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7.2.1 Operating Density

When the Boussinesq approximation is not used, the operating density ρ0 appears in the body-force
term in the momentum equations as (ρ − ρ0)g.
This form of the body-force term follows from the redefinition of pressure in ANSYS FLUENT as

ps0 = ps − ρ0gx
The hydrostatic pressure in a fluid at rest is then

ps0 = 0
(a) Setting the Operating Density

By default, ANSYS FLUENT will compute the operating density by averaging over all cells. In some
cases, you may obtain better results if you explicitly specify the operating density instead of having the
solver compute it for you. For example, if you are solving a natural-convection problem with a pressure
boundary, it is important to understand that the pressure you are specifying is p0s in Equation 13.2-3.
Although you will know the actual pressure ps, you will need to know the operating density ρ0 in order
to determine p0s from ps. Therefore, you should explicitly specify the operating density rather than
use the computed average. The specified value should, however, be representative of the average
value.
In some cases the specification of an operating density will improve convergence behavior, rather than
the actual results. For such cases use the approximate bulk density value as the operating density and
be sure that the value you choose is appropriate for the characteristic temperature in the domain.
Note that if you are using the Boussinesq approximation for all fluid materials, the operating density
ρ0 does not appear in the body-force term of the momentum equation. Consequently, you need not
specify it.

7.3 Shell Conduction Considerations

By default, ANSYS FLUENT treats walls as zero thickness presenting no thermal resis-tance to heat
transfer across them. If a thickness is specified for walls then the appropriate thermal resistance
across the wall thickness is imposed, although conduction is considered in the wall in the normal
direction only. There are applications, however, where conduc-tion in the planar direction of the wall
is also important. For these applications, you have two options: you can either mesh the thickness or
you can use the shell conduction approach. Shell conduction can be used to model thin sheets without
the need to mesh the wall thickness in a preprocessor. When the shell conduction approach is utilized,
you have the ability to easily switch on and off conjugate heat transfer on any wall. When you specify a
thickness for the wall, a material property, and enable Shell Conduction in the Wall dialog box, then
during the solution process ANSYS FLUENT automatically grows a layer of prism cells or hex cells for
the wall, depending on the type of face mesh that is utilized.

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