© University of Reading 2008 www.reading.ac.uk 18 December 2012

Wind speed profiles over Greater London, UK Daniel Drew, Janet Barlow and Siân Lane

2

Introduction

• Vertical wind speed profiles are required to address a

number of wind engineering problems:

• Dispersion of pollution

• Designing tall buildings

• Several theoretical and empirical models:

• Power law

• Log law

• Deaves and Harris model

Wind speed profile models

3

• Log law (Eurocode)

𝑈 𝑧 =𝑢∗

𝜅ln

𝑧

𝑧0 z0=0.8 m for an urban surface (Cook, 1985).

• Deaves and Harris Model (UK, Australia)

𝑈 𝑧 =𝑢∗𝑘

𝑙𝑛𝑧

𝑧0+ 5.75

𝑧

ℎ− 1.88

𝑧

ℎ

2

− 1.33𝑧

ℎ

3

+ 0.25𝑧

ℎ

4

h, the height of the boundary layer is assumed to equal 3250 m.

4

Introduction

• Vertical wind speed profiles are required to address a

number of wind engineering problems:

• Dispersion of pollution

• Designing tall buildings

• Several theoretical and empirical models:

• Power law

• Log law

• Deaves and Harris model

• Little validation of models, particularly in urban areas.

• 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

•Measures horizontal and vertical components of wind.

•Sampling frequency = 20 Hz

Instruments at BT Tower (190 m)

Observations analysed to estimate:

𝑈∗2 = 𝑢′𝑤′2 + 𝑣′𝑤′2

𝐿 =−𝑢∗

3𝑇

𝜅𝑔 𝑤′𝑇′

Halo Photonics Streamline pulsed Doppler lidar

•Fully programmable scanner

•Doppler Beam Swinging Method

•Gate length = 30 m

•80 measurement gates

•Instrument location = 20 m above ground level

•Min. measurement height = 90 m above lidar (110 m above ground)

•Profile every 2 minutes

•21st May 2011 – 6th Jan 2012

Doppler beam swinging method

• Three-beam wind-profiling method

• Derives wind speeds from one vertical and two tilted beams

• 2 s of data taken consecutively in each direction (40,000 pulses)

• Short scan time means flow will not change much over scan period.

• Interval between scans = 120 s

• See Pearson et al. (2009) for comparison with other methods in a rural setting.

θ = 15°

Halo Photonics Streamline pulsed Doppler lidar

•Fully programmable scanner

•Doppler Beam Swinging Method

•Gate length = 30 m

•80 measurement gates

•Instrument location = 20 m above ground level

•Min. measurement height = 90 m above lidar (110 m above ground)

•Profile every 2 minutes

•21st May 2011 – 6th Jan 2012

30th September 2011

11

Mean wind speed profile

• Derived from 5500 hours of observations

• Compared with the 3 models

12

Surface dependent

parameters (Cook, 1997)

z0=0.8 m

h=3250 m

α=0.32

Stability

• 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

• Derived from log law using

u* observed at BT tower.

16

• Morphological values determined in Wood et al. (2010).

z0mean= 0.6 m

z0mean= 0.9 m

Power law exponent, α

• Derived from wind profile

observations.

• Good agreement for

westerly winds.

17

𝛼 =1

ln(𝑧1𝑧2)

0.5

𝑧0

αmean= 0.23

Boundary layer height, h

18

ℎ =𝑈∗

6𝑓

hmean= 1050 m

Model comparison

19

Conclusions

20

Future Work

• Presented wind speed profiles derived from lidar observations.

• High wind speeds occur during neutral conditions. • High wind speed profile shows reasonable fit with

model profiles (log law and Deaves and Harris).

• 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

• Three-beam wind-profiling method

• Derives wind speeds from one vertical and two tilted beams

• 2 s of data taken consecutively in each direction (40,000 pulses)

• Short scan time means flow will not change much over scan period.

• Interval between scans = 120 s

• See Pearson et al. (2009) for comparison with other methods in a rural setting.

θ = 15°

No

. of d

ata

po

ints

Lid

ar w

ind

sp

eed

(m

s-1)

Wind speed (60 minute average)

•60 minute average used to include sufficient data from lidar.

•Some of RMSE can be explained by standard error (average SE = 0.4 ms-1).

•Some difference likely due to large separation between instruments.

Y=0.98x+0.56

RMSE=1.4

Weighted best fit