Hi,

I think you can try to enclose your present solid part using surfaces from all sides. Once you do that you can create a material point in the void space. I believe, now when you make a mesh, you get mesh for fluid and solid separate.

Or 

In the same way, enclose with surface and you can FILL operation available in ANSYS meshing.

//Once you do that you can create a material point in the void space. I believe, now when you make a mesh, you get mesh for fluid and solid separate.//

I have tried to do this, but when a fluid(or void) is created as a body by material point, it (newly created fluid) covers all over the domain(walls, solid and void part), in effect fluid now covers entire domain and solid covers just solid part( the latter part is desirable in my case)

I did not understand .."it (newly created fluid) covers all over the domain(walls, solid and void part)"

What you can try is make separate material point for solid(provided the solid is continuous) and fluid.

Try to mesh it with OpenFOAM setting in the snappyHexMesh one time the reference point inside the fluid- and the second time inside the solid-domain. So you can create two meshed that are geometrically conformal and couple it with an interface. The OpenFOAM format can be easily caonverted into the Ansys .msh format.

Franco

After X-ray tomography you have the freedom to select (by masking) your different density domain from histogram by selecting/specifying the grey values. In the present case I believe that you have 2 phases. Solid and air(open void)can be separated individually and simultaneously from the histogram. It is available in image processing softwares like scanip(pretty much sure) and in volumegraphics. After masking you have option to mesh your solid and air domain separately.