Find Iso surface to impose BC

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,

(I asked a similar question some time ago, but nobody could help me - maybe you can now? Extract Iso Surface in 3D)

Without math formulation No idea,

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

mesh3 ThSol = trunc( Th3, T>=Tsol*0.7);
fespace Uh(ThSol,P1);
Uh Tiso;
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,