Introduction
At the beginning, the title of this BES was Phoenics, name of a
software used here to study the Rayleigh Benard convection. But
actually, most of the simulations described in this report were done
with Fluent. Consequently, we renammed this project BES Rayleigh
Benard. This is aiming to:
 study an example of dynamics fluid instabilty.
 use a CFD tool such as Fluent, i.e., simulate problems
with adapted parameters and, above all, understand and analyse the
results.
Before to start any simulations, we have to list the main
numerics parameters that influence the results :
 the size and the structure of the grid ; this will be
developped later.
 the number of sweep, i.e., iterations per slab ; thus high
value would make the simulation converge but , at the same time,
increase its period .
 the relaxation ; this is based on virtual time steps so as to
avoid computations oscillations . This enables us to choose the
dependance of a X(t) on X(tdt). Consequently, increasing the
relaxation means delaying the convergence but avoiding numerics
discrepancies. Moreover, a problem highly realaxed must have a lot
of sweeps not to stay too near the initial solution.
In all this project , the option PRESTO! of fluent
will always be used for all quadrilateral and hexahedral meshes .The
different numerics controls are :

PRESTO!
Linear interpolation is the default procedure for computing
the face pressure from cell pressures. For problems
involving large body forces and high pressure gradients, we
may wish to use PRESTO! Typical problems include flows with
high swirl numbers, highRayleighnumber natural convection,
high speed rotating flows and flows in strongly curved
domains. This may only be used with qudrilateral and
hexahedral meshes.

The fluid is liquid water. It is defined by :