~D
("4
( uJ
E
Z jqO!aq
C
r,
0 4~ U~
o
o
~
o
o
~ ~
o {3
mo •
•
•
•
0
"~= o
0 i
I
I C-4
|
° o
o
(u~)Z 4q6!aq
I
,
|
I
-4"
o (",4
o
c.
0
o
o~
Z
o
139
o
o
o
o
0
-~ <
•
,~
I
~
I
E
I I
o
0
o
l
o
o
o
o
o
0
0
E
~
0
0
(w) Z ~,qO!aq
91
o
~o
o
0
0
%
%
t.-,
~
0
0
~4
d
,
~o
o~
~
~_ I o
~
~
I
E
I I
o
(w) Z 446!a4
I
o
140
5.2.2. Longitudinal,component energy spectra The longitudinal components of the energy spectra for point 40 at a height of 20 m are presented in Fig. 17. It is clear that the general shapes of the spectra provide very good agreement. The full-scale results have been adjusted by the 1:4000 scale-factor for comparison; however, this presumes that turbulence simulation has been achieved at exactly 1:4000, which is not very probable and therefore introduces some freedom of spectral location along the frequency axis. For this reason the 1:8000 scale results have been plotted at their measured frequencies and not normalised. It is clear that the empirical spectral model proposed by Harris [11] fits the measured results very well. The constant L in this model has been adjusted from the full-scale value of 1800 m to accommodate both model scales and, as shown, results in a relatively small spectral spread. 10 symboi ~aI'e o 1C000
~ m : ~.000 ^ /
=
/ ~
Lm
I 8000
BO(]O
.....
o' Z ] / I ~ \ [ ~ , ~ - ~
Lo
nS~ln)o i
="
:
full s(:ate
1800m
-~
nSu(n)
2/:I U10
+ ~g~o ] J 00l 0
1
I00
I000
10,000
frequency (Hz)
Fig. 17. Longitudinal component of energy spectrum for point 40 at 20 m height.
The full-scale and modelled spectra for point 12 are presented in:Fig. 18 and also show excellent agreement in terms of shape. However, flatness o f the spectra is also apparent when compared with the Harris empirical spectrum. Not a great deal can be said about this comparison, as the empirical model is strictly valid only for flat homogeneous terrain. Modelled spectra for point 25 on the leeward side o f the saddle are compared in Fig. 19 and, despite the ~ situation, are seen to agree very well with the Harris empirical spectrum. 5.2.3. Length scales Lu~ Length scales Lu~ based on the full.scale and modelled spectral data were calculated from the spectra using the relationship proposed by Teunimmn [121 :
141 10 symbol L_~ - /*000 Lm -
I t-
/1' ~ T f,l ~ /1 / I~ 811~ • il, o eo nSu(n) {~u2
•
Ol
•
~ A
~
/'~'WL*.¢7~\ .
~k~/
L -~_
o *
: 8000
i r u l l scale
i ....
I ', ~
~
scale
1/.000 18000
I--'--Iresu#s I I
II
o
o
0 01 10
I 10
i 100 frequency (Hz)
10100
10,000
Fig. 18. Longitudinal component of energy spectrum for point 12 at 20 m height. 10
Lo Lm = /*000
nSu(n)
LQ :8000 im
symbol scale [ o 1 4000 " 1 8000
01
0 011 0
10
100
10 0
10,000
frequency (Hz)
Fig. 19. Longitudinal component of energy spectrum for point 25 at 20 m height.
Lux = 0 . 1 4 6 / k p ,
(1)
w h e r e kpL' is t h e spectral-peak w a v e n u m b e r . L e n g t h scales w e r e also calculated using Taylor's h y p o t h e s i s and t h e autoc o r r e l a t i o n c o e f f i c i e n t , w h i c h p r o v i d e t h e relationship
Lu x
= u TE
w h e r e T E is t h e Eulerian t i m e c o n s t a n t and w a s t a k e n as t h e t i m e f o r t h e a u t o c o r r e l a t i o n c o e f f i c i e n t t o fall t o a value o f e - ' .
