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Old   February 14, 2004, 04:45
Default Grid size and combustion simulation
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versi
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In simulating chemcially reacting flow, if I do not want to resolve the chemical time scale, I use implict scheme, selecting the marching time step solely based on CFL number. My Question Is: Is there any restriction on the spatial grid size that is linked with chemical time scale similar to CFL? It seems that the chemical souce terms do not provide any spatial scale.
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Old   February 15, 2004, 12:09
Default Re: Grid size and combustion simulation
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RMG
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Waht kind of wall-treatment/approach are you using?
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Old   February 16, 2004, 03:04
Default Re: Grid size and combustion simulation
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versi
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My case is inviscid Euler equations. It does not concern with wall law.
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Old   February 16, 2004, 05:33
Default Re: Grid size and combustion simulation
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jdc
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Hi Versi,

Since chemical reactions occur on a local basic (eg the reaction rate depends only on the local mass fractions), the chemical reaction does not limit your time-step through a "CFL like" relation. However, you should bare in mind the following:

- even using an implicit scheme, your time-step should still be small enough to capture the relevant time-scale of your chemical reaction scheme.

- since the mesh resolution needs to be fine enough to correctly capture the reaction zone, the diffusion limit (called Fourier criteria) may also limit your time-step.

Have fun. Julien
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Old   February 16, 2004, 07:42
Default Re: Grid size and combustion simulation
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Salvador
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Everything depends on what are you studying. In combustion processes usually a lof of species and reactions are involved. Some of the reactions are very fast others are very slow, all this makes the treatment of the chemical source terms very stiff, which means that your time step is reduced ENOURMOUSLY. Usually an implicit scheme is performed for the chemical source terms to alleviate these symptoms. Again, everyhtnig depends, if you are using some equilibrium assumptions or infinitely fast chemistry, this restriction changes. In non-premixed combustion you can used pre-computed flamelets, for example. Usually, in RANS, hundreds of chemical iterations are needed for just one flow field step. Again, if you want to solve everyhting (DNS),usually chemical time scales are shorter than Kolmogorov (turbulent) scales, the reaction zone is thinner. Check you Damokhler and Reynolds numbers
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