  The goal of this work is to follow the displacement and possible deformation
of a free particle in a fluid flow in 2D axi-symmetry, 2D or 3D using the
classical finite elements method without the usual drawbacks finite elements
bring for fluid-structure interaction, i.e. huge numerical problems and strong
mesh distortions. Working with finite elements is a choice motivated by the
fact that finite elements are well known by a large majority of researchers and
are easy to manipulate. The method we describe in this paper, called the camera
method, is well adapted to the study of a single particle in a network and most
particularly when the study focuses on the particle behaviour. The camera
method is based on two principles: 1/ the fluid structure interaction problem
is restricted to a neighbourhood of the particle, thus reducing drastically the
number of degrees of freedom of the problem; 2/ the neighbourhood mesh moves
and rotates with the particle, thus avoiding most of the mesh distortions that
occur in a standard ALE method. In this article, we present the camera method
and the conditions under which it can be used. Then we apply it to several
examples from the literature in 2D axi-symmetry, 2D and 3D.
