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.

Contactben.moat@soc.soton.ac.uk

www.soc.soton.ac.uk/JRD/MET/cfd_shipflow.php