Damping of structural vibrations by vorticity production

An experimental investigation has been conducted to examine the influence of vorticity production on the damping of elastic plates and airfoils vibrating in nominally steady, high Reynolds number mean flow. Previous theoretical and experimental studies suggest that damping of structural vibrations can be enhanced by causing the vibrating structure to increase the rate at which vorticity is produced. This can be done by perforating all or part of the vibrating structure with small apertures, and forcing the mean flow to pass through and over the perforates. Vorticity is generated in the perforates and at the trailing edge of the airfoil or plate, and is subsequently convected away (together with its kinetic energy) in the mean flow. The vibrations supply the energy required to produce vorticity.; Two different series of experiments were performed to examine the likely efficiency of the damping mechanism. The first made use of a vibrating, perforated plate aligned with a mean flow in a low speed water channel. Vorticity is produced in the small circular apertures by the unsteady pressure loading of the plate. Significant damping was found to occur within certain regions of the Strouhal number ωR/U, defined by the radian frequency ω of the vibration, aperture radius R and the mean stream velocity U, between 0.2 and 2.1. The peak observed attenuation was about 5 dB relative to that exhibited by an identical, unperforated plate.; The second set of experiments involved a perforated flap excited by a large-scale vortex wake in a wind tunnel. These experiments were motivated by the need to understand and control the premature fatigue failure of aerodynamic control surfaces (such as an external nozzle engine flap) that have been observed on certain military aircrafts, but the results will be relevant to the alleviation of structural resonances in variety of other practical applications. Measurements were made of vibration damping occurring when the vorticity is generated by a bias flow through the apertures in the upper and lower surfaces of the airfoil. Damping was measured for aperture Strouhal numbers between 0.01 and 0.4. The average observed attenuation was on the order of 2 dB.