[evgenii]

Hi, all!

I've got a question about directionMixed or mixed b.c.
I don't understand how to use it, because I need to specify four values:
refValue
refGradient
valueFraction
value
I haven't met any description of it neither in Programmers Guide, neither in Users Guide(only very brief).
I'll be very grateful if one can give me advice.
Thank you

[hjasak]

Ok, here goes:

1) a mixed boundary condition is a combination of a fixedValue and a fixedGradient boundary condition, controlled by the valueFraction variable.

For valueFraction = 1, the mixed b.c. behaves as fixedValue; for valueFraction = 0 you get the fixedGradient. Values in between are a blend of the two.

2) The mixed b.c. is therefore specified by three fields:

- a refValue field, giving the valud for the fixedValue part
- a refGrad field, giving the gradient for the fixedGradient part
- a valueFraction, telling the mixed b.c. what to do.

So far, so good.

For vectors and tensors in the mixed b.c., the "mix" will be the same on all components of a vector or a tensor.

3) the directionMixed boundary condition is done along the same lines, but it also allows the directional decomposition. Thus, for a vector, the b.c. is decomposed into the normal and tangential component.

For the normal component, the directionMixed b.c. behaves like the mixed condition (above), but only on the normal component. For the tangential component, the condition is always a fixedGradient b.c.

Finally, the value part of the b.c. codes in the current boundary value.

4) If this is not enough for you (there is another possibility), I have written a newDirectionMixedFvPatchField boundary condition, with 4 components. Unlike the above one, this b.c. allows both the normal and tangential condition to be mixed separately between the fixedValue and fixedGradient b.c. with two blending factors. You need this kind of thing for contact problems with friction and large deformation and similar. In my version, the stuff lives in:

OpenFOAM-1.2/applications/solvers/newStressAnalysis/materialModels/newDirectionM ixed

Enjoy,

Hrv

[evgenii]

Thank you so much for such a detailed answer!
I realise that I need 4th possibility, I mean to set conditions on normal and tangential components separately. Unfortunately I don't have the directory you've mentioned. Maybe because I have an old version of OpenFoam. Where can I find this files?

Thank you, Eugene

[mehulkumar]

Hi..guys,
I have a fixed grid over an airfoil naca0012 in o-topology in circular domain.

I want to specify boundary condition farField kind, so depending the velocity with angle of attack, it should decide the inlet and outlet for the case.

I found that patch -> mixed / directionMixed type can help me in such case.

Can you give me suggestion?

Thanks...

[mehulkumar]

Hi ..guys...pls help...I am stuck here.
I am not getting how to use these BCs.
Pls guide me.

Thanks

[fgal]

Hi,
Is it possible to force zeroGradient and a fixedValue for the temperature field in an inlet for example?
Because I would like to prevent the energy flux go out through a fuel inlet for a combustion and fo fix the temperature of the inlet gases.
Thanks

Francois

[ziemowitzima]

Hi all,
Could anyone of you give me an example of usage directionMixed type boundary condition ?
I would like to see exemplary syntax .
I tryied for example like that:
{
type directionMixed;
refValue uniform (0 0 0);
refGradient uniform (1000 0 0);
valueFraction 1;
value uniform (1000 0 0);
}

but I got error .

Thanks

[poplar]

Originally Posted by ziemowitzima

Hi all,
Could anyone of you give me an example of usage directionMixed type boundary condition ?
I would like to see exemplary syntax .
I tryied for example like that:
{
type directionMixed;
refValue uniform (0 0 0);
refGradient uniform (1000 0 0);
valueFraction 1;
value uniform (1000 0 0);
}

but I got error .

Thanks

Hi,ziemowit
valueFraction must be symmTensorField, not a scalar.


I have a question on Hrvoje's comment that " For the normal component, the directionMixed b.c. behaves like the mixed condition (above), but only on the normal component.". In directionMixedFvPatchField.C:
....
normalValue = transform(valueFraction_, refValue_);
gradValue = pif + refGrad_/this->patch().deltaCoeffs();
transformGradValue = transform(I - valueFraction_, gradValue);
return (normalValue + transformGradValue - pif)*this->patch().deltaCoeffs();
........

