Tagged: 2019 R3, cfx, Convergence Problems, fluid-dynamics, General, source-terms, Species/Reactions
January 25, 2023 at 7:17 amFAQParticipant
For details and an example see also Tutorial 16 “Reacting Flow in a Mixing Tube” of ANSYS CFX. https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v195/cfx_tutr/cfxReacDefiForm.html A source S is fully specified by an expression for its value . A source coefficient C is optional, but can be specified to provide convergence enhancement or stability for strongly-varying sources. The value of C may affect the rate of convergence but will not affect the converged results. If no suitable value is available for C, the solution time scale or time step can still be reduced to help improve convergence of difficult source terms. Important: C must never be positive. (If a positive C is specified, CFX will automatically change it to be negative so that the calculation remains stable.) An optimal value for C when solving an individual equation for a positive variable Phi with a source S whose strength decreases with increasing Phi is C = d S / d Phi Where this derivative cannot be computed easily, C = S / Phi may be sufficient to ensure convergence. Another useful formula for C is C = -rho / tau where tau is a local estimate for the source time scale. Provided that the source time scale is not excessively small compared to flow or mixing time scales, this may be a useful approach for controlling sources with positive feedback ( d S / d Phi > 0) or sources that do not depend directly on the solved variable Phi.
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