(2)
142
It must be appreciated that owing to the flatness of the spectra it was difficult to ascertain precise values for kp, : therefore the results presented here (Table 1) are based on the averages of the ranges into which the peaks could reasonably fall. There is uniform agreement of the length scales for point 40, at ~ 200 m, which is in agreement with the 178 m sugge~ed by Counihan [13] for a similar terrain situation: There is a wide range of results for point 12, for which there is no easy explanation. The increased velocity found at point 12 in the 1 : 8000 simulation explains a small part of the discrepancy, but the author believes the major factor to be that of spectral flatness and the failure of Taylor's hypothesis in such a situation. TABLE 1 Length Point
scales,Lux for points 40 and 12 Length scales Lux (m) Field
40 12
1:4000 scale
1:8000 scale
Spectral
Autoeorrelation Spectral
Autocorrelation Spectral
Aut0correlation
200 350
215 250
213 400
290 800
200 500
242 800
6. Conclusions The results of this study have answered several questions arising from an earlier 1:4000 scale simulation of the Gebbies Pass region reported by Neal [2], the major points of note being: (1) wind-tunnel blockages as high as 7% do not significantly influence flow directions or magnitudes in the lower 50% of the boundary layer, this being demonstrated by using normalised velocities in the form of speed-up/down ratios to show the very high degree of correlation (0.91--0.96} between the two models considered; (2) the results suggest that it is possible to segment a model for analysis and produce data that are representative of the area as a whole; (3) the velocity and local turbulence-intensity profiles have been shown to be in excellent agreement between the two models considered, particularly in the lower 100 m of the bounda~¢ layer; there is a significant variation in the magnitude of the local turbulence intensity as the flow passes over the saddle; in all cases there is good agreement between the models and excellent agreement with the full-scale data; (4) Tala velocity profiles compare most favourably with those measured at the equivalent model sites; (5) it has been shown that at 1:8000 scale the accuracy of modetlingis
143 e x t r e m e l y i m p o r t a n t , p a r t i c u l a r l y in t h e f o r m i n g o f t h e t o p s o f i s o l a t e d hills; ( 6 ) t h e s h a p e s o f t h e m o d e l l e d a n d f u l l - s c a l e s p e c t r a a r e in v e r y g o o d a g r e e ment and although they generally compare well with the empirical spectrum p r o p o s e d b y H a r r i s [ 1 1 ] t h e y d o e x h i b i t a d e f i n i t e f l a t n e s s w h i c h is n o t a feature of the empirical model; ( 7 ) t h e l e n g t h s c a l e Lux o f t u r b u l e n c e h a s b e e n c a l c u l a t e d a n d s h o w s e x cellent agreement between both models and full-scale measurements for point 4 0 ; l i m i t a t i o n s d u e t o s p e c t r a l f l a t n e s s a r e e v i d e n t in t h e a n a l y s i s f o r p o i n t 12, o n t h e s a d d l e c r e s t ; h o w e v e r , all m e a s u r e m e n t s a t t h i s p o i n t s u g g e s t t h e e x p e c t e d i n c r e a s e in Lux as t h e t u r b u l e n c e is e f f e c t i v e l y " s t r e t c h e d " in t h e uc o m p o n e n t d i r e c t i o n in p a s s i n g o v e r t h e s a d d l e . References 1 R.N. Meroney, A.J. Bowen, D. Lindley and J.R. Pearse, Wind characteristics over complex terrain : laboratory simulation and field measurements at Rakaia Gorge, New Zealand, Rep. TH/FL 102/78, Department of Mechanical Engineering, University of Canterbury, Christchurch, New Zealand, 1978. 2 D. Neal, Wind flow and structure over Gebbies Pass, New Zealand: a comparison between wind tunnel simulation and field measurements, Ph.D. thesis, University of Canterbury, Christchurch, New Zealand, 1979. 3 J.D. Holmes, G.R. Walker and W.E. Steen, Some effects of an isolated hill on wind velocities near ground level, 7th Aust. Conf. on Hydraulics and Fluid Mechanics, Brisbane, August 18--22, 1980. 4 H.C. Chien, R.N. Meroney and V.A. Sandborne, Sites for wind power installations: physical modelling of the wind field over Kanuku Point, Oahu, Hawaii, U.S. Dept. of Energy, Contract No. DE AS06-77ET20292 A004, 1979. 5 D. Neai, Full scale measurement of the wind regime and structure over a saddle and their correlation with wind tunnel tests over a 1:4000 scale model, Boundary-Layer Meteorol., 22 (1982) 351--371. 6 D. Neal, D.C. Stevenson and D. Lindley, A wind tunnel boundary simulation of wind flow over complex terrain: effect of terrain and model construction, Boundary-Layer Meteorol., 21 (1981) 271--295. 7 J.R. Pearse, A data acquisition system for an atmospheric boundary layer wind tunnel based on the use of a Hewlett--Packard digital computer, Rep. T H / F L 101/ 78, Department of Mechanical Engineering, University of Canterbury, Christchurch, New Zealand, 1978. 8 C.F. Shieh and W. Frost, Tether analysis for kite anemometer, Contract No. AC0676ET20242, 1976. 9 R.G.J. Flay, Structure of a rural atmospheric boundary layer near the ground, Ph.D. Thesis, University of Canterbury, Christchurch, New Zealand, 1978. 10 ESDU, Characteristics of atmospheric turbulence near the ground. Part II. Single point data for strong winds (neutral atmosphere), Data Item 74031, Eng. Sci. Data Unit, London, 1974. 11 R.I. Harris, The nature of wind, Proc. Semin. on the Modern Design of WindSensitive Structures, Inst. Cir. Eng. London, June 18, 1970, Construct. Ind. Res. Inf. Assoc., 1971, pp. 29--55. 12 H.W. Teunissen, Characteristics of the mean wind and turbulence in the planetary boundary layer, UTIAS Rev., 32 (1970). 13 J. Counihan, Adiabatic atmospheric boundary layers: a review and analysis of data from the period 1880--1972, Atmos. Environ., 9 (1975) 871--905.