From above codes, normalValue　is a vector component normal to transformGradValue. So the normalValue is not normal to B.C., is it right?

and in pressureInletOutletVelocityFvPatchVectorField.C which is derived from directionMixed:
........
valueFraction() = neg(phip)*(I - sqr(patch().nf()));
........
normalValue = transform(valueFraction(), refValue());
transformGradValue = transform(I - valueFraction(), pvf);
operator=(normalValue + transformGradValue);
........

So the valueFraction() is a tensor to calculate the component normal to nf(). Then normalValue[=refV-(n & refV)*n=(I-nn)*refV] is normal to nf() and transformGradValue[=nn*pvf=(n & pvf)*n] is parallel to nf()?

"normalValue" give me the first guess is that it is normal to B.C., but now I am confused by above codes. If "normalValue" is not normal to B.C., then what does tangentialVelocity mean? tangential to nf()?

in pressureInletOutletVelocityFvPatchVectorField.C:
setTangentialVelocity(const vectorField& tangentialVelocity)
{
tangentialVelocity_ = tangentialVelocity;
vectorField n = patch().nf();
refValue() = tangentialVelocity_ - n*(n & tangentialVelocity_);
}

These codes let me confused. If tangentialVelocity_ is tangential to B.C., then it normal to n, so (n & tangentialVelocity_) should be zero. one gets refValue() = tangentialVelocity_. If tangentialVelocity_ is tangential to n, n*(n & tangentialVelocity_) = tangentialVelocity_, then one gets refValue() = 0.

Briefly,my question is:
normalValue is normal to which one, nf() or B.C.?

[T.D.]

hi,
what if i need to apply a certain value of a VectorField on one direction only, let's say if:

volVectorField V;

and i need to apply a BC where Vy =0 only;
????


help please

[ziemowitzima]

Hi TD,
U can use DirectionMixed bc.
here u have working example:

patch_name
{


type directionMixed;
refValue uniform (0 0 0);
refGradient uniform (1000 0 0);
valueFraction uniform (0 0 0 0 0 1);
}

here it means that bc are as follows v=(v1,v2,v3):
1) dv1/dz = 1000 (v1 is tangent vector component to the boundary and z is normal direction to the boundary),

2) dv2/dz = 0 (v2 is second tangent vector component to the boundary and z is normal direction to the boundary)

3) v3=0 (v3 is normal vector component, dirichlet type bc).

In general you have 3 directions and 3 vector component, thats why there is tensor:
valueFraction uniform (0 0 0 0 0 1)

Best
ZM

[T.D.]

Originally Posted by ziemowitzima

Hi TD,
U can use DirectionMixed bc.
here u have working example:

patch_name
{


type directionMixed;
refValue uniform (0 0 0);
refGradient uniform (1000 0 0);
valueFraction uniform (0 0 0 0 0 1);
}

here it means that bc are as follows v=(v1,v2,v3):
1) dv1/dz = 1000 (v1 is tangent vector component to the boundary and z is normal direction to the boundary),

2) dv2/dz = 0 (v2 is second tangent vector component to the boundary and z is normal direction to the boundary)

3) v3=0 (v3 is normal vector component, dirichlet type bc).

In general you have 3 directions and 3 vector component, thats why there is tensor:
valueFraction uniform (0 0 0 0 0 1)

Best
ZM

hi thanks a lot,
but can you specify please what each term means and inside brackets in
refValue uniform (0 0 0);
refGradient uniform (1000 0 0);
valueFraction uniform (0 0 0 0 0 1);

why dv1/dz=1000 ? shouldn't be dv1/dx=1000 ?

help

thanks

[ziemowitzima]

Hi,
This was just example I used to work with.
It could be dv1/dx as well, depending what ones need.
for detailed description see hjasak post at the top.

ZM

[T.D.]

thanks a lot

[AcfdO]

Hi everyone,

I have read all the posts but I am still confused about mixed and directionMixed boundary conditions. I would be grateful if anyone could help me understand this matter by simply give an example about how I can set only du/dy=0 (I don't want to set other gradients to zero)
velocity field is ( u, v,w)
direction is: (x,y,z)
Thank you
Amin

[gpextra]

Dear Foamers,

During the last two days I have intensively struggled to understand the role of the valueFraction keyword in the directionMixed boundary condition. I have read all the posts about it, I have run several cases with the most diverse situations but no, the role of each one of the 6 components of valueFraction remains a mystery.

