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-   -   What does "traction free" mean? (http://www.cfd-online.com/Forums/main/11908-what-does-traction-free-mean.html)

Yang August 1, 2006 04:12

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

Thanks a lot!


ag August 1, 2006 07:53

Re: What does "traction free" mean?
 
Traction-free = no surface traction = no tangential stress component.

Yang August 1, 2006 22:04

Re: What does "traction free" mean?
 
Thanks for the reply! The boundary condition is, sigma(u).n-p.n=0 in the paper I read. I don't know the mean. no tangential stress component and no normal stress? Thanks a lot!

rt August 2, 2006 02:01

Re: What does "traction free" mean?
 
generally traction is used for stress bc in literature.

>sigma(u).n-p.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


Tom August 2, 2006 04:08

Re: What does "traction free" mean?
 
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.

rt August 2, 2006 05:16

Re: What does "traction free" mean?
 
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?


Tom August 2, 2006 06:03

Re: What does "traction free" mean?
 
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)).

rt August 2, 2006 09:16

Re: What does "traction free" mean?
 
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?

Tom August 2, 2006 09:51

Re: What does "traction free" mean?
 
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).


rt August 2, 2006 12:31

Re: What does "traction free" mean?
 
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)

Yang August 2, 2006 20:25

Re: What does "traction free" mean?
 
Thanks for your replies! The air treated as pseudio-fluid 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]

Tom August 3, 2006 04:03

Re: What does "traction free" mean?
 
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 stress-free surface and not traction free one.

psb November 9, 2009 06:43

Using traction free boundary condition in Fluent?
 
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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