Domain partitioning; ‘exporting’ and ‘importing’

1. Domain partitioning; ‘exporting’ and ‘importing’ . Domain-partitioning reduces a large calculation to a succession of smaller ones • It is useful for computer simulation of flow phenomena. characterised by a predominant direction of flow, as for example when several chemical-plant vessels are connected in series. • A similar situation arises when it is necessary to simulate the flow over an extensive tract of terrain, for example a complete city or a wide forest. Partitioning is then possible because usually the direction of wind varies little from place to place. • Upstream partitions are simulated first; their results are ‘dumped’ as ‘export objects’ which are treated as ‘import objects’ by the next-downstream partitions. • The computations are carried out successively. • How this can be done with PHOENICS will now be explained.

2. Using transfer objects for import and export • The idea is simple to understand; but implementation has to be made easy for users. • Therefore ‘Transfer Objects’ have been introduced into PHOENICS by providing two new keywords for In-Form, namely:(EXPORT and (IMPORT • The first causes the PHOENICS solver module, EARTH, to write a transfer-object file at the end of its run; and the second causes EARTH to read such a file at the start of its run. • Transfer objects can be created by placing in the Q1 file an In-Form statement such as: (EXPORT in NAME_OF_TRANSFER_OBJECT at PATCH_NAME) , or (EXPORT in NAME_OF_TRANSFER_OBJECT at OBJECT_NAME) . • Some tests now follow.

3. Transfer-object tests, 1 • This 2D test of steady laminar convective flow shows how one gets the same answer whether one partitions the domain (B) or not (A) when the flow is uni-directional, and the Reynolds number is much larger than 1. • This is Library case 856 . • The variable is H1 .

4. Transfer-object tests, 2 • This 3D example shows partitioning in two directions. • It represents a steady atmospheric boundary layer with a point source of pollutant. • The results with (i.e. B) and without (i.e. A) partitioning are in close agreement. • Library case 858.

5. Transfer-object tests, 3 • This example concerns unsteady spread of a finite release of pollutant into the atmosphere. • With (bottom) and without (top) partitioning, the concentration distribution at a fixed time is much the same, • Library case 859.

6. Transfer-object tests, 4 • The partitions may be connected in more complex ways. • For example, the first might be used to compute the flow and heat transfer within, and the output from, a computer cabinet; then the second might comprise a computer room with several identical computers within it, • Or the first might be a room with a smoke-producing fire in it, the second the space around the building, and the third another room into which smoke enters through open windows. • Both of these will be illustrated in what follows.

7. Transfer-object tests, 5 • Here is the result of computing the temperature distribution within, and the heat output from, a (highly idealised) computer cabinet. • Its output is ‘exported’ to its environment via transfer objects at its fan inlets and outlets. • The library case is 863.

8. The cabinet enlarged

9. Transfer-object tests, 6 • This is the result of the subsequent simulation of the temperature distribution in a room containing several identical computers. • Their effects are ‘imported’ via the ‘export’ objects of the previous calculation, • This is library case 864.

10. The computer room enlarged

11. Transfer-object tests, 7 • Now for the smoke-producing fire in a room. • It ‘exports its smoke through open windows. • This is library case 860. • It is treated as steady, which is not realistic; but it suffices to show how transfer objects can be used.

12. Transfer-object tests, 8 • The second computation shows how the smoke is ‘imported’ into the surroundings which ‘export’ some of it to other rooms in the building’. • This is library case 861.

13. Transfer-object tests, 9 • Simulation number 3 shows how the adjoining room ‘imports’ smoke through its open windows. • This is library case 862. • Of course, the simulation could have been carried out in a time-dependent manner; • And all the rooms in the building could have been treated in the same way. • Finally, if two-way interactions between rooms are suspected, it is necessary to iterate.

14. How to learn abouttransfer objects • Look in the PHOENICS Encyclopaedia. • Try the tutorial (It’s quite good). • Look in the library (you won’t find any, because the search engine has not yet been told what to look for!) • Create some examples for yourself. • Send them to CHAM for use by all. ------------ The End -------------------------