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Convergence of User Defined Scalar
There might be a need to solve for User Defined Scalar(UDS) in Fluent6.1.22. It has been found that in some instances, using zero diffusivity for UDS leads to NAN. Also, first-order discretisation results in diverging of the run after initial convergence. Following suggestions might help in overcoming the problem.
1)Instead of zero diffusivity, use a very small number, say, 1e-12.
2)Use double precision solver.
3)Importantly "power law" discretisation helps in overcoming divergence of UDS equation.