Lattice Boltzmann Method For Bluff Body Aerodynamics

Modern bridges span over longer distances and are more susceptible to wind actions, partic-ularly vortex-induced vibration and flutter, which emphasize the importance of research of aero-dynamic performance and aeroelasticity. Conventional Computational Fluid Dynamics (CFD) tools are not adequate because their inefficiency and lower numerical stability for complex flows around bluff bodies at high-Reynolds number. In Chapter 2, the state-of-the-art of bridge wind engineering is reviewed, and disadvantages of conventional numerical methods in engineering practice are summarized.This dissertation focus on the development of a Lattice Boltzmann Method (LBM) for bluff-body aerodynamics based on the kinetic theory. In Chapter 3, mathematical background and the-oretical fundamentals of LBM are presented. The lattice Boltzmann equation is directly derived from the Boltzmann equation by discretizing it in both time and phase space. The derivation directly connects the LBE to the Boltzmann equation, thus the framework of the LBE can be built on the established foundation of the Boltzmann Equation and the rigorous results of the Boltzmann equation can be extended to the LBE. A numerical LBM algorithm for bluff body aerodynamics is proposed, which includes the computation of exterior incompressible flows, boundary implementations, evaluation of aerodynamic forces, as well as parallel implementa-tion with General Purpose Graphical Processing Units (GPGPU). A new three point Lagrange interpolation scheme is proposed for arbitrary solid wall boundaries. Finally, numerical simula-tions of Poiseuille flow and 2D exterior flows around circular cylinder is conducted. The results agree well with theoretical and previous numerical results.In Chapter 4, an incompressible LBM scheme with multiple relaxation time collision model is developed and its superior numerical stability and efficiency are demonstrated through various case studies. Numerical simulation of boundary layer of flat plate demonstrate the high accuracy of MRT-LBE for steady flows at high Reynolds number. In addition, numerical computations of unsteady flows around several bluff bodies are conducted, which includes circular cylinder, rectangular cylinder, H section, and general girder section of long-span bridge. Results of both the flow structures and aerodynamic forces compare favorably with experimental data.In Chapter 5, MRT-LBM is extended to allow incorporation of traditional turbulence mod-els. Implementation of Large Eddy Simulation (LES) model and RNG k-εtwo-equation model in conjunction with a wall model are presented. Because the validity of using a log-law type of wall model in a separated region is highly questionable, only LES-MRT is validated through studies of turbulent flows around the decks of several long-span bridges, which includes the monobox deck of Great Belt bridge and of Sutong bridge, the dualbox deck of Xihoumen bridge, and the tribox deck of Messina bridge. It is shown that the proposed LES-MRT-LBM is both accurate and efficient.Chapter 6 investigates the validity and efficiency of coupling the LES-MRT and structural dynamics to simulate transient 2D fluid-structure interaction problems or aeroelastic instabili-ties of bridge decks. In the proposed algorithm, the moving interface was tracked and updated continuously on fixed lattice sites, and the fluid solver and structural solver are run in an alter-native manner. Details about the force evaluation, displacement transfers and the algorithm used to couple these completely different solvers are described. Validation studies of forced vibration and self-excited vibration of bridge decks are simulated. Both flutter derivatives and flutter crit-ical velocity obtained demonstrate that aeroelastic phenomena can be efficiently reproduced by this setting.Chapter 7 apply the numerical tools developed to study the aerodynamics and aeroelasitic-ity of a super-long multi-span suspension bridge proposed for Qiongzhou Strait. Static loading coefficients and flutter critical velocity are determined numerically, and the aerodynamic perfor-mance and structural feasibility of the bridge are evaluated preliminarily.