  Three-dimensional direct numerical simulations of an incompressible open
square cavity flow are conducted. Features of the permanent (non-linear) regime
together with the linear stability analysis of a two-dimensional steady base
flow are discussed. Spanwise boundary conditions are periodic and control
parameters set such that the shear layer is stable against Kelvin-Helmholtz
modes. Three branches of destabilising modes are found. The most destabilising
branch is associated with steady modes, over a finite range of spanwise
wavenumbers. The two other branches provide unsteady modes. Features of each
branches are recovered in the permanent regime: wavelength of the most powerful
spanwise Fourier mode, swaying phenomenon, angular frequencies, indicating that
modes of each branches are selected and interact in the permanent flow.
