Session: FSI-02-06 Selected Topics in FSI

Paper Number: 106667

106667 - Measuring Damping in Linear and Nonlinear Systems 

Establishing the damping of a structure exposed to flow-induced vibration is usually a necessity when making an investigation or assessment. Damping models the energy absorbed during vibration and may be due to a linear or nonlinear mechanism. It cannot usually be predicted but must be obtained from measurements. This paper looks at accurate methods for measuring damping. The approach used is to measure the free vibration response of a structure which is typically a decaying wave form. Usually, this wave form is contaminated by a superposition of several modes and noise. The approach taken is to extract individual modes by using a filtering method in which the decaying time history is reversed before passing it through a narrow band filter. The result is a decay wave form from a single degree of freedom system.  It is shown that if the damping mechanism is linear then the damping and natural frequency of the wave form are unaffected by the filtering. For nonlinear behaviour the wave form clarifies the nonlinearity but needs to be deconvolved from the effects of the filter. Methods for performing deconvolution are described. Examples of damping extracted from fluid systems that are linear and nonlinear are used to illustrate the method.

Presenting Author: Hugh Goyder Cranfield University

Presenting Author Biography: Dr Hugh Goyder is a specialist in fluids and structures and their interaction. He gained his PhD from the Institute of Sound and Vibration at Southampton University in 1978 where he worked on vibration and in particular structure borne sound. Hugh then worked on flow-induced vibration problems in civil nuclear power at the Atomic Energy Research Establishment Harwell. Here he also worked on vibration, acoustic and fluid problems in process plant. Since leaving Harwell he has worked at Cranfield University where he continues to undertake research into vibration and fluid problems extending this to ballistics and gun dynamics. Recent work has included investigation of nonlinear vibration and damping both experimentally and theoretically. He is a member of the ASME committed on dynamics of bolted joints.

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