PhD Thesis - staffweb - University of Greenwich

PhD Thesis by John Ewer.satisfactory degree due to the low number of configured sweeps per time-step. The only set ofacceptable parameters used was from user specified set #1.2 which only failed to converge ontwo of the time steps and then only by a relatively small factor. User specified set #1.3 wasreasonably stable but failed to reach convergence in 25% of the time steps whereas userspecified set #1.4 had too much under-relaxation that caused the time steps to do very littleuseful processing with none of the time steps actually converging.Initial control setTABLE 6.2.1-3 : Processing timings for the various control regimes.Cumulativenumber of sweepsTotal processingtime (seconds)Non-convergedtime steps#1.1 Safe initial controlparameters#1.2 Expert user specifiedcontrol parameters#1.3 Expert user specifiedcontrol parameters#1.4 Expert user specifiedcontrol parameters#1.5 Expert user specifiedcontrol parameters10750 13641 0 of 1107728 9806 2 of 11010900 13831 28 of 11016329 20720 98 of 1103267 4145 110 of 110The configurations used above were selected by a small, but hopefully quite representative,group of researchers with various degrees of familiarity with the particular CFD engine used butall with considerable familiarity with CFD techniques in general or specific alternate CFD codes.This highlights another problem, which is the unique behaviour of different classes of CFD codeto the initial configurations. When the users were informed of the quality of the controlspecification, that they provided, most were surprised that the settings they would have used intheir usual CFD system did not work well in the prototype interactive system. This lack oftransferability of set-up knowledge means that users are forced to learn the idiosyncrasies ofeach new class of code (e.g. staggered mesh, unstructured) and the behaviour due to theparticular combinations of approximations, solvers, boundary condition handling and empiricaltechniques used by each CFD system.6-94