Wind speed profiles over Greater London, UKbl_met/actual/non_protect...ย ยท Wind speed profile...
Transcript of Wind speed profiles over Greater London, UKbl_met/actual/non_protect...ย ยท Wind speed profile...
ยฉ University of Reading 2008 www.reading.ac.uk 18 December 2012
Wind speed profiles over Greater London, UK Daniel Drew, Janet Barlow and Siรขn Lane
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Introduction
โข Vertical wind speed profiles are required to address a
number of wind engineering problems:
โข Dispersion of pollution
โข Designing tall buildings
โข Several theoretical and empirical models:
โข Power law
โข Log law
โข Deaves and Harris model
Wind speed profile models
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โข Log law (Eurocode)
๐ ๐ง =๐ขโ
๐ ln
๐ง
๐ง0 z0=0.8 m for an urban surface (Cook, 1985).
โข Deaves and Harris Model (UK, Australia)
๐ ๐ง =๐ขโ๐
๐๐๐ง
๐ง0+ 5.75
๐ง
โโ 1.88
๐ง
โ
2
โ 1.33๐ง
โ
3
+ 0.25๐ง
โ
4
h, the height of the boundary layer is assumed to equal 3250 m.
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Introduction
โข Vertical wind speed profiles are required to address a
number of wind engineering problems:
โข Dispersion of pollution
โข Designing tall buildings
โข Several theoretical and empirical models:
โข Power law
โข Log law
โข Deaves and Harris model
โข Little validation of models, particularly in urban areas.
โข Assessed wind speed profile over Greater London.
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The surrounding surface is very heterogeneous (parks, urban, river)
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Gill instruments R3-50 ultrasonic anemometer
โขMeasures horizontal and vertical components of wind.
โขSampling frequency = 20 Hz
Instruments at BT Tower (190 m)
Observations analysed to estimate:
๐โ2 = ๐ขโฒ๐คโฒ2 + ๐ฃโฒ๐คโฒ2
๐ฟ =โ๐ขโ
3๐
๐ ๐ ๐คโฒ๐โฒ
Halo Photonics Streamline pulsed Doppler lidar
โขFully programmable scanner
โขDoppler Beam Swinging Method
โขGate length = 30 m
โข80 measurement gates
โขInstrument location = 20 m above ground level
โขMin. measurement height = 90 m above lidar (110 m above ground)
โขProfile every 2 minutes
โข21st May 2011 โ 6th Jan 2012
Doppler beam swinging method
โข Three-beam wind-profiling method
โข Derives wind speeds from one vertical and two tilted beams
โข 2 s of data taken consecutively in each direction (40,000 pulses)
โข Short scan time means flow will not change much over scan period.
โข Interval between scans = 120 s
โข See Pearson et al. (2009) for comparison with other methods in a rural setting.
ฮธ = 15ยฐ
Halo Photonics Streamline pulsed Doppler lidar
โขFully programmable scanner
โขDoppler Beam Swinging Method
โขGate length = 30 m
โข80 measurement gates
โขInstrument location = 20 m above ground level
โขMin. measurement height = 90 m above lidar (110 m above ground)
โขProfile every 2 minutes
โข21st May 2011 โ 6th Jan 2012
30th September 2011
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Mean wind speed profile
โข Derived from 5500 hours of observations
โข Compared with the 3 models
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Surface dependent
parameters (Cook, 1997)
z0=0.8 m
h=3250 m
ฮฑ=0.32
Stability
โข Data filtered by stability derived from BT tower observations.
13 UQ25 UQ50 UQ75
High wind speeds
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LOW:
U<UQ25
MEDIUM:
UQ25<U<UQ50
HIGH:
UQ50<U<UQ75
VERY HIGH:
U>UQ75
Terrain dependent parameters
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Roughness length, z0
โข Derived from log law using
u* observed at BT tower.
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โข Morphological values determined in Wood et al. (2010).
z0mean= 0.6 m
z0mean= 0.9 m
Power law exponent, ฮฑ
โข Derived from wind profile
observations.
โข Good agreement for
westerly winds.
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๐ผ =1
ln(๐ง1๐ง2)
0.5
๐ง0
ฮฑmean= 0.23
Boundary layer height, h
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โ =๐โ
6๐
hmean= 1050 m
Model comparison
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Conclusions
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Future Work
โข Presented wind speed profiles derived from lidar observations.
โข High wind speeds occur during neutral conditions. โข High wind speed profile shows reasonable fit with
model profiles (log law and Deaves and Harris).
โข Lack of Doppler lidar observations below 90 m restricts potential to assess wind loading models- potential for Sodar.
Extra slides
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Doppler beam swinging method
โข Three-beam wind-profiling method
โข Derives wind speeds from one vertical and two tilted beams
โข 2 s of data taken consecutively in each direction (40,000 pulses)
โข Short scan time means flow will not change much over scan period.
โข Interval between scans = 120 s
โข See Pearson et al. (2009) for comparison with other methods in a rural setting.
ฮธ = 15ยฐ
No
. of d
ata
po
ints
Lid
ar w
ind
sp
eed
(m
s-1)
Wind speed (60 minute average)
โข60 minute average used to include sufficient data from lidar.
โขSome of RMSE can be explained by standard error (average SE = 0.4 ms-1).
โขSome difference likely due to large separation between instruments.
Y=0.98x+0.56
RMSE=1.4
Weighted best fit