  Oscillatory instability of buoyancy convection in a laterally heated cube
with perfectly thermally conducting horizontal boundaries is studied. The
effect of the spanwise boundaries on the oscillatory instability onset is
studied. The problem is treated by Krylov-subspace-iteration based Newton and
Arnoldi methods. The Krylov basis vectors are calculated by a novel approach
that involves the SIMPLE iteration and a projection onto a space of functions
satisfying all linearized and homogeneous boundary conditions. The finite
volume grid is gradually refined from 1003 to 2563 finite volumes. A
self-sustaining oscillatory process responsible for the instability onset is
revealed, visualized and explained.
