Air flow measurement by vortex shedding from multimode and monomode optical fibres

Wind tunnel experiments to investigate flow-induced vibration by vortex shedding from both multimode and monomode optical fibres are described, covering Reynolds numbers from 30 to 240. The fibre vibration frequency was a linear function of air flow speed over the range 2 to 8 m s^-1. Multimode fibre vibration was detected by the modulated bend loss at the fibre clamping points. A fibre Mach-Zehnder interferometer enabled the strain, and hence vibration amplitude, of a monomode fibre to be determined under similar flow conditions. It was thus shown that for vortex shedding flowmeters with fibre sensing elements, interferometric techniques offer no significant performance advantages over simpler bend loss modulated devices.