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Old   December 30, 2013, 06:32
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AKHILESH S L
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HI EVERYONE,
i need to find the shear stress in the fluid flow. by default there is only shear strain and wall shear in the cfx post. i need to write the user defined variable for finding the shear stress. can anybody please guide me how to write a user defined variable for shear stress? (tau = mu*du/dy)
please

thanks in advance
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Old   December 30, 2013, 16:04
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Glenn Horrocks
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Do you mean you want all components of the shear stress tensor? That can be evaluated from a built in variable operator which can give the derivative of the velocity vector (eg velocity.grad_x).
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Old   December 31, 2013, 01:03
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THANKS ghorrocks
i need to calculate the total shear stress tensor itself. how can i get that? how can i input the variable which you have send? can u please explain me in a few steps please?
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Old   December 31, 2013, 05:18
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CEL are scalar variables, so to generate a tensor you need to store the components as individual variables. So you will have velocity u.grad_x, velocity u.grad_y, velocity u.grad_z, velocity v.grad_x.... etc.

And assuming your viscosity is constant you can just multiply by viscosity to get the stress tensor.

These are just standard CEL variables so you can use them as any other CEL variable. Have a look at the tutorials for examples of CEL expressions.
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Old   December 31, 2013, 06:30
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thank u ghorrocks,
can i ask you something? can i get shear stress by multiplying the shear strain (which is already available in CFX post) with dynamic viscosity? tau = mu*(du/dy)?
this is more simple way na? if is possible how can i write a user defined variable for shear stress in terms of "mu" and shear strain?
can you help me in writing this code?
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Old   December 31, 2013, 06:34
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This is really basic CEL expressions - have a look at the tutorials for how to do this.

An expression something like ShearStress_u_x = velocity u.grad_x will do it, then repeat with the other combinations of u/v/w, x/y/z for all the other tensor components.
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