“Nested Iterations” option when monitoring mass conservation for gas mixture simulation (update)
“Nested Iterations” option when monitoring mass conservation for gas mixture simulation
If we need to have mass conservation for gas mixture simulation (with only initial condition defined but with no boundary condition specified), it is necessary to use “Nested Iterations” option:
These iterations allow treating mutual influence of flow parameters and non-linear terms in equations and boundary conditions and are incorporated within each time step and repeated until a solver convergence is obtained or until a Maximum number of nested iterations is reached or each conservation law.
It is possible also to adjust time step size and some parameters controlling maximum number of nested iterations and target residual values if necessary. This option activates different type of transient solver, which accounts properly transient compressibility effects and mass conservation for gas components (considering more terms in equation including dP/dt & dRho/dt).
For correct steady state results, we need to be sure that stationary solution exists for given conditions and this solution doesn’t depend on the initial condition. Otherwise, stationary solver will give incorrect oscillating solution, solution depending on initial condition (this is also incorrect), or even don’t give any converged solution at all. So, If the aim is calculating the mixing of two gases in stationary case, then stationary (using only initial conditions) set up is incorrect. This is because the task should have inlet and outlet boundary conditions. Without defining BCs, the solution will depend on initial condition and this is incorrect. Therefore, your task with mixing should be solved in transient.