PhD Thesis - staffweb - University of Greenwich

PhD Thesis by John Ewer.performs in the various groups. For instance, the maximum number of iterations in an "Inactivegroup" will be considerably smaller than the number for an "Active group". As the solutiondevelops, cells can migrate to and from groups, thus receiving more or less computationalattention. The overall convergence criteria are still configured as for conventional problems sothere should be no significant difference in the quality of the converged solution.Group solvers are a novel feature of the CFD component introduced during the software reengineering.In traditional CFD codes, solver type and control apply to all the cells in thesolution domain. Group solvers allow the solution domain to be split into a collection of groupsof-cells.A group is defined as a collection of cells that has its own independent controlparameters. A group solver is used for a particular variable on a particular sub-region of thedomain. The group solver makes use of standard low-level numerical solution matrix solvermethods such as JOR or SOR.There are several different criteria which may be used to determine the cell groupings."Geometric groups" have membership with cells grouped by geometric location (e.g. a near wallgroup, a fire group or a "dead" region group). Such geometric groups are intended to keep theircell membership throughout a simulation. Conversely cells may be dynamically assigned togroups whose membership may change during the solution process. These are so called"dynamic membership groups". The membership assignment process is triggered by preconfiguredselection criteria which are dependent on the magnitude of particular variables. Foreach group, there is a lower and upper value of the trigger-variable(s) which define anacceptance band for membership of that group. When the chosen value(s) in a cell comply withthe entry criteria, the cell will be transferred to the matching group.Typically one could define four base groups for dynamic membership, namely: "Active","Moderate", "Inactive", and "Void". "Active" has the upper value range for flow or heat,"Moderate" has the medium value range and "Inactive" has the lowest value range. "Void" isused for areas in the geometry that are not part of the flow domain and there is a fixed constantvalue of the variable (e.g. regions that have been meshed for convenience but are not part of theflow domain for all, or part, of the simulation) that does not require iterative re-calculation.7-103