[Karne de Boer]

Hi Guys

Sorry for the basic question, but I would like to know how the Reynolds number in the average scale information is calculated. It is for a pipe flow and I've calculated it to be a factor of 10 less then that shown in the average scale information.

Any Thoughts??
Have a great day
Karne

[rogbrito]

Hi Sir,

use the calculator in CFX Post and compute the average velocity in a section of the pipe. Compare with Reynolds number from CFX with yours. Give a look on this:

http://rapidshare.de/files/48238559/ReynoldNumber.pdf.html or

http://www.4shared.com/file/12861259...oldNumber.html

with kind regards,

Rogerio.

Originally Posted by Karne de Boer

Hi Guys

Sorry for the basic question, but I would like to know how the Reynolds number in the average scale information is calculated. It is for a pipe flow and I've calculated it to be a factor of 10 less then that shown in the average scale information.

Any Thoughts??
Have a great day
Karne

[ghorrocks]

I think CFX calculates the Re in the solver based a length scale from the cube root of the total volume and a velocity, density and viscosity from mass weighted averages in the domain. This is totally different to the definition you use when you calculate Re based on frontal area, inlet speed, density and viscosity. That's why the difference.

[Karne de Boer]

Thanks rogerio - I had already tried this and my predicted amount matches that calculated in post.

Thanks for the answer Glen - that explains why I cannot get the same answer. Do you know if this would influence the Schiller Nauman drag coefficient correlation which is based on the particle Reynolds number????

Have a great day
Karne

[rogbrito]

You´re welcome!

Originally Posted by Karne de Boer

Thanks rogerio - I had already tried this and my predicted amount matches that calculated in post.

Thanks for the answer Glen - that explains why I cannot get the same answer. Do you know if this would influence the Schiller Nauman drag coefficient correlation which is based on the particle Reynolds number????

Have a great day
Karne

[ghorrocks]

Particle drag is calculated from a Re number based on the particle area and local slip velocity. This is totally different to the overall simulation Re number reported in the out file. They are not linked in any way - this is all explained in the documentation, sounds like you should have a read of it.

[Karne de Boer]

Hi Glen

I agree with your answer totally and I don't know if you noticed it but I refered to the "particle reynolds number" in my post not the overall reynolds number.

I must share with you that I have read the documentation as well as other papers from which these are derived. On the basis of this documentation I don't quite agree with your statement

" Particle drag is calculated from a Re number based on the particle area and local slip velocity"

This is true for the Shiller Naumann drag coefficient correlation but not true for the Ishii-Zuber correlation.

I have been doing some investigation into this in a particular multiphase model that I am working on and couldn't work out why the surface tension had no effect on the "drag force" when using the Ishii-Zuber correlation. After doing some calculations I realised that the Ishii-Zuber correlation was being over-ridden by the spherical and cap limits.

I am doing some further investigation into this now.

Please correct me if I am wrong

Have a great day
Karne

[ghorrocks]

OK, no problem. I am no expert in particle drag models. It sounds like you are on the case and are getting in the details of how they work, good work.

[Karne de Boer]

Thanks Glen, lot's of reading to do