125
Elsevier Science Publishers B.V., A m s t e r d a m - - P r i n t e d in T h e N e t h e r l a n d s
THE INFLUENCE OF MODEL SCALE ON A WIND-TUNNEL SIMULATION OF COMPLEX T E R R A I N
D. N E A L
Vickers Dawson, Crayford, Kent (Gt. Britain) (Received N o v e m b e r 18, 1 9 8 1 ; a c c e p t e d O c t o b e r 14, 1 9 8 2 )
Summary T h e results o f w i n d - t u n n e l s i m u l a t i o n s involving 1 : 4 0 0 0 and 1 : 8 0 0 0 scale m o d e l s of G e b b i e s Pass (New Z e a l a n d ) are p r e s e n t e d and c o m p a r e d w i t h full-scale m e a s u r e m e n t s . T h e results f r o m t h e t w o m o d e l s are c o m p a r e d to investigate t h e possible effects o f w i n d - t u n n e l blockage and to test t h e validity o f s e g m e n t i n g a m o d e l for analysis. Speedu p / d o w n ratios are used to c o m p a r e t h e m e a n wind speeds over t h e t w o m o d e l s at a fullscale h e i g h t o f 12 m and result in high c o r r e l a t i o n s , in t h e range 0 . 9 0 - - 0 . 9 6 . Full-scale w i n d - s t r u c t u r e m e a s u r e m e n t s were m a d e to a h e i g h t o f 20 m at selected sites in t h e region. T h e full-scale and m o d e l results are c o m p a r e d in t e r m s o f velocity and turb u l e n c e - i n t e n s i t y profiles, energy s p e c t r a and length scales. In a d d i t i o n , velocity-profile d a t a collected by m e a n s of Tala kites are p r e s e n t e d and c o m p a r e d w i t h t h e results obt a i n e d in t h e w i n d - t u n n e l simulations. In all cases t h e r e is a high degree o f c o m p a t i b i l i t y , w h i c h suggests t h a t w i n d - t u n n e l m o d e l l i n g as for t h e G e b b i e s Pass c o m p l e x t e r r a i n region is a viable tool in t h e evaluation of p o t e n t i a l w i n d - e n e r g y sites.
1. Introduction One of the options available to investigators wishing to evaluate a potential wind-energy site involves physically modelling the area in a planetary boundary-layer wind tunnel. Although physical modelling offers advantages in terms of speed of data collection, ease of data handling and relatively low cost, there are other aspects that must be considered;these include: (1) t y p e of model surface finish; (2) selection of a suitable model scale; (3) blockage effects and their significance; (4) knowledge of approach-flow characteristics. This last factor becomes increasingly important if the wind regime does n o t meet the conditions required for a neutrally stable flow; however, some windtunnel facilities can model thermally stratified flow situations. Studies by Meroney et al. [ 1 ] , Neal [ 2 ] , Holmes et al. [3] and Chien et al. [4] produced data from wind-tunnel simulations which were shown to be in good agreement with full-scale measurements of the wind regimes over
0304~3908/83/$03.00
© 1 9 8 3 Elsevier Science P u b l i s h e r s B.V.
126
the actual terrains modelled. The study reported by Neal [2] involved simulation of the wind flow over a saddle region of complex terrain, Gebbies Pass, located in the South Island of New Zealand. A schematic diagram showing the major terrain features is given in Fig. 1; a full description of the saddle region and the approach terrain has been given by Neal [5].
Heights:-
Port Hills 600m 5ebbies Saddle 300m Mt Herbert Range 500m
Fig. 1. Terrain features of the Gebbies Pass region (viewed from the NE).
In the earlier work [5], Gebbies Pass was physically modelled at a scale o f 1:4000, resulting in a model 2.4 m wide and 3.6 m long which required subdividing to fit into the 1.2 m square cross-section of the wind tunnel; a photograph showing the three models produced from the region and their orientations is given in Fig. 2. The influence of the type of model surface finish in a complex terrain situation was considered by Neal et al. [6] who showed a terraced surface finish to be unsatisfactory for surface wind analysis. Other aspects o f this study involved the correlation of model data with full-scale measurements, and comparisons of several wind-structure parameters. The model results left some important questions unanswered, the major of these concerning: (1) the influence of wind-tunnel blockage (up to 7%) generated by the presence of the model itself; (2) the validity of subdividing the model and performing the analysis in sections; (3) the effects of removing significant portions of terrain from both sides of the area under analysis (this being the case for Model B). The influence of blockage will, of.course, vary from one terraia situation to another, and therefore the results of this aspect are applicable only to the Gebbies Pass situation, albeit a severe one.
