|Title:||Rain-wind-induced cable vibration through wind tunnel tests and numerical analysis|
|Subject:||Cable-stayed bridges -- Vibration.|
Cables -- Vibration.
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Civil and Environmental Engineering|
|Pages:||xxviii, 205 pages : color illustrations|
|Abstract:||The large vibration of stay cables has been observed in many cable-stayed bridges under the simultaneous occurrence of rain and wind, which is referred to as rain-wind-induced vibration (RWIV). The RWIV has become a great concern to bridge engineering and wind engineering communities over past three decades. Although extensive researches have been conducted to investigate the RWIV through field measurements, wind tunnel tests, and theoretical analyses, the excitation mechanism of the RWIV remains unclear and many divergences and debates exist. The wind tunnel test is an effective method to investigate the excitation mechanism of the RWIV because many parameters such as the wind speed and direction could be well controlled. In this thesis, the RWIV is investigated in detail through wind tunnel testing and numerical analysis, aiming to better understand the excitation mechanism of the RWIV. RWIV of a cable model with the external diameter of 160 mm was successfully reproduced in a wind tunnel using simulated water rivulets. The effects of different parameters such as the wind speed, rivulets, and damping ratio on the RWIV have been investigated. The wind speed in the wake field of the stationary cable model was measured to study the effect of water rivulets on the wake flow. The digital image processing technique is developed to identify the rivulet's movement and thickness during the RWIV for the first time. The cable displacement is also extracted simultaneously. The relationship between the cable vibration and rivulet oscillation is thus obtained and investigated.|
A numerical model is derived based on the quasi-static assumption using the measured upper rivulet information. Numerical analyses are conducted using the aerodynamic force coefficients measured by Prof. Y.L. Xu and other researchers considering the three dimensionality of the cable geometry. The numerical results are compared with the wind tunnel testing results. From the obtained rivulet information and cable vibration, a new excitation mechanism of the RWIV is proposed based on the interaction between the upper rivulet, air boundary layer, and cable vibration. When the cable vibrates upward, the upper rivulet, as an obstacle, changes the cable's cross-section and induces the air boundary layer to asymmetrically attach on the upper side of the cable, generating the significant upward aerodynamic force. The aerodynamic force excites the cable to vibrate at large amplitude. The large cable vibration, in turn, changes the attacking angle of the wind and provides inertial force on the upper rivulet, exciting the upper rivulet to steadily and circumferentially oscillate on the cable surface. Besides, the cable vibration also enhances the coherence of the upper rivulet along the cable and harmonizes the attachment of the air boundary layer along the cable, resulting in a larger resultant force on the cable. Finally, a single degree of freedom numerical model is established to verify the proposed excitation mechanism and an energy-based criterion is developed to predict the steady amplitude of the RWIV.
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