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Wind stress measurements from the open ocean corrected for air flow distortion by the ship

Wind stress measurements from the open ocean corrected for air flow distortion by the ship
Wind stress measurements from the open ocean corrected for air flow distortion by the ship
A large dataset of wind stress estimates, covering a wide range of wind speed and stability conditions, was obtained during three cruises of the RRS Discovery in the Southern Ocean. These data were used by Yelland and Taylor to determine the relationship between 10-m height, neutral stability values for the drag coefficient, and the wind speed, and to devise a new formulation for the nondimensional dissipation function under diabatic conditions. These results have been reevaluated allowing for the airflow distortion caused by the ship. The acceleration and vertical displacement of the flow have been modeled in three dimensions using computational fluid dynamics (CFD). The CFD modeling was tested, first by comparison with wind tunnel measurements on models of two Canadian research ships and second, by analysis of data from four anemometers on the foremast of the RRS Charles Darwin. Originally, the four anemometers gave drag coefficient values that differed by up to 20% from one to another and were all unexpectedly high. The CFD results showed that the airflow had been decelerated by 4%–14% and displaced vertically by about 1 m. These effects caused the original drag coefficient results to be overestimated by up to 60%. After correcting for flow distortion effects, the results from the different anemometers became consistent, which gave confidence in the quantitative CFD-derived corrections.

The CFD modeling showed that the anemometer position on the RRS Discovery was much less affected by airflow distortion. For a given wind speed the CFD corrections reduced the drag coefficient by about 6%. The resulting mean drag coefficient to wind speed relationship confirmed that suggested by Smith from a more limited set of open ocean data.

The effects of flow distortion are sensitive to changes in the relative wind direction. It is shown that much of the scatter in drag coefficient estimates may be due to variations in airflow distortion rather than to the effect of changing sea states. The Discovery wind stress data is examined for evidence of a sea-state dependence: none is found. It is concluded that a wave-age-dependent wind stress formulation is not applicable to open ocean conditions.
CFD, wind speed error
0022-3670
1511-1526
Yelland, M.J.
3b2e2a38-334f-430f-b110-253a0a835a07
Moat, B.I.
497dbb18-a98f-466b-b459-aa2c872ad2dc
Taylor, P.K.
3ef27388-c14e-4f4b-85fc-97bb17d089d2
Hutchings, J.
f09af1c8-4b33-45de-a007-9190d832dc42
Cornell, V.C.
0773aff4-9ce8-4ac6-9492-dae1443463dd
Yelland, M.J.
3b2e2a38-334f-430f-b110-253a0a835a07
Moat, B.I.
497dbb18-a98f-466b-b459-aa2c872ad2dc
Taylor, P.K.
3ef27388-c14e-4f4b-85fc-97bb17d089d2
Hutchings, J.
f09af1c8-4b33-45de-a007-9190d832dc42
Cornell, V.C.
0773aff4-9ce8-4ac6-9492-dae1443463dd

Yelland, M.J., Moat, B.I., Taylor, P.K., Hutchings, J. and Cornell, V.C. (1998) Wind stress measurements from the open ocean corrected for air flow distortion by the ship. Journal of Physical Oceanography, 28 (7), 1511-1526. (doi:10.1175/1520-0485(1998)028<1511:WSMFTO>2.0.CO;2).

Record type: Article

Abstract

A large dataset of wind stress estimates, covering a wide range of wind speed and stability conditions, was obtained during three cruises of the RRS Discovery in the Southern Ocean. These data were used by Yelland and Taylor to determine the relationship between 10-m height, neutral stability values for the drag coefficient, and the wind speed, and to devise a new formulation for the nondimensional dissipation function under diabatic conditions. These results have been reevaluated allowing for the airflow distortion caused by the ship. The acceleration and vertical displacement of the flow have been modeled in three dimensions using computational fluid dynamics (CFD). The CFD modeling was tested, first by comparison with wind tunnel measurements on models of two Canadian research ships and second, by analysis of data from four anemometers on the foremast of the RRS Charles Darwin. Originally, the four anemometers gave drag coefficient values that differed by up to 20% from one to another and were all unexpectedly high. The CFD results showed that the airflow had been decelerated by 4%–14% and displaced vertically by about 1 m. These effects caused the original drag coefficient results to be overestimated by up to 60%. After correcting for flow distortion effects, the results from the different anemometers became consistent, which gave confidence in the quantitative CFD-derived corrections.

The CFD modeling showed that the anemometer position on the RRS Discovery was much less affected by airflow distortion. For a given wind speed the CFD corrections reduced the drag coefficient by about 6%. The resulting mean drag coefficient to wind speed relationship confirmed that suggested by Smith from a more limited set of open ocean data.

The effects of flow distortion are sensitive to changes in the relative wind direction. It is shown that much of the scatter in drag coefficient estimates may be due to variations in airflow distortion rather than to the effect of changing sea states. The Discovery wind stress data is examined for evidence of a sea-state dependence: none is found. It is concluded that a wave-age-dependent wind stress formulation is not applicable to open ocean conditions.

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More information

Published date: July 1998
Keywords: CFD, wind speed error
Organisations: National Oceanography Centre,Southampton

Identifiers

Local EPrints ID: 39165
URI: http://eprints.soton.ac.uk/id/eprint/39165
ISSN: 0022-3670
PURE UUID: 866ca60c-33b5-42ad-a574-71ac60760a9f

Catalogue record

Date deposited: 27 Jun 2006
Last modified: 15 Mar 2024 08:11

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Contributors

Author: M.J. Yelland
Author: B.I. Moat
Author: P.K. Taylor
Author: J. Hutchings
Author: V.C. Cornell

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