# User sclar boundary condition.

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 February 13, 2006, 04:18 User sclar boundary condition. #1 shuqin Guest   Posts: n/a 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?

 February 13, 2006, 11:15 Re: User sclar boundary condition. #2 Ynot Guest   Posts: n/a the flux of the scalar is d(Phi)/dn

 February 14, 2006, 05:14 Re: User sclar boundary condition. #3 shuqin Guest   Posts: n/a yeah, thank you for correcting !

 February 15, 2006, 03:56 Re: User sclar boundary condition. #4 shuqin Guest   Posts: n/a 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!

 February 15, 2006, 12:32 Re: User sclar boundary condition. #5 Ynot Guest   Posts: n/a 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.

 February 16, 2006, 02:41 Re: User sclar boundary condition. #6 shuqin Guest   Posts: n/a thank you ,Ynot!I see now!

 February 16, 2006, 03:47 Re: User sclar boundary condition. #7 Kharicha Guest   Posts: n/a 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?

 February 17, 2006, 01:00 Re: User sclar boundary condition. #8 shuqin Guest   Posts: n/a 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?

 February 17, 2006, 02:06 Re: User sclar boundary condition. #9 Kharicha Guest   Posts: n/a Magnetohydrodynamic=MHD! So I wonder what is the purpose of your study if it is not MHD ?

 February 17, 2006, 07:03 Re: User sclar boundary condition. #10 Arvind Phate Guest   Posts: n/a Can you tell me what scalar you are defining with UDS.

 February 19, 2006, 23:01 Re: User sclar boundary condition. #11 shuqin Guest   Posts: n/a not MHD,but fuel cell

 February 22, 2006, 21:58 Re: User sclar boundary condition. #12 shuqin Guest   Posts: n/a Electronic potential,I've mentioned.

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