Study On Extreme Wind-induced Response Of Trees

Every year in our country,a large number of trees suffer heavy losses due to strong winds.Wind damage of trees is not the result of a single factor,which is often complex and uncertain,which brings many difficulties for scholars to study the wind resistance of trees.Based on the wind-induced response of trees measured in the field,this paper aims to summarize the law of wind-induced response and provide a new idea for wind damage analysis of trees.Firstly,the wind field in front of willow tree and the acceleration response of its trunk were measured in situ,and the distribution law of the wind-induced acceleration response at each height of the trunk was explored under different wind speed conditions.The results show that there are significant differences between the probability distribution characteristics of windinduced acceleration response and the wind-induced acceleration response of the tree.The wind-induced acceleration response of the tree is approximately Gaussian,and the probability distribution of wind-induced acceleration response of the tree is obviously non-Gaussian with sharp peaks.By comparing the dimensionless power spectrum characteristics of downwind pulsating wind speed and downwind acceleration,it can be seen that both of them have a main peak.The frequency corresponding to the main peak of the former is less than 0.1Hz,while the frequency corresponding to the main peak of the trunk's downwind acceleration is in the range of 0.55-0.65 Hz,which is much higher than that of the downwind pulsating wind speed.Secondly,under different guarantee rates,Gumbel algorithm,peak factor method,improved peak factor method and generalized Pareto method were used to calculate the extreme acceleration response of trees,and the accuracy of the four algorithms was compared.The generalized Pareto extremum algorithm with the best fitting degree was selected to calculate the extremum acceleration response of willow tree.When the time interval was 180 s,the correlation between the front mean wind speed and the extreme acceleration response in the downwind direction of willow trunk was the highest.The response of extreme acceleration at different height of the trunk under different mean wind speed in front of the tree was analyzed and predicted by fitting.It was found that the downwind extreme acceleration of the trunk had a quadratic nonlinear relationship with the mean wind speed in front of the tree.Finally,the tree model of willow was simplified into 8 particles to calculate the extreme wind-induced response of willow.The extreme acceleration at each particle was calculated by using the fitting extreme acceleration formula,and the dynamic wind-induced response of willow was obtained by combining the extreme acceleration with the mass of each particle.The static wind response of willow was calculated according to the resistance coefficient,upwind area,average wind speed and other parameters obtained by calculation and analysis,and then the total extreme wind-induced response of willow was obtained by superposition of dynamic response and static response.Through literature review,the value range of bending strength was determined,and the situation of tree trunk wind damage was analyzed.It was obtained that the critical damage wind speed range of willow was between 28.3m/s and 40m/s,and the damage location was close to the bottom of the tree trunk.The study of extreme wind-induced response of common willow is of great significance for the study of wind resistance of trees.