# Cell zone condition

Hello,

I am interested in simulating a conjugate heat transfer problem in Fluent and I am using Trelis to generate the mesh. For sake of simplicity, consider air flow through a pipe with another pipe located within it (see .jou below). The inner pipe has a constant thermal source term and the boundary condition on the inner wall must be coupled. For this type of problem, Fluent requires multiple cell zones. In this case, the first cell zone represents the fluid and the second zone represents the heated pipe. How does one create an additional cell zone in Trelis on the inner pipe surface (i.e., wall_inner)?

Thanks,
Markus

## CREATE GEOMETRY:

create Cylinder height 30 radius 5
create Cylinder height 30 radius 1
move Volume 2 x 0 y 1.5 z 0 include_merged
subtract volume 2 from volume 1

## CREATE MESH

curve 6 interval 20
curve 6 scheme bias factor 1.0
mesh curve 6
curve 2 interval 50
curve 2 scheme bias factor 1.0
mesh curve 2
surface 9 size auto factor 5
mesh surface 9
volume 1 size auto factor 5
mesh volume 1

## BOUNDARY CONDITIONS

create cfd_bc name ‘inlet’ velocityinlet on surface 9
create cfd_bc name ‘outlet’ pressureoutlet on surface 8
create cfd_bc name ‘wall_outer’ wall on surface 1
create cfd_bc name ‘wall_inner’ wall on surface 7

Is this the kind of thing you are looking for? I assume that if you want another cell zone that you actually want fluid in the heated pipe. In that case, we don’t want to completely remove solid with the boolean operation, we want to keep it around. However, Trelis doesn’t know which solid to keep so you have to make the determination of which one to delete after the boolean subtraction is performed. I used the compress operation from the command line just to keep my ids all starting at 1 after the delete operation. I would also presume that you will need to add new boundary conditions to the heated pipe.

reset

## CREATE GEOMETRY:

create Cylinder height 30 radius 5
create Cylinder height 30 radius 1
move Volume 2 x 0 y 1.5 z 0 include_merged
subtract volume 2 from volume 1 keep
delete vol 1
compress
imprint all
merge all

## CREATE MESH

curve 2 interval 20
curve 6 interval 50
volume all size auto factor 5
mesh volume all

## BOUNDARY CONDITIONS

create cfd_bc name ‘inlet’ velocityinlet on surface 7
create cfd_bc name ‘outlet’ pressureoutlet on surface 6
create cfd_bc name ‘wall_outer’ wall on surface 5
create cfd_bc name ‘wall_inner’ wall on surface 1

The solution to this problem is quite simple: in Fluent, cell zones refer to various regions within the domain that may represent different materials, regardless of the combination of fluid(s) and solid(s). Simply define the regions within Cubit/Trelis as blocks.