FLUID DAMPING IN AN OSCILLATING ARRAY OF CYLINDERS SUBMITTED TO AXIAL FLOW

Axial flows are known to generate a high level of damping with respect to the lateral oscillations of cylinder arrays. The evaluation of these dissipative forces is crucial for seismic design of fuel assemblies. The present study is focused on the case of an array of 40 cylinders oscillating laterally in an axial flow. An experiment is performed in a water tunnel with an rectangular array of 8 x 5 cylinders. The cylinder diameters are D = 10mm, and their length to diameter ratio is L/D = 56. The cylinders are equally spaced with a gap of 3 mm. The distance from the array of cylinders to the wall of the water tunnel is equal to 14mm in the lateral direction and 24.5mm in the direction of the oscillations. The axial flow velocity varies between 0.5m/s and 4.5m/s. The array of cylinders is rigidly clamped to a rod connected to a motoreductor system which enforces a sine-like displacement. The oscillation frequency ranges from 3Hz to 7Hz and the amplitude from 5.4mm to 16mm. An impedance head and an optical micrometer are used to measure the lateral force and the displacement. The added mass and damping coefficients are estimated from the lateral force signal for different axial flow velocities, oscillation amplitudes and frequencies. Results show that the added mass coefficient is almost constant and equal to the potential value proposed by Chen. The damping coefficient depends on only one non dimensional parameter which is the ratio between the oscillation velocity and the axial flow velocity. Two force regimes are also observed : for very low oscillation velocities to axial flow velocity ratios, the damping force is proportional to the oscillation velocity and quadratic otherwise.