Posts from Jasak and Malecha, above, have been of great help, but did not entirely clarified the matter. Also similar requests from other users have been systematically ignored.

So, is it possible to someone kindly post exactly how this tensor-like valueFraction should be set for any case? Or, in other words, which values correspond to the normal component, which correspond to the tangential components?

To be even more specific, my case is of an impermeable wall (zero normal velocity) but with a given shear-stress (i.e., given velocity gradient). During my several tries I noticed that, even when the results seem to be +/- correct, there are streamlines dying at and originating from the wall, which is unexpected.

Regards.
G.Paulo.

[bigphil]

Hi G. Paulo,


I occasionally use directionMixed for stress simulations where I want a fixedValue (displacement) in the patch normal direction and some fixedGradient (traction) in the patch tangential direction.

So to achieve a fixedValue in the normal direction, and a fixedGradient in the tangential direction on a patch, then the valueFraction (which is a symmTensor, where the components of a symmTensor are (xx xy xz yy yz zz) ) should be set:
valueFraction = symm(n*n)
where n is the patch normal.

If your patch faces all have the same normal then you can calculate symm(n*n) by hand and put it in your initial conditions, eg:
for a patch with n = ( 1 0 0 )
then the valueFraction = symm( (1 0 0)(1 0 0) ) = ( 1 0 0 0 0 0)
then refValue will be enforced in the patch normal direction and refGradient will be enforced in the patch tangential direction.

If your patch faces have all different normals then you will have to set the patch valueFraction in the solver (using refCast etc.) to be symm(n*n).

So to sum up, if you want a fixedValue in the patch normal direction and a fixedGradient in the patch tangential direction then it looks like this:
refValue uniform (fixed_value_you_want)
refGradient uniform (shear_gradient_you_want)
valueFraction uniform (symm(n*n))

directionMixed can also be used for other situations:
it can act just like a fixedValue BC if valueFraction = (1 0 0 1 0 1),

Or, it can act just like a fixedGradient BC if valueFraction = (0 0 0 0 0 0).

If you look at the directionMixed BC source code, you can see it calculates it.


Hope it helps,
Philip

[gpextra]

Thank you for your post, Philip. Things are clear now.

G.Paulo.

[nlc]

Hi Philip, Thank you for your great post. I would like to fix different gradient value on each tangential direction. Is it possible to do that with directionMixed or an other tool in OpenFoam. Or should I make a new mixed boundary condition to do something like this ??

Or maybe, I just need to change the value of refGradient to achieve this.

Thanks in advance for the answer

Nicolas L.

[bigphil]

Hi Nicolas,

Yes it is possible, using directionMixed, to give every face on a boundary patch a different gradient in the tangential direction. I would do something like this in a header file at the start of a time-step (although it might be possible to use groovyBC but I am not sure):

label patchID = mesh.boundaryMesh().findPatchID("patchOfInterest");
if(patchID == -1)
 {
    Info << "patch not found" << endl;
    return 0;
 }

directionMixedFvPatchVectorField& Upatch =
        refCast<directionMixedFvPatchVectorField>
        (
         U.boundaryField()[patchID]
         );

//- patch face normals      
vectorField n = mesh.boundary()[loadingIndex].nf();
      
//- set value fraction so as to have fixed value in patch normal direction
Upatch.valueFraction() = symm(n*n);

//- set fixed value
Upatch.refValue() = vector (0, 0, 0); //- put what ever vectorField you want here

//- set fixed gradient (as a function of position and time as an example)
const vectorField& patchFaceCentres = mesh.boundaryMesh()[patchID].faceCentre();
Upatch.refGradient() = patchFaceCentres * runTime.value();

By the way, "U" is my solution variable above.

Hope this helps,
Philip

[nlc]

Thank you for the fast answer. I will try it in a new Boundary condition. Base on directionMixed this is very nice of you.

An other question: In the constructors is it possible to call the "grand mother" constructor instead of the mother constructor in order to fix a default value for the mother class ???

For example in a class that depend on fixed Gradient I put

fvPatchFieldScalar(p, iF, dict),
gradient_(dict.lookupOrDefault<scalar>("gradient",0))

instead of
fixedGradientFvPatchScalarField(p, iF, dict),

in one of the constructor.

Regards