need help for calculating friction factor

rahulv · October 20, 2010, 03:11

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, 05:46

Hi,

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

rahulv · October 20, 2010, 06:05

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, 07:07

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, 08:13

thx ghorrock
i will go through it and it should help

ashgun · September 24, 2012, 14:50

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 ?

monkaeydadde · February 28, 2018, 08:39

the problem here is that u dont know the physics of the problem. check for lamina flow development...

October 20, 2010, 03:11	need help for calculating friction factor	#1
rahulv Member Rahul Vaibhav Join Date: Nov 2009 Location: India Posts: 33 Rep Power: 17	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

October 20, 2010, 05:46		#2
pavitran Member Dynampally Pavitran Join Date: Mar 2010 Location: India Posts: 74 Rep Power: 16	Hi, The pressure should be the difference in the total pressure at the inlet and outlet. I guess your pipe length is long enough for the flow to be fully developed (usually it should be 10 times the inlet diameter). Lastly and the most important are your getting the recommended y-plus(according to CFX manual) values for the mesh.

September 24, 2012, 14:50	regarding friction factor	#6
ashgun New Member abhi Join Date: Sep 2012 Posts: 11 Rep Power: 14	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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October 20, 2010, 06:05		#3
rahulv Member Rahul Vaibhav Join Date: Nov 2009 Location: India Posts: 33 Rep Power: 17	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.

October 20, 2010, 07:07		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Why are my results inaccurate? Been asked a million times, check out the FAQ: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

October 20, 2010, 08:13		#5
rahulv Member Rahul Vaibhav Join Date: Nov 2009 Location: India Posts: 33 Rep Power: 17	thx ghorrock i will go through it and it should help

February 28, 2018, 08:39		#7
monkaeydadde New Member Davide Join Date: Feb 2017 Posts: 6 Rep Power: 9	the problem here is that u dont know the physics of the problem. check for lamina flow development...