Fluid-coupling coefficients in an array of hexagonal prisms

The study is related to the seismic analysis of fast breeder reactor cores. The first part of the paper describes an efficient method to compute the added mass coefficients in an array of slender bodies immersed in a fluid. It is assumed that the fluid is incompressible and non-viscous and that the flow is bidimensional. The Finite Element model uses one-dimensional elements with parabolic pressure field (three nodes per element) in the areas with small gaps, and two-dimensional elements elsewhere. The method is applied to an array of hexagonal prisms. The second part of the paper deals with the experimental determination of fluid-coupling coefficients in an array of hexagonal prisms. Both inertial and viscous couplings are investigated. It is found that the fluid damping depends critically on the gap size and that the cross-damping terms are small. The added mass coefficients are consistent with the numerical results of the 2D model. They are, however, 20 to 25% smaller, which is likely to result from axial leaks in the tests. Finally, the equation of motion of a seismically fluid-coupled system is briefly discussed. Some difficulties arising from the fluid are pointed out.