Preparing computation

The computation of the chemical reactions and of the species transport can be executed in the coupled and decoupled mode with the fluid dynamics model.

The coupled mode in the backward coupling sense is activated, if at least one of two conditions is satisfied: either the checkbox effect on gas convection activated or endothermic or exothermic reactions are available. If the chemical model is coupled, then the execution of the chemical model can start either by selecting of the actual variable group Chemistry or Temperature or Convection or turbulent_Convection and pressing   button. In all cases the chemical model will be computed alternately with the treatment of the global heat transport. The convection will be iteratively computed each time after the chemical model is updated for all variable groups excepted Temperature. The decoupled chemical model is computed after the variables group Chemistry was selected and the item Computation > Start Computation is selected or also by clicking on   button.

Before the computation in the decoupled mode begin, first the divergence free solution for the fluid flow in the computational domain of the chemical model is checked. The balance of the mass fluxes is computed. If the mass conservation in the fluid flow satisfies to the conditions of the chemical model, then the chemical model assembly and execution start immediately.

The total mass flux balance error in all fluid domains where the chemistry should be executed, should be less than 1.e-10. Otherwise the program first is going to improve the mass balance. A wrong mass balance leads to the artificial effects as flow sources and sinks. These numerical effects are dangerous because they can be interpreted as effects of chemical reactions. On the other hand sometimes it is very difficult to get a deep convergence for the fluid flow. A typical situation is the gas flow in the complex environment by high Re and Gr numbers. Therefore a special hint is implemented in CrysMAS.

For solution of the divergence free flow field problem the SIMPLEC algorithm is utilized. The momentum equations are assembled once and the coefficients for the pressure correction equation are stored. Than the pressure correction equation is assembled block wise. The matrix equations for the unknown pressure correction are copied then from each numerical block into the global matrix. The boundary conditions for the pressure correction at the symmetry axis and at the outflow boundary are accounted in the the global matrix equation implicitly. The assembled pressure correction equation is passed to the selected solver in the dialog window Computation -> Numerical Parameters -> Forward dialog tab. Usually the GSSV direct solver is applied. The pressure correction equation is solved with precision allowed by the computer accuracy (remaining error about 1.e-15). Then the mass fluxes are corrected according to the SIMPLEC procedure. The remaining mass fluxes balance error corresponds to the precision of the pressure correction solution and has the same order of magnitude.

The corrected mass fluxes distribution corresponds to a slightly changed velocity field. The magnitude of the change is dependent on the residual error of the mass balance before the chemical model starts. Only mass fluxes will be changed by the correction, the velocity distribution remains not changed.

Only one step of the pressure correction procedure is required and the pressure correction equation should be solved once thanks to its linearity. As a result overall fluxes correction procedure takes only few seconds even for a large size 2D fluid dynamics problem.