The effect of ship shape and anemometer location on wind speed measurements obtained from ships

1
The effect of ship shape and anemometer location
on wind speed measurements obtained from ships

• B I Moat1, M J Yelland1, A F Molland2 and R W
  Pascal1
• Southampton Oceanography Centre, UK
• School of Engineering Sciences, Ship Science,
• University of Southampton, UK

4th International Conference on Marine CFD,
University of Southampton, 30-31 March 2005.
NOTE as of 1st May 2005 Southampton Oceanography
Centre becomes National Oceanography Centre,
Southampton
2

• Wind speed measurements can be severely biased by
  the presence of the ship
• CFD can be used to predict/correct wind speed
  measurements

3
OUTLINE

• Background
• Description of the CFD code
• CFD code validation
• Results
• research ships (individual ships)
• tankers/bulk carriers/general cargo ships
  (generic modelling approach)
• Container ships
• Conclusions

4
Background

• Research ships limited coverage, but measurements
  of high quality.
• Merchant ships routinely report meteorological
  parameters at sea surface (wind speed and
  direction)
• Data used in satellite validation, ocean
  atmosphere modelling forcing and climate research

5
Background impact of flow distortion on climate
studies

• 10 error in mean wind speed
• 27 bias in the momentum exchange
• 10 bias in the heat exchange

6
CFD code description

• Commercial RANS solver VECTIS
• Mesh generation
• Non-uniform Cartesian mesh
• (generate 500,000 cells/hour)
• 3-dimensional and isothermal
• MEAN FLOW ONLY (STEADY STATE)
• RNG turbulence model
• Simulations based on up to 600,000 cells
• All results normalised by the wind speed profile
  at the measurement site

7
VALIDATION

• Comparison to 2 previous wind tunnel studies
• Martinuzzi and Tropea (1993)
• Minson et al. (1995)
• Comparison to in situ wind speed measurements
  made from a ship
• Moat et al. (2005)

8
Validation channel flow over a surface mounted
cube
tunnel roof
accelerated flow
H cube height Re105
decelerated flow
z/H
cube top
normalised wind speed

• Good comparison with RNG

9
Validation boundary layer flow over a surface
mounted cube
decelerated flow
H cube height Re4x104
z/H
accelerated flow
normalised wind speed

• Good comparison with RNG

10
Validation In situ wind speed measurements from
RRS Charles Darwin
Measurements were made using 6
anemometers. Instruments were located on a 6 m
mast. Only beam-on wind speed data used.
Wind speed profile measured above a block like
ship.
11
Validation comparison with in situ wind speed
measurements
decelerated flow
H bridge to sea level height Re1.3x107
z/H
accelerated flow
normalised wind speed

• Agreement to within 4

12
Accuracy of CFD simulations

• Comparisons of simulations show variations of
• Mesh density (1 )
• Turbulence model (2 )
• Scaling the geometry (3 )
• Wind speed profile (4 )
• VECTIS agrees to 4  or better with in situ wind
  speed data

13
RESULTS research ships

• Project running since 1994
• Over 11 ships have been studied
• American, British, Canadian, French and German
• Present results from well exposed anemometers in
  the bow of 2 UK ships
• RRS Discovery
• RRS Charles Darwin

14
Results RRS Discovery
typical anemometer location
length overall 90 m

• Wind speed measurements are biased by about 5

15
Results RRS Charles Darwin
typical anemometer location
length overall 70 m

• Wind speed measurements are biased by about 10

16
Results research ships
RRS Discovery
bow
Wind speed bias ()
port starboard
RRS Charles Darwin
Relative wind direction

• Streamlined superstructure needed
• Locate anemometers as high as possible above the
  platform, not in front

17
Research ship design RRS James Cook
Anemometer location
First steel cut 26th January 2005

• CFD will be used to determine the best sensor
  locations

18
RESULTS tankers, bulk carriers and general cargo
ships
Typical anemometer location
www.shipphotos.co.uk
Large number of ships. Cannot be studied
individually. The ships are large complex shapes
19
Results A generic ship model
bow stern

• Ship dimensions from RINA publication Significant
  ships (1990-93)
• Tankers/bulk carriers/general cargo ships can be
  represented by a simple shape.

