Fixing relaxation factor for conserved turbulent variables in SST model

[C-Perron]

Proposed Changes

This fixes an issue introduced in #2525 where the under-relaxation factors for $k$ and $\omega$ were calculated incorrectly with the SST model.

Since the update for $k$ and $\omega$ is in terms of conserved transport quantities, their stored value must be multiplied by the local flow density when compared to the non-linear update. This was not the case previously, and the solver could overestimate the relative change and compute a much stricter relaxation factor.

This is particularly relevant to incompressible flow, such as the steam periodic flow tutorial case, where the density is several orders of magnitude. The consequence of this issue was a stalled convergence of the turbulence variables as discussed in #2594.

This issue did not affect problems using the SA model, which doesn't use a conserved quantity in its formulation, or where the normalized density is near unity.

Related Work

Related to #2525 and #2594

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[pcarruscag]

LGTM

[bigfooted]

Thanks! Can you update the residual values in the Testcases/*.py regression files with the new values as reported above in the failed regression tests?