Abstract
The present paper investigates the fluid–structure interaction (FSI) of a wing with two degrees of freedom (DOF), i.e., pitch and heave, in the transitional Reynolds number regime. This 2-DOF setup marks a classic configuration in aeroelasticity to demonstrate flutter stability of wings. In the past, mainly analytic approaches have been developed to investigate this challenging problem under simplifying assumptions such as potential flow. Although the classical theory offers satisfying results for certain cases, modern numerical simulations based on fully coupled approaches, which are more generally applicable and powerful, are still rarely found. Thus, the aim of this paper is to provide appropriate experimental reference data for well-defined configurations under clear operating conditions. In a follow-up contribution these will be used to demonstrate the capability of modern simulation techniques to capture instantaneous physical phenomena such as flutter. The measurements in a wind tunnel are carried out based on digital-image correlation (DIC). The investigated setup consists of a straight wing using a symmetric NACA 0012 airfoil. For the experiments the model is mounted into a frame by means of bending and torsional springs imitating the elastic behavior of the wing. Three different configurations of the wing possessing a fixed elastic axis are considered. For this purpose, the center of gravity is shifted along the chord line of the airfoil influencing the flutter stability of the setup. Still air free-oscillation tests are used to determine characteristic properties of the unloaded system (e.g. mass moment of inertia and damping ratios) for one (pitch or heave) and two degrees (pitch and heave) of freedom. The investigations on the coupled 2-DOF system in the wind tunnel are performed in an overall chord Reynolds number range of 9.66×103≤Re≤8.77×104. The effect of the fluid-load induced damping is studied for the three configurations. Furthermore, the cases of limit-cycle oscillation (LCO) as well as diverging flutter motion of the wing are characterized in detail. In addition to the DIC measurements, hot-film measurements of the wake flow for the rigid and the oscillating airfoil are presented in order to distinguish effects originating from the flow and the structure.
Highlights
Modern computing capabilities allow the accurate simulation of complex coupled problems such as fluid–structure interaction (FSI)
This study focused on the large-amplitude oscillations’’ (LAO) type including very large pitch angles up to 97◦
The occurring time-dependent displacements of the airfoil were measured by high-speed three-dimensional digital-image correlation taking three configurations with a variable center of gravity G and a fixed elastic axis E into account
Summary
Modern computing capabilities allow the accurate simulation of complex coupled problems such as fluid–structure interaction (FSI). The validation is often based on reference data taken from comparable experimental studies For this purpose, all relevant parameters and boundary conditions of the observed physical system have to be accurately determined in the experiment and transferred to the numerics to ensure comparability. All relevant parameters and boundary conditions of the observed physical system have to be accurately determined in the experiment and transferred to the numerics to ensure comparability This complementary experimental–numerical approach has been successfully applied to several well-established benchmark cases for FSI (Breuer et al, 2012; De Nayer et al, 2014; De Nayer and Breuer, 2014; Gomes and Lienhart, 2010; Gomes et al, 2010; Gomes and Lienhart, 2013; Kalmbach and Breuer, 2013). The dominant FSI mechanisms involved were associated with vortex-induced vibrations caused by the formation and shedding of large-scale vortices
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