127
A
\ \ \ \ \ \ \
Fig. 2. 1 : 4 0 0 0 scale m o d e l of G e b b i e s Pass.
C
"1
128
2. The model In response to the above questions, a 1:8000 scale model o f the same region was constructed for ~ . M ~ 1.2 m wkle by 1:8 m long, it was able to fit into the wind tunnel in one piece. simut tion. board 9 ram thick were obtained by le region and printin glued together uaing a P V A glue and r e ~ in a model witlt a terraced finish. A polyfiUer n o n ~ t r i n k material was ~ t o m in the ~ to produce the contoured model, shaping of the hills~and ~ being aided by photographs of the ~ o n . A series of tests w e r e have been given pl~vioualy by Neal [2]. The fmal water-based shelter.belts (typically 20 m high) and scrub areas {typically 3-~5m high). Wool was used to represent the shelt~-belt~ and an open form o f hessian for the scrub areas on the i:4000 scale model, and twine for the aheltar-belts and lint for the scrub areas on the 1:8000 ~ m o d e l A futI ~ r i p t i o n and justification for the use o f these materialshavebeen given by Neal [2] ; a photograph of the resulting 1:8000 scale model is given in Fig. 3,
The analysis of Model B, as shown in F~. 2, w a s o f particular interest, for two masons: (1) this model resulted in the saddle being positioned almost perpendicular to the general flow direction; (2} the resulting SSW alignment was found to be more representative of the full-scale weather patterns, which tended to be more southerly than southwesterly.
129 A cross-section through the saddle indicating the locations of the measurement points both for the models and for fun-scale data collection is given in Fig. 4. A dual¢hannel Thermo Systems hot-film anemometer was used to record wind velocity and turbulence. The hot-film cylindrical probe (Type 1210-20) has an effective length of 1.0 mm and a diameter of 0.051 mm. This type of sensor has the following inherent advantages over more traditional hot-wire systems: (1) better frequency response (when controlled electronically) than a hot wire of the same diameter, owing to the sensitive part of the hot-film sensor being distributed on the surface rather than including the entire cross-sectional area; (2) low heat conduction to the supports (end loss) for a given length-todiameter ratio, owing to the low thermal conductivity of the substrate material; a shorter sensing length can thus be used; (3) less susceptibility to fouling and greater ease of cleaning: a thin quartz coating on the surface resists accumulation of foreign material. model fu~Lscale daf~ dafo 40 &O 12 12 25 37
f{ow dlrechon
point 12
[point26 point 25 1 t -
~"~"
~
~
\ s o d d ~
po,nt31 point 37 PO,)t z*O
t
t
Fig. 4. Locations of m e a s u r e m e n t points on longitudinal cross-section through Gebbies Pass saddle.
The o u t p u t from the anemometer was passed to a Hewlett--Packard 2100A minicomputer for processing and extraction of various wind-structure parameters; full details of the data handling have been given by Pearse [7]. A traversing system supported on rails above the model was designed and commissioned for the Gebbies Pass study, the traverser being driven by stepperm o t o r and providing the following probe (sensor) positional accuracy: (1) longitudinal direction, + 2.0 mm; (2) vertical direction, + 0.1 mm; (3) lateral direction, + 1.0 mm; (4) angular (yaw about z-axis), +0.5 ° .
130 In conjunction with the hot-film anemometry, flow-visualisation techniques involving polystyrene beads and m i n i a t u ~ ~ were used tegether witha five. hole pressure probe to determine flow directions.
