Wind Tunnel And Analytical Investigations Into The Wake Galloping Of Parallel Twin Cables

The length of cable increases with the increase of bridge span, and its arrangment form presents a diversification characteristic, which may lead to the problem of wind-induced vibration of long cable that seriously affect the comfortableness of driving, and even threaten safety. Wake galloping of parallel superlong cables have become the focus in the wind engineering of bridge for its complicated mechanism.Therefore, this paper gives a systematic investigation of the typical wake galloping phenomenon. Firstly modal analysis was carried out on parallel superlong cables, and based on aerodynamic force experiments, theoretical analysis method was used for predicting the critical velocity and the instability regions of wake galloping of downstream cable. Then using traditional full aeroelastic cables model wind tunnel test investigated on the characteristic of wake galloping. Eventually, standing on the perspective of aerodynamic forces do work on downsteam cable and using the energy analysis method, the mechanism of the wake galloping was studied by forced vibration wind tunnel tests.This thesis is mainly concerned with the following aspects:(1) The current status of flow around two circular cylinders, research methods and control methods of wake galloping are reviewed in detail in the first chapter. Based on the past research, major contents, channels and method of the thesis is proposed.(2) Three kinds of finite element modes(FEM), including spatial beam model(SBM), spatial plate-beam model(SPBM) and spatial shell model(SSM), and single cable FEM for modal analysis are described, and the modal frequencies for wind tunnel tests are calculate in the second chapter. It is show that the results from the four modes are basically the same. Because of the obvious effects of sag and its own big gravity stiffness, the first-order lateral frequency compared to the vertical vibration frequency is larger, but both of them are still small, which may lead to occuring with the second-order anti-symmetric modal wake galloping under natural wind load. After adding connectors between parallel superlong cables, local frequency which is helpful to improve the critical velocity of wake galloping increase greatly.(3) By taking twin parallel cables length of 420 meters of one bridge as background, based on flexibly-mounted rigid section model in wind tunnel, the twin cylinder's aerodynamic coefficients in different wind attack angle were measured. Ac-cording to the results of dynamometry, the characteristics of cable wake galloping were analysised by theoretical analysis method. The classic galloping theory and the two degrees of freedom of quasi-steady theory analysis results in the third chapter show that the critical velocity of downstream cable is 18m/s and instability regions of wind attack angles is about 15°.(4) Using systematic full aeroelastic cables model wind tunnel test, the wake galloping characteristics and the suppress measures are studied in chapter four. Research results show that the results of theoretical analysis and tests are basically identical, but increasing structural damping impacts on the critical velocity not so obvious. The direction of motion of the downstream cable is always upstream near the wake centre and downstream near the outer edge of the wake. When the downstream cable occurs wake galloping, it move in a elliptical path, the semi-major axes are inclined at a small angle to the horizontal and the angle of inclination varied with wake position and free stream velocity.(5) By force vibration device reproduced this phenomenon for the classical problem of wake galloping of parallel superlong cables, and calculated the work done by aerodynamic forces during single motion cycle of downstream under different conditions by the time domain integral equation method. Research in chapter five shows that wake galloping of parallel superlong cables is closely associated with relative position. Wake galloping occurs only when the instability regions are placed into middle wind attack angle, and it has the identified orientation and direction. Drag do positive work to downstream cable and make the horizontal vibration intensified. While lift almostly do negative work and make the vertical vibration suppressed. Again, the results agree well with the theoretical analysis and full aeroelastic cables model wind tunnel test, which can be used to estimate the occurrence possibility of wake galloping in practical parallel cables. Moreover, the time-history curves of work done by drag and lift appear identical positive and negative part, and their phase differences with their section displacement-time curves are all almost 90 degrees when wake galloping occurs. And the signs(positive and negative)of work done by drag and lift are always reversed.