Improved species solver robustness for coarse grids

[EvertBunschoten]

Simulations of flames in large domains is difficult because volumetric source terms blow up in large cells, causing the species solver to diverge. I implemented a damping term which scales the species source terms in cells with a length scale above a user-defined threshold. This allows for the flame to propagate through regions of the domain with large cells (such as in the far-field) without diverging, while the source terms in refined regions remain unaffected.

To illustrate how this feature improves robustness, I ran a 2D simulation of a circular flame propagating from a refined region in the middle domain towards the edges where the mesh is significantly more coarse.

The image below shows the temperature field after 30 iterations as computed with the previous implementation (left), and the new implementation with a flame length scale specified as 1e-4 m (right). The solution is unaffected while the flame is within the refined region of the mesh.

However, the instance the flame reaches the coarser cells, the solver quickly diverges without damping. With the damping active, the flame propagates through the entire domain without an issue.

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[pcarruscag]

Hmmm but then you are not solving the same equations anymore.
If the issue is "propagation speed" it sounds like something should be done about the CFL.

[EvertBunschoten]

Hmmm but then you are not solving the same equations anymore. If the issue is "propagation speed" it sounds like something should be done about the CFL.

It is not about accurately modeling the propagation of the flame, it is just to allow the flame to propagate through the coarse regions of the domain without the solver diverging. For cells larger than the flame length scale, we can't accurately resolve the propagation of the flame front anyway because the diffusive time scales and reaction source time scales differ at those length scales.

[pcarruscag]

So is it possible to adjust the diffusion coefficients in those regions?
Numerical diffusion as a way of dealing with instabilities is a more established practice than disabling source terms.

[bigfooted]

I agree with Pedro, it is great that it doesn't blow up, but the solution is not physical anymore. You can look into 'thickened flame' models. In these models, the propagation speed is physically correct, but the flame is artificially thickened by increasing the diffusion coefficient.

[bigfooted]

It's mentioned in Poinsot & Veynante btw.