Calculation of laminar / viscous sublayer
Calculation of laminar / viscous sublayer
(OP)
Hi there,
I'm interested in determining whether or not a pressure pipe can be considered to hydraulically smooth or not however i've only found one unreferenced equation for calculating the laminar sublayer thickness:
http://wiki.answers.com/Q/What_is_meant_by_Hydraul...
t=5D*(8^0.5)/((f^0/5)*Re
There are no examples of this equation in my fluids book, on wikipedia, or on engineeringtoolbox, so i'm not happy to blindly plug this equation into my model!
Is this easy enough to calculate this for a particular turbulent flow based on reynolds number and roughness or something?
Thanks for reading
Regards
Chris
I'm interested in determining whether or not a pressure pipe can be considered to hydraulically smooth or not however i've only found one unreferenced equation for calculating the laminar sublayer thickness:
http://wiki.answers.com/Q/What_is_meant_by_Hydraul...
t=5D*(8^0.5)/((f^0/5)*Re
There are no examples of this equation in my fluids book, on wikipedia, or on engineeringtoolbox, so i'm not happy to blindly plug this equation into my model!
Is this easy enough to calculate this for a particular turbulent flow based on reynolds number and roughness or something?
Thanks for reading
Regards
Chris





RE: Calculation of laminar / viscous sublayer
RE: Calculation of laminar / viscous sublayer
You could probably come to some analytical solution by using one of the explicit friction factor equations rather than using Colebrook-White. The results weren't very important to what I was doing, so I did not perservere very far with it.
I think that any attempt at trying to calculate the exact thickness of the boundary sublayers would involve so many assumptions and approximations that you couldn't have much confidence in the answers anyway.
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Calculation of laminar / viscous sublayer
btrueblood... i need to calculate friction factor in spreadsheet so cant use charts... I still cant find a reference for that equation (using massey mechanics of fluids)!
katmar... sure i figured it would be kind of approximate.
I only need to estimate whether rough because apparently:
"At high Reynolds numbers, the sub layer thickness becomes very small and the friction factor f
becomes independent of Re and depends only on the relative roughness height. In this case the pipe is
a hydraulically rough pipe, and Von Karman found that the friction factor f:"
(colebrook-white formula without Re/f term)
I just wanted to do a check in order to use the appropriate equation when calculating f, though maybe it is unnecessary and I can just use C-W for all turbulent cases?
Regards
Chris