During the solution process you can monitor the convergence dynamically by checking residuals. At the end of each solver iteration, the residual sum for each of the conserved variables is computed and stored, thus recording the convergence history. This history is also saved in the data file. The residual sum is defined below.
On a computer with infinite precision, these residuals will go to zero as the solution converges. On an actual computer, the residuals decay to some small value ("round-off") and then stop changing ("level out"). For "single precision" computations (the default for workstations and most computers), residuals can drop as many as six orders of magnitude before hitting round-off. Double precision residuals can drop up to twelve orders of magnitude. Guidelines for judging convergence can be found in Judging Convergence.
After discretization, the conservation equation for a general variable φ at a cell P can be written as
 | (40–169) |
Here aP is the center coefficient, anb are the influence coefficients for the neighboring cells, and b is the contribution of the constant part of the source term Sc in S = Sc + SPφ and of the boundary conditions. In Equation 40–169,
 | (40–170) |
The residual Rφ computed by Ansys Icepak is the imbalance in Equation 40–169 summed over all the computational cells P. This is referred to as the "unscaled" residual. It may be written as
 | (40–171) |
In general, it is difficult to judge convergence by examining the residuals defined by Equation 40–171 since no scaling is employed. This is especially true in enclosed flows such as natural convection in a room where there is no inlet flow rate of φ with which to compare the residual. Ansys Icepak scales the residual using a scaling factor representative of the flow rate of φ through the domain. This "scaled" residual is defined as
 | (40–172) |
For the momentum equations the denominator term aPφP is replaced by aP , where vP is the magnitude of the velocity at cell P.
The scaled residual is a more appropriate indicator of convergence, and is the residual displayed by Ansys Icepak.
For the continuity equation, the unscaled residual is defined as
 | (40–173) |
The scaled residual for the continuity equation is defined as
 | (40–174) |
The denominator is the largest absolute value of the continuity residual in the first five iterations.