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Old   April 13, 2015, 02:06
Default Mesh Motion is not consistent
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Hello guys I need to simulate an airfoil undergoing pitch + plunge as is done in the paper kinsey and dumas.

The pitch and plunge equations are as follows.

Code:
H = h *sin(wt)
theta = theta*cos(wt)
I used the following dynamicMeshDict file.

Code:
dynamicFvMesh      dynamicMotionSolverFvMesh;

motionSolverLibs ("libsixDoFRigidBodyMotion.so");

solver            sixDoFRigidBodyMotion;

sixDoFRigidBodyMotionCoeffs
{
    patches         (wing);
    innerDistance   0.3;
    outerDistance   14;

    mass            10000;
    centreOfMass    (0.3 0  0.125);
    momentOfInertia (1.958864357 3.920839234 10000);
    orientation
    (
        1 0 0
        0 1 0
        0 0 1
    );
    angularMomentum (0 0 -1579.136704); //605.3357367);
    velocity	    (0 0 0); //0.1507964474 0);
    g               (0 0 0);
    rhoName         rhoInf;
    rhoInf          2.5;
    report          on;

    constraints
    {
        yLine
        {
            sixDoFRigidBodyMotionConstraint line;
            centreOfRotation    (0.3 0 0.125);
            direction           (0 1 0);
        }

        zAxis
        {
            sixDoFRigidBodyMotionConstraint axis;
            axis                (0 0 1);
        }
    }

    restraints
    {
        verticalSpring
        {
            sixDoFRigidBodyMotionRestraint linearSpring;

            anchor          (0.3 -1 0.125);
            refAttachmentPt (0.3 0  0.125);
            stiffness       227.3956854;
            damping         0;
            restLength      2;
        }

        axialSpring
        {
            sixDoFRigidBodyMotionRestraint linearAxialAngularSpring;

            axis            (0.3 0 1);
            stiffness       227.3956854;
            damping         0;
            referenceOrientation $orientation;
        }
    }
}
I find that the resulting motion is not repeated consistently. Click here to check the video and you can observe that the trailing edge doesn't reach the initial position and steadily moves farther away. Any suggestions on how to fix that?



Thanks,
Pruthvi.

Last edited by pruthvi1991; April 16, 2015 at 13:05. Reason: Added color to highlight mass, moment of inertia and stiffness
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Old   April 14, 2015, 08:35
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How did you do this?

Does the mesh topology change? Did you use topology change dynamic mesh?
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Old   April 15, 2015, 08:26
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Hello Tar,

Yes the mesh topology changes. Look at this video . I used sixDofRigidBodyMotion.

Thanks,
Pruthvi.
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Old   April 16, 2015, 07:28
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Hi pruthvi,

Is w really the same for H an for theta?

For me it seems that H has an higher frequency, and therefore the airfoil reaches highest/lowest point before being horizontal again.

regards
Alex
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Old   April 16, 2015, 13:50
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Hello Alex,

I think omega is the same for both. Check out the dynamicMeshdict file I posted. The mass, MOI and stiffness are highlighted in red.

Thanks,
Pruthvi.
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Old   April 19, 2015, 12:00
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I cannot understand what you are trying to model here.

Is it forced oscillation motion like T. Kinsey and G. Dumas paper 2008? OR you are trying to model a response problem?

Because the equations of motion ,which are mentioned here, are for prescribed boundary motion, on the other hand you are using sixDoFRigidBodyMotion which calculate the displacement based on forces and moments every time step.
Sorry, I think, I am missing something here.

Bw,
Hassan



Quote:
Originally Posted by pruthvi1991 View Post
Hello guys I need to simulate an airfoil undergoing pitch + plunge as is done in the paper kinsey and dumas.

The pitch and plunge equations are as follows.

Code:
H = h *sin(wt)
theta = theta*cos(wt)
I used the following dynamicMeshDict file.

Code:
dynamicFvMesh      dynamicMotionSolverFvMesh;

motionSolverLibs ("libsixDoFRigidBodyMotion.so");

solver            sixDoFRigidBodyMotion;

sixDoFRigidBodyMotionCoeffs
{
    patches         (wing);
    innerDistance   0.3;
    outerDistance   14;

    mass            10000;
    centreOfMass    (0.3 0  0.125);
    momentOfInertia (1.958864357 3.920839234 10000);
    orientation
    (
        1 0 0
        0 1 0
        0 0 1
    );
    angularMomentum (0 0 -1579.136704); //605.3357367);
    velocity	    (0 0 0); //0.1507964474 0);
    g               (0 0 0);
    rhoName         rhoInf;
    rhoInf          2.5;
    report          on;

