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 rahulv October 20, 2010 02:11

need help for calculating friction factor

Hi

I want to predict pressure drop over a model on a given flow rate.
For benchmarking i took the cylinderical pipe and study the pressure drop for laminar and turbulent flow at various Re.
i am trying to calculate fricition factor from moody chart (f=64/Re for laminar flow) and comparing it with cfx ( f = -(dp/dx)(2d/(rho*V^2)).

but cfx friction factor is almost double in all the cases. i am could not find reason behind it.

my BC

inlet- mass flow rate (based on Re)
outlet- static pressure =0

Thanks and Regards
rv

 pavitran October 20, 2010 04:46

Hi,
1. The pressure should be the difference in the total pressure at the inlet and outlet.
2. I guess your pipe length is long enough for the flow to be fully developed (usually it should be 10 times the inlet diameter).
3. Lastly and the most important are your getting the recommended y-plus(according to CFX manual) values for the mesh.

 rahulv October 20, 2010 05:05

Hi Pavitran

Thank for the suggestiong you gave.
Indeed i am not sure about the y+ values for turbulent flow so i am studying pressure drop of laminar flow in a pipe.
I have taken a sufficient lenght of the pipe to make sure the flow become fully developed and i am calculating the pressure drop of fully developed region. Still i am not getting the right answer.

 ghorrocks October 20, 2010 06:07

Why are my results inaccurate? Been asked a million times, check out the FAQ:

http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

 rahulv October 20, 2010 07:13

thx ghorrock
i will go through it and it should help :)

 ashgun September 24, 2012 13:50

regarding friction factor

even if we take total pressure difference is same as static pressure difference
my value is also deviating from expected value i don't know y ?
expected is 0.128 and i am getting 0.145 but i am doing it in fluent plz tell me where am i going wrong ?

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