Mechanism Study On Longitudinal Ribs Mitigation Of Rain-Wind Induced Vibration Of Stay Cable

Stay cable of cable-stayed bridge may suffer large amplitude under certain wind and rain,and this kind of phenomenon is called Rain Wind Induced Vibration(RWIV).During all the wind-induced vibraitons,cable amplitude of RWIV is the biggest one,thus it may bring serious damage.Stay cable has many features,such as big flexibility,small damping and low weight,thus they could occur RWIV easily.As a result,the service life of stay cable may be deduced,and the cable-stayed bridge would be unsafe.Nowadays in order to mitigate RWIV,researchers bring forward many countermeasures,such as aerodynamic countermeasures,mechanical countermeasures and cross-ties countermeasures.Actually aerodynamic countermeasures is the most common measures to hinder RWIV.However,the mitigation mechanism of aerodynamic countermeasures is still uncertain,and it is really blind to use the aerodynamic countermeasures in the practical engineering.Besides,the main research method is using wind tunnel test.It is difficult to study the dynamic characteristics of water film and wind load,and numerical method could remedy this.Based on it,Lubrication theory,One-degree vibration theory and Computational fluid mechanics are combined to build a new theoretical model,which could be used to study the inner mechanism of aerodynamic countermeasures.Firstly,a RWIV mitigation theoretical model is put forward.The coupling effect among stay cable,water film and wind load is considered for the first time.This model could be used to analyse stay cable with an aibitrary 2D shape and study the mitigation mechanmism of 2D aerodynamic countermeasures.Based on Lubrication theory and One-degree vibration theory,water film evolution equation and cable vibtiaon equation are deduced.The continuous water film around the stay cable is assumed,thus the geometrical features of water film around the stay cable with different time step could be traced.Based on Computational fluid mechanics,effect of water film evolution on wind load features around the stay cable is considered.Wind load coefficients varying with time are calculated.The finite difference method is used to calculate water film evolution equation and the Runge-Kutta method is used to calculate cable vibration equation.The calculation accuracy is enough and the calculation efficiency is high.Secondly,the two-degree segmental cable numerical model is used to study the stay cable with and without longitudinal ribs.Three conditions,incluing gravity,wind,gravity and wind at the same time are considered.Cable vibration response,aerodynamic lift variation,water film evolution and frequency analysis are studied.Numerical results are compared with experimental results to verify the precision of the theoretical model.Mitigation effect of longitudianal ribs are analysed.Thirdly,stay cable model with two longitudinal ribs is considered.Cable vibration response,aerodynamic lift variation and water film evolution are studied.Besides,numerical results are compared with the results of cable without ribs to study the mitigation mechanism and inner essence of longitudinal ribs deeply.Fourthly,stay cable with several longitudinal ribs are considered.The mitigation theoretical model is used to study the effect of longitudinal ribs' number,longitudinal ribs' height,inclination angle and deviation angle of wind on RWIV.Numerical results are compared with that of stay cable without ribs.Cable vibration response,wind characteristics around the cable,water film geometry and dynamic characteristics,the forces between stay cable and water film and the coupling variation law among stay cable,wind load and water film are studied to analyse the worst calculation conditions,thus to bring theoretical base for practical engineering.Lastly,the interference effect of two tandem stay cables are considered.A new numerical model which could analyse the interference effect of two tandem stay cables is built.Three typical wind speeds are considered to study two tandem stay cables with and without longitudinal ribs.Cable vibration characteristics,aerodynamic lift variation,water film evolution and frequency analysis are studied to analyse the interference effect of two tandem stay cables on RWIV.In this thesis,the theoretical mitigation model are used to study all the contents above in detail to show the mitigation mechanism of longitudinal ribs.Many important parameters are analysed to provide the basis for practical engineering.The results show that: aerodynamic countermeasures could change the original flow path of water film around the cable and hinder the formation and oscillation of upper rivulet,thus the periodic variation of aerodynamic lift is affected and the large amplitude of stay cable is mitigated;among all the wind speeds,two longitudinal ribs around the stay cable do not trigger any other kinds of unstable vibrations,and the mitigation effect is good;if the longitudinal ribs' height is too low and the longitudinal ribs' number is too little,it may intensify the unstable vibration of stay cable;the lower one of two tandem stay cables could aggravate unstable vibration of the upper one.