    constraints
    {
        yLine
        {
            sixDoFRigidBodyMotionConstraint line;
            centreOfRotation    (0.3 0 0.125);
            direction           (0 1 0);
        }

        zAxis
        {
            sixDoFRigidBodyMotionConstraint axis;
            axis                (0 0 1);
        }
    }

    restraints
    {
        verticalSpring
        {
            sixDoFRigidBodyMotionRestraint linearSpring;

            anchor          (0.3 -1 0.125);
            refAttachmentPt (0.3 0  0.125);
            stiffness       227.3956854;
            damping         0;
            restLength      2;
        }

        axialSpring
        {
            sixDoFRigidBodyMotionRestraint linearAxialAngularSpring;

            axis            (0.3 0 1);
            stiffness       227.3956854;
            damping         0;
            referenceOrientation $orientation;
        }
    }
}
I find that the resulting motion is not repeated consistently. Click here to check the video and you can observe that the trailing edge doesn't reach the initial position and steadily moves farther away. Any suggestions on how to fix that?



Thanks,
Pruthvi.
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Old   April 20, 2015, 20:24
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Hello Hassan,

I'm trying to do exactly what kinsey and dumas did. Forced oscillations. I'm not interested in the response of the system. I want to see the flow caused by the prescribed motion. Which other solver is apt for this kind of problem? I'm not very familiar with moving mesh techniques. Any help is appreciated in accurately describing the motion.

Thanks,
Pruthvi.
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Old   April 21, 2015, 07:42
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Hi Pruthvi,


There are two BC in OF for pointDisplacement which you can combine. I already used this approach before to test the dynamicMesh solvers in OF.
I will describe it as a tutorial instead just uploading the files.They are oscillatingDisplacement and angularOscillatingDisplacement. The source code is located in
Code:
$FOAM_SRC/fvMotionSolver/pointPatchFields/derived/
oscillatingDisplacement is for heave motion "H" which is harmonic oscillation
Code:
Field<vector>::operator=(amplitude_*sin(omega_*t.value()));
angularOscillatingDisplacement is for pitch motion "theta" which is harmonic as well
Code:
    scalar angle = angle0_ + amplitude_*sin(omega_*t.value());
    vector axisHat = axis_/mag(axis_);
    vectorField p0Rel(p0_ - origin_);

    vectorField::operator=
    (
        p0Rel*(cos(angle) - 1)
      + (axisHat ^ p0Rel*sin(angle))
      + (axisHat & p0Rel)*(1 - cos(angle))*axisHat
    );
To combine both the Heave and pitch degrees of freedom assuming that they are uncoupled around the elastic axis, you can do the following;
(I will try to make it as simple as possible to be suitable for new OF users. Please excuse me if you found it boring but I am trying to make it as useful as possible.)


1 - copy the "angularOscillatingDisplacement" to your run folder and rename it, lets call it for instance "twoDOscillatingDisplacement". Also replace "angularOscillatingDisplacement" with "twoDOscillatingDisplacement" inside the new twoDOscillatingDisplacementPointPatchVectorField.H and .C files.

2 - Now you have to add two new private data to .H file
Code:
    // added for H

        vector amplitudeH_; //Heave motion amplitude (m)
        scalar omegaH_;      // Heave motion frequency (rad/s)
3 - Then you have to add these variables in the four constructors in .C file (the red part)
Code:
//first
    p0_(p.localPoints()),
    amplitudeH_(vector::zero),
    omegaH_(0.0)   

//second
    omega_(readScalar(dict.lookup("omega"))),
    amplitudeH_(dict.lookup("amplitudeH")),
    omegaH_(readScalar(dict.lookup("omegaH")))

//Third
    p0_(ptf.p0_, mapper),
    amplitudeH_(ptf.amplitudeH_),
    omegaH_(ptf.omegaH_)

//fourth
    p0_(ptf.p0_),
    amplitudeH_(ptf.amplitudeH_),
    omegaH_(ptf.omegaH_)
4. this new BC in pointDisplacement should be defined as
Code:
     wing
    {
        type            twoDOscillatingDisplacement;
        axis            (0 0 1); // pitching axis
        origin          (0.14944506531 -0.01289078948 0.125); // elastic axis
        angle0          0; // mean theta
        amplitude       -0.10; // theta amplitude (rad +ve C.C.W)
        omega           100; // frequency for theta (rad/s)
        value           uniform (0 0 0);
        amplitudeH      (0 0.1 0); // H amplitude
        omegaH           100;   // H frequency
    }
5. modify the update function to add the Heave motion

Code:
    vectorField::operator=
    (
        p0Rel*(cos(angle) - 1)
      + (axisHat ^ p0Rel*sin(angle))
      + (amplitudeH_*sin(omegaH_*t.value()))
      + (axisHat & p0Rel)*(1 - cos(angle))*axisHat
    );
6. modify the write function

