Fiber Bragg grating tether used to measure drag forces in neutral buoyancy flow tank tests

The drag exerted on neutrally buoyant tethered spheres in a flow tank was measured as a function of flow rate. A unique solution to the problem was achieved using an optical fiber including a Bragg grating sensor as part of the tether. Measurements of the strain on the tether taken at flow rates between 0.14 and 0.33 m/s, were used to determine drag forces for spheres with diameters ranging from 40 to 100 mm. Vortex-induced vibration was observed in tests performed at Reynolds numbers from 5×103 to 4.5×104. The drag coefficients for these tests were found to range from 0.51 to 0.77.