The effect of ship shape and anemometer location on wind speed measurements obtained from ships B I Moat 1, M J Yelland 1, A F Molland 2 and R W Pascal 1 1) 1)Southampton Oceanography Centre, UK 2) 2)School of Engineering Sciences, Ship Science, University of Southampton, UK 4th International Conference on Marine CFD, University of Southampton, March NOTE: as of 1st May 2005 Southampton Oceanography Centre becomes National Oceanography Centre, Southampton becomes 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 speed z/H acceleratedflow decelerated flow H = cube height Re=10 5 tunnel roof cube top

Good comparison with RNG normalised wind speed z/H decelerated flow acceleratedflow H = cube height Re=4x10 4 Validation: boundary layer flow over a surface mounted cube

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.

Validation: comparison with in situ wind speed measurements Agreement to within 4% normalised wind speed z/H decelerated flow accelerated flow H = bridge to sea level height Re=1.3x10 7

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 % typicalanemometerlocation length overall = 90 m

Results: RRS Charles Darwin Wind speed measurements are biased by about 10% typicalanemometerlocation length overall = 70 m

Results: research ships Streamlined superstructure needed Locate anemometers as high as possible above the platform, not in front Relative wind direction Wind speed bias (%) port starboard RRS Charles Darwin RRS Discovery bow

Research ship design: RRS James Cook CFD will be used to determine the best sensor locations Anemometer location First 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

Results: A generic ship model Ship dimensions from RINA publication Significant ships ( ) Tankers/bulk carriers/general cargo ships can be represented by a simple shape. bow stern

Results: A generic ship model Perform CFD simulations over the simple geometry Bridge anemometers Flows directly over the bow bow stern bridgeanemometers

Wind tunnel: flow visulisation mean flow direction Standing vortex in front of the deck house

Wind tunnel: flow visulisation Decelerated region increases with distance from the leading edge mean flow direction Standing vortex in front of the deck house Vortices produced above the bridge top

Wind tunnel: flow visulisation Complex flow pattern mean flow direction Standing vortex in front of the deck house Less disturbance with increase in height Vortices produced above 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 speed z/H deceleration and recirculation bow stern Normalised wind speed profile Wind speed accelerated by about 10 % Decelerated by up to 100 % H

Region of high velocity gradients Normalised wind speed deceleration and recirculation bow stern Normalised wind speed profile z/H H

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 height, z (m) Distance from leading edge, x (m) Anemometer position Bow Bridge Depth of the recirculation region

Container ships More complex shape than a typical tanker Irregular container loading ??? Anemometerlocations

Container ships: General flow patternaccelerated accelerated accelerated accelerated decelerated decelerated decelerated bow bridge (Moat et al. 2005) container ship

Container ships: General flow pattern Bow influences the bridge flow Complex flow and the subject of future work accelerated accelerated accelerated accelerated decelerated decelerated decelerated bow bridge container ship typical tanker (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 sec

FUTURE WORK Good representation of atmospheric turbulence in the wake region of a ship LES code GERRIS time = 3 sec Iso-surface of wind speed at 90% of the inflow velocity

Acknowledgements Partial funding from Meteorological Service of Canada and the Woods Hole Oceanographic Institution, USA. Contact