| Because of the characteristics of convenient construction,short construction period and light weight,the cable-stayed bridge with ?-shaped main girder has been widely used in recent years,and the main span is usually about 200?600 m.However,due to its open section,it has obvious blunt body characteristics.Under the action of wind loading,periodic vortex shedding is easy to occur in the wake.Moreover,because of its light weight and small torsional stiffness,the aerodynamic stability of the bridge is so poor that it is prone to vortex-induced vibration(VIV)at low wind speed.Although VIV is a kind of limited amplitude vibration,and it will not cause destructive damage to the structure,excessive amplitude may cause structural fatigue problems and safety and comfort problems of pedestrians.Therefore,it is of great significance and engineering value to study the mechanism of VIV and aerodynamic suppression measures of cable-stayed bridge with ?-shaped main girder.Based on wind tunnel test and numerical simulation,the mechanism of VIV and aerodynamic suppression measures of a cable-stayed bridge with ?-shaped main girder are systematically studied.(1)Segmental model wind tunnel test was carried out on a cable-stayed bridge with ?-shaped main girder.The results showed that the amplitudes of vertical and torsional VIV in completion state exceed the allowable value in a certain wind speed range,and the response of vertical VIV is the largest at the-5-degrees wind attack angle,while the response of VIV in construction state is far lower than the allowable value,and VIV performance is good.(2)Based on wind tunnel test,a two-dimensional CFD(Computational Fluid Dynamics)numerical model of the main girder was established,and the static flow characteristics and pressure distribution characteristics around the ?-shaped main girder was calculated and analyzed.From the analysis,it can be seen that there are significant flow separation and reattachment phenomena on the surface at the ?5-degrees wind angle attack.The scale of the vortex on the sub-deck decreases with the increase of the wind attack angle,and the scale of the vortex on the supper-deck increases with the increase of the wind attack angle,and vortex shedding is both formed in the wake.At-5-degrees wind attack angle of,the overall change of pressure on windward side of supper-deck is small,and the pressure on leeward side fluctuates in a small range,while the negative pressure on sub-deck is large,and the position is constantly changing.With the increase of wind attack angle,the pressure of supper-deck gradually changes from positive to negative,and the pressure of sub-deck changes from negative to positive.(3)Using the method of UDF(User-Defined Functions)secondary development program embedded in Fluent,the numerical simulation of VIV of the ?-shaped main girder was carried out at the-5-degrees wind attack angle.The mechanism of VIV was analyzed,and the influence of auxiliary facilities on the performance of VIV was studied.The results showed that the separated airflow on windward side does not form a vortex after attaching to deck of the main girder,and a local vortex is formed on the leeward side.In the vortex region,there are large-scale vortices in the inner wall of the sub-deck,and the separated airflow during the forward movement of the vortices can greatly promote the development of the leeward side vortices,leading to periodic vortex shedding in the wake,which leads to the VIV of the main girder;the pressure field on the supper-deck of the main girder is relatively stable,the change of surface pressure is small,and there is a negative pressure zone on the sub-bridge deck,and the pressure fluctuation value is large,and the dynamic effect is obvious.The formation and development of the vortices on the surface of the main girder without auxiliary facilities are not obvious,and no significant vortices falls off in the wake,which indicates that the existence of auxiliary facilities will increase the scale of the vortices in the wake and increase the amplitude of the VIV.(4)Based on the analysis of the mechanism of VIV of the ?-shaped section,three aerodynamic optimization measures are adopted: Three sub-stabilizing plates,40-degrees sharp nozzles and skirt plate with 0.5D cross-bridge length(D is the height of the main girder).The restraining effects of different optimization schemes were calculated and analyzed by Fluent.From the analysis,it can be concluded that setting three sub-stabilizing plates on the main beam section can separate the space of the lower bridge deck,disturb the development of vortex on the lower bridge deck to a certain extent,and reduce the maximum response of VIV by about 23%.However,the stabilizer plate has little influence on the formation of leeward side vortices,periodic vortices still fall off in the wake,and the amplitude of VIV of the main girder is still large,which indicates that the three sub-stabilizing plates have limited effect of suppressing VIV of the ?-shaped main girder.(5)When 40-degrees sharp nozzle is installed on the main girder,the linearity of cross-section flow can be enhanced,the air flow transition can be smoother,and the wake vortex can be damaged better.There are only a few separate bubbles at the leeward side nozzle,and the scale decreases significantly.The response amplitude of VIV of the main beam decreases by about 80%.The vibration wind speed increases slightly,and the performance of VIV is improved greatly.(6)With the addition of 0.5D length skirt plate,the formation,development and shedding of vortices are not disturbed,and the response of VIV is almost the same as that of the original section,which indicates that the skirt plate has no suppression effect on VIV.The reason is that the size of the skirt plate is too small in the transverse direction,and it does not play a role in the core area of the vortex shedding.After extending the length of skirt plate transverse bridge to 1.5D,the skirt plate separates the upper and lower airflow on the leeward side and guides its smooth transition.Large-scale vortices cannot be formed in the wake,and the response amplitude of VIV decreases by about 77%.It shows that the length of skirt plate transverse bridge has a great influence on the suppression of VIV,and the appropriate length can effectively suppress the VIV of the ?-shaped main girder. |
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