Airflow distortion at instrument sites on the ODEN during the ACSE project

Abstract

Wind speed measurements obtained from anemometers mounted on ships are prone to systematic errors caused by the distortion of the air flow around the ship's hull and superstructure. This report describes the results of simulations of the air flow around the ODEN made using the computational fluid dynamics (CFD) software VECTIS. The airflow distortion at anemometer sites used during the ACSE project has been quantified at a wind speed of 7 ms-1 for a wide range of wind directions: every 10 degrees from bow on to 120 degrees off the bow, and an additional run was undertaken at 150 degrees off the bow. The anemometers used in this study were located in the bows of the ship. The vertical displacements of the airflow at the anemometer sites and at a location of an aerosol intake are included. Wind speed profiles above a motion-stabilised doppler lidar were also obtained.

For bow-on flows the anemometers in the bows of the ship experienced relatively small flow distortion. At these sites the flow was decelerated by about 3% of the free stream wind speed. Over the full range of relative wind directions the flow to the METEK sonic is generally accelerated with the largest wind speed biases at flows directly over the beam. The vertical displacement of the airflow increases from around 3 m for flows directly over the bow, to around 6 m for flows over the ship's beam as the lockage of the airflow by the ship becomes greater.

The vertical displacement at the aerosol intake location varied from 6m for flows directly over the bow, to around 16 m for flows over the ship's beam. The ship imposes a significant obstacle to the flow and forces a strong vertical velocity in the lowest few tens of meters above the lidar.

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