2. Introduction

Many industrial components or structures are in contact with fluid media, which may also be in flow. These surrounding fluid environments disturb the vibratory characteristics of structures, in particular their modal characteristics. This action of the fluid on the structure results in fluid/structure coupling effects.

Here, the surrounding fluid medium is assumed to be incompressible, perfect and at rest (without flow). We will show that then, a structure that vibrates with a small amplitude by sharing a border with this fluid domain modifies the pressure field in the fluid at rest, and therefore experiences a fluctuating pressure force, proportional to its acceleration. The proportionality coefficient is a mass. It describes the inertial effect of the fluid on the structure: this is why we call this mass added mass of the fluid on the structure.

The fluid domain can also have fixed boundaries or free surfaces, but in what follows, we will not consider the action of gravity on the free surface, and therefore not the sloshing modes.

When several structures are in contact with the same fluid domain, when one of the structures starts to vibrate, it not only feels the inertia of the fluid, but it changes the fluctuating pressure field around the interfaces with the fluid of all the other structures. The forces that each one experiences are proportional to the acceleration of the vibrating structure: again, the proportionality coefficients are masses called coupled masses.

The added masses are established on the basis of the natural modes of the structures considered in the absence of fluid. We can then re-perform the modal analysis of the structures taking into account these added masses, and then possibly return to the finite element base (« physical » base).