I have problem in expression in specified shear of boundary condition
 

hello
i'm doing graduation project
but i got a problem
when i try to input the formula
sheart stress = dynamic viscosity * dvelocity/dy
on the expression of boundary condition,
the message on the image keeps coming up.
i also attach the image,
please answer the question~

Scalar and Vector
 

You are trying to use an expression that returns a vector for a field that is a scalar. Since you are providing value for the component, expression should return a component and not the whole vector.

how can i make it scalar?
 

thank you very much for the answer
but i still have the problem
I also thought the same way and i tried to make it absolute value
but another message comes up.
i guess dVelocitydy is the vector
how can i make it scalar?
i attach the image...

Vector
 

Yes, dvelocity/dy is a vector because velocity is a vector. You have to use velocity component, say, xvelocity or velocity-x, whichever is correct format.

thank you
 

thank you for the answer
but i don't know if i can change the component
i can only select the component of pascal
i will try to find if i have another measure to solve it.
one more question though,
is it right that if specified shear is zero wall doesn't affect the velocity of fluid?

Clarity
 

Your query is not very clear to me. Whether you use velocity vector or scalar, result is always in Pa. A value of 0 implies slip boundary condition; it is different from what you are trying to apply now. You should use the condition that is appropriate for your case.

what im trying to do
 

yes what you're saying is right
once i simulated with zero shear stress on the wall
the streamline seemed like the wall doesn't affect the velocity of the fluid.
in real situation fluid gradually loses its speed.
so i thought it isn't realistic.
so i though shear stress would make it realistic, but putting in the expression, i have problem as the images i attached before.
if result comes to pascal from both vector and scalar, why there's an error?
i can't understand...

Expression
 

Why do you want to apply an expression? Just select no-slip condition.

no slip condition
 

in no silip condition stream line stops right at the wall
but in my project, i'm trying to improve the flow of fluid in smoking booth
so stream line matters..

Wall
 

Walls are walls; they are supposed to stop the flow. If you want a slip, then you have to find out how much slip should be there.

no slip
 

now i think no slip condition is right
i just simulated again with no slip condition
and found out that fluid near the wall doesn't stop.
the simulation that i did the other day stopped all the fluid soonner because every stream line was on the wall almost right after it's come out from the inlet ( the shape was different from now).
thank you

Streamlines
 

Those are not streamlines, but pathlines. And whether those stop due to wall or not depends on number of steps used for pathline calculation. Default is 500. Use higher value, such as, 20000 or higher, until most of the pathlines have reached outlet.