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

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I want to input the wall slip law below with the limit yield stress above which there is a tangantial slip velocity : if if 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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April 20, 2012, 10:36 

#2 
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dengjiajia
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what is mean of your t?
it is time ? 

April 20, 2012, 10:57 

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April 22, 2012, 01:44 

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dengjiajia
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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. 

April 22, 2012, 02:55 

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Lucky Tran
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April 22, 2012, 06:41 

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see below for the message.


April 22, 2012, 06:45 

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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 */ #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 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... 

April 22, 2012, 11:35 

#8  
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Lucky Tran
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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. 

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 . 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. 

April 23, 2012, 07:49 

#10 
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Lucky Tran
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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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