THE SELF-INDUCED FREE-SURFACE “SWELL FLAPPING” CAUSED BY THE INTERACTION AMONG A JET, A FREE SURFACE AND A STRUCTURE

Abstract

A new type of self-induced free-surface oscillation (swell flapping) was discovered in a system with a jet, a free surface and a structure. An upward round jet was injected into a cylindrical tank from the bottom centre. The jet impinged on the bottom of an upper-inner-structure (UIS) in a LMFBR nuclear reactor, which was set just beneath the free surface and above the jet inlet channel. The jet was separated at the edge of the UIS. In the cylindrical tank system, the effects of the geometric parameters on the occurrence and period of the swell flapping were investigated. The swell height during the oscillation was thought to be important; therefore, the experiments were made with a rectangular tank system in order to measure the swell height. Swell flapping was also observed in the rectangular tank. At a certain jet inlet velocity and UIS depth, the separated jet oscillated, forming a swell of the free surface around the UIS. The maximum swell height from the bottom edge of the UIS increases proportionately with the jet inlet velocity, independently of the UIS depth. The period of oscillation increases with increasing swell height and velocity. The period of oscillation was divided into a swell rising component and a swell breaking component. The time required for swell rising depended on the jet inlet velocity, while that required for swell breaking corresponded to the wave propagation period.