The suggestions below are provided to help you enhance convergence for turbulent
Starting with excessively crude initial guesses for mean and turbulence
quantities may cause the solution to diverge. A safe approach is to start your
calculation using conservative (small) under-relaxation parameters and (for the
density-based solvers) a conservative Courant number, and increase them gradually as
the iterations proceed and the solution begins to settle down.
It is also helpful for faster convergence to start with reasonable initial
guesses for the
) fields. Particularly
when the enhanced wall treatment is used, it is important to start with a
sufficiently developed turbulence field, as recommended in
12.21, to avoid the need for an excessive number of
iterations to develop the turbulence field.
When you are using the RNG
model, an approach that might help you
achieve better convergence is to obtain a solution with the standard
model before switching to the RNG model. Due to the additional
non-linearities in the RNG model, lower under-relaxation factors and (for the
density-based solvers) a lower Courant number might also be necessary.
Note that when you use the enhanced wall treatment, you may sometimes find
during the calculation that the residual for
is reported to be zero.
This happens when your flow is such that Re
is less than 200 in the entire
flow domain, and
is obtained from the algebraic formula
12.10-25) instead of from its transport equation.