Partie 5 : Generation of the two levels convection
 

Grid

The hypothesis is that the upper rolls need more room to developp, so we mqde a higher mesh.
 


 
 
 

Viscosity

/*user define function*/
/* udf pour viscosite*/

#include "udf.h"

DEFINE_PROPERTY(viscosite,cell,thread)
{
real nu;
real temp;

temp=C_T(cell,thread);

if (temp<300.5)
   nu=0.1;
else if (temp<300.75)
    nu=-0.396*temp+119.098;
else
    nu=0.001;
 
 

return nu;
}

Initial Velocity, |v|=1, delta T=1


 

If we simulate this case without any viscosity gradient, the convergence is to 2 rolls, one above the other.
 

Velocity

We can notice the developpement  of the lower roll and the motion of the fluid above, but still not one real roll.
 


 

 Velocity, |v|=10, delta T=1
 

Even with a greater velocity initialisation, the convergence is similar to the last one.

 Velocity, |v|=10, delta T=10
 

The solution could have been to take a bigger Rayleigh number, but , as we can see, the researched case is unstable and with a greater Rayleigh, the case converges to one single roll.

Velocity, |v|=1, delta T=5


 

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