Thunderstorm-induced mean wind velocities and accelerations through the continuous wavelet transform

By analysing a set of thunderstorm-induced events acquired on the Italian north-west coast, the separation of the relevant wind velocity signals into a time-varying mean and a residual fluctuation is considered. It is assumed that, within a structural design process, the former is used for a quasi-steady evaluation of the aerodynamic loads and the latter to estimate the dynamic response. A procedure based on the continuous wavelet transform is proposed to obtain the time-varying mean velocity and its outcomes are compared with those of different moving average techniques. The wavelet filtering appears to provide a time-varying mean that better describes the main time variations of the original velocity signals; a decrease of the correlation between mean and residual velocities is also found. The derivative of the time-varying mean wind speed leads to a time-varying mean acceleration, whose characteristics and possible role in a design process are discussed. An estimate of the acceleration-induced contribution to the aerodynamic loads on a slender body is also carried out through the Morison equation. The force term directly related to acceleration is shown to generally have a limited influence on the total design loads, except for bodies that are considerably elongated in the flow direction.