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Mixing Determination

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Old   March 31, 2000, 16:20
Default Mixing Determination
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J.Spence
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Thanks to Sung-Eun Kim earlier. Would also like to know whether the cells Reynold's no. would give an indication of 'level of mixing' - higher no. greater level of mixing?!
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Old   March 31, 2000, 18:08
Default Re: Mixing Determination
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Sung-Eun Kim
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"Cell Reynolds number" is normally a reserved word for numeric guys meaning "Peclet number" (u*delx/alpha, where u is the mean velocity, delx the cell size, and alpha a diffusivity). Needless to say, Peclet number is not even remotely related to the level of mixing. I suspect you didn't mean this Reynolds number.

Turbulent Reynolds number (q*l/nu) is strongly related to turbulent mixing, where q is a turbulent velocity scale and l a turbulence length scale. In the frame work of k-epsilon model, turbulent Reynolds number can be written as as k*k/(nu*epsilon). One of many physical meanings we can attach to this turbulent Reynolds number is "the ratio of turbulent viscosity (diffusivity) to molecular viscosity". So you can make a general statement that turbulent Reynolds number has to do a lot with turbulent mixing. in FLUENT, you have "Turbulent Visclsity Ratio" as a postprocessing variable.

However, turbulent viscosity or diffusivity) is not a direct meansure of "mixedness" or level of mixing. It's just an entity that makes mixing possible "in our model". This can be seen in the diffusion term d/dx_j(nu_t*dc/dx_j)). Turbulent diffusion can occur only when you have concentration gradient. A direct measure of mixedness, a consequence of turbulent mixing (where turbulent viscosity plays a key role in our model), is reflected in the very concentrantion fields of the species or whatever scalar fileds you're modeing.
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