Aerodynamic and Structural Resonance of an Elastic Airfoil Due to Oncoming Vortices

Aerodynamic and structural resonance of an elastic airfoil in a uniform stream with oncoming vortices was investigated experimentally and numerically. An experiment was designed to create two parallel rows of vortices that serve as external excitation for the symmetric airfoil (NACA 0012). The vortices were produced by two identical side-by-side circular cylinders of diameter D in a uniform stream located at a fixed distance ahead of the airfoil. The whole arrangement was placed symmetrically about the midplane in the test section of a wind tunnel. Reynolds numbers ranging from similar to8 X 10(4) to similar to2 X 10(5) were selected because the Strouhal number was essentially constant in this range. A range of D was selected to provide a corresponding range of shedding frequencies f(s) that could lead to both aerodynamic and structural resonance of the airfoil. A hot-wire anemometer and a dual-beam laser vibrometer were used to measure the wake pattern and the vortex convection velocity, and the airfoil response, respectively. The airfoil displacement amplitude at structural resonance increases by many fold compared to its value far away from resonance and the airfoil goes into a limit-cycle oscillation behavior. As D decreases, near-aerodynamic resonance is also observed. These results were used to verify a boundary element method (BEM) developed to treat vortex-airfoil interaction problems. Aerodynamic and structural resonance of an elastic airfoil in a uniform stream with oncoming vortices was correctly replicated by the BEM, thus showing that the BEM model is appropriate for fluid-structure interaction problems.