Indicial functions or flutter derivatives: an evaluation for time domain simulations

Even more than in the past, bridge aerodynamics is a crucial topic, due both to the increase of the span length and to the trend towards very light and stylish structural solutions for smaller bridges. This paper focuses on self-excited forces, where the mixed frequency-time domain model via aeroelastic flutter derivatives is compared to the pure time domain model via indicial functions. The theoretical equivalence of the two approaches is demonstrated and computational advantages of indicial functions are pointed out. As experimental procedures for obtaining indicial functions are not well established, analytical approximations by parametric exponential filters are considered here. Filter coefficients can be related to aeroelastic derivatives and therefore identified by means of non-linear optimization techniques. Different algorithms are tested in order to define a robust numerical identification procedure. Some examples are presented and verified by comparison with experimental data, proving the feasibility of the approach. According to this first study, identified indicial functions can be confidently used as long as they lead to good approximation of flutter derivatives. Though further investigations are required, different sets of coefficients, which well approximate measured derivatives, seem in fact to produce also good results in time-domain simulations.