Wind Gust Factors with Various Turbulence Intensities and Length Scales

Abstract

Wind velocity data are often analysed to determine gust (peak) and mean (average) statistics for design to sustain in extreme conditions and for control to monitor these extreme levels through continuous measurement of the related means over a period of time. The relationship between the gust and the mean is simply expressed as the ratio of the gust to mean wind speeds, termed as the Gust Factor, G. The shorter the averaging period for the mean, the closer of the measured mean to the gust wind speed this becomes. The gust maximum value itself would also be higher when averaged over a shorter gust duration, t, or measured with higher turbulence intensity Iu or higher longitudinal length scale Lux. Therefore, it is important in the description of a Gust Factor to qualify the averaging duration both for gust and for mean wind speeds as well as the turbulence characteristics in the wind flow. This paper presents an analysis of wind tunnel data to relate the gust wind speed measurements and hence the Gust Factors for various averaging durations in neutral wind flow conditions of various turbulence intensities and longitudinal length scales. Some full scale wind data are also compared. The proposed relationship with respect to the hourly mean wind speed is as follows: G = 1 + 0.075 • Iu • Lux 0.25 • [ ln(t/3600) ]2 • [ 0.3 + 0.02 ln(t/3600) ]