Normalized Phase Mass Conservation Error

This report is available only for Eulerian Multiphase (EMP) simulations. Monitoring the normalized mass conservation error allows you to assess the quality of the solution, identify the section with higher error in mass conservation, and define stopping criteria based on a specified conservation target.

In EMP simulations, numerical phase mass conservation error is related to the convergence of the simulation. If convergence is not fulfilled, mass might not be conserved for a specific phase and a mass error occurs. In unsteady simulations, adequate inner iterations within a time-step lead to good convergence and preserved mass. However, the necessary iterations per time-step for convergence can decrease as the simulation progresses. Thus, maintaining a fixed number of inner iterations per time-step throughout the simulation, is not necessary as it can impact the overall runtime. Instead, adaptive stopping criteria are recommended to account for the dynamic nature of convergence.

where

• ${\stackrel{\cdot }{m}}_{Imbalance}$ is the mass imbalance of the selected phase. Mass imbalance is computed when solving the transport equations. For more information, see General Transport Equation.
• $\mathrm{\Delta }t$ is the current time-step size (in steady simulations, $\mathrm{\Delta }t=1$ ).
• $m_{phase}$ is the mass of the selected phase.
• $\sum$ describes a sum over all cells of all regions belonging to the specified phase.

You can use the Normalized Phase Mass Conservation Error report to create the following:

• A monitor plot that displays the normalized phase mass conservation error throughout the simulation.
• An adaptive stopping criterion for automatically adjusting the number of inner iterations based on the value of the normalized phase mass conservation error to optimize the computational time of your EMP simulation.

The adaptive stopping criterion can be defined, for example, to stop the simulation when the desired minimum convergence level, or steady asymptotic values for mass conservation error are reached. You are advised to set a minimum value greater than zero for the criterion to avoid excessive number of inner iterations. Alternatively, you can use this criterion alongside another, for example, the maximum number of iterations, to optimize the runtime of your simulation.

To create an adaptive stopping criterion the general Simcenter STAR-CCM+ methods for defining monitors as stopping criteria apply. You can then assess how the adaptive stopping criterion changes the number of inner iteration with the iteration per time-step report.