Topology change occurs before the time-increment and hence the oldest time
field (old-time in the case of 1st order time schemes, old-old-time in the case
of 2nd-order time schemes) is not actually needed as it is replaced by the
current time-field after time-increment so there is no purpose to mapping this
field. However, it is necessary to keep track of the existence of the
oldest-time field to ensure the correct number of old-time fields are cached for
the time-scheme. This development allows fvMesh to delete the redundant
oldest-time fields in such a manner that GeometricField can reinstate them
correctly after time-increment which is more efficient and more reliable than
attempting to map them and done previously.
Additionally fvMesh movement, which occurs after time-increment, now ensure all
old-time fields are up-to-date before NCC stitcher mapping so that both fields
and their old-time values are mapped consistently. This removes the need for
old-time field caching calls in MapGeometricFields, fvMeshAdder and
fvMeshStitcher, thus simplifying the code and improving maintainability.
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