# Find Iso surface to impose BC

**URL:** <https://community.freefem.org/t/find-iso-surface-to-impose-bc/83>\
**Category:** General Discussion\
**Created:** [June 25, 2019, 12:15pm UTC](https://community.freefem.org/t/find-iso-surface-to-impose-bc/83 "2019-06-25T12:15:06Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Jonas\_Zille](https://avatars.discourse-cdn.com/v4/letter/j/53a042/32.png) [@Jonas\_Zille](https://community.freefem.org/u/Jonas_Zille)\
**Post date:** [June 25, 2019, 12:15pm UTC](https://community.freefem.org/t/find-iso-surface-to-impose-bc/83/1 "2019-06-25T12:15:06Z")

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Hi fellow FreeFEM’er,  
I have a 3D mesh on which I solve a PDE (time=i), for the solution in the next timestep i+1, I need to impose additional boundary conditions on the iso surface through the 3d volume (time: i).  
How can this be handeled efficiently? (and preferably with methods supported by FreeFEM)

Thank your for your help in advance,  
Jonas

(I asked a similar question some time ago, but nobody could help me - maybe you can now? [Extract Iso Surface in 3D](https://community.freefem.org/t/extract-iso-surface-in-3d/36))

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**Author:** ![frederichecht](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/frederichecht/32/15_2.png) [@frederichecht](https://community.freefem.org/u/frederichecht)\
**Post date:** [June 26, 2019, 12:27pm UTC](https://community.freefem.org/t/find-iso-surface-to-impose-bc/83/2 "2019-06-26T12:27:40Z")

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Without math formulation No idea,

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**Author:** ![xlzheng](https://avatars.discourse-cdn.com/v4/letter/x/2acd7d/32.png) [@xlzheng](https://community.freefem.org/u/xlzheng)\
**Post date:** [June 27, 2019, 8:17am UTC](https://community.freefem.org/t/find-iso-surface-to-impose-bc/83/3 "2019-06-27T08:17:44Z")

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define a func f(x,y,z), and on the boundry, u=f.  
I don’t know if it help, maybe you could try

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**Author:** ![Jonas\_Zille](https://avatars.discourse-cdn.com/v4/letter/j/53a042/32.png) [@Jonas\_Zille](https://community.freefem.org/u/Jonas_Zille)\
**Post date:** [June 10, 2020, 9:45am UTC](https://community.freefem.org/t/find-iso-surface-to-impose-bc/83/4 "2020-06-10T09:45:04Z")

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mesh3 ThSol = trunc( Th3, T\>=Tsol\*0.7);  
fespace Uh(ThSol,P1);  
Uh Tiso;  
Tiso=T-Tsol;  
ThSol = mmg3d(ThSol,metric=Tiso[],iso=1,ls=0,hausd=0.003,hgrad=2.3,verbose=10);  
plot(ThSol,cmm=“ThSol mmg3d bdmeshed”,wait=1);   
ThSol = buildBdMesh(ThSol);   
meshS ThSSol = ThSol.Gamma;   
ThSSol = trunc(ThSSol,region==10);  
ThSSol = mmgs(ThSSol,hausd=0.003,hgrad=2.3);

Thanks, with the new version it works!!

Best regards,  
Jonas