Gust Magnitudes Research Articles

Development and Application of a Physical Approach to Estimating Wind Gusts

A new wind gust estimate method (denoted WGE method) is proposed. Contrary to most techniques used in operational weather forecasting, the determination of gusts in this approach is fully based on physical considerations. The main motivation for developing such an approach is to improve the knowledge of the physical processes that control the determination of gusts. The proposed approach assumes that surface gusts result from the deflection of air parcels flowing higher in the boundary layer, which are brought down by turbulent eddies. The WGE method takes into account the mean wind and the turbulent structure of the atmosphere. Moreover, this method includes the computation of a bounding interval around the gust estimate, which provides a range of likely gust magnitudes. The WGE method has been tested on two explosive cyclogenesis events that were satisfyingly simulated with the Modele Atmospherique Regional mesoscale model nested in the ECMWF analysis. Daily maximum gusts are predicted with good accuracy, while the hourly temporal evolution of estimated gusts depends strongly on the accuracy of the meteorological fields generated by the model. Typical error range for the gust estimates range is about 5 m s(-1). The bounding interval is useful for determining the uncertainty around estimated gusts. Statistical evaluation of the WGE method shows that the main features of the climatology of gusts during the period from January to March 1990 are reproduced, even though estimated gusts have a negative bias (from 3% to 10%) compared to observations. An interesting aspect of the WGE method is the reliability of the bounding interval, with 73% of the predicted daily gusts lying in this interval. Compared to other approaches, the WGE method is as good as other methods used in weather forecasting although extensive testing remains to be done.

Wind conditions for wind turbine design proposals for revision of the IEC 1400-1 standard

Abstract The International Electrotechnical Commission (IEC) TC88 formed a subcommittee to re-evaluate the external wind condition models defined in the standard for “Wind Turbine Generator Systems, Part I: Safety Requirements” 1400-1. These models define design turbulence conditions, extreme gust transients, extreme wind direction transients and extreme shear transients as well as other normal and extreme conditions. The TC88 subcommittee solicited extreme wind condition data from countries throughout the world reporting such parameters as maximum gust magnitudes, minimum gust rise times, and extreme wind shears. These data were used to modify the IEC models to be more representative of typical field conditions. Some data used for comparison together with the new models are presented.

The influence of buoyancy on third-order turbulent velocity statistics within a deciduous forest

The influence of atmospheric stability on the behaviour of the third moment of flow velocities observed inside a deciduous forest canopy is examined. Results suggest that buoyancy plays a dominant role in dictating the magnitude of gusts observed inside tall vegetation. Furthermore, an examination of the turbulence recorded throughout leaf fall inside the same forest indicates that larger velocity skewnesses are observed inside a canopy in full leaf than inside a sparse canopy. The behaviour of the measured terms in the non-dimensionalized rate equation of the third moment of canopy flow velocities is also examined. Turbulent diffusion and turbulence gradient interaction terms are largest in stable conditions in the upper canopy layer while these are most important in unstable conditions in the lower canopy layer. In all stability regimes, the turbulent diffusion term is the main source of skewness. The turbulence gradient interaction term, the residual and buoyant production terms all contribute to destroy skewness in stable conditions.

Roll response of a ship under the action of a sudden excitation

This paper examines the importance of roll damping on the gust response of a ship. As a mathematical model an ad hoc roll equation is used and the action of the wind gust is represented by means of a step function. A solution of the equation of motion is developed by using the asymptotic method of Bogoliubov and Mitropolsky . Computational results on the influence of roll damping, initial conditions and the gust magnitude are given for a stern trawler. Results clearly indicate the importance of roll damping. In the light of this investigation it is suggested that in determining the gust response of a ship in a weather criterion the effect of roll damping should be taken into account.