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This is a small, simple tutorial for testing the implementation of large, incomplete data bases. In this tutorial case, rStatAnalysis is used as post-processor and it uses a data base in memory, which is smaller than the number of snapshots on disk.
79 lines
2.9 KiB
Markdown
79 lines
2.9 KiB
Markdown
# Test large, incomplete data bases
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This tutorial tests the use of data bases (in memory), which are smaller
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than the number of snaphots in the data base (on disk). This distinction
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becomes important for cases with large meshes, a large number of snapshots,
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or cases with both of these properties. In such cases, the memory (RAM) of
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the computer might be too small to fit all snapshots.
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In such a case - with *M*, the number of snapshots in memory, being smaller
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than *N*, the number of snapshots on disk - we use the recurrence model
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designed for large, complete data bases.
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This tutorial is based on the *laminar flow over cylinder* tutorial. It is
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rather small, but is nevertheless used to demonstrate the use of the large,
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incomplete data bases feature.
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## Case description
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This case is run for a small amount of time, and then, the tool `rStatAnalysis`
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is used as a post-processing tool to compute the recurrence matrix [1].
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In this case, `rStatAnalysis` is run with a data base (in memory) smaller than
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the number of snapshots on disk.
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## Results
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If we monitor the log file written by `rStatAnalysis`, we fill find the line
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similar to the following:
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`Checking all 20 fields`
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This line tells us the total number of snapshots on disk, which will all be
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checked for existence and readabiltiy.
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Afterwards, when it is enured that all expected fields are present, the data
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base in memory is filled with fields read from disk. However, as the data base
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in memory is smaller than the number of snapshots on disk, the data base in
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memory will be incomplete.
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Thus, we will encounter a line similar to the following one, which tells us
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that a number of fields is being read.
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`Reading fields for 11 dataBase slots`
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When computing the recurrence matrix, all *N* snapshots need to be compared
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with each other. As the data base holds only *M* snapshots, with *M<N*, at
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some point, the snapshots in the data base will need to be replaced with
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other ones, wich have not been read at that point.
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Such an operation is indicated by a line as shown below, provided the switch
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`verboseVacate` is enabled in `recProperties`.
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` --> vacate dataBase : 5`
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After the recurrence matrix has been computed, the recurrence model prints
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the total number of reads-from-disk to the Terminal. This is shown below:
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`Nr. of reads from disk : 30; compared to a theoretical minimum of 20 reads.`
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This indicates, that the total number of reads-from-disk is necessarily
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larger that the number of snapshots. The total number of reads-from-disk
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depends on the relative sizes of *N* and *M*. However, it will always be larger
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than the theoretical minimum, which is achieved, when *M* is equal to *N*, i.e.
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each field is read only once.
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## Tested
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This collection of cases has been tested with:
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* OpenFOAM-5.0
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## References
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[1] T. Lichtenegger and S. Pirker. Recurrence CFD – A novel approach to simulate
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multiphase flows with strongly separated time scales. Chemical Engineering Science,
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153:394-410, 2016.
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