Results of the unsteady simulation
The aim of this part is to define the time that a fluid needs to have an established flow. The results that are developped below are obtained with Fluent and as it is not a real experiment the results have to be manipulated with caution. They are not precise and are just an order.
It is going to be shown that transition time depends on the Prandtl number.
 
Transition time
The times of establishing are given in the following table.
 
Fluid
Prandtl
Transition time
1
7
350-400 s 
2
0.74
200-240 s
3
0.02
250-280 s
4
13460
7000-8000 s
5
100
750-800 s

It seems that the transition times increase when the Prandtl number is high. But they seem to tend to a certain value when Pr->0 whereas they quickly increase when Pr-> infinity. Anyway it is logical because a very viscous fluid ( so with a high Prandtl number) is physically hard to set off.