I have been examining the available drag models available and I have a number of questions. Your time in addressing these is most appreciated.
When examining the Schiller Naumann drag model implementation, the drag term is given by;
drag = 0.125*Cd*rho*M_PI*ds*ds*magUr*Ur
- why is the scalar term 0.125 and not 0.5?
When examining the Koch Hill drag model implementation, the Reynolds number term is given by;
Rep = ds/scale_*voidfraction*magUr/(nuf+SMALL)
- is scale_ = 2? or if not, what is this term?
- what is the SMALL term?
When examining the Koch Hill drag model implementation, there is a term scalar beta given by;
scalar beta = 18*nuf*rho*voidfraction*voidfraction*volumefraction/(ds/scale_*ds/scale_)*(F0 + 0.5*F3*Rep)
- is this the interphase momentum transfer coefficient?
- if so, why does it not take the form;
beta = 18*nuf*voidfraction*volumefraction/(ds*ds)*(F0 + F3*Rep) ?
When examining the DiFelice drag model implementation, the Cd term is given by;
Cd = sqr(0.63 + 4.8/sqrt(Rep))
- what drag coefficient correlation is this?
When examining the DiFelice drag model implementation, the drag term is given by;
drag = 0.125*Cd*rho*M_PI*ds*ds*pow(voidfraction, (2-Xi))*MagUr*Ur
- similarly to the question above regarding the Schiller Naumann drag correlation, why is the scalar term 0.125 and not 0.5?
- why is the exponent of voidfraction 2-Xi and not -Xi?
When examining the Gidaspow drag model implementation, there is a term KslLab, which is commented as the momentum exchange of the particle and given by;
KslLag = 0.75*((1-voidfraction)*rho*voidfraction*CdMagUrLag/(ds*Foam::pow(voidfraction, 2.65)))
- if so, why does it not take the form;
beta = 18*nuf*voidfraction*volumefraction/(ds*ds)*Cd*(Re/24)*pow(voidfraction, -1.65) ?
ahobbs | Wed, 08/27/2014 - 17:16
Reference for the Di Felice modification
Christophe, can you provide the reference that shows the modification to the Di Felice exponent to 2-Xi. Most of the Tsuji and Yu papers I've found use 1-Xi.
Thanks, Andrew
jtvanlew | Fri, 11/14/2014 - 22:52
KH Drag...
I spent quite some time looking at the Koch Hill Drag used in the cfdem code too. It is indeed the interphase momentum transfer coefficient -- the way it's written into the solver does match up, eventually, with the equations given in the paper:
Maureen S. van Buijtenen, Willem-Jan van Dijk, Niels G. Deen, J.A.M. Kuipers, T. Leadbeater, D.J. Parker, Numerical and experimental study on multiple-spout fluidized beds, Chemical Engineering Science, Volume 66, Issue 11, 1 June 2011, Pages 2368-2376, ISSN 0009-2509, http://dx.doi.org/10.1016/j.ces.2011.02.055.
I believe the cfdem team took some liberties with moving around voidfractions and volumefractions into different terms (e.g. voidfractions on Reynolds # or volumefractions on the F terms). But if you plug everything in and carry through some multiplications, they eventually match up.
the scale_ term is defined in your dictionary for KochHill. It's default 1 but you can set it to a value > 1 if you want. I'm not clear why you'd want to do this but I did track it down through the documentation as such.
the SMALL term is just a small value defined in the code that helps with the computer doing stable numerics (i believe).