[Gmsh] Meshing using attractor

[Julian Dean]

Hi gmsh, 

I’ve found a few comments on using attractors and thresholds, which have been helpful but my problem is the other way around to most. 

What i’d like to do is create a sphere in a box and mesh around the surface of the sphere with a controllable fine mesh that extends a little bit inside and outside the sphere.
Inside the sphere it should extend to be coarse, but not just all elements collapsing to a shared centre point. 
Outside the box a coarse mesh as well. 

I have used an attractor and threshold, stopping the threshold at the radius of the sphere but i’ve got a strange effect around the sphere now, where the mesh goes quite large, going back to the box mesh size by the time it gets to the boundary. Its not quite what i’m after as my simulation’s main information will be around the sphere, 

Any suggestions what i’m doing wrong? Should i be using a boundary layer - and if so how do you use such a meshing algorithm?

cheers

Julian

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