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how can I input a wall slip law? Which Boundary Condition ? |
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April 20, 2012, 09:28 |
how can I input a wall slip law? Which Boundary Condition ?
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#1 |
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Hi,
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 |
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#2 |
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dengjiajia
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what is mean of your t?
it is time ? |
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April 20, 2012, 10:57 |
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#3 |
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April 22, 2012, 01:44 |
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#4 |
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dengjiajia
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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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April 22, 2012, 02:55 |
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#5 |
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Lucky
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Quote:
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April 22, 2012, 06:41 |
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#6 |
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see below for the message.
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April 22, 2012, 06:45 |
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#7 | |
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Quote:
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 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 : 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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April 22, 2012, 11:35 |
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#8 | |
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Lucky
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Quote:
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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April 23, 2012, 04:10 |
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#9 |
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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. |
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April 23, 2012, 07:49 |
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#10 |
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Lucky
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Quote:
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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