Research On The Mechanism And Quantitative Analysis Of Mode Localization Of Wind Turbine Blades

In recent years,with the continuous improvement of living standards,energy demand is also growing.As a relatively important new energy,wind energy has been recognized by many countries due to its renewable.However,due to the particularity of wind turbine blades’ natural environment,it is prone to various forms of damage.In order to determine that whether wind turbine blades are damaged,location of damage and the degree of damage of wind turbine blades,various structural health monitoring(SHM)technologies are required to monitor the status of wind turbine blades in real time.In the field of structural health monitoring,the modal parameter of service structure need to be identified,so acquiring the mode of the intact and damage state of structure is very important to evaluate the true state of wind turbine blades.The structure of wind turbine blades has the symmetry of 120 degree rotation,which is a typical circular symmetry structure.In the design stage of the wind turbine blades,it is generally assumed that the structure is in an ideal state.However,due to manufacturing or construction errors,material defects and structural damage,there is always a certain deviation between the actual wind turbine blades and the ideal one(called detuning).Due to the existence of detuning,the circulatory symmetry of the wind turbine blades is destroyed and thus becomes detuning structure.In general,the small detuning of circularly symmetric structures results in obvious modal localization.By studying the influence of different detuning type,degree of detuning on the model localization,finding the sensitive areas of the model localization.Taking these areas as the key site of monitoring,we can judge that whether the wind turbine blades is damage,the degree of damage and even the damage location by observing that whether the mode localization phenomenon appeared,and the degree of mode localization,so the overall state of wind turbine blades is assessed.First of all,using ANSYS to model the parametric modeling of the wind turbine blades structure,the modal analysis is carried out by ANSYS software.The vibration modes and frequencies of the structure are obtained,and finding that the harmonic wind turbine blades structure has the phenomenon of repeated frequencies.At the same time,the absolute displacement values of all the nodes in the cylindrical coordinate system of the first-six order mode shapes are summed.The displacement components of the first-six order mode shapes are analyzed,and the displacement amplitudes of the three blades in the directions of UY and UZ are extracted.Secondly,the existence of modal localization of wind turbine blades structure is analyzed,and the mechanism of modal localization of wind turbine blades structure is discussed by using perturbation theory.It is found that perturbation theory can only qualitatively analyze the mechanism of modal localization,but cannot achieve the result of quantitative analysis.In this paper,on the basis of perturbation theory,the mechanism of modal localization is described quantitatively in combination with detuning degree,modal density degree and modal confidence criterion.At the same time,MATLAB software is used to establish the calculation model of the spring particle structure with 26 degrees of freedom,and,the stiffness detuning is simulated according to its structural characteristics.The modal analysis of the structure is carried out to obtain its mode shapes and frequencies,and the superposition method of harmonic mode shapes is used to reproduce the detuning mode.Finally,by changing the material density,elastic modulus and blade element rotation angle,mass detuning,stiffness detuning and geometry detuning are simulated respectively.At the same time,by comparing four different local factors,the applicability of the improved local factors in evaluating the degree of modal localization of wind turbine blades is obtained.By changing the degree of the detuning,the influence of different detuning degree on the mode shapes of the wind turbine blade structure is studied,and the variation law of mode localization of the first-three order mode shapes with the degree of detuning is obtained.The variation law of the components of the harmonic mode shapes in the detuning ones under the different kinds of detuning and different detuning degree.At the same time,mass and stiffness detuning at six different positions is simulated from blade tip to blade root,and the sensitivity of degree of the first three mode shapes to different detuning positions is studied.