The airflow distortion at instruments sites on the RRS "James Cook"

Abstract

Wind speed and air-sea flux measurements made from instrumentation on ships are affected by the airflow distortion created by the presence of the ship. The airflow can be either
accelerated or decelerated depending on the shape of the ship and the location of the anemometer. The computational fluid dynamics (CFD) package VECTIS was used to examine
the extent of the flow distortion at potential anemometer locations on the foremast platform of the RRS "James Cook". This technique has been previously used to study the airflow over many research ships, but this is believed to be the first time it has been applied to a research ship in the design/build stage.

CFD modelling of the airflow over the ship showed that the foremast platform of the RRS "James Cook" is a good location to locate instrumentation and make high quality air-sea flux measurements. The wind speed is decelerated by about 2 % of the freestream wind speed for bow-on flows at well-exposed anemometer sites on the foremast platform. For relative wind directions up to ±30° of the bow the airflow is accelerated by up to 5 %.

The ship’s anemometers are located on the main mast and are relatively close to the ship’s large satellite communication radome. For winds within 15° of the bow the wind speeds at these anemometer sites are accelerated by up to about 7 %. For wind directions at ±30° the satellite radome has a significant effect on the flow and the wind speeds will be severely biased, with the magnitude of the bias varying rapidly with wind direction and the angle of pitch of the ship. It is strongly recommended that these anemometers be moved higher up and further away from the mast.

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