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Β© University of Reading 2008 www.reading.ac.uk 18 December 2012
Wind speed profiles over Greater London, UK Daniel Drew, Janet Barlow and SiΓ’n Lane
2
Introduction
β’ Vertical wind speed profiles are required to address a
number of wind engineering problems:
β’ Dispersion of pollution
β’ Designing tall buildings
β’ Several theoretical and empirical models:
β’ Power law
β’ Log law
β’ Deaves and Harris model
Wind speed profile models
3
β’ Log law (Eurocode)
π π§ =π’β
π ln
π§
π§0 z0=0.8 m for an urban surface (Cook, 1985).
β’ Deaves and Harris Model (UK, Australia)
π π§ =π’βπ
πππ§
π§0+ 5.75
π§
ββ 1.88
π§
β
2
β 1.33π§
β
3
+ 0.25π§
β
4
h, the height of the boundary layer is assumed to equal 3250 m.
4
Introduction
β’ Vertical wind speed profiles are required to address a
number of wind engineering problems:
β’ Dispersion of pollution
β’ Designing tall buildings
β’ Several theoretical and empirical models:
β’ Power law
β’ Log law
β’ Deaves and Harris model
β’ Little validation of models, particularly in urban areas.
β’ Assessed wind speed profile over Greater London.
5
6
The surrounding surface is very heterogeneous (parks, urban, river)
7
Gill instruments R3-50 ultrasonic anemometer
β’Measures horizontal and vertical components of wind.
β’Sampling frequency = 20 Hz
Instruments at BT Tower (190 m)
Observations analysed to estimate:
πβ2 = π’β²π€β²2 + π£β²π€β²2
πΏ =βπ’β
3π
π π π€β²πβ²
Halo Photonics Streamline pulsed Doppler lidar
β’Fully programmable scanner
β’Doppler Beam Swinging Method
β’Gate length = 30 m
β’80 measurement gates
β’Instrument location = 20 m above ground level
β’Min. measurement height = 90 m above lidar (110 m above ground)
β’Profile every 2 minutes
β’21st May 2011 β 6th Jan 2012
Doppler beam swinging method
β’ Three-beam wind-profiling method
β’ Derives wind speeds from one vertical and two tilted beams
β’ 2 s of data taken consecutively in each direction (40,000 pulses)
β’ Short scan time means flow will not change much over scan period.
β’ Interval between scans = 120 s
β’ See Pearson et al. (2009) for comparison with other methods in a rural setting.
ΞΈ = 15Β°
Halo Photonics Streamline pulsed Doppler lidar
β’Fully programmable scanner
β’Doppler Beam Swinging Method
β’Gate length = 30 m
β’80 measurement gates
β’Instrument location = 20 m above ground level
β’Min. measurement height = 90 m above lidar (110 m above ground)
β’Profile every 2 minutes
β’21st May 2011 β 6th Jan 2012
30th September 2011
11
Mean wind speed profile
β’ Derived from 5500 hours of observations
β’ Compared with the 3 models
12
Surface dependent
parameters (Cook, 1997)
z0=0.8 m
h=3250 m
Ξ±=0.32
Stability
β’ Data filtered by stability derived from BT tower observations.
13 UQ25 UQ50 UQ75
High wind speeds
14
LOW:
U<UQ25
MEDIUM:
UQ25<U<UQ50
HIGH:
UQ50<U<UQ75
VERY HIGH:
U>UQ75
Terrain dependent parameters
15
Roughness length, z0
β’ Derived from log law using
u* observed at BT tower.
16
β’ Morphological values determined in Wood et al. (2010).
z0mean= 0.6 m
z0mean= 0.9 m
Power law exponent, Ξ±
β’ Derived from wind profile
observations.
β’ Good agreement for
westerly winds.
17
πΌ =1
ln(π§1π§2)
0.5
π§0
Ξ±mean= 0.23
Boundary layer height, h
18
β =πβ
6π
hmean= 1050 m
Model comparison
19
Conclusions
20
Future Work
β’ Presented wind speed profiles derived from lidar observations.
β’ High wind speeds occur during neutral conditions. β’ High wind speed profile shows reasonable fit with
model profiles (log law and Deaves and Harris).
β’ Lack of Doppler lidar observations below 90 m restricts potential to assess wind loading models- potential for Sodar.
Extra slides
21
Doppler beam swinging method
β’ Three-beam wind-profiling method
β’ Derives wind speeds from one vertical and two tilted beams
β’ 2 s of data taken consecutively in each direction (40,000 pulses)
β’ Short scan time means flow will not change much over scan period.
β’ Interval between scans = 120 s
β’ See Pearson et al. (2009) for comparison with other methods in a rural setting.
ΞΈ = 15Β°
No
. of d
ata
po
ints
Lid
ar w
ind
sp
eed
(m
s-1)
Wind speed (60 minute average)
β’60 minute average used to include sufficient data from lidar.
β’Some of RMSE can be explained by standard error (average SE = 0.4 ms-1).
β’Some difference likely due to large separation between instruments.
Y=0.98x+0.56
RMSE=1.4
Weighted best fit
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