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August 1, 2006, 04:12 
What does "traction free" mean?

#1 
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What does "traction free" mean? Is it a boundary condiction that there's no stress action on that boundary? For example, the pseudiofluid is free to leave the mold wall as pushed out by the advancing liquid.
Thanks a lot! 

August 1, 2006, 07:53 
Re: What does "traction free" mean?

#2 
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Tractionfree = no surface traction = no tangential stress component.


August 1, 2006, 22:04 
Re: What does "traction free" mean?

#3 
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Thanks for the reply! The boundary condition is, sigma(u).np.n=0 in the paper I read. I don't know the mean. no tangential stress component and no normal stress? Thanks a lot!


August 2, 2006, 02:01 
Re: What does "traction free" mean?

#4 
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generally traction is used for stress bc in literature.
>sigma(u).np.n=0 n probably mean normal vector to boundary and .n mean normal component of those vector, as p (probably pressure) is scaler the better expression is pI.n 

August 2, 2006, 04:08 
Re: What does "traction free" mean?

#5 
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Strictly the zero traction condition is
(Tn).s = 0 where T is the stress tensor, n is the normal to the surface and s is the surface tangent (in 3D there are two surface tangents). Note that, since n.s =0 the pressure does not contribute to this calculation and u.n=0 for no flow through the surface. For n=k ans s=i then this is simplifies to the usual conditions (laminar Newtonian fluid) w=0, u_z=0. 

August 2, 2006, 05:16 
Re: What does "traction free" mean?

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Note that traction free surface mean no stresses normal stress and tangent one,
the general stress bc condition for an arbitrary 3d surface is (in fluid dynamics): i. (S.n).n = normal stress e.g. surface tension = sigma k/We S= pI + mu/Re[(grad U)+ (grad U)^T] (stress tensor) where n is local normal vector, mu is viscosity, p is pressure, U is velosity vector, k is kurvature, Re is renold number, We is weber number ii. (S.n).m1=0 and (S.n).m2=0 where m1 and m2 are local tangential vectors, note that n, m1 and m2 make independent basis for 3d space Finally traction free mean condition leads to: (S.n).n =0. (S.n).m1 =0. (S.n).m2 =0. Is it clear yang? 

August 2, 2006, 06:03 
Re: What does "traction free" mean?

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The condition (S.n).n =0 will over prescibe the system if you require u.n=0 on a solid surface. The only time you would employ a condition on (S.n).n is if you had a surface which could deform under the motion of the fluid (even in this case it's really a condition on p rather than on S.n).n)).


August 2, 2006, 09:16 
Re: What does "traction free" mean?

#8 
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i don't knowe what is your argue about correctness of previous massage, (S.n).n =0 in solid mechanics this condition is usually natural on free boundaries of solid but in fluid dynamics, normal component usually balance with pressure force (ambient pressure+surface tension pressure) i.e.
(S.n).n = P_ambient + sigma k is it correct? 

August 2, 2006, 09:51 
Re: What does "traction free" mean?

#9 
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My point is that if you have a "traction free" or freeslip rigid surface then u.n = 0 is the relevant boundary condition and it does not in general imply (Sn).n=0; i.e. there is a normal pressure force on the surface even if there is no traction (think inviscid flow past an aerofoil  there is no drag but there is lift).


August 2, 2006, 12:31 
Re: What does "traction free" mean?

#10 
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well, you mean traction free as only tangent (or shear) stresse=0., althouth i don't see this expression in CFD literature but in solid mechanics it is general based my nowledge (normal+tangent components)


August 2, 2006, 20:25 
Re: What does "traction free" mean?

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Thanks for your replies! The air treated as pseudiofluid is free to leave the mold wall as pushed out by the advancing liquid.The traction free boundary condition is T.n=0 at the interface between the wall and the air. Where, T=pI+2*mu*D,D=0.5*[grad(u)+grad(u)^T]


August 3, 2006, 04:03 
Re: What does "traction free" mean?

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In fluid mechanics the normal condition is generally only applied to the pressure (on a material surface); i.e. constant atmospheric pressure or a pressure jump related to surface tension  this is why it is usually referred to as a stressfree surface and not traction free one.


November 9, 2009, 07:43 
Using traction free boundary condition in Fluent?

#13 
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Hi..
I want to implement the condition –p+0.0035 (∂u/∂x)=0 stated as the traction free boundary condition at the outlet of a pipe. How to implement this in Fluent? Is there any such option in Fluent or it will require a udf? 

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