BES Fluent
Instability of RayleighBenard
At first, we want to study the difference between
the steady option and the unsteady one.
We take the same boundary conditions and the same initial
conditions. (Difference of temperature between the two walls = 0,2 . Initial
uniform field of temperature = 305 K)
The boundary conditions impose the formation of two contrarotative
rolls.
During the unsteady simulation, we iterated a very long time to obtain identical results between steady and unsteady simulations. (Time simulation = 1075 s)
Along this study, we always compared the two results obtained in parallel.
The only one problem we had, it was that the two rolls
obtained with unsteady are not turning in the same way than with steady
option.
This is possible because, in theory, we have an infiny
domain in xdirection and the simulation results must give an infinity
succesion of contrarotative rolls turning in the two ways.
If you put a window only on two consecutive rolls, you
could obtain two sorts of rotative directions.
Velocity profiles1. Velocity magnitude




Temperature profiles2. Temperature




3. Pressure




We had compared different profiles to be sure they were identical.4. Results
All plots and all profiles of velocity, temperature and pressure are identical.
We could see that, for this example, the two methods gave the same results.
But we had to iterate a very long time with unsteady option to find exactly the same results.
So, we recommend to use unsteady option only to study
the transitional time of a phenomenon.