3. Fuli~cale data collection A mobile survey technique was used to provide mean wind speed data over the region at a height of 10 m. Mean wind speeds were collected over fiveminute intervals using Rimco three-cup fast-response anemometers, Datarecording start times were synchronised with those at a reference tower at a site upstream of the region modelled, thus allowing the use of normalised velocities for comparisons; a full description of the technique and the justification for its use have been given b y Neal [ 2 ] . Tala kites were used to collect wind-velocity data to a height of 300 m. Forces exerted on the aerodynamically lifting kite, of sled design, produce tension on the flying line which is connected to a calibrated spring to give velocities directly. Recordings of the length of the flying line (nonstretch kelvar) and the angle of inclination of the kite were required in order to apply the manufacturer's formula to obtain the actual altitude of the kite; the accuracy of this formula has been verified by Shieh and Frost [8]. Orthogonally arranged propeller anemometers, aerodynamically similar to the Gill propeller anemometer, were supported on a lattice t o w e r at various intervals to a height of 20 m and the o u t p u t collected on seven-track magnetic tape for analysis using a Burroughs 6700 computer. Details of the anemometry, data collection and software have been given by Flay [9]
4. Approach-flow characteristics The model approach-flow conditions for both the 1:4000 and 1:8000 scale models were established by simulation of the following parameters: (1) velocity profile; (2) local turbulence-intensity profile; (3) roughness length; (4) longitudinal c o m p o n e n t of energy spectrum; (5) boundary-layer depth. The modelling of these parameters was based on data for flat rural terrain presented by ESDU [10] and, where possible, on full-scale measurements made on the p r o t o t y p e approach terrain. In obtaining the high degree o f compatibility in the parameters discussed it was found that the boundarylayer height was significantly higher than the initial specification o f Z~ = 400 m (50 ram). In the 1:4000 simulation the choice of a boundary-layer h e i g h t o f 400 m was based on available literature and the earlier work o f M e r o n e y et al. [ 1 ] . Subsequent to the 1:4000 study, local Meteorological Office measurements suggested that owing to the nature of the southwesterly weather patterns
131 this parameter could be as high as 1000 m. Therefore, owing to this uncertainty, it was considered reasonable to accept the modelled boundary height in the approach flow of Z~ --- 640 m in a t r a d e ~ f f in order to hold the other parameters to the required values. 5. Results The results are presented in two sections. The first shows the influence of blockage and model sectioning, by means of data correlations. The second considers Model B in more detail using data correlations between the model and full-scale data, together with wind-structure comparisons between the model and full-scale measurements. 5.1. R e s u l t s f o r M o d e l s A a n d C
If the wind-tunnel blockage of 6--7% for the 1:4000 scale model had been a significant factor it would have resulted in compression of the flow streamlines and a subsequent increase in the velocity close to the model surface, together with possible changes in flow direction. These effects would be superimposed on the velocity and direction changes induced by the topography itself. Flow direction tests using miniature flags and the pressure probe showed that there were in fact no significant variations in the flow directions between the two models, thus eliminating this aspect of the blockage influence. The speed-up/down ratio for each analysis point relative to the approachflow velocity was calculated for the heights Z p = 12, 20 and 40 m, and used to assess the possible influence of blockage on the wind-velocity regime. These data were used to produce a sample correlation coefficient which, based on 23 data sets, ranged from 0.812 at Z p = 12 m to 0.833 at Z p = 40 m. The percentage differences between the 1:4000 and 1:8000 data at Z p = 12, 20 and 40 m were calculated and an arbitrary differential of 15% set to account for any small modelling discrepancies and to allow for experimental errors. It was apparent that five of the data points were significantly above this value, with 18--40% differences. With these exceptions all data fell well within the 15% difference level, and in the majority of cases the difference was in the region of 5%. A detailed comparison of these points provided clear explanations for the larger discrepancies, which were in all cases due to modelling errors. Although care was taken in the construction of both models it will be appreciated that at a scale of 1:8000 very small model errors can represent large full-scale discrepancies. The feature most prone to this was the formation of hilltops, where there is often an absence of contours for guidance. Indeed, four of the five points concerned were situated on isolated hilltops. It should also be noted that in several cases where the percentage difference was 20% at Z p = 12 m, this dropped to ~10% at Z p = 40 m, thus illustrating the localised influence that small model discrepancies can produce. With the extreme data removed, the sample correlation coefficient r for
132
Zp = 12 m improved to 0.958, as shown on the scatter diagram in Fig. 5, which also includes the linear regresaion curves. The scatter d ~ in Fig. 6 represents the Zp = 40 m data, for which r has the value o f 0.918. The high correlations obtained between the two models suggest the following: (a) the blockage introduced by the 1:4000 scale model (up to 7%) does not significantly influence the flow directions or magnitudes; (b) the results obtained by the analysis o f Models A and C separately are representative of the area as a whole. 22 2-0 1-B
1.6 1"4. o ~ o
1"2 1.0 0.8 0.6
, ~/.~/
n:18
0-~ 0.2 0
0.2 0./., 0"6 0-8 1-0 1.2 1./+ 1.6 1"8 2"0 2.2 O
Regression tines 0
~6/~ooo = 1°s6 "~6/8ooo-o.o79
Corretafion coefficient r= 0-95B Fig. 5. Scatter diagra.-~ for Models A and C at Zp = 12 m.
