Studies On Dynamic Characteristics Of Trees Measured In Field And Wind Tunnel Tests Of An Aeroelastic Model Tree

Trees are very important for humans.However,disasters about trees caused by strong wind with global climate changing appear frequently and intensively.Compared with bluff structures,the porous canopy structure easily causes large deflection under wind load,showing obvious streamlining effects.The resistance of trees to wind is closely related to the streamlining effect of the canopy,which shows the influence on the aerodynamics,dynamic characteristics and wind-induced responses.Field measurements about trees are rarely conducted because of the limitations of environment,acquisition system,testing period and data acquisition capacity.The mismatch between model and prototype due to lack of prototype details cannot satisfy similarity criteria about wind tunnel test,which makes it challenging to transfer the measured results into full-scale dimensions.Meanwhile,most studies about tree aerodynamics focus on average characteristics,and lack fluctuation characteristics.Therefore,it is of great academic value for studying resistance of trees to wind to make a scaled,aeroelastic model tree based on the prototype details that meets similarity criteria,which can be used to study aerodynamics,dynamic characteristics and windinduced responses reflecting prototype.The camphor tree as a typical decurrent tree(which is characterized by a short trunk and a crown supported by radial branches)was selected as the research object for studies on dynamic characteristics of trees measured in field and wind tunnel tests of an aeroelastic model tree.The modal parameters of five camphor trees were measured in field.The aeroelastic model tree was designed and made based on the prototype details(including dimensions,crown morphology and modal parameters)and similarity criteria.The aerodynamics,dynamics and wind-induced responses of the model,and terrain effects on the model were studied in the wind tunnel tests.The following research results have been achieved in this paper.(1)The modal parameters of five camphor trees were identified in the field measurement,which were validated by Morlet wavelet transform.The first three modal parameters of the trunk(including frequencies,damping and mode shapes)were obtained.One of the camphor trees was selected as the prototype of the aeroelastic model.The geometry and crown morphology were measured.An apporoach to estimate leaf mass was developed by comparing fundamental trunk frequency between field measurement and finite element model.(2)The similarity criteria and parameters about wind tunnel test of the aeroelastic model tree were estimated based on that the blockage ratio should be below about 5%.The models of the trunk,branches and leaves were designed based on the requiremetns of similarity parameters and the prototype details(including geometry,crown morphology and modal parameters),respectively.The eight distinct crown configurations with different quantities of leaves were constructed.The model was validated by comparing crown morphology and distributions of mass and stiffness between the model and the prototype.(3)The base overturning moments,crown frontal area,wind speed and crown center displacement of the model were measured synchronously in the wind tunnel tests,in which aerodynamics were analyzed in time and frequency domain.The mean and fluctuating aerodynamics of the model were obtained.The different incoming wind effects on base overturning moments,base overturning moment coefficients and crown center displacement were also analyzed.(4)The wind speed,crown center displacement,trunk displacement,accelaration of trunk and branches of the model were measured synchronously in the wind tunnel tests,in which the modal parameters of the model were identified by NEx T-ERA modal identification method.The crown center displacement effects on the damping at crown frequency,and the trunk displacement effects on the damping at trunk frequency were analyzed.The mean and fluctuating characteristics of wind-induced displacement of the model were obtained.The probability distribution characteristics of acceleration and the characteristics of extreme acceleration fo the model under different incoming winds were obtained.