Self-induced oscillations of upward plane jet impinging on free surface

Two types of self-induced oscillations were observed in a rectangular tank having a submerged plane water jet impinging on the free surface. The jet entered the test tank from the bottom center and impinged vertically on the free surface. Then the flow forked there, and flowed out through underflow gates on the both sides of the main section to sub-sections. The water depths in these sections were controlled by overflow weirs attached to the sub-sections. The flow pattern was two-dimensional and symmetric with respect to the jet axis if no oscillation existed. The jet oscillated when the velocity exceeded a certain value. In case of shallow water, the free surface swelled by the impingement, making a ridge with hydraulic jumps by the impingement, making a ridge with hydraulic jumps on the both sides. The jet fluttered and the ridge oscillated horizontally, generating waves propagating in the tank. The frequency was that of a liquid column in a U-shaped tube with the same depth. Jet instability is considered to supply energy to the jet flutter. When water was deep, sloshing without the ridge was induced in the fundamental mode of the main section. The frequency was about the same as the eigenvalue of stagnant fluid, which was affected little by the steady circulating flow. The coupled effect with the level oscillation in the sub-section seems to play an important role in the sloshing growth. The paper discusses application to the fluid-structure interactions in the reactor vessel of a liquid metal fast breeder reactor under seismic influence