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Why is “Passive Region” Used?

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Old   May 19, 2024, 07:42
Default Why is “Passive Region” Used?
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Hello everyone,

In Converge CFD software, there is a section called “passive”. In particular, it is used in the G-equation combustion model, one of the combustion models, and I don't understand exactly what it is used for. Can anyone explain this clearly?

Other ICE modeling programs (such as Forte) do not do this. Can anyone explain comparatively why we need to model it this way in the G-equation combustion model in Converge CFD software?
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Old   May 19, 2024, 13:09
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Killian Whyte
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Hi Ibrahim, and thanks for your question.

The G equation combustion model is a simplified combustion model in which the flame-front and burned mass are assumed not to affect the material properties of the fluid it is in. This assumption allows us to de-couple the transport equation for G from the other transport equations for momentum, energy, etc.
In other words, while the main physical quantities like momentum and energy affect the transport of G, there is no feedback in the other direction, where G affects the energy and momentum. That is to say, the results of the simulation for velocity, temperature, pressure, etc would look exactly the same whether or not we activate the G equation model.
In CONVERGE, and in mathematics and science in general, a physical quantity such as G which is affected by, but does not affect, the momentum, is known as a *passive*.
I do not know the specifics of how Forte talks about the G equation, but the assumption that G is passive is fundamental to the model itself, whether or not Forte chooses not to call it a passive. The reason I would assume they do not call it a passive is because it does not obey a typical advection-diffusion equation - rather, not only is a diffusive term not present, but due to the contribution of the laminar flamespeed, there is also an appearance of a Hamilton-Jacobi like term on the right hand side.
I would suggest looking at this short presentation which derives the G equation and also getting a copy of the CONVERGE manual from to see how it is implemented in our software.
Please let me know if you have more questions. Best regards,
-Killian (Applications Engineer at Convergent Science)
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