Dynamic strain measurements of a circular cylinder in a cross flow using a fibre Bragg grating sensor

 A fibre Bragg grating (FBG) sensor was proposed as an alternative to strain gauges to measure the strain ɛ of a vibrating cylinder in a uniform cross flow. In order to validate the measurements of the FBG sensor, the transverse fluctuating bending displacement Y of the cylinder was also measured using a laser vibrometer. The two measurements were found to be consistent in terms of the natural frequency of the fluid–structure system and the vortex shedding frequency. The spectral coherence between ɛ and Y at the same point of the cylinder attains 1 at these frequencies, thus indicating a near perfect correlation between the two quantities. When the transverse bending displacement is small, the measured ɛ and Y are linearly related. Therefore, the results indicate that the FBG sensor can be used with confidence to measure the fluctuating strain arising from the vortex-induced forces on a structure in a uniform cross flow. As such, it can be used in conjunction with a laser Doppler anemometer to study fluid–structure interactions in flow-induced vibration problems. Furthermore, it is expected that the FBG sensor, because of its physical uniqueness, will have an important role to play in the study of fluid–structure interaction problems with multiple structures arranged in an array. Received: 17 August 1998 / Accepted 27 January 1999