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-   -   Shear stress from velocity profile (http://www.cfd-online.com/Forums/main/70028-shear-stress-velocity-profile.html)

baranii2000 November 12, 2009 07:46

Shear stress from velocity profile
 
Dear all,

I was trying to calculate the shear stress due to fluid flow inbetween two parallel plates from the velocity profile taken from the middle of the flow channel (along vertical axis 'y') using comsol. The solution i got is two times the analytical solution i have calculated. The analytical equation i have used here is
Tou=viscosity*(Umax/h)

where tou is shear stress, umax is the max velocity
h is half of the distance between plates.

The calculation with velocity profile from comsol is done by

Tou= viscosity*(du/dy)
du/dy, i calculated with excel from the u,y values in the velocity profile.

the velocity profile is parabolic and looks laminar.
is that i am doing something wrong?


Thanks

Arun

sbaffini November 12, 2009 08:04

I don't remember the formulas but, if i understand well your problem, it is impossible that for the parabolic velocity profile the derivative at wall is equal to the one from the linear profile. Hence you should check your analytical formula for the stress (just take by yourself the derivative of the analytic velocity profile)

Charles Rundle November 16, 2009 21:16

Hello,

I agree with the previous poster. I think your analytical solution is incorrect. Using a back of the envelope calculation I get a significantly different solution for the shear stress. I am assuming a laminar parabolic flow so depending on conditions my solution may not apply. Also since I mean the back of envelope quite literally I may have made an error.

Assuming your analytical solution is correct I would consider using a finer mesh. I would suggest that you plot your velocity profile against the analytical solution. The profile can look parabolic without truly being parabolic. The problem may simply be that grid resolution or insufficient convergence.

Lastly, your analytical solution may not valid for your Re.

I hope this is helpful. Have a good day.

CAR

baranii2000 November 18, 2009 10:20

Thanks very much Paolo & Charles, i think i did a mistake in my analytical equation...it is supposed to be Tou=2*Umax/h*viscosity i dont know in many books they just refer to Umax/h...

Thanks
Arun

malolagp November 11, 2015 05:08

Shear stress in fluids
 
Hi
I'm trying to plot the shear stress in Comsol for the following problem from the velocity profile that I have previously approximated with Comsol.
Do somebody know how to do that?
Thanks in advance
Dolores

Consider two immiscible and viscous liquids, water and transformer oil, flowing at 300 k in the z-direction in a horizontal thin slit of length 0.5 m and width 0.1 m under the influence of a
pressure gradient. Both liquids can be considered as incompressible. At the inlet both liquids can be assumed to have the same pressure of 0.02 Pa. Determine the corresponding velocity profile for each fluid.

MBdonCFD November 11, 2015 16:43

rather than revive a thread that has been dead for 6 years, try creating a new thread. and mention COMSOL in the title so that people will know its a software issue you want resolved and not a general question about shear stress from velocity profile.

if there is a COMSOL user forum, that would be the best place to post. if not, then you can post it here.

malolagp November 12, 2015 04:40

Thank you very much! You are right!


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