Numerical simulation of fluid-structure interaction phenomena

In this thesis, a new method for the investigation of aeroelastic phenomena for long-span bridges is proposed: the aerodynamic fields and the motion of structure are simulated simultaneously and in a coupled manner. The structure is represented as a bidimensional elastically suspended rigid body with two degrees of freedom whose natural frequencies correspond to those of the fundamental flexural and torsional modes of vibration of the structure. The aerodynamic fields are simulated by numerically integrating the Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations with a finite volume scheme on moving grids which adapt to the structural motion. The URANS equations are completed by the turbulent closure relations which are expressed as a function of the turbulent kinetic energy, the turbulence frequency and the strain tensor according to the k- SST approach. The presented model is used in order to identify the critical flutter wind velocity of the Forth Road Bridge deck, and the numerical results are compared with those of an experimental campaign. For wind velocities equal or greater than the critical wind flutter velocity, the deck starts to oscillate increasingly. It is demonstrated that the reason for the onset of the torsional-branch coupled flutter lies in the fact that, within each of the first oscillation cycles, there is a portion of the cycle in which the energy supplied by the aerodynamic field to the deck motion is more than the energy extracted in the rest of the cycle. Then it is shown that the reason for the amplification of the instability resides in the drifting of large vortical formations along the deck surface. The numerical model is also used to test the effect, on the aeroelastic stability of the Forth Road Bridge deck, of the introduction of a couple of sloping barriers at the windward and leeward bridge deck edges. It is demonstrated that the aerodynamic modifications produced by the introduction of such barriers is effective in increasing the critical flutter velocity and mitigating the vibration amplitudes which develop during the flutter instability.