An investigation into the effect of freeplay on the nonlinear aeroelastic behavior of a wing via higher-order spectra

Higher-order spectra analysis is used to analyze the nonlinear flutter characteristics of the AGARD 445.6 wing benchmark model, modified to include freeplay structural nonlinearities in pitch and plunge at the wing root. High-fidelity coupled CFD-CSD fluid-structure interaction simulations are conducted and bispectral densities are computed from selected aeroelastic wing dynamic response signals to identify the presence of quadratic nonlinearities within the available time histories. Initially, various freeplay conditions are investigated, before focusing on the intensity of nonlinear behavior at various wing spanwise stations. The considerable effect of freeplay is clearly demonstrated as the wing enters a bounded limit cycle oscillation (LCO) with amplitudes several orders of magnitude larger than that of the case without freeplay. Nonlinear coupling is demonstrated via self-interaction of the LCO frequency and coupling between the LCO frequency and the first torsional mode, this is shown to be strongest at the root where the pitch / plunge mechanism is installed and torsion is most prominent, dissipating spanwise from the pitch / plunge mechanism as the torsional motion becomes less significant. Modal analysis, alongside analysis of the leading-trailing edge phase change and wing deformation shapes supports the findings of the HOS, generating understanding of the underlying physical attributes leading to the observed nonlinear aeroelastic phenomena.