# Variable Boundary Condition

 Register Blogs Members List Search Today's Posts Mark Forums Read

 June 20, 2005, 13:27 Variable Boundary Condition #1 Malcolm Guest   Posts: n/a I am interested in creating a wall boundary condition that can switch from no-slip to free-slip depending on temperature. Is this possible with cfx 5.7? Any advice would be appreciated. Malcolm

 June 20, 2005, 14:13 Re: Variable Boundary Condition #2 Rui Guest   Posts: n/a Hi, In CFX, the B.C. type cannot be changed: you cannot switch from no-slip to free-slip, as you cannot, for example, switch from inlet to wall. But there may be other ways to implement a simmilar behaviour. You may define the walls as free-slip, create a subdomain (1 element thick) next to the walls and define a source of momentum as:Sm=-C*U*step(T-Ts) Sm will be equal to -C*U for T above Ts, and 0 for T below Ts. If you set C to a large value, the source term -C*U will provoque a zero velocity at the walls when T>Ts. While the for T below Ts, the wall will be a free-slip wall. Are you performing a steady-state or a transient simulation? And why would a wall switch from no-slip to free-slip? Regards, Rui

 June 20, 2005, 15:14 Re: Variable Boundary Condition #3 Malcolm Guest   Posts: n/a Hi Rui, Thanks for the advice. Currently I'm performing a steady-state simulation. I want to switch the wall from no-slip to free-slip because I am modeling solidification. At some points liquid is in contacting with the mold while at other locations a 'moving solid' is present. I am using a single phase model where the moving solid is simulated using temperature dependent momentum sources. Malcolm

 June 21, 2005, 10:45 Re: Variable Boundary Condition #4 stex Guest   Posts: n/a you can set a no slip wall with a wall velocity function of the temperature e.g. wall velocity u= step(T-T*) * YOUR_velocity if T step(T-T*)= 0 ---> u=0 ---> wall no slip if T>T* ---> step(T-T*)= 1 ---> u=YOUR_velocity ---> wall moving

 June 21, 2005, 11:39 Re: Variable Boundary Condition #5 Rui Guest   Posts: n/a Hi, That's interesting. But how would you define YOUR_velocity to simulate a free-slip wall? Do you think it should be the tangencial component of the fluid velocity, Vt, at the wall? Do you think that defining YOUR_Velocity as: YOUR_velocity = Vt = V - Vn = V - (V dot n) * n would work?Are the variables Normal X, Normal Y and Normal Z available in CFX-Pre and Solver? Or are they only avaialble in Post? Rui

 June 21, 2005, 18:37 Re: Variable Boundary Condition #6 Nepal Guest   Posts: n/a Hi , Just more description about step() function : 1. if a < b then step(a-b)=0 2. if a = b then step(a-b)=0.5 3. if a > b then step(a-b)=1

 June 22, 2005, 08:10 Re: Variable Boundary Condition #7 stex Guest   Posts: n/a Normal X, Normal Y and Normal Z are NOT available in CFX-Pre... Maybe the problem is that you have a curved wall? If you have a plane wall you can use obviously the velocity component u,v,w.

 June 22, 2005, 10:38 Re: Variable Boundary Condition #8 Rui Guest   Posts: n/a Hi, But would you define YOUR_velocity = (u,v,w), or in case a wall is perpendicular to the y-axis would you define it as (u,0,w)? Have you ever tried to implement this wall definition? Are there problems in obtaining convergence? Regards, Rui

 June 22, 2005, 10:54 Re: Variable Boundary Condition #9 stex Guest   Posts: n/a Never tried Rui, also because defining a velocity of the wall equal to the velocity of the flow is not really like having a free slip.....I mean...is quite good but I don't think is the same...it depemnds from the geometry of the problem... but I've got no other ideas......so this is the best setup ...

 June 23, 2005, 06:20 Re: Variable Boundary Condition #10 Rui Guest   Posts: n/a Hi, Probably you're rigth when you say "defining a velocity of the wall equal to the velocity of the flow is not really like having a free slip".However if you define the wall velocity (which corresponds to the hybrid value of velocity) equal to the tangencial component of the flow velocity (which corresponds to the conservative value of the tangencial component of the velocity - the mean velocity of the control volume next to the wall) it will result in zero shear stress --> free-slip condition But it sounds like it will lead to convergence difficulties, as the flow velocity will "push" the wall and the wall velocity will "push" the flow, but I'm not sure. I think it would worth experimenting. But itīs a shame you cannot use Normal X, Y and Z in CFX-Solver. Regards, Rui

 June 23, 2005, 06:27 Re: Variable Boundary Condition #11 stex Guest   Posts: n/a I agree when you say that the wall with a tangential velocity should push the flow and that is not physically right. I agree also that is a shame you cannot use Normal X, Y and Z , and in my case massFlow and massFlowAve too. Regards stex

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post creddy_trddc CFX 3 September 21, 2011 07:44 Eli CFX 2 April 4, 2010 10:19 normunds OpenFOAM Running, Solving & CFD 2 June 4, 2007 06:45 flga CFX 2 May 30, 2007 03:14 Matt Umbel Main CFD Forum 0 January 11, 2002 11:06

All times are GMT -4. The time now is 10:48.