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[FAQ]

The CFX solver is responding to a failure of the Newton Rhapson iterations to converge for temperature and pressure. These errors are triggered when instability is detected, and the underlying assumption is that the thresholds and iterations limits for T and P are set appropriately. This error can come up for any compressible fluid flow that is experiencing convergence difficulties. The key to resolution is in understanding the reason for the divergence. In migrating a combustion case from Fluent to CFX, or vice-versa, one potential source of instability is the direct transcription of Arrhenius rate coefficients. the conventions for the 2 solvers are different as outlined below: Activation Energy (E): Fluent — [J kmol^-1] CFX — [J mol^-1] Pre-exponential (A) Fluent [time^-1 (mol m^-3)^(n-1)] CFX [time^-1 (kg m^-3)^(n-1)] Direct transcription therefore often results completely different reaction set and it is this that leads to instability. On the subject of the parameters offered up by the solver, other solutions, such as 2040318, that discuss how to change these parameters. In the situation outlined above, however, the suggested change is inappropriate. There are 6 Newton Rhapson control parameters in total, defaulted as follows: Constitutive Relation Iteration Limit = 100 Constitutive Relation Temperature Criterion = 0.01[K] Constitutive Relation Under Relaxation = 1.0 Newton Pressure Criterion = 1[Pa] Newton Pressure Iteration Limit = 150 Newton Pressure Under Relaxation = 1.0 Of these, it is Constitutive Relation Temperature Criterion = 0.01[K] and Newton Pressure Criterion = 1[Pa] that are problematic for combustion. The error message about under relaxation gets triggered when these convergence criteria are unmet. For combustion, these defaults should be increased to 50 [K] and 100 [Pa] respectively, or even higher (about 1% of total P and T range). Underrelaxing to meet such strict criteria makes no sense, as normal temperature and pressure ranges, as well as oscillations, for combustion simulations can be very large.