BES Fluent

Instability of Rayleigh-Benard

II. Comparison between steady and unsteady options

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 x-direction 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.

1. Velocity magnitude
Velocity profiles

2. Temperature
Temperature profiles

3. Pressure

Pressure profiles


4. Results
We had compared different profiles to be sure they were identical.


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.