CFX enables you to create solid regions in which the equations for heat transfer are solved. This is known as conjugate heat transfer, and the solid regions are known as solid domains.
Within solid domains, the conservation of energy equation can account for heat transport due to solid motion, conduction, and volumetric heat sources:
 | (1–128) |
where 
, 
, and 
are
the enthalpy,
density, and thermal conductivity of the solid, respectively, and 
is an optional volumetric heat source.
The reference frame for a solid domain is determined by the Domain Motion setting in CFX-Pre, and can be either Stationary or Rotating. In equation Equation 1–128, none of the terms depend on the reference
frame. As a consequence, the Domain Motion setting
has no effect on the solution field in a solid domain. However, this
setting is still required in some situations, as shown in the examples
presented below.
The term that has the solid velocity 
is an optional term that accounts for motion of
the solid with respect to the reference frame, and corresponds to
the Solid Motion setting in CFX-Pre. For details,
see Solid Motion in the CFX-Pre User's Guide. This is an advection term applied to the temperature field. The
solid domain mesh does not actually move, but because the temperature
field is moved, this setting can be used to model solid motion that
is tangential to the solid boundaries (you cannot advect the temperature
field through a solid boundary and outside the solid domain). Examples
of when solid motion can be used include:
A continuous sheet of material moving along a conveyor belt
A material being continuously extruded
A solid that rotates about its symmetry axis
In each of these examples, the solid domain must have the form of a simple extruded or revolved volume to satisfy the tangential motion condition.
A rotating solid that is adjacent to a fluid domain can be modeled in different ways:
If the rotating solid is connected to a rotating fluid domain, and both have the same rotational speed (such as for a turbine blade case), then the Domain Motion setting for both domains should be set to
Rotating. Solid motion should not be used in this case. In addition, if a periodic sector is used for the fluid and solid domain, and the periodic boundaries do not line up circumferentially, a Frozen Rotor interface should be used.Note that, in this case, you can also set the solid Domain Motion setting to
Stationarybecause Equation 1–128 continues to apply even when the solid Domain Motion setting is changed. Although this is a valid setup, it can be confusing, and CFX-Pre will issue a warning. You can ignore the warning message for this specific case, but for clarity it is recommended that you set the solid Domain Motion setting toRotatingand specify the same rotational velocity as for the adjacent fluid domain.If the rotating solid is a body of revolution (that is, the fluid solid interface is a surface of revolution), such as a solid disk spinning in a fluid, and the fluid domain is stationary, then there are two valid approaches:
For the solid domain, you can set the Domain Motion setting to
Rotating. As was stated above, this setting does not change the equation solved in the solid domain, but it makes it clear that a frame change model is required to account for the reference frame change. If the Frozen Rotor model is used, then the solution will be the same as for a stationary solid. If there is non-axisymmetric flow in the fluid domain, such as a hot jet impinging on the disk surface (not at the axis of rotation) then a transient simulation with a transient rotor stator interface should be used, which is equivalent to rotating the solid domain mesh as the solution proceeds. The hot jet will effectively "see" a different part of the solid disk as the simulation progresses.For the solid domain, you can set the Domain Motion setting to
Stationary, then select the Solid Motion option and specify a velocity to account for the rotation of the solid. This can be done with a steady-state solution because it will activate the term involving
as mentioned above. In this case, the hot jet will
always "see" the same part of the solid disk, but the
temperature field will be advected around the solid disk to account
for the solid motion. When a rotating solid has a stationary heat
source applied, such as a brake disk where the brake pad is modeled
as a heat source, it is best to use this approach because it is computationally
less expensive than a Transient Rotor Stator approach. A hot jet impinging
on the disk can also be considered a stationary heat source.
Note that applying a Domain Motion setting
of Rotating to a solid domain, or adding solid
motion, will not have any effect on the fluid momentum equations,
and therefore it is necessary to specify a rotational wall velocity
on the fluid side of a domain interface whenever a solid is rotating
in a stationary fluid domain (that is, whenever there is a frame change
across the fluid-solid interface).
Additional information on plotting variables at a solid-fluid interface is available in Solid-Fluid Interface Variable Values in the CFX Reference Guide.