User sclar boundary condition.
 

Hi!Does anyone know wall boundary conditon for a UDS,thi, as:

d/dn(thi)=0 and d/dn(thi)=S;

According to the mannual you can only set:

1)thi=fixed value and

2)flux of scalar = fixed value

Since I can access grandiant of scalar through C_UDSI_G(c,t,i)[x] where x= 0,1,2;

but how should I set the gradiant of scalar as fixed value S?

or should I have to solve two scalar equations and couple them?

Does anyone know how to deal with this problem?

Re: User sclar boundary condition.
 

the flux of the scalar is d(Phi)/dn

Re: User sclar boundary condition.
 

yeah, thank you for correcting !

Re: User sclar boundary condition.
 

the UDS equation I am going to solve is a laplace equation.

the scalar thi represents Electric potential.

and convective flux is not to be computed and the equation as the UDF mannual is:

-d[(diff*(d(thi)/dxi)]/dxi=S ;

and at one wall ,I have to define the gradiant of scalar as a specified value,and how shall I resolve it?

any suggestions or comments would be welcomed!

Thanks in advance!

Re: User sclar boundary condition.
 

As i said in my previous reply, you just set d(Phi)/dn at the boundary.

For scalar equations, the boundary conditions are either specified value, specified flux, or mixed. It is confusing when u say "gradient of scalar as a specified value".

In any case, you can either specify the value of Phi or the flux of Phi at the given boundary. When you open the boundary conditions setting in Fluent, you will have the option to do that.

Hope that helps.

Re: User sclar boundary condition.
 

thank you ,Ynot!I see now!

Re: User sclar boundary condition.
 

The flux is not simply a gradient,

The flux is always defined as FLUX =-Diff*d(phi)/dn

Be carefull : n is the direction normal to your boundary and directed INTO the flow. Do not forget the sign (-)...

Do you have a constant flux, or is it a function of something else ?

Are you modeling some MHD flow?

Re: User sclar boundary condition.
 

thank you,Kharicha !

the flux should be : FLUX =-Diff*d(phi)/dn ;

the flux is not constant ,but a function of the internal conditions of the fluid zone;

by the way,what is MHD mean?

Re: User sclar boundary condition.
 

Magnetohydrodynamic=MHD!

So I wonder what is the purpose of your study if it is not MHD ?

Re: User sclar boundary condition.
 

Can you tell me what scalar you are defining with UDS.

Re: User sclar boundary condition.
 

not MHD,but fuel cell

Re: User sclar boundary condition.
 

Electronic potential,I've mentioned.