the effect of ship shape and anemometer location on wind speed measurements obtained from ships b i...
Post on 22-Dec-2015
218 views
TRANSCRIPT
The effect of ship shape and anemometer location
on wind speed measurements obtained
from shipsB I Moat1, M J Yelland1, A F Molland2 and R W Pascal1
1) Southampton Oceanography Centre, UK
2) 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 CentreNOTE: as of 1st May 2005 Southampton Oceanography Centre becomes National Oceanography Centre, Southamptonbecomes National Oceanography Centre, Southampton
• Wind speed measurements can be severely biased by the presence of the ship
• CFD can be used to predict/correct wind speed measurements
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
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
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
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
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)
Validation: channel flow over a surface mounted cube
• Good comparison with RNG
normalised wind speednormalised wind speed
z/Hz/H
accelerated accelerated
flowflow
decelerated flowdecelerated flow H = cube H = cube heightheight
Re=10Re=1055
tunnel rooftunnel roof
cube topcube top
• Good comparison with RNGnormalised wind speednormalised wind speed
z/Hz/H
decelerated flowdecelerated flow
accelerated accelerated
flowflow
H = cube H = cube heightheight
Re=4x10Re=4x1044
Validation: boundary layer flow over a surface mounted cube
Validation: In situ wind speed measurements from RRS Charles
DarwinMeasurements 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.
Validation: comparison with in situ wind speed measurements
• Agreement to within 4%
normalised wind speednormalised wind speed
z/Hz/H
decelerated flowdecelerated flow
acce
lera
ted
flo
wac
cele
rate
d f
low H = bridge to H = bridge to
sea level heightsea level height
Re=1.3x10Re=1.3x1077
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
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
Results: RRS Discovery
• Wind speed measurements are biased by about 5 %
typical typical anemometeranemometer
locationlocation
length overall = 90 mlength overall = 90 m
Results: RRS Charles Darwin
• Wind speed measurements are biased by about 10%
typical typical anemometeranemometer
locationlocation
length overall = 70 mlength overall = 70 m
Results: research ships
Streamlined superstructure neededLocate anemometers as high as possible above the
platform, not in front
Relative wind directionRelative wind direction
Win
d s
pe
ed
bia
s (%
)W
ind
sp
ee
d b
ias
(%)
port starboardport starboard RRS Charles DarwinRRS Charles Darwin
RRS Discovery RRS Discovery bow bow
Research ship design: RRS James Cook
• CFD will be used to determine the best sensor locations
Anemometer locationAnemometer location
First steel cut 26th January 2005First steel cut 26th January 2005
RESULTS: tankers, bulk carriers and general cargo
ships
Large number of ships. Cannot be studied individually.
The ships are large complex shapes
Typical anemometer location
www.shipphotos.co.ukwww.shipphotos.co.uk
Results: A generic ship model
• Ship dimensions from RINA publication Significant ships (1990-93)
• Tankers/bulk carriers/general cargo ships can be represented by a simple shape.
bow sternbow stern
Results: A generic ship model
• Perform CFD simulations over the simple geometry
• Bridge anemometers • Flows directly over the bow
bow sternbow stern
bridgebridgeanemometersanemometers
Wind tunnel: flow visulisation
mean flow directionmean flow direction
Standing vortex Standing vortex in front of the in front of the
deck housedeck house
Wind tunnel: flow visulisation
• Decelerated region increases with distance from the leading edge
mean flow directionmean flow direction
Standing vortex Standing vortex in front of the in front of the
deck housedeck house
Vortices produced Vortices produced above the bridge topabove the bridge top
Wind tunnel: flow visulisation
• Complex flow pattern
mean flow directionmean flow direction
Standing vortex Standing vortex in front of the in front of the
deck housedeck house
Less disturbance Less disturbance with increase inwith increase in
heightheight
Vortices produced Vortices produced above the bridge topabove the bridge top
Tanker
Flow direction
CFD: Airflow above the bridge
3D simulation of the airflow over the tanker.(RNG turbulence closure)
accelerated flow
decelerated flow with recirculation.
Qualitatively, the numerical model reproduces the general flow pattern quite well.
Tanker
Flow direction
CFD: Airflow above the bridge
3D simulation of the airflow over the tanker.(RNG turbulence closure)
accelerated flow.
decelerated flow with recirculation.
Qualitatively, the numerical model reproduces the general flow pattern quite well.
Normalised wind speedNormalised wind speed
z/Hz/Hdeceleration and deceleration and
recirculationrecirculation
bow sternbow stern
Normalised wind speed profile
• Wind speed accelerated by about 10 %
• Decelerated by up to 100 %
HH
Region of high velocity gradients
Normalised wind speedNormalised wind speed
deceleration and deceleration and recirculationrecirculation
bow sternbow stern
Normalised wind speed profile
z/Hz/HHH
RESULTS: typical merchant ships
• Anemometers will be less distorted in the bow• Locate anemometers as high above the deck as
possible and above the leading edge
hei
gh
t, z
(m
)h
eig
ht,
z (
m)
Distance from leading edge, x (m)Distance from leading edge, x (m)
Anemometer positionAnemometer position
BowBow
BridgeBridgeDepth of the Depth of the
recirculation regionrecirculation region
Container ships
• More complex shape than a typical tanker• Irregular container loading ???
Anemometer Anemometer
locationslocations
www.shipphotos.co.ukwww.shipphotos.co.uk
Container ships: General flow pattern
acceleratedaccelerated
acceleratedaccelerated
acceleratedaccelerated
acceleratedaccelerated
decelerateddecelerated
dec
eler
ated
dec
eler
ated
decelerateddecelerated
1.01.0
1.01.0
1.01.0
1.01.01.01.0
bow bridgebow bridge
(Moat et al. 2005)(Moat et al. 2005)
container shipcontainer ship
Container ships: General flow pattern
• Bow influences the bridge flow• Complex flow and the subject of future work
acceleratedaccelerated
acceleratedaccelerated
acceleratedaccelerated
acceleratedaccelerated
decelerateddecelerated
dec
eler
ated
dec
eler
ated
decelerateddecelerated
1.01.0
1.01.0
1.01.0
1.01.01.01.0
bow bridgebow bridge
container shipcontainer ship
typical tanker typical tanker
(Moat et al. 2005)(Moat et al. 2005)
APPLICATION OF RESULTS: MERCHANT SHIPS
• To predict the wind speed bias– Ship type– Ship length– Anemometer position
• Parameters are now available (WMO-47)
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
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
FUTURE WORK
• How does the turbulence structure change with ship shape ?
time = 3 sectime = 3 sec
FUTURE WORK
• Good representation of atmospheric turbulence in the wake region of a ship
LES code GERRISLES code GERRIS
time = 3 sectime = 3 sec
Iso-surface of
wind speed
at 90% of the
inflow velocity
AcknowledgementsPartial funding from Meteorological Service
of Canada and the Woods Hole Oceanographic Institution, USA.
www.soc.soton.ac.uk/JRD/MET/cfd_shipflow.php