Fluid and Energy Mass Balance

Mass balance errors are displayed in the postprocessor to give an indication of the accuracy with which the finite difference matrix equations are being solved. Mass balance error alone, however, may not be a valid indicator of simulation accuracy. A small mass balance error may not indicate a high degree of accuracy, but a large mass balance error is usually indicative of low accuracy. As an example, backward differencing in solution of the transport equation is mass conservative and can produce low mass balance errors, but for problems involving sharp temperature fronts this scheme will not be highly accurate because of the introduction of artificial numerical dispersion.

An error of 0% would indicate that mass is being perfectly conserved. As general rule, it is desirable to have the Total Simulation Mass Balance Error less than about 1%. Errors greater than that may indicate that the the matrix solver is not solving the matrix equation to the desired accuracy.

The mass balance error can usually be decreased by reducing the "Closure criterion for temperature" in the solver options under the "Model Options" tabbed dialog box. Other factors that may affect the mass balance are grid spacing and time step size. Reducing these values should lead to an improved mass balance.

There are situations when a relatively large mass balance error may not be indicative of solution error. This occurs when changing constant head boundaries for a recharge period. If a finite difference cell is changed from a no-flow condition (with pressure head h0) to a constant head condition (with pressure head h1), then if h0 is not equal to h1, the model sees this as an instantaneous change in head. Accompanying this head change will be an instantaneous change in water stored within this cell. Since this increase (or decrease) in storage was not accounted for in the previous time step (or by the initial conditions) the model treats it as a discrepancy in the total mass balance.

The most common occurrence of this apparent discrepancy is on the very first time step of a simulation when there is an inconsistency between initial and boundary conditions. Such an occurrence is manifested by a large mass balance error for the first time step. Users should be aware of this apparent discrepancy and should not be overly concerned about the mass balance error. NOTE, the same type of apparent discrepancy can occur in the Energy Mass Balance when fixed temperature boundaries are assigned in recharge periods.

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