Forced excitation and active control for the measurement of fluid-elastic forces

Fluid forces acting on a tube bundle are commonly divided into random buffeting excitation and fluidelastic excitation. Fluidelastic forces, which are motion-dependent, may be derived from an analysis of the vibrating response of the structure to the buffeting forces. At low velocities, buffeting forces may be insufficient to make the tube vibrate significantly and to allow a proper analysis. On the other hand, the structure may become unsteady for high flow velocities: fluidelastic forces make the damping of the fluid-structure System falling toward zero. Two experimental methods are proposed here in order to extend the flow velocity range explored. An added excitation force allows to increase the vibratory level of the tube to improve the signal to noise ratio when flow velocities are low. When the tube is subject to fluidelastic instability, an artificially added damping using active vibration control allows to stabilize the tube. The methods are carried into effect under water and air-water cross-flows using a flexible tube inserted in a bundle, the tubes of which are rigid. Two technologies of actuator are used: an electromagnetic shaker and piezoelectric actuators. Using the added excitation method, it is found that fluidelastic effects remain negligible at low velocities in single-phase flow. The active control method allows to carry out tests over the fluidelas tic instability. A stabilizing phenomemum is then displayed in two-phase flow for low void fractions. The new results obtained are compared with results of the literature in terms of fluidelastic coupling and buffeting excitation. (author)