  Most of deterministic solvers for rarefied gas dynamics use discrete velocity
(or discrete ordinate) approximations of the distribution function on a
Cartesian grid. This grid must be sufficiently large and fine to describe the
distribution functions at every space position in the computational domain. For
3-dimensional hypersonic flows, like in re-entry problems, this induces much
too dense velocity grids that cannot be practically used, for memory storage
requirements. In this article, we present an approach to generate automatically
a locally refined velocity grid adapted to a given simulation. This grid
contains much less points than a standard Cartesian grid and allows us to make
realistic 3-dimensional simulations at a reduced cost, with a comparable
accuracy.
