Experimental evaluation of the use of embedded fiber Bragg gratings to measure steady and unsteady flow-induced marine propeller blade deformation

This paper presents an experimental study that investigates the deformation of a composite propeller blade subjected to a uniform inflow. An array of fiber Bragg grating sensors were embedded inside the blade, allowing for real-time monitoring of strain. Measurements were carried out in the towing tank at Centrale Nantes, France. An original experimental setup was developed to support and fully submerge the blade in the hydrodynamic facility, which reduced the effect of the free surface during testing. The blade, with a root chord of 0.3 m and a span of 0.5 m, was instrumented with embedded fiber Bragg grating (FBG) sensors on both the pressure and suction sides to measure the deformation field. The FBG measurements were dynamically synchronized with a six degree-of-freedom hydrodynamic balance to measure the loads and moments on the blade. Visualizations of the boundary layer regime were observed to determine the flow detachment regions at the surface. The carriage velocities utilized were between 1 and 4m/s and the angle of attack of the blade, based on the zero-lift angle of attack, ranged between 0 to 12^∘. After determining the natural frequencies and modes of the blade using laser doppler vibrometry, the forces and deformations were investigated under hydrodynamic flow for both averaged and unsteady data. For averaged results, a good correlation is observed between the lift force and deformations, which demonstrates the accuracy of the measurement system. The fluid–structure interaction dynamic is analyzed using the frequency spectra of drag, lift, pitching moment, and deformations. The interaction between Strouhal frequencies of vortex shedding resulting from flow detachments at the blade surface is identified with the flow visualization, and the modal response of the blade is demonstrated.