5.2. Results for Model B Since a more detailed analysis was ~ e d o u t for the Model B results, the analysis points used are presented on the map in ~ g . 7. The procedure o f correlating the s ~ - u p / d o w n ratios was alsoapptied to
133 22 20 1.8 1'6 1.z,
§
1"2
Oo
10 0-8
//
0.6
n=
18
0.4 02 I
0
I
I
I
i
0"2 04., 0"6 08
I
I
I
i
I
,,
l
1'0 1"2 1"/., 1"6 1"8 2.0 2'2
°/, U&
000
Re~]ression Lines
O
/,ooo o°/
& 8000
Corre[otion coefficient
g
bU
-0'028
000
+ 0"197
r = 0.918
Fig. 6. Scatter diagram for Models A and C at Zp = 40 m. the Model B data: based on 15 data sets, values varying from r = 0.912 at Zp = 12 m to r = 0.925 at Zp = 40 m were obtained, as shown by the scatter diagrams in Figs. 8 and 9. The 1:4000 scale model results were shown by Neal [5] to correlate very well with full-scale data, producing correlation coefficients in the range 0.73-0.90. It follows that there exists an implied high correlation between the 1:8000 scale mo de l and the full-scale data.
5.2.1. Velocity and local turbulence-intensity profiles Nondimensional velocity and local turbulence-intensity profiles are presented in Figs. 10 and 11 f or points 40 and 31, which are situated on the flat terrain approaching the saddle region. In Fig. 10, for point 40, full-scale local turbulence-intensity data collected to a height o f 20 m are included and show excellent agreement with the model data. The model velocity data have been
134
:¢ modelled but not photogra~ed Fig. 7. Measurement points for analysis of Model B.
135
c-4
o
~
o
o
d:,
.'9 ÷
~_
÷
o o
o
I.~ o--
I:::~ II~
i:::) II:)
II c_
d~
tl
II
~t o
o
c-4
N
I
•
.
cM
t'~
.
o
.
c_ o kJ
.
o
o
o
Q4 N o
d~ o
o
~
,9
<:~ 0
n
II
II
,-
0
O
N
m
r~
E N
o ~
o
12~ Ii2~
~,
ap o
o,
"9 o
~y o
t
I
,~
N
I
I
I
I
I
I
0 000~/911
I
o
~ |
/ - -
r - -
I
J
I \~I~
0
o
~6
136
-5 O
"~
@
O
O
•
O
O
~..
CO
O
O
"-0
,4D
0
0
i
0
o
o
-4"
c'~
CD
0
Cr~
0
O
0
@
0
@
0
c I
o
o
o
0
0
0
'?
""
~
~
I
I
o
o
c~
(w)z 4q6!aq
o ~
i ~
o
o
N
i ~
8
M
N
~p
5
e~
E
0
•
0
• .
0
IPl
?1,="
~
0
4.J
~
o
_o _o
o
C
o
0
-~
_o _o
o
d
~
6
o o
c 0
0
d~,
~
t-
o
o
o
Z
Z
o
--
~
r,,.
r,,
0 o
•
o o•
[]
"7"
o I
I
o
o~
(wlZ
446!a4
0
137
normalised by the velocity at the gradient height. Figure 12 presents similar information for point 12 and shows clearly the uniform nature of the velocity profile at this point and the significant reduction in the local turbulence values. Again there is excellent agreement with full-scale local turbulenceintensity data to a height of 20 m. The results for point 26 are presented in Fig. 13: although there is good agreement in terms of slope, there is a small variation in magnitude between the two models. In order to provide some comparisons with full-scale measurements the Tala velocity data were used in normalised form. The velocity at 160 m height was used for normalisation, since this was the highest c o m m o n denominator for all measurement points. The resulting profiles are presented in Figs. 14-16 for points 40, 12 and 25, together with the profiles for both model scales. In all cases there is general agreement with the Tala profiles in terms of slope, which is most encouraging from the modelling point of view. a
160
•
•
o
sccl[e
symb0[ 14.0
120
UZ
o
1:4000
0 ~
•
1:8000
cru
[]
I:/+000
UZ
m
1:8000
100 N
.E
Ell
80
•
0
.c
60
40
13
•
0
Q
20
O
manful[- scale [l
0 0 I
I
I
X
0
I
I
.I
0-1 0-2 0"3 0.4 05 I
I
• o
I
I
I
I
I
l
I
|.
~ F-Iz
0.6 0-7 0-8 0.9 10_--I
I
t
1
I
0-04 0.06 008 0.10 0-12 0.14 0.16 0.18 0'20 0,22 0"24 O-U
0z
Fig. 12. Nondimensiona] velocity and local turbulence-intensity profiles for point 12.
138
b'
o
o
o
r-,IW
4
I
o
"--
,5
o
I
I
,.6
I
~D
("4
( uJ
E
Z jqO!aq
C
r,
0 4~ U~
o
o
~
o
o
~ ~
o {3
mo •
•
•
•
0
"~= o
0 i
I
I C-4
|
° o
o
(u~)Z 4q6!aq
I
,
|
I
-4"
o (",4
o
c.
