Discussion Forum

Hi

Are you sure you do not have enough COMSOl variables already defined to extract the shear stress directly in the GUI ?

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Good luck
Ivar

Correct me if I am wrong, the direction of my flow velocity is in the y-axis and if I want to calculate the shear stress for an xy plane slice along a z-axis coordinate, then I input the expression "spf.mu*(vx+uy)" with spf.mu is the dynamic viscosity to give shear stress in Pa?

Hi

well within a minus sign that is the COMSOL internal stress tensor variable:

spf.K_stress_tensorxy

check your expressions, so I would say its not far away ;)


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Good luck
Ivar

i don't quite understand what you mean by the previous statement because when I enter that tensor variable expression I get the same results as before.

Hi Cindy,

I am interested in the same problem, have you found an answer?

Thanks in advance,

Gautier

Hi

my comment was just to remind you about all the predefined internal variables of COMSOL, unfortunately I do not know of any exhaustive list, but you can find them in the pull down menus of the Derived Variables, (or with the CNTRL Space) and in the equation view nodes

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Good luck
Ivar

Hi Ivar,

I know this is an old post, but I wanted to thank you for mentioning the spf.K_stress_tensorxy variable. I never would have found it otherwise.

As a general comment, it seems far more difficult than need be to find some variables. For example, one place I've looked extensively is in the drop-down lists where you can add variables to a probe. Those lists do not seem complete. I checked my "Derived" list also and the only things in there are the probes I defined. Then there is the "Equation View," which sometimes reveals a useful variable, but it does not seem complete either. And finally, there is the documentation -- you can search every manual, and there is no mention of "stress_tensorxy". There is a discussion of the stress tensor, but it doesn't tell you what to call it when you need it.

Any idea why these lists can't be comprehensive? And how does one find these "secret" variables -- is there somewhere I have not looked?

Thanks,
James

By the way, if I assume spf.K_stress_tensorxy is shear stress (which is applied to the boundary of a solid via a liquid) I get very unrealistic results. Anyone know what the physical interpretation of this variable is?

More generally, other than unit differences, it is unclear to me how, at a basic conceptual level, viscous shear stress, traction, and shear rate (since I would think it would be directly proportional to shear stress via dynamic viscosity) really differ. I'm trying to use them as the constraint to set to 0 in a global equation for a stationary study and none of them (plus others I've tried) are giving me answers that make sense. Any insight would be greatly appreciated.