Quantifying the effects of airflow distortion on anemometer wind speed measurements from merchant ships
Anemometers on Voluntary Observing Ships (VOS) are usually located above the bridge in a region where the effects of air flow distortion, created by the presence of the ship, may be large. Until now it was not known whether measurements from such anemometers would be biased high or low, and the possible magnitude of any such bias was not known. Investigations into the airflow above merchant ships have been carried out experimentally using a low-speed wind tunnel and numerically using a commercial Computational Fluid Dynamics (CFD) code VECTIS. The investigations examined the airflow over simple block models of VOS shapes. The results of the investigations were compared to wind speed measurements made from the RRS Charles Darwin. Experimental and CFD techniques have been used to devise scaling rules that predict the effects of the flow distortion. Both techniques have shown that the pattern of the flow distortion above the bridge scales with the ‘step height’, H, of the model. In the case of a tanker, H is the ‘bridge to deck’ height, i.e. the height of the accommodation block above the deck, for bow-on flows. Close to the top of the bridge the flow is severely decelerated and may even reverse in direction. Using the upwind edge of the bridge as the origin of the scaled co-ordinate system, there is a definite line above the decelerated region along which the speed of the flow is equal to the undistorted wind speed. Above this ‘line of equality’ the wind speed increases to a maximum and then decreases with increased height to a free stream wind speed. Simple equations have been devised to predict the positions of the ‘line of equality’, the maximum wind speed and the minimum wind speed within the decelerated region. Comparisons of the results with wind speed data obtained from field measurements made using a number of anemometers located on the RRS Charles Darwin agreed well and have predicted a maximum wind speed increase of approximately 15 ±5 %. Comparisons with the field data have confirmed that CFD models can be used to predict the effects of airflow distortion above merchant ships. The investigation has demonstrated the ability of the wind tunnel and CFD approaches employed to provide a better understanding of the airflow over merchant ships. Both methods have contributed to improve the understanding of how the wind speed at anemometer sites on merchant ships is affected by the ships hull and superstructure.