0
o
o~
Z
o
139
o
o
o
o
0
-~ <
•
,~
I
~
I
E
I I
o
0
o
l
o
o
o
o
o
0
0
E
~
0
0
(w) Z ~,qO!aq
91
o
~o
o
0
0
%
%
t.-,
~
0
0
~4
d
,
~o
o~
~
~_ I o
~
~
I
E
I I
o
(w) Z 446!a4
I
o
140
5.2.2. Longitudinal,component energy spectra The longitudinal components of the energy spectra for point 40 at a height of 20 m are presented in Fig. 17. It is clear that the general shapes of the spectra provide very good agreement. The full-scale results have been adjusted by the 1:4000 scale-factor for comparison; however, this presumes that turbulence simulation has been achieved at exactly 1:4000, which is not very probable and therefore introduces some freedom of spectral location along the frequency axis. For this reason the 1:8000 scale results have been plotted at their measured frequencies and not normalised. It is clear that the empirical spectral model proposed by Harris [11] fits the measured results very well. The constant L in this model has been adjusted from the full-scale value of 1800 m to accommodate both model scales and, as shown, results in a relatively small spectral spread. 10 symboi ~aI'e o 1C000
~ m : ~.000 ^ /
=
/ ~
Lm
I 8000
BO(]O
.....
o' Z ] / I ~ \ [ ~ , ~ - ~
Lo
nS~ln)o i
="
:
full s(:ate
1800m
-~
nSu(n)
2/:I U10
+ ~g~o ] J 00l 0
1
I00
I000
10,000
frequency (Hz)
Fig. 17. Longitudinal component of energy spectrum for point 40 at 20 m height.
The full-scale and modelled spectra for point 12 are presented in:Fig. 18 and also show excellent agreement in terms of shape. However, flatness o f the spectra is also apparent when compared with the Harris empirical spectrum. Not a great deal can be said about this comparison, as the empirical model is strictly valid only for flat homogeneous terrain. Modelled spectra for point 25 on the leeward side o f the saddle are compared in Fig. 19 and, despite the ~ situation, are seen to agree very well with the Harris empirical spectrum. 5.2.3. Length scales Lu~ Length scales Lu~ based on the full.scale and modelled spectral data were calculated from the spectra using the relationship proposed by Teunimmn [121 :
141 10 symbol L_~ - /*000 Lm -
I t-
/1' ~ T f,l ~ /1 / I~ 811~ • il, o eo nSu(n) {~u2
•
Ol
•
~ A
~
/'~'WL*.¢7~\ .
~k~/
L -~_
o *
: 8000
i r u l l scale
i ....
I ', ~
~
scale
1/.000 18000
I--'--Iresu#s I I
II
o
o
0 01 10
I 10
i 100 frequency (Hz)
10100
10,000
Fig. 18. Longitudinal component of energy spectrum for point 12 at 20 m height. 10
Lo Lm = /*000
nSu(n)
LQ :8000 im
symbol scale [ o 1 4000 " 1 8000
01
0 011 0
10
100
10 0
10,000
frequency (Hz)
Fig. 19. Longitudinal component of energy spectrum for point 25 at 20 m height.
Lux = 0 . 1 4 6 / k p ,
(1)
w h e r e kpL' is t h e spectral-peak w a v e n u m b e r . L e n g t h scales w e r e also calculated using Taylor's h y p o t h e s i s and t h e autoc o r r e l a t i o n c o e f f i c i e n t , w h i c h p r o v i d e t h e relationship
Lu x
= u TE
w h e r e T E is t h e Eulerian t i m e c o n s t a n t and w a s t a k e n as t h e t i m e f o r t h e a u t o c o r r e l a t i o n c o e f f i c i e n t t o fall t o a value o f e - ' .
