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 ε 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 ε 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 ε 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.