Cell Reynolds number in Multiphase

Zaktatir · February 9, 2011, 05:30

How can i find the cell Reynolds Number in multiphase problems.

I need that number ( Reynolds Number) in order to calculate the interfacial friction coefficient (free surface flows)

Thanks Jessica

ghorrocks · February 9, 2011, 06:37

Simply define a user variable with the definition of Cell Reynolds number in it. You can do this in CFD-Post providing you included enough variables in your output file.

Zaktatir · February 9, 2011, 06:59

i don t need this in post.. i need it in CFX PRE in order to calculate locally this friction coefficient.

ghorrocks · February 9, 2011, 19:24

It will work in the solver as well. What do you want to do with it? Look at it in the results file or use it to calculate some flow thing?

Zaktatir · February 10, 2011, 04:34

Please how shall i write it in piece of which hydraulic diameter? I need this as said already two times before, to calculate the friction coefficient on the interphase. My Problem is this hydraulic diameter pr any specific length .

By means oh that coefficient i can furthermore estimate the drag coeffcient for the mixture model.

ghorrocks · February 10, 2011, 05:13

There is no universal definition of cell Reynolds number. Also, cell Reynolds number is usually used to assess the accuracy of numerical schemes (ie Peclet number) so I have no idea why you are trying to use it for a physical model.

Zaktatir · February 10, 2011, 05:20

then i need any kind of Reynlods number forget about the cell one.

ghorrocks · February 10, 2011, 07:01

I think you need to read up about Reynolds Number. Time to get out the Fluid Mechanics textbook, or even wikipedia.

Zaktatir · February 10, 2011, 07:52

??? i am only asking.. do you think i dunno the definition of the Reynolds number.

I am making my Phd MR know better than you..

I am discovering that if you don t have a reply to a question you start to think that the others are beginners or something...

thanks for your reply, however.

Jessica: Engineer at RWE Power.

ghorrocks · February 10, 2011, 18:07

Puzzling...

Anyway, Reynolds number is simply Density*Velocity*Length/Viscosity. You can use any definition of density, velocity, length and viscosity you like. For flow over a cylinder taking the diameter as the length scale and the free stream velocity as the velocity makes sense. For flow over a plate, a length scale of the length along the plate makes sense.

So there is no universal definition of Reynolds number. It has to be defined in context with what you are doing. And comparisons of Reynolds number where the definitions are different is meaningless.

February 9, 2011, 05:30	Cell Reynolds number in Multiphase	#1
Zaktatir Senior Member Join Date: Jan 2010 Location: Germany Posts: 268 Rep Power: 17	How can i find the cell Reynolds Number in multiphase problems. I need that number ( Reynolds Number) in order to calculate the interfacial friction coefficient (free surface flows) Thanks Jessica

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February 9, 2011, 06:37		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	Simply define a user variable with the definition of Cell Reynolds number in it. You can do this in CFD-Post providing you included enough variables in your output file.

February 9, 2011, 06:59		#3
Zaktatir Senior Member Join Date: Jan 2010 Location: Germany Posts: 268 Rep Power: 17	i don t need this in post.. i need it in CFX PRE in order to calculate locally this friction coefficient.

February 9, 2011, 19:24		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	It will work in the solver as well. What do you want to do with it? Look at it in the results file or use it to calculate some flow thing?

February 10, 2011, 04:34		#5
Zaktatir Senior Member Join Date: Jan 2010 Location: Germany Posts: 268 Rep Power: 17	Please how shall i write it in piece of which hydraulic diameter? I need this as said already two times before, to calculate the friction coefficient on the interphase. My Problem is this hydraulic diameter pr any specific length . By means oh that coefficient i can furthermore estimate the drag coeffcient for the mixture model.

February 10, 2011, 05:13		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	There is no universal definition of cell Reynolds number. Also, cell Reynolds number is usually used to assess the accuracy of numerical schemes (ie Peclet number) so I have no idea why you are trying to use it for a physical model.

February 10, 2011, 05:20		#7
Zaktatir Senior Member Join Date: Jan 2010 Location: Germany Posts: 268 Rep Power: 17	then i need any kind of Reynlods number forget about the cell one.

February 10, 2011, 07:01		#8
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	I think you need to read up about Reynolds Number. Time to get out the Fluid Mechanics textbook, or even wikipedia.

February 10, 2011, 07:52		#9
Zaktatir Senior Member Join Date: Jan 2010 Location: Germany Posts: 268 Rep Power: 17	??? i am only asking.. do you think i dunno the definition of the Reynolds number. I am making my Phd MR know better than you.. I am discovering that if you don t have a reply to a question you start to think that the others are beginners or something... thanks for your reply, however. Jessica: Engineer at RWE Power.

February 10, 2011, 18:07		#10
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	Puzzling... Anyway, Reynolds number is simply DensityVelocityLength/Viscosity. You can use any definition of density, velocity, length and viscosity you like. For flow over a cylinder taking the diameter as the length scale and the free stream velocity as the velocity makes sense. For flow over a plate, a length scale of the length along the plate makes sense. So there is no universal definition of Reynolds number. It has to be defined in context with what you are doing. And comparisons of Reynolds number where the definitions are different is meaningless.