Numerical Simulation For The Rain Wind Induced Vibration Of Stay Cables

A fluid structure instability phenomenon frequently occurs in the subcritical Reynolds regimes multiphase flow system, and rain-wind-induced vibration(RWIV) is taken as an example in civil engineering to characterize the aeroelastic instability caused by fluid-structure interactions. RWIV of stay cable affected the safety of the entire bridge even-tually, and most have focused on these phenomena from the field observation, analytical model and wind tunnel experiments aspects, but rarely on numerical investigation aspect. However, due to the complicated interactions mechanisms in the liquid-gas-solid system, the mechanism of RWIV has not been thoroughly solved and recognized by the previous researchers. Both the aerodynamic characteristics of stay cables and the rainwater mor-phology evolution along the circumference of stay cable can be obtained by numerical simulations. Therefore, the numerical investigations have been made in this thesis and the specified contents are summarized as follows,Firstly, the high precision numerical model is proposed to investigate the flow struc-ture, separation characteristics, and the mechanism of RWIV, based on the assumptions that the rivulets regarded as a fixed solid surface attach on the cable to simulate the rain effects on the aerodynamic characteristics of stay cable. To avoid the low efficiency and divergence problems in solving the Poisson equations by traditional commercial soft-ware, the projection methods and multigrid solvers based on the adaptive mesh refinement (AMR) framework using classical algorithms on regular Cartesian grids of different reso-lutions arranged hierarchically have been implemented. In addition, high-precision spatial discretization scheme, second-order temporal discretization scheme, and monotone inte-grated large eddy simulation (MILES) have been incorporated in to capture the vortex shedding structure in the near wake of stay cable, precisely and accurately. Based on this model, the position of artificial rivulet effects on the vortex shedding structure, pressure distribution around the cable surface, and the lift force dominant frequency are conducted and analyzed.Next, the high precision numerical multiphase model is proposed to capture de-tailedly and accurately the rainwater morphology evolution along the circumference of cable surface, the rivulet effects on the pressure distribution around the cable surface, the aerodynamic characteristics of cable, and the vortex shedding behind the cable. To con- sider the mass, momentum, energy and turbulence transport phenomena across the multi-phase interface in previous numerical models, and to avoid the lower accuracy for tracking the multiphase interface, large deflections of the evaluation for the curvature of interface, and unbalanced force term in momentum equation in traditional volume of fluid method; the high precision numerical multiphase model adopts tree-based adaptive mesh refine-ment framework, high accurate volume of fluid (VOF) method, height function method to evaluate the curvature of interface; and continuous surface force based on the balanced force model to track the nonlinear multiphase interface between wind and rainwater. In addition, the adaptive scheme for the grid system is based on the gradient of the interface curvature, the error of the horizontal velocity, and the gradient of volume fraction. Both the rainwater rivulets evolution along the circumference of cable and the aerodynamic characteristics of stay cable can be captured and obtained from the present numerical mul-tiphase model.Thirdly, multiphase multiscale model (MMM) is proposed to investigate the mech-anism for RWIV and simulate the rain effects in a more straight way. MMM is based on the computational domain decomposition. Direct numerical simulations (DNS) and large eddy simulations (LES) are combined to simulate this complicated process. The Lagrange point particle method is used to simulate rain droplets in the LES zone, far away from the cylinder surface. When droplets fall into the DNS zone, i.e., near the cable surface, the particles are converted into liquid droplets. The volume-of-fluid (VOF) method is then adopted to capture the morphology of the interface of the liquid-gas system. Addition-ally, a simplified numerical rain model has been proposed to approximate the natural rain from physical aspect, and to reduce the computational resource from numerical aspect. The accuracy, reliability, and efficiency have been validated in comparison with wind tunnel experimental results.Finally, based on MMM, the high precision numerical investigations is conducted, including the rainwater morphology evolution along the circumference of cable surface, the rivulet effects on the pressure distribution around the cable surface, the aerodynamic characteristics of cable, and the vortex shedding behind the cable during RWIV. Rainwater morphology evolution along the circumference of cable is mainly characterized by the wind inflow velocity. The circumferentially periodically vibrating upper rivulet across the cable surface is observed at the critical wind inflow velocity with a lower frequency. The circumferentially periodically vibrating upper rivulet has a significant effect on the dominant frequency of lift force, causing the lift force dominant frequency of the cable shifting to a much lower value compared with the convectional evaluation. When the natural frequency of stay cable, the dominant frequency of circumferentially periodically vibrating upper rivulet and the dominant frequency of Von Karman vortex shedding behind the stay cable lock-in with each other, RWIV occurs.