Experimental Investigation on a Single NACA Airfoil’s Nonlinear Aeroelasticity in Wake Induced Vibrations

Abstract

In order to meet the economic and environmental requirements, turbomachine blades and aircraft wings are becoming more light and flexible, and bearing more mechanical and aerodynamic loads. However aerodynamic excitation would bring more variables into the structural vibration, and becoming an aeroelasticity problem. Unlike mechanical resonance vibration, the structure would interact with the aerodynamic excitation, and the aerodynamic excitation frequency would lock into structural natural frequency even the frequency margin is more than 10%. This phenomenon extends the high amplitude response range and should be noticed in safety design in order to deal with the margin in specific resonance conditions. In this paper, the aerodynamic excitation induced forced response is investigated with experimental setup including upstream cylinder and a downstream single NACA airfoil in wind tunnel. The upstream cylinder generates the vortices imposed on the NACA airfoil, brings periodic excitation on the flexible blade. Flow velocity is measured with hot wire anemometer (HWA) at upstream and downstream of the blade synchronously. Numerical simulation is conducted based experimental condition and verified by the measurements. Proper Orthogonal Decomposition (POD) is applied to obtain the major flow structure at one typical flow condition. The structural properties of the airfoil including natural frequency and damping are evaluated through finite element analysis and hammer test. Based on the fluid and structure properties, coupled test and analysis can be conducted. The vibration characteristics of NACA airfoil at 1st and 2nd order modes are explored by altering the freestream velocity and cylinder diameter. The forced vibration of 1st order mode has the lock-in phenomenon, and the maximum amplitude point is not at the resonance point. But 2nd order mode shows typical resonance behavior.