[comingdaytime]

Dear CFDers,

I already know that if we wanna to construct the mean value of mixture fraction from LES simulation result, we can use the atomic mass fraction and use the mean value of the species mass fraction to calculate. Like it is indicated in the FLUENT user guide here:
https://www.sharcnet.ca/Software/Flu...ug/node627.htm

now here is the question:

what if i wanna to get the rms value of mixture fraction? I should use the rms value of the species and using the same equations for the mean calculation? I tried it in my code, but it seems that it doesn't match with the exp value at all. Therefore I think this method is not right. but is there another method except solving the transport equation of mixture fraction?

Thanks a lot

Litchy

[sbaffini]

Not an expert of mixture fractions here, but...

you can't use the same equation of course, you need to scale it properly (take a look here, https://en.wikipedia.org/wiki/Variance, under Basic properties, the third and fourth one). However, this only works for your linear scaling (and all the other parameters in the transformation are considered constant). More in general, once you have a time history of the instantaneous mixture fraction (either from its own equation or from any other mean), your best shot is to simply apply the definition of rms (and mean) to that time history.

Note that this obviously relates to the resolved mixture fraction only. the non resolved part, if needed, still requires a SGS model.

[comingdaytime]

Hey Paolo,

thanks for your reply, i'll take a look at the link you sent.

best

Litchy

[FMDenaro]

I think you could get help by reading the papers of Oefelein.
This one (Combustion and Flame 155 (2008) 54–69) is an example but you can find help in many others.