(2)
142
It must be appreciated that owing to the flatness of the spectra it was difficult to ascertain precise values for kp, : therefore the results presented here (Table 1) are based on the averages of the ranges into which the peaks could reasonably fall. There is uniform agreement of the length scales for point 40, at ~ 200 m, which is in agreement with the 178 m sugge~ed by Counihan [13] for a similar terrain situation: There is a wide range of results for point 12, for which there is no easy explanation. The increased velocity found at point 12 in the 1 : 8000 simulation explains a small part of the discrepancy, but the author believes the major factor to be that of spectral flatness and the failure of Taylor's hypothesis in such a situation. TABLE 1 Length Point
scales,Lux for points 40 and 12 Length scales Lux (m) Field
40 12
1:4000 scale
1:8000 scale
Spectral
Autoeorrelation Spectral
Autocorrelation Spectral
Aut0correlation
200 350
215 250
213 400
290 800
200 500
242 800
6. Conclusions The results of this study have answered several questions arising from an earlier 1:4000 scale simulation of the Gebbies Pass region reported by Neal [2], the major points of note being: (1) wind-tunnel blockages as high as 7% do not significantly influence flow directions or magnitudes in the lower 50% of the boundary layer, this being demonstrated by using normalised velocities in the form of speed-up/down ratios to show the very high degree of correlation (0.91--0.96} between the two models considered; (2) the results suggest that it is possible to segment a model for analysis and produce data that are representative of the area as a whole; (3) the velocity and local turbulence-intensity profiles have been shown to be in excellent agreement between the two models considered, particularly in the lower 100 m of the bounda~¢ layer; there is a significant variation in the magnitude of the local turbulence intensity as the flow passes over the saddle; in all cases there is good agreement between the models and excellent agreement with the full-scale data; (4) Tala velocity profiles compare most favourably with those measured at the equivalent model sites; (5) it has been shown that at 1:8000 scale the accuracy of modetlingis
143 e x t r e m e l y i m p o r t a n t , p a r t i c u l a r l y in t h e f o r m i n g o f t h e t o p s o f i s o l a t e d hills; ( 6 ) t h e s h a p e s o f t h e m o d e l l e d a n d f u l l - s c a l e s p e c t r a a r e in v e r y g o o d a g r e e ment and although they generally compare well with the empirical spectrum p r o p o s e d b y H a r r i s [ 1 1 ] t h e y d o e x h i b i t a d e f i n i t e f l a t n e s s w h i c h is n o t a feature of the empirical model; ( 7 ) t h e l e n g t h s c a l e Lux o f t u r b u l e n c e h a s b e e n c a l c u l a t e d a n d s h o w s e x cellent agreement between both models and full-scale measurements for point 4 0 ; l i m i t a t i o n s d u e t o s p e c t r a l f l a t n e s s a r e e v i d e n t in t h e a n a l y s i s f o r p o i n t 12, o n t h e s a d d l e c r e s t ; h o w e v e r , all m e a s u r e m e n t s a t t h i s p o i n t s u g g e s t t h e e x p e c t e d i n c r e a s e in Lux as t h e t u r b u l e n c e is e f f e c t i v e l y " s t r e t c h e d " in t h e uc o m p o n e n t d i r e c t i o n in p a s s i n g o v e r t h e s a d d l e . References 1 R.N. Meroney, A.J. Bowen, D. Lindley and J.R. Pearse, Wind characteristics over complex terrain : laboratory simulation and field measurements at Rakaia Gorge, New Zealand, Rep. TH/FL 102/78, Department of Mechanical Engineering, University of Canterbury, Christchurch, New Zealand, 1978. 2 D. Neal, Wind flow and structure over Gebbies Pass, New Zealand: a comparison between wind tunnel simulation and field measurements, Ph.D. thesis, University of Canterbury, Christchurch, New Zealand, 1979. 3 J.D. Holmes, G.R. Walker and W.E. Steen, Some effects of an isolated hill on wind velocities near ground level, 7th Aust. Conf. on Hydraulics and Fluid Mechanics, Brisbane, August 18--22, 1980. 4 H.C. Chien, R.N. Meroney and V.A. Sandborne, Sites for wind power installations: physical modelling of the wind field over Kanuku Point, Oahu, Hawaii, U.S. Dept. of Energy, Contract No. DE AS06-77ET20292 A004, 1979. 5 D. Neai, Full scale measurement of the wind regime and structure over a saddle and their correlation with wind tunnel tests over a 1:4000 scale model, Boundary-Layer Meteorol., 22 (1982) 351--371. 6 D. Neal, D.C. Stevenson and D. Lindley, A wind tunnel boundary simulation of wind flow over complex terrain: effect of terrain and model construction, Boundary-Layer Meteorol., 21 (1981) 271--295. 7 J.R. Pearse, A data acquisition system for an atmospheric boundary layer wind tunnel based on the use of a Hewlett--Packard digital computer, Rep. T H / F L 101/ 78, Department of Mechanical Engineering, University of Canterbury, Christchurch, New Zealand, 1978. 8 C.F. Shieh and W. Frost, Tether analysis for kite anemometer, Contract No. AC0676ET20242, 1976. 9 R.G.J. Flay, Structure of a rural atmospheric boundary layer near the ground, Ph.D. Thesis, University of Canterbury, Christchurch, New Zealand, 1978. 10 ESDU, Characteristics of atmospheric turbulence near the ground. Part II. Single point data for strong winds (neutral atmosphere), Data Item 74031, Eng. Sci. Data Unit, London, 1974. 11 R.I. Harris, The nature of wind, Proc. Semin. on the Modern Design of WindSensitive Structures, Inst. Cir. Eng. London, June 18, 1970, Construct. Ind. Res. Inf. Assoc., 1971, pp. 29--55. 12 H.W. Teunissen, Characteristics of the mean wind and turbulence in the planetary boundary layer, UTIAS Rev., 32 (1970). 13 J. Counihan, Adiabatic atmospheric boundary layers: a review and analysis of data from the period 1880--1972, Atmos. Environ., 9 (1975) 871--905.