Study On Galloping Stability Of Single Pylon With Variable Section Of Cable-stayed Bridge

This paper was supported by the national natural science fund project“Study on aerodynamic admittance function of main girder section of long-span suspension bridge in mountain city”（No.51778093）.Taking Gerald Desmond Cable-stayed Bridge as an engineering background,the galloping stability of variable cross-section pylon under independent state was studied by CFD numerical simulation,and the results were compared with the wind tunnel test values.It can provide some references for actual engineering and scientific research.In this paper,the Ansys software was used to calculate the dynamic characteristics of cable-stayed bridge in completed state,maximum single cantilever state,maximum double cantilever state and independent state.The frequencies and modal characteristics of cable-stayed bridge in different states were analyzed.The results show that the modal characteristics of cable-stayed bridges are quite different.The completed state and the construction state should be considered separately when calculating wind-induced vibration of cable-stayed bridges.The first-order frequency of the pylon is the lowest under the condition of maximum double cantilever,and the pylon under this condition is very prone to having wind-induced vibration.Based on the quasi-steady theory,the CFD numerical simulation was used to calculate the critical galloping wind speed of the pylon according to the Den Hartog criterion and an improved method which Considering the influence of modal,section and wind profile.The results show that the critical galloping wind speed calculated by Den Hartog criterion are very different from the wind tunnel test results.Den Hartog criterion can not be applied to the tall structures.After considering the influence of modal,section and wind profile,the results are slightly smaller than wind tunnel test values,and the error is within the allowable range of practical engineering.Therefore,the new method can greatly improve the accuracy of calculating the critical galloping wind speed.The galloping performance of square cylinder is not necessarily improved by corner cut.When the corner cut ratio is less than 15%,the galloping instability of the square section is increased.Only when the corner cut ratio is more than 27%,the galloping performance of the square section can be significantly improved.Considering the unsteady aerodynamic effect,the critical galloping wind speed of pylon was calculated through the flutter derivative.Using dynamic mesh technique and forced vibration method,the flutter derivative₁^*of cross section under different dimensionless wind speeds was identified,and the influence of amplitude was considered.The aerodynamic characteristics of cross section were analyzed in detail.The results show that the linear flutter derivative theory can only describe the cross-wind aerodynamic forces around the pylon when the dimensionless wind speed is small.For the case of large dimensionless wind speed,the aerodynamic force around pylon contains a large number of non-linear components.When the vortex shedding frequency is"captured"by the non-linear component,the amplitude of the non-linear component will suddenly increase,sometimes it far bigger than that of the linear component.With the increase of dimensionless wind speed,the amplitude of linear aerodynamic component decreases gradually,and the amplitude of non-linear components in aerodynamic force tends to increase.With the increase of amplitude of forced vibration,the amplitude of the non-linear aerodynamic component tends to increase.Therefore,the non-liner aerodynamic force must be consider for the bluff body,and the result of critical wind speed of pylon calculated by the linear flutter derivative theory is unreliable.In this paper,the critical galloping wind speed of pylon was analyzed from the point of view of energy,and the minimum damping ratio of pylon was calculated,which was compared with the test results.The results show that the critical galloping wind speed calculated by energy method is slightly larger than the test value of wind tunnel.The result is unsafe,but it can provide some references for galloping analysis calculated by energy method.The energy method can calculate the minimum damping ratio restraining the increase of amplitude of pylon.The calculated results are in good agreement with the test result of wind tunnel.