Development of Eddy Current Damping Device for Wind Tunnel Test

Abstract

As the length of the bridge becomes longer, a vibration problem due to wind becomes important. Wind-induced vibrations, particularly flutter and vortex-induced vibration (VIV), are a key point in bridge design. VIV phenomenon is an important design factor in connection with serviceability problems because VIV phenomenon generally occurs in low wind speed. Wind tunnel test is generally selected for testing serviceability problems. It is generally known that the amplitude of VIV is inversely proportional to the number of Scruton number when evaluating the serviceability criterion in wind tunnel experiments. Scruton number is proportional to the damping ratio, so it is necessary to set the damping ratio precisely. However, in general, the oil damper used in the wind tunnel test cannot be set to exact damping ratio because of the nonlinearity depending on the vibration amplitude. There is a risk of incorrect estimation of the vibration amplitude. For precise vibration amplitude estimation, this paper developed an eddy current damping device using magnets. Unlike oil dampers, eddy current damping devices are non-contact type, and force is generated in proportion to speed, so there is no nonlinearity due to vibration amplitude. In order to design the eddy current damping device, the damping coefficients of several magnets were calculated by using a simple shear building model, then, we apply the eddy current damping device in the free vibration test by utilizing wind tunnel test setup, and compared the actual measured value with the predicted one. The validity of the damper can be verified by comparing the two results. By creating a general manual that can be applied to various wind tunnel test settings, wind tunnel experiments can be developed more accurately and conveniently.