# Reynolds number

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 March 26, 2013, 13:00 Reynolds number #1 New Member   Kishore Join Date: Mar 2013 Posts: 20 Rep Power: 6 Sponsored Links Dear all I am doing flow through a venturi pipe. I have calculated the Reynolds number theoritically at the throat was 2800 which was transiiton between laminar to turbulant.But in the upstream and down stream the reynols number is less than 2000. When i do the setup, I specified as laminar. Is the way correct or shall I specify k-omega or SST ? When I run the solution, No warning occurs during laminar to turbulant transisiton . Why? 2. How to calculate the reynolds number in cfx post? ( I did this way: I have created a plane at the throat and use the function calculater to find the areaAve velocity and areAve dynamic viscosity on the plane . Then I apply the Re equation . But I got very less value. Why this different value? Please any one help me to point out my mistakes Thank you all

 March 26, 2013, 17:57 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,735 Rep Power: 106 Be careful with this flow - the expansion of the venturi might induce turbulence at a lower Re than for straight pipe flow. But if it is turbulent it is going to be low Re turbulence. So I would use a laminar flow model. What do you mean by lam-turb transition warning? Calculating Re - what did you use for the length scale? And when you calculated it by hand what values did you use?

 March 27, 2013, 07:28 #3 Member   Thiagu Join Date: Oct 2012 Location: India Posts: 59 Rep Power: 6 apple to apple comparison # check the variables one by one From your Re handcalutaion to CFX expression. # what is your ultimate objective of analysis

March 27, 2013, 12:34
#4
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Kishore
Join Date: Mar 2013
Posts: 20
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Quote:
 Originally Posted by ghorrocks Be careful with this flow - the expansion of the venturi might induce turbulence at a lower Re than for straight pipe flow. But if it is turbulent it is going to be low Re turbulence. So I would use a laminar flow model. What do you mean by lam-turb transition warning? Calculating Re - what did you use for the length scale? And when you calculated it by hand what values did you use?
so , my laminar setup is correct? The flow is z direction. I have created a xy plane and found the areaAve velocity and areaAve dynamic viscosity on the plane for Re calculation. I am not using length scale.
Actually this is our assignment. we are modelling a artery and area restriction is 90%.
We suppose to analyse shear layer instability in the model? literally i know its meaning. when I bring to this to our study i don't know how to discuss shear layer instability. Is there any way to find or discuss shear layer instability in cfx.

Thank you

March 27, 2013, 17:56
#5
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Glenn Horrocks
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Location: Sydney, Australia
Posts: 13,735
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Quote:
 I am not using length scale.
You need a length scale for the Re number calculation. Where did it come from?

What exactly do you want to analyse in the shear layer instability?

March 28, 2013, 08:51
#6
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Kishore
Join Date: Mar 2013
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Quote:
 Originally Posted by ghorrocks You need a length scale for the Re number calculation. Where did it come from? What exactly do you want to analyse in the shear layer instability?
Dear Mr Glenn

For Reynolds number calculation, when can we apply diameter of the pipe?
when can we apply length scale?

I fix the throat length in our case 0.75 mm and Diameter at the throat 1 mm. In such case Re value changes. Now what is the appropriate value.

we applied transient velocity at the inlet. we fix non Newtonian flow.

how to find shear layer instability in low reynolds turbulance?

Thank you

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