No abstract

Fluidelastic effects which are responsible for fluidelastic instabilities may be indirectly measured through the analysis of the vibrating motion of a system under flow. In this paper, piezoelectric actuators are used to increase the vibratory level when buffeting forces which excite tube vibration are low, and to improve the measurement of fluidelastic forces. The proposed method based on an added excitation allows the study of the added mass and provides a better accuracy on the measurement of the vibrating characteristics and, thereby, of the fluidelastic forces. This added excitation method is compared with a standard indirect approach on a tube underwater crossflow. The influence of the level of piezoelectric excitation forces is discussed, as well as the range of application of this technique.

A new method based on active vibration control is proposed to investigate fluidelastic coupling effects beyond fluidelastic instability. This active control method allows to extend the range of flow velocity explored for single input-single output control systems. The method is applied on a flexible tube inserted in a rigid bundle in water and air-water cross-flows. This structure becomes unstable for high flow velocities, fluidelastic forces then causing the damping of the fluid-structure system to fall towards zero. The active control method allows to carry out tests beyond the fluidelastic instability. The flow velocity range explored is doubled in two-phase flow.

Fluidelastic effects may be responsible for instabilities of heat exchanger tubes when the fluid flow reaches the critical velocity. The fluidelastic phenomenon is usually studied on experimental mock-ups, which may display only one critical velocity. In this paper, a method based on active vibration control is proposed in order to derive several critical velocities for fluidelastic instability corresponding to several different values of damping, which is artificially varied on the same mock-up. Experimental tests are performed on a flexible tube equipped with piezoelectric actuators in a rigid array under air-water cross-flow. It is shown that the reduced critical velocities thus obtained fit well in a classical stability map. [S0739-3717(00)01603-2]

Due to a new design, a multi-hole orifice was arranged on the Spent Fuel Pit Cooling system (SFPC) of a French nuclear power plant. Serious pipe vibrations and a high level of noise were generated during the qualification testing. On-site measurements showed that a pure tone appeared at 640 Hz for a precise value of the mean flow. Furthermore, when the multi-hole orifice was replaced by another one with a different set of holes, the pure tone frequency was shifted to 520 Hz. The Strouhal number of the orifice holes is close to unity in the high-vibration regime. Besides, the acoustic boundary conditions appear highly reflecting. A fully-coupled finite element analysis of the piping system shows that several natural modes in the 500–700 Hz range exhibit simultaneously a shell deformation of the orifice, a pattern of plane acoustic waves upstream the orifice and a shell deformation of the piping system. The phenomenon is explained as a lock-in between vortex shedding inside the multi-hole orifice and the acoustic cavity, enhancing shell deformation of the pipe.