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how can I input a wall slip law? Which Boundary Condition ?

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Old   April 20, 2012, 09:28
Default how can I input a wall slip law? Which Boundary Condition ?
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Hi,

I want to input the wall slip law below with \tau_0 the limit yield stress above which there is a tangantial slip velocity :
u_{wall}=0 if \tau<\tau_0
u_{wall}=\mu*\frac{\partial u_z}{\partial r} if \tau\geq\tau_0

Can you advice me which boundary condition do I have to use ?

***A wall boundary condition with a moving wall and (?) shear rate
or
***A velocity boundary condition with the velocity as a component
or
*** An other way

My problem is a circlar pipe with a newtonian or non newtonian fluid which flow inside it.

Thank you all...
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Old   April 20, 2012, 10:36
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what is mean of your t?
it is time ?
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Old   April 20, 2012, 10:57
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Quote:
Originally Posted by dunga82 View Post
what is mean of your t?
it is time ?
\tau is the yield stress.
When then yield stress is above the limit \tau_0 on the wall, there is a wall slip velocity given by
u_{wall}=\frac{mu}{c_f}*\frac{\partial u_z}{\partial r}
where \mu is the dynamic viscosity and c_f is the friction coefficient.

(I have omitted c_f in my previous message )

So what do you think about the boundary condition ?
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Old   April 22, 2012, 01:44
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Wall Motion Based on Velocity Components
For problems that include linear or non-linear translational motion of the wall boundary you can enable the Components option and specify the X-Velocity, Y-Velocity, and Z-Velocity of the wall. You can define non-linear translational motion using a boundary profile or a user-defined function for the X-Velocity, Y-Velocity, and/or Z-Velocity of the wall

I think this condition is that you want.
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Old   April 22, 2012, 02:55
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Quote:
Originally Posted by tsi07 View Post
Hi,

I want to input the wall slip law below with \tau_0 the limit yield stress above which there is a tangantial slip velocity :
u_{wall}=0 if \tau<\tau_0
u_{wall}=\mu*\frac{\partial u_z}{\partial r} if \tau\geq\tau_0

Can you advice me which boundary condition do I have to use ?

***A wall boundary condition with a moving wall and (?) shear rate
or
***A velocity boundary condition with the velocity as a component
or
*** An other way

My problem is a circlar pipe with a newtonian or non newtonian fluid which flow inside it.

Thank you all...
You will need a UDF. Your basic wall properties will not handle what you are trying to do. Maybe you can dig one up. You will basically need to program Fluent (using the UDF) to assign the slip velocity to the slip-wall boundary condition each iteration according to the criteria you described.
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Old   April 22, 2012, 06:41
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see below for the message.
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Old   April 22, 2012, 06:45
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Quote:
Originally Posted by LuckyTran View Post
You will need a UDF. Your basic wall properties will not handle what you are trying to do. Maybe you can dig one up. You will basically need to program Fluent (using the UDF) to assign the slip velocity to the slip-wall boundary condition each iteration according to the criteria you described.
Thank you for your answer both.

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 */
#define TAU0 0.2            /* limit yield stress for slipping */
 
DEFINE_PROFILE(wall_shear_x, thread, position)
{
face_t f;
Thread *t0;
cell_t c0;
double TAUxy, mu;
double dudy;
begin_f_loop(f, thread)
{
t0 = THREAD_T0(thread); /* adjacent cell thread to f */
c0 = F_C0(f, thread); 
dudy = C_DUDY(c0,t0);
mu = C_MU_L(c0,t0); /* viscosity */
TAUxy=mu*dudy;
F_PROFILE(f, thread, position)=TAUxy;
}
end_f_loop(f, thread)
}
 
DEFINE_PROFILE(wall_velocity_x, thread, position)
{
    face_t f;
    Thread *t0;
    cell_t c0;
    double VC[ND_ND];
    double TAUxy, VC_MAG, vg, mu;
    double dudy;
    begin_f_loop(f, thread)
        {
        t0 = THREAD_T0(thread);        /* adjacent cell thread to f */
        c0 = F_C0(f, thread); 
        dudy = C_DUDY(c0,t0);
        mu = C_MU_L(c0,t0);                        /* viscosity */
VC[0]=-mu*dudy/Cf;
        VC[1]=0;                /* stress vector's y componant*/
VC_MAG=NV_MAG(VC);                    /* stress vector's magnitude */
vg=VC[0];
        if ( VC_MAG < TAU0)
            {
      F_PROFILE(f, thread, position)=0;
            }
        else       {F_PROFILE(f, thread, position)=vg;
            }
        }
    end_f_loop(f, thread)
}
And I check wall motion and apply udf wall_velocity_x in X-VELOCITY
and I check specified shear and I put udf wall_shear_x in X-VELOCITY.

Is it right ? I will test tomorrow but I wonder if I follow exactly the wall-slip model below with this boundary condition :

u_{wall}=0 if \tau<\tau_0
u_{wall}=\frac{\mu}{c_f}*\frac{\partial u_z}{\partial r} if \tau\geq\tau_0

where \mu is the dynamic viscosity and c_f is the friction coefficient.

and with \overline{\tau}=\mu*\overline{\gamma}
where \overline{\tau} is the stress tensor and \overline{\gamma} 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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Old   April 22, 2012, 11:35
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Quote:
Originally Posted by tsi07 View Post
Thank you for your answer both.

What do you think about that ? Is the boundary condition appropriate to the model ??

Thank for your answer...
You are going to apply the slip condition to the entire wall. So your criteria for slip or not, should be based on overall wall shear. Is that how you are checking?

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 wall-slip 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 x-wall-shear 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.
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Old   April 23, 2012, 04:10
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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 \tau_0.
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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Old   April 23, 2012, 07:49
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Quote:
Originally Posted by tsi07 View Post
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 \tau_0.
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.
Newtonian or non-newtonian that is a simple fluid property. Whether you have newtonian or non-newtonian the wall simply does not move the way you are trying to make it move. How can you have a section of wall moving in one location and non-moving in another? This is a boundary-condition problem, not a fluid property problem. The fact that you are trying to simulate a non-newtonian flow is immaterial to the fact that you are trying to make the wall move. For non-newtonian flow you just need to patch the viscosity based on the localized strain-rate. Recall again that you called it "wall" slip, not localized wall motion or portions of wall slip, it is simply [entire] wall slip. There is no need to model this type of behavior since it simply does not occur, the wall is bound by physical kinematic constraints.

The steady flow, the wall shear stress is again stored in a simple variable wall-shear-x 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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