what is mean of your t?
it is time ? 
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When then yield stress is above the limit on the wall, there is a wall slip velocity given by where is the dynamic viscosity and is the friction coefficient. (I have omitted in my previous message ) So what do you think about the boundary condition ? 
Wall Motion Based on Velocity Components
For problems that include linear or nonlinear translational motion of the wall boundary you can enable the Components option and specify the XVelocity, YVelocity, and ZVelocity of the wall. You can define nonlinear translational motion using a boundary profile or a userdefined function for the XVelocity, YVelocity, and/or ZVelocity of the wall I think this condition is that you want. 
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see below for the message.

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Here is my code below. If I understand well I have to put two component udf: wall_shear_x and wall_velocity_x. Code:
#define Cf 1 /* friction coefficient */ and I check specified shear and I put udf wall_shear_x in XVELOCITY. Is it right ? I will test tomorrow but I wonder if I follow exactly the wallslip model below with this boundary condition : if if where is the dynamic viscosity and is the friction coefficient. and with where is the stress tensor and is the strain tensor. What do you think about that ? Is the boundary condition appropriate to the model ?? Thank for your answer... 
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In other words, you calculate wall shear for each individual cell next to the walls and then integrate/add to get the total force, and then use that total wall shear to compare to your slipping wall criteria. Put in other words, you cannot change the wallslip velocity locally for each cell since the entire wall must have the same slip velocity. Also, is it necessary to calculate TAUxy? Fluent should already have a variable called xwallshear or something that you should be able to access directly and not calculate through UDF. I am not the best at UDF so I wanted to ask you to make sure. 
I don't find anywhere how to access directly the wall shear stress.
So, in my case I have to calculate the element of the strain tensor in each cell to calculate the wall shear stress and to be able to compare this value with the limit stress . This comparison has to be made in each cell to know if there is a wall slip or not. My real purpose is to have a numerical wall slip model for non newtonian flow. That's why I have to calculate in each cell. With a newtonian flow it's not very usefull to make a loop on each cell. You say that it's impossible to have different slip velocity for each cell with a wall motion boundary condition (?). So how can I make this model ? Thank you. 
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The steady flow, the wall shear stress is again stored in a simple variable wallshearx y and z. Since it can be outputted to file / printed to screen, that variable should be accessible. Anyway, you can calculate if you really want to spend that effort. 
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