Observation of the atmospheric boundary layer over the Atlantic and its effects for wind propulsion

Abstract

Good knowledge of the atmospheric boundary layer in open seas is crucial for the development of wind-propelled and wind-assisted ships. Still, very little data is available away from the shores, and one needs to rely on modelling to estimate the wind conditions at sea. This paper presents experimental measurements of the vertical wind profile over the North-Atlantic ocean made on board a cargo ship during its normal operation. The study focuses on two aspects: the estimation of the “undisturbed” atmospheric boundary layer profile and the influence of the hull on the flow. One of the methods often used to describe the evolution of wind speed with height is by using a power law, with a typical value for the exponent of 1/7 ($\simeq 0.14$). The results of this study however show a significantly smaller value, around 0.035, to be representative of the predominant conditions, and highlight that the 1/7 exponent overestimates by 50% the amount of kinetic energy compared to the predominant conditions. The results also show the very large variability of the power law exponent. At the same time, the flow disturbance is clearly visible above the deck up to one or two times the hull height, with a strong dependency on the apparent wind angle, which can lead to wind speed variations up to 20% compared to the power law profile and direction changes of more than 10 degrees compared to the undisturbed wind.