Experimental and numerical modelling for flag flutter

In the present work the flutter instability of a cantilevered flag-plate subjected to an axial flow on both surfaces has been experimentally and numerically studied. This instability mechanism, which can lead to self-sustained oscillation, is nowadays at the core of prototypes or concept of flow energy harvesters. In this work, three different numerical models are used for examining the instability and the post-critical behaviour and compared with obtained experimental results. Indeed, the flutter boundary, the aeroelastic modes, and the amplitude of limit cycle oscillation (LCO) predicted by the different models are compared with experimental data provided by wind tunnel tests. Beginning from a linear 2D theory, the analytical model is then extended through a linear numerical one in which the flutter modes are found to be again two dimensional although the potential flow, discretized by a lifting surface theory (namely, a doublet lattice method, DLM), is 3D modelled. Thus, the post-critical behaviour is numerically analysed through a nonlinear (both structural and aerodynamic) numerical model where an unsteady free-wake vortex lattice method (UVLM) is used. Numerical and experimental results are in good agreement for the flutter onset with the literature results, including the critical flow velocity at which the aeroelastic system becomes unstable, as well as the aeroelastic modes of oscillation and frequency. Significant differences between the nonlinear numerical model and experiments for LCO amplitudes have been found. The comparison between the numerical aeroelastic mode and experimental one, provided by a high resolution video measurement system, is done by using modal assurance criterion parameter. Moreover, a large hysteresis loop is experimentally observed suggesting a subcritical bifurcation, which is not numerically predicted.