At the end of each run, a summary of global balances is reported in the CFX-Solver Output file. For details, see Global Conservation Statistics in the CFX-Solver Manager User's Guide. You can also monitor Flows, Forces and general CEL expressions during the run. In most cases, when the residuals have converged sufficiently, the global balances will be met. However, if there is a process whose time scale is very large relative to other physical processes, it is possible to have converged residuals before the global balances are met. An example might be the dissipation of heat from a cup of coffee into a large room.
Flow balances are evaluated while assembling the conservation equations, and are therefore based on the solution from the previous timestep. For transient runs, if convergence within a timestep is achieved in one coefficient iteration, this can lead to unexpected diagnostics for the accumulation term. By default, the initial guess within a timestep is taken from the end of the previous timestep. For the First Order Backward Euler transient scheme, this leads to an accumulation term of zero, and for the Second Order Backward Euler scheme, the accumulation term has the opposite sign from expectation. However, if the Second Order Backward Euler scheme is used with extrapolation as the initialization option, then the accumulation term is reasonable after one coefficient. For details about the initialization setting, see Transient Scheme.
A quantity may be produced in one region of a domain, and destroyed in another region of a domain. For example, an intermediate combustion species, may have a small overall source resulting from the difference of significant production and destruction. In order to handle this case appropriately, the positive and negative sources are accumulated separately.
In case a transport equation has production and destruction terms that contribute simultaneously at the same location, details of the implementation determine whether the positive and negative contributions are balanced separately, or whether just the local net source is balanced. In the latter case, the effective local source values are spread into the positive and negative domain source totals.
To ensure the global balances are met, you may additionally apply a target imbalance for the conservation equations (that is, a global balance criterion). In such a case, the solver will only stop before the maximum number of iterations if the residual criteria are met and global balances are also met. The value you specify is the fractional imbalance, where the default value is 0.01 (that is, 1%). The global imbalances for the hydrodynamic equations should at least be less than 1% before you can consider a solution converged.