Limit-Cycle Oscillations of a Supercritical Airfoil

Abstract

Non-linear effects, either of structural or aerodynamic nature, are of high importance in aeroelasticity. These effects can cause limit-cycle oscillations (LCOs), which result in high loads on the oscillating structure. Possible sources of non-linearity in the flow are trailing-edge/shock-induced separation or shock dynamics. Investigations into these sources are limited due to the high computational costs of coupled fluid-structure simulations using Computational Fluid Dynamics (CFD). In this work LCOs are analysed using fluid-structure interaction (FSI) simulations and forced motion simulations of a two-degree-of-freedom airfoil system. The mean flow conditions from which LCOs might develop, are determined from the moment polar. The LCO analysis shows a slightly non-linear behaviour of the aerodynamic power. Furthermore, comparison of the coupled FSI simulations and the forced motion simulations shows that forced motion oscillations are useful for LCO modelling and amplitude estimation. For the five test cases investigated in this work it was found that the LCO amplitudes obtained using forced motion oscillations agrees fairly well with the LCO amplitude obtained from coupled FSI simulation. That is, if the complex mode shape of the LCO is used in the forced motion simulation. The power delivered by the lift is shown to be stongly influenced by the phase difference between pitch and plunge.