Fix d s_hat

[emaberman]

Proposed Changes

The current limiting of (\hat{S}) in the source term of the Spalart-Allmaras (SA) model follows the guidelines from "Modifications and Clarifications for the Implementation of the Spalart-Allmaras Turbulence Model" (Note 1, option c) source. This approach introduces two conditional branches for defining (\hat{S}). However, the derivative of (\hat{S}) used in the Jacobian is inconsistent with these branches.

We propose to modify this to ensure consistency between (\hat{S}) and its derivative in the Jacobian.

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[pcarruscag]

@emaberman are you planning to update the regression tests?

[emaberman]

@emaberman are you planning to update the regression tests?

yes. I haven"t forgotten about this, I will get to it..