Wall shear stress

andrewburns · February 12, 2008, 18:16

Hello, I'm trying to figure out the dimensions of openFoam's wall shear stress. I'm trying to compare the same mesh run for the same conditions in both fluent and openFoam, however when I use wallShearStress to find the stress on the openFoam case I get numbers that are much different to those from fluent.

I know fluent calculates wall shear stress in pascals, however when I look at the dimensions in the shear stress file generated by openFoam I see this:

[0 2 -2 0 0 0 0]

Wouldn't this mean the units for shear stress in openFoam are m^2/s^2 ? I don't understand how this can be converted to pascals for comparison purposes.

For an idea of the differences I'm talking about, fluent reports the maximum shear stress as 14 pascals, while openFoam reports it as 53.6.

dmoroian · February 13, 2008, 02:44

For an incompressible flow, all the equations are divided with density. If you divide [Pa] with r, it should go like this:
Pa/r = N/m2/kg/m3 = kg*m/s2/m2/kg/m3 = m2/s2

I hope this is helpful!
Dragos

andrewburns · February 13, 2008, 17:40

Thanks that's perfect. I thought it would be something like that but I wasn't able to work out if it made sense dimensionally.

pedroxramos · November 4, 2019, 05:25

is it the same in case of a turbulent flow?

because here they make a difference between laminar and turbulent:
http://aboutcfd.blogspot.com/2017/05...-openfoam.html

February 12, 2008, 18:16	Hello, I'm trying to figure ou	#1
andrewburns Member Andrew Burns Join Date: Mar 2009 Posts: 36 Rep Power: 17	Hello, I'm trying to figure out the dimensions of openFoam's wall shear stress. I'm trying to compare the same mesh run for the same conditions in both fluent and openFoam, however when I use wallShearStress to find the stress on the openFoam case I get numbers that are much different to those from fluent. I know fluent calculates wall shear stress in pascals, however when I look at the dimensions in the shear stress file generated by openFoam I see this: [0 2 -2 0 0 0 0] Wouldn't this mean the units for shear stress in openFoam are m^2/s^2 ? I don't understand how this can be converted to pascals for comparison purposes. For an idea of the differences I'm talking about, fluent reports the maximum shear stress as 14 pascals, while openFoam reports it as 53.6. Luttappy, MaySea and sourav90 like this.

February 13, 2008, 02:44	For an incompressible flow, al	#2
dmoroian Senior Member Dragos Join Date: Mar 2009 Posts: 648 Rep Power: 20	For an incompressible flow, all the equations are divided with density. If you divide [Pa] with r, it should go like this: Pa/r = N/m2/kg/m3 = kg*m/s2/m2/kg/m3 = m2/s2 I hope this is helpful! Dragos pedroxramos, Luttappy, faiazk and 1 others like this.

February 13, 2008, 17:40	Thanks that's perfect. I thoug	#3
andrewburns Member Andrew Burns Join Date: Mar 2009 Posts: 36 Rep Power: 17	Thanks that's perfect. I thought it would be something like that but I wasn't able to work out if it made sense dimensionally.

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November 4, 2019, 05:25		#4
pedroxramos Member Pedro Ramos Join Date: Mar 2012 Location: Belgium Posts: 81 Rep Power: 14	is it the same in case of a turbulent flow? because here they make a difference between laminar and turbulent: http://aboutcfd.blogspot.com/2017/05...-openfoam.html