Improving solution for heat transfer with tetrahedral mesh
The solution accuracy for heat transfer prediction for laminar flow in channels with full tetrahedral mesh can be lower compared to a full hexahedral mesh. This is true even when the second order discretization scheme is used. This can be a problem in modeling complex geometries for heat transfer calculations where a full hexahedral mesh or even a boundary layer approach is not practical.
Fluent uses a cell-centered Finite Volume scheme and, in order to go
second order, one performs a solution reconstruction from the cell-center
towards the integration points (the cell's face-center). This reconstruction
uses the cell-center value and the reconstruction gradient of that variable
to compute the face center values (on either sides of any cell-face).
Well, the flow field can have discontinuities or even large local spatial variations,
so the local gradients can be big, and the reconstruction can fail to produce the
expected results. To 'fix' this problem one uses a limiter. The
limiter is built such that it satisfies the so called "maximum principle". Basically, the limiter will switch to a first order reconstruction any time it detects a 'discontinuity'.
Effectively, although the user specifies to
use a second order scheme, at least for temperature the limiter can switch the
scheme almost everywhere to first order, the scheme with extra dissipation.
The Fluent default limiter works fine for regular hex grids
but obviously not that well for tetrahedral meshes. To alleviate this problem,
a new limiter is implemented in Fluent 6.1, called "Multidimensional Limiter",
which does the same job of enforcing the 'maximum principle', but it will not introduce
extra dissipation for smooth flows.
Solution. Use TUI and type:
> sol <enter>
> set <enter>
> slo <enter>
and choose the MD limiter . The results for heat transfer with full tet mesh with this limiter will be closer to the solution obtained for full hex mesh.