Numerical Simulation Study On Unsteady And Nonlinear Galloping Force Of Typical Bridge Tower Section

All the time,the study on galloping of bridge tower mainly follows the quasi-steady theory summarized from the translational vibration of iced conductors,also the stability checking of galloping is mostly limited to the initial instability discrimination under the action of linear aerodynamic force.Although this method has been widely applied,it ignores the unsteady effect of flows around body.Recently,dubious of the quasi-steady theory,some scholars found that the estimations of the galloping stability through quasi-steady theory are often on the side of disadvantages and the responses of the experiment are more characterized by the form of "soft" galloping through wind tunnel tests.After the summarization of current status of galloping research,taking the typical bridge section in engineering as the research object,this dissertation has analyzed and discussed the unsteady and nonlinear characteristics of galloping force through numerical simulation methods.The main research contents of this article are as follows:(1)The quasi-steady theory and unsteady analysis method based on the linear galloping force model are disserted.A nonlinear galloping force model based on unsteady analysis method is developed by Taylor polynomial,and the corresponding parameter identification method is given.Aiming at the established nonlinear galloping force model,the solving method of galloping response is proposed and deduced.The accuracy of the solving method is verified by an example.(2)The aerodynamic forces of several typical bridge tower sections under static and vibration conditions is calculated by FLUENT,and the corresponding parameters of quasi-steady and unsteady linear galloping force is identified.The results show that the quasi-steady galloping force coefficients are much smaller than that of the unsteady ones when the dimensionless wind-speed is not large enough,as the wind speed increases,the both become consistent gradually.Finally,the difference between the parameters of quasi-steady and unsteady galloping forces is analyzed through vorticity.(3)The frequency spectrums of aerodynamic forces at different wind speeds and amplitudes of the square section are analyzed.It is found that the high-frequency components of the aerodynamic force,corresponding to higher wind speed and greater amplitude,are more obvious.The time histories of aerodynamic force with two different amplitudes are used to fit the parameters of the nonlinear galloping force model,and it is found that they were different from each other.The difference between the galloping responses calculated by the two sets of parameters shows it is irrational to calculate galloping response using the nonlinear aerodynamic force obtained through forced-vibration method,in addition,the reason of the irration was analyzed.Finally,whether the galloping response calculated through the aerodynamic force obtained from forced-vibration is rational or not,the effects of every component of galloping force on response are discussed from the perspective of the system damping.