[Gmsh] Non-uniform mesh in a multi-STL geometry

[Einat Lev]

Hello all
I have a GEO file which merges two STL surfaces, which together create a single-surface closed volume.(following the helpful advice given in http://geuz.org/pipermail/gmsh/2010/005275.html)
gMsh has no problem making a 3D volume mesh for the combined surface.

What I'm trying to do now is to have the volume mesh fine near one surface (the outer shell of the volume) and coarser in the interior.
I followed instructions for using an Attractor field and a Threshold field and then a resulting field that takes the minimum. This strategy have worked for me before for simple surfaces created inside gMsh (e.g. a pipe, a box -- see attached picture). But now when I am trying it for the STL model, I get errors saying that:
Error   : Cannot interpolate discrete surface

Is there any other way to do this sort of refinement?

All the best,
Einat 

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