Study of bluff body flow fields and aeroelastic stability using a discrete vortex method
A two dimensional discrete vortex method has been developed to simulate the unsteady, incompressible flow field and aerodynamic loading on bluff bodies. The method has been validated successfully on a range of simple bluff geometries, both stoic and oscillating, and has also been validated on a wider range of problems including static and oscillating suspension bridge deck sections. The results have been compared with experimental data and demonstrate good qualitative and quantitative agreement, and also compare favourably with other computational methods. Most notably, the method has been used to study the aeroelastic stability of a recent bridge deck, with accurate predictions of the critical flutter velocity. The basis of the method is the discretisation of the vorticity field into a series of vortex particles, which are transported in the flow field that they collectively induce. In the method presented herein, the time evolution of the system of particles is calculated by solving the vorticity transport equation in two stages: employing the Biot-Savart law to calculate particle velocities and random walks to simulate flow diffusion. The Lagrangian approach to the calculation avoids the necessity for a calculation grid, and therefore removes some of the problems associated with more traditional grid based methods. These include numerical diffusion and difficulties in resolving small scale vortical structures. In contrast, vortex methods concentrate particles in areas of vorticity, and can provide high quality representations of these small scale structures. Dispensing with a calculation mesh also eases the task of modelling a more arbitrary range of geometries. In particular, vortex methods are well suited to the analysis of moving body problems. Results of the validation exercise are firstly presented for a range of simple bluff geometries to give confidence in the results before moving on to more complex geometries. These results include the effect of incidence on the aerodynamic loading for a stationary square cylinder, and also a study of the effect on aspect ratio for rectangular cylinders. This includes the limiting case of a flat plate. Vortex lock-in is studied on a square cylinder undergoing a forced transverse oscillation, for a range of frequencies and amplitudes. The results in each of these cases are in good agreement with experimental data.