20
Results A generic ship model
bridge anemometers
bow stern

• Perform CFD simulations over the simple geometry
• Bridge anemometers
• Flows directly over the bow

21
Wind tunnel flow visulisation
mean flow direction
Standing vortex in front of the deck house
22
Wind tunnel flow visulisation
mean flow direction
Vortices produced above the bridge top
Standing vortex in front of the deck house

• Decelerated region increases with distance from
  the leading edge

23
Wind tunnel flow visulisation
mean flow direction
Less disturbance with increase in height
Vortices produced above the bridge top
Standing vortex in front of the deck house

• Complex flow pattern

24
CFD Airflow above the bridge
accelerated flow
3D simulation of the airflow over the
tanker. (RNG turbulence closure)
decelerated flow with recirculation.
Tanker
Flow direction
Qualitatively, the numerical model reproduces
the general flow pattern quite well.
25
CFD Airflow above the bridge
accelerated flow.
3D simulation of the airflow over the
tanker. (RNG turbulence closure)
decelerated flow with recirculation.
Tanker
Flow direction
Qualitatively, the numerical model reproduces
the general flow pattern quite well.
26
Normalised wind speed profile
deceleration and recirculation
z/H
H
bow stern
Normalised wind speed

• Wind speed accelerated by about 10 
• Decelerated by up to 100 

27
Normalised wind speed profile
deceleration and recirculation
z/H
H
bow stern
Normalised wind speed
Region of high velocity gradients
28
RESULTS typical merchant ships
Anemometer position
Bridge
height, z (m)
Depth of the recirculation region
Bow
Distance from leading edge, x (m)

• Anemometers will be less distorted in the bow
• Locate anemometers as high above the deck as
  possible and above the leading edge

29
Container ships
Anemometer locations
www.shipphotos.co.uk

• More complex shape than a typical tanker
• Irregular container loading ???

30
Container ships General flow pattern
accelerated
1.0
1.0
accelerated
container ship
decelerated
1.0
bow
bridge
accelerated
1.0
1.0
accelerated
decelerated
decelerated
(Moat et al. 2005)
31
Container ships General flow pattern
accelerated
1.0
1.0
accelerated
container ship
decelerated
1.0
bow
bridge
accelerated
1.0
1.0
typical tanker
accelerated
decelerated
decelerated
(Moat et al. 2005)

• Bow influences the bridge flow
• Complex flow and the subject of future work

32
APPLICATION OF RESULTS MERCHANT SHIPS

• To predict the wind speed bias
• Ship type
• Ship length
• Anemometer position
• Parameters are now available (WMO-47)

33
CONCLUSIONS Research ships

• CFD is a valid research tool to examine the mean
  airflow over ships
• anemometers biased by about 10 or less (highly
  dependent on position)
• Streamlined superstructure needed for accurate
  wind speed measurements

34
CONCLUSIONS Tankers/bulk carriers/general cargo

• anemometers biased high by 10 and low by 100
• Position anemometers as high as possible above
  the deck
• If possible locate anemometers in the bows of
  the ship

35
FUTURE WORK
time 3 sec

• How does the turbulence structure change with
  ship shape ?

36
FUTURE WORK
LES code GERRIS
Iso-surface of wind speed at 90 of the inflow
velocity
time 3 sec

• Good representation of atmospheric turbulence in
  the wake region of a ship

37
Acknowledgements Partial funding from
Meteorological Service of Canada and the Woods
Hole Oceanographic Institution,
USA. Contact ben.moat_at_soc.soton.ac.uk www.soc.sot
on.ac.uk/JRD/MET/cfd_shipflow.php