# Shear stress from velocity profile

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 November 12, 2009, 06:46 Shear stress from velocity profile #1 New Member   Arun Join Date: Sep 2009 Posts: 3 Rep Power: 16 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

 November 12, 2009, 07:04 #2 Senior Member     Paolo Lampitella Join Date: Mar 2009 Location: Italy Posts: 2,163 Blog Entries: 29 Rep Power: 39 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)

 November 16, 2009, 20:16 #3 New Member   Charles Rundle Join Date: Oct 2009 Posts: 9 Rep Power: 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

 November 18, 2009, 09:20 #4 New Member   Arun Join Date: Sep 2009 Posts: 3 Rep Power: 16 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

 November 11, 2015, 04:08 Shear stress in fluids #5 New Member   Dolores Join Date: Nov 2015 Posts: 1 Rep Power: 0 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.

 Tags comsol, fluid flow, shear stress