Command Default

Although the command is not required for solution, if you do not issue VFUP, view factors are not updated at the substep level. Consequently, results of thermal radiation modeling may be inaccurate because they do not account for changes in the view factors of radiation surfaces as they deform or move.

Notes

Deformation and/or motion of radiating surfaces during an anlysis can cause inaccuracies in radiation calculations if the view factors are not updated to account for these changes. Issue VFUP,DEFINE,ON to enable view factor updates at the substep level and improve simulation accuracy for an analysis that includes all of the following conditions:

Updates occur only if all of these conditions are met:

When view factor updating is enabled (VFUP,,ON), radiation facet areas are recalculated along with the view factors according to the specified criteria. To specify the updating criteria, you must re-issue the VFUP command and choose one of the following:

Controlling the frequency of view factor updates

The full solution method (THOPT,FULL) with radiosity has three nested loops, which occur for each load step: the substep (or time step) loop, the multipass loop, and the Newton-Raphson (NR) loop. Displacements and temperatures, in addition to any other DOFs that may be included in your coupled-field analysis, are computed in the NR loop. The view factors / radiation areas can be recalculated at different points during the solution, as depicted in Figure 33: View Factor Update Timing. There are three timing strategies, Explicit, Implicit, and Implicit-1, and the implications of each timing strategy are detailed in the following discussion.

Figure 33: View Factor Update Timing

As seen in the figure, implicit updates occur within the multipass loop, and updated view factors are used in the current substep. Instead, explicit updates occur at the end of the time step, and updated view factors are used in the next time step. An additional update occurs before entering the substep loop for a restart (ANTYPE,, RESTART) or if you have imposed initial conditions on the displacements (IC) as indicated.

There is a trade-off between accuracy and computational cost as updating the view factors at the substep level improves accuracy but also increases the solution time. To control the timing of view factor updates and whether updates are based on geometric changes to the radiation facets due to kinematic motion or deformation during solution, you must choose one of these options:

There are three timing strategies:

Kinematic-based algorithm

For Opt = KNEX, KNIM, or KNI1, the kinematic-based criteria are used in the algorithm depicted below to test and update view factors.

Figure 34: Kinematic-based Algorithm

In most practical situations, S₂ will be found and the 2-surface test will be done. In some situations, when the facet only radiates to the environment, the 1-surface test is required. Two examples of when a 1-surface test is encountered are shown in the following figure. For a 2D square with four radiating surfaces around it, the 4x4 view factor matrix is zero, and only single surface tests will be done as the body translates (A in the figure). The second example (B) is two radiating surfaces, one fixed and one rotating. When the rotating surface reaches an 180 degree rotation, the view factor matrix becomes zero, and the 1-surface test will be encountered.

Figure 35: Examples Scenarios that Require a 1-Surface Test

Notes on kinematic-based and frequency-based updates

Kinematic-based updates (Opt = KNEX, KNIM, or KNI1) are sensitive to motion while frequency-based updates (Opt = FRQU) are not. For example, consider a 2 plate problem with total loadstep time = 100. Plate 1 is fixed and plate to moves only from time 10-20. Frequency-based updates will occur from time = 0 - 100, but kinematic-based updates will occur from time 10-20.

If an enclosure is undergoing rigid-body translation or rotation, and there is no change to the radiation facets:

When you have no knowledge about body deformation prior to the simulation, kinematic-based updates are a good option since view factor updates will only be triggered if the geometry of the radiation surfaces move or deform significantly.

For kinematic-based updating, when bodies are moving away from each other, updates occur less often. Vice-versa, when bodies move toward each other, updates occur more frequently.

Restrictions / limitations

Enclosure status change and penetration must be avoided  —  Simulations using the VFUP to update view factors at the substep level are invalid if an enclosure status changes or the surface of a radiating body penetrates the surface of another radiating body. Figure 36: Examples of Radiation Problems that Undergo an Enclosure Status Change illustrates some examples of enclosure status changes that must be avoided. For example, if two radiating bodies in close proximity penetrate each other due to thermal expansion during the analysis, their view factors become zero, and results using the VFUP command to update view factors are unpredictable and inaccurate. It is better to model bodies within close proximity using thermal contact radiation (contact and target elements: CONTA172, CONTA174, CONTA175, CONTA178,TARGE169, and TARGE170).

You must also be careful to avoid an enclosure status change, for example if movement or deformation in the analysis causes an open enclosure to become closed or vice versa. If an enclosure is undergoing a status change in your analysis, split the load step into multiple load steps such that:

Figure 36: Examples of Radiation Problems that Undergo an Enclosure Status Change

View factor Updates and Reciprocity  —  For explicit updates, view factors are recalculated at the end of the substep and applied in the next substep. Consequently, they violate the reciprocity rule in the next substep as the radiating areas change since the view factors were calculated from the areas of the previous substep. This may lead to energy imbalance issues. The same problem exists if you specify a substep (OPT2) or pass (OPT3) frequency other than one. Figure 37: View Factor Updates and Reciprocity illustrates this issue for different updating strategies. Reciprocity / row sum properties are only satisfied in substeps where view factors are updated in every pass (OPT3 = 1) provided you have used the VFSM command appropriately (see View Factors in the Theory Reference).

Figure 37: View Factor Updates and Reciprocity

Supported/restricted commands at the substep level with VFUP,DEFINE,ON

The following commands are not supported at the substep level:

The following commands may be used to query and print view factors or average view factors at the substep level:

The following radiation commands are valid at the substep level, and may be used with view factor updating enabled (VFUP,DEFINE,ON):

Example Usage

Example of a Structural-Thermal Analysis with Substep-Level View Factor Updating in the Thermal Analysis Guide

VM321 in the Ansys Mechanical APDL Verification Manual

Menu Paths