why low reynolds number?

Lea · December 12, 2012, 05:13

Hey,

I simulate a water stream through a pipe with 20mm bore. I chose the realizable k-e Model and enabled enhanced-wall -treatment. In boundary conditions I chose the ke model with k=e=1 as turbulence specification. The incomming flow rate is 2 m/s.

I calculated that there must be a turbulent flow with a flow rate of 0,0755m/s so that 2 m/s should bring reynolds numbers over 2300. But the reynoldsnumbers are maximal about 800.

Maybe you can help me and tell me what I do wrong.

Thank you=)

Far · December 12, 2012, 06:07

which Reynolds numbers you are talking about?

Lea · December 12, 2012, 06:11

The reynolds numbers in the whole pipe

Far · December 12, 2012, 06:13

Where you are seeing them?

Reynolds number is a reference quantity and depends on the reference you are taking !!!

Lea · December 12, 2012, 06:19

I see them when I go to Results->Graphics and Animations->Contours and then Counters of Velocity-> cell Reynolds Number

Far · December 12, 2012, 06:25

I guess it is Peclet Number related to local cell velocity and cell length scale. No need to look on it.

LuckyTran · December 13, 2012, 23:12

Quote:

Originally Posted by Far

I guess it is Peclet Number related to local cell velocity and cell length scale. No need to look on it.

Pe=Re*Pr

It is not quite Peclet number but similar.

Quote:

Originally Posted by Lea

I see them when I go to Results->Graphics and Animations->Contours and then Counters of Velocity-> cell Reynolds Number

The cell Reynolds number is different than the Reynolds number of the pipe.

This is a local Reynolds number. It uses the local cell velocity, length scale and properties. The length scale is the cube root of the volume of the cell.

Far · December 14, 2012, 03:27

Quote:

Originally Posted by LuckyTran

Pe=Re*Pr

It is not quite Peclet number but similar.

The cell Reynolds number is different than the Reynolds number of the pipe.

This is a local Reynolds number. It uses the local cell velocity, length scale and properties. The length scale is the cube root of the volume of the cell.

Thanks

It was a long time when I read them in CFD course. Thanks again.

Lea · December 14, 2012, 05:02

Thank you both for your help=)

Lucky Train, can you tell me what I have to do to see the "real" reynolds number so that I can see if the flow is laminar or turbulent?

Far · December 14, 2012, 05:07

Real Reynolds number is what you have specified

Lea · December 14, 2012, 05:11

but I haven't specified the reynoldsnumber because that is my variable. I want to see how the reynoldsnumber changes when I have a different geometry.

Far · December 14, 2012, 05:13

Then calculate reynolds number from the formula

RodriguezFatz · December 14, 2012, 06:02

Re = density * velocity * length / viscosity.
Density and viscosity are material properties. Take them from the material you chose. Velocity is what you set as inlet velocity of your pipe.
"Lenght" is a typical length scale of your domain. For pipes one usually takes the diameter of the pipe. Now calculate.

What people here were trying to explain: Fluent can't tell you "the Reynold's number" because there is no single Reynold's number. It allways depends on the length-scale you choose. You could also take the radius for a pipe. It's more or less arbitrary what you take...

Lea · December 18, 2012, 05:44

Thank you all for your help!

I found the solution: When you look on the turbulent reynolds number Re_y it is already turbulent with a value about 200.

So my flow is turbulent=)

December 12, 2012, 05:13	why low reynolds number?	#1
Lea New Member Join Date: Dec 2012 Posts: 10 Rep Power: 13	Hey, I simulate a water stream through a pipe with 20mm bore. I chose the realizable k-e Model and enabled enhanced-wall -treatment. In boundary conditions I chose the ke model with k=e=1 as turbulence specification. The incomming flow rate is 2 m/s. I calculated that there must be a turbulent flow with a flow rate of 0,0755m/s so that 2 m/s should bring reynolds numbers over 2300. But the reynoldsnumbers are maximal about 800. Maybe you can help me and tell me what I do wrong. Thank you=)

December 14, 2012, 06:02		#13
RodriguezFatz Senior Member Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 26	Re = density * velocity * length / viscosity. Density and viscosity are material properties. Take them from the material you chose. Velocity is what you set as inlet velocity of your pipe. "Lenght" is a typical length scale of your domain. For pipes one usually takes the diameter of the pipe. Now calculate. What people here were trying to explain: Fluent can't tell you "the Reynold's number" because there is no single Reynold's number. It allways depends on the length-scale you choose. You could also take the radius for a pipe. It's more or less arbitrary what you take... __________________ The skeleton ran out of shampoo in the shower.

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December 12, 2012, 06:07		#2
Far Super Moderator Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,553 Blog Entries: 6 Rep Power: 54	which Reynolds numbers you are talking about?

December 12, 2012, 06:11		#3
Lea New Member Join Date: Dec 2012 Posts: 10 Rep Power: 13	The reynolds numbers in the whole pipe

December 12, 2012, 06:13		#4
Far Super Moderator Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,553 Blog Entries: 6 Rep Power: 54	Where you are seeing them? Reynolds number is a reference quantity and depends on the reference you are taking !!!

December 12, 2012, 06:19		#5
Lea New Member Join Date: Dec 2012 Posts: 10 Rep Power: 13	I see them when I go to Results->Graphics and Animations->Contours and then Counters of Velocity-> cell Reynolds Number

December 12, 2012, 06:25		#6
Far Super Moderator Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,553 Blog Entries: 6 Rep Power: 54	I guess it is Peclet Number related to local cell velocity and cell length scale. No need to look on it.

December 14, 2012, 05:02		#9
Lea New Member Join Date: Dec 2012 Posts: 10 Rep Power: 13	Thank you both for your help=) Lucky Train, can you tell me what I have to do to see the "real" reynolds number so that I can see if the flow is laminar or turbulent?

December 14, 2012, 05:07		#10
Far Super Moderator Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,553 Blog Entries: 6 Rep Power: 54	Real Reynolds number is what you have specified

December 14, 2012, 05:11		#11
Lea New Member Join Date: Dec 2012 Posts: 10 Rep Power: 13	but I haven't specified the reynoldsnumber because that is my variable. I want to see how the reynoldsnumber changes when I have a different geometry.

December 14, 2012, 05:13		#12
Far Super Moderator Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,553 Blog Entries: 6 Rep Power: 54	Then calculate reynolds number from the formula

December 18, 2012, 05:44		#14
Lea New Member Join Date: Dec 2012 Posts: 10 Rep Power: 13	Thank you all for your help! I found the solution: When you look on the turbulent reynolds number Re_y it is already turbulent with a value about 200. So my flow is turbulent=)