Code:
    os.writeKeyword("amplitudeH")
        << amplitudeH_ << token::END_STATEMENT << nl;
    os.writeKeyword("omegaH")
        << omegaH_ << token::END_STATEMENT << nl;
7. To compile this BC you need create Make folder with files and options files

files
Code:
twoDOscillatingDisplacementPointPatchVectorField.C


LIB = $(FOAM_LIBBIN)/libmy2DOF

options
Code:
EXE_INC = \
    -I$(LIB_SRC)/triSurface/lnInclude \
    -I$(LIB_SRC)/meshTools/lnInclude \
    -I$(LIB_SRC)/dynamicMesh/lnInclude \
    -I$(LIB_SRC)/finiteVolume/lnInclude \
    -I$(LIB_SRC)/fileFormats/lnInclude 

LIB_LIBS = \
    -ltriSurface \
    -lmeshTools \
    -ldynamicMesh \
    -lfiniteVolume
I think you will not need to add anything else to the options file.

8. finally try to complie it using wmake libso, then include the library in yourcase controlDict

9. modify dynamicMeshDict

Code:
dynamicFvMesh      dynamicMotionSolverFvMesh;

motionSolverLibs ("libfvMotionSolvers.so");

solver            displacementLaplacian;

displacementLaplacianCoeffs
{
    diffusivity       inverseDistance (wing); // try different diffusivity  
}
10. You can test the mesh and BC without the physical solver, just use moveDynamicMesh.


This is one approach to solve this problem but it isn't the only one. You can use Non-Conforming AMI Patches with solidBodyMotion like in the paper and in turbo-machinery applications (relative reference frames). I am not familiar with this approach therefore I cannot give you any tips about it.


Best wishes,
Hassan
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Old   May 1, 2015, 01:23
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Hi Pruthvi,

Which type of mesh motion solver do you use?

I mean, how did you use topoChanging mesh and sixDofRigidBodyDisplacement together?

Regards,
Thaw Tar
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Old   May 1, 2015, 01:26
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@Hassan Many thanks for your solution. I will try to compile it and tell you about the results.

@Thaw I just followed the wing motion tutorial in pimpleDyMFoam. I changed the values of stiffness and mass to get the desired motion.
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Old   May 1, 2015, 01:37
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Hi Pruthvi,

Thanks for your answer. Actually, I am doing 2D cylinder under vortex induced vibrations (viv) using sixDoFRigidBodyDisplacement. But if the motion amplitude is too high, the mesh distorts too much and the computation blows up.

Thus, I wanted to use some kinds of topoChangerMesh together with sixDoFRigidBody motion solver.

Regards
thaw tar
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Old   May 1, 2015, 01:47
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Hello Thaw,

Whats the max amplitude that you may face? I could go upto 1 chord length without any computational issues.
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Old   May 1, 2015, 04:09
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@ThawTar, it is expected that the dynamic mesh solver cannot handle too high amplitude. But before trying topoChangMesh, try to use prescribed BC (oscillating displacement) with high amplitude. Solve the mesh only using moveDynamicMesh solver and try different diffusion methods. Also make sure that in your case, you are starting from convergent solution for fixed cylinder.
Bw
Hassan
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Old   May 1, 2015, 07:41
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Hello Hassan,

I followed your instructions. They were pretty clear considering that I have little C++ knowledge. I couldn't compile it due to an error.

Code:
wmakeLnInclude: linking include files to ./lnInclude
make: *** No rule to make target `twoDOscillatingDisplacementPointPatchVectorField.dep', needed by `Make/linux64GccDPOpt/dependencies'.  Stop.
I'm not sure how to fix it. I uploaded the folder to github. You can take a look to see what I did.

Quote:
Also replace "angularOscillatingDisplacement" with "twoDOscillatingDisplacement" inside the new twoDOscillatingDisplacementPointPatchVectorField.H and .C files.
I could not find a mention of angularOscillatingDisplacement inside the .H file.
Code:
#ifndef oscillatingDisplacementPointPatchVectorField_H
#define oscillatingDisplacementPointPatchVectorField_H

#include "fixedValuePointPatchField.H"

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

namespace Foam
{

/*---------------------------------------------------------------------------*\
        Class oscillatingDisplacementPointPatchVectorField Declaration
\*---------------------------------------------------------------------------*/

class oscillatingDisplacementPointPatchVectorField
:
    public fixedValuePointPatchField<vector>
{
    // Private data

        vector amplitude_;
        scalar omega_;
   
    // added for H

        vector amplitudeH_; //Heave motion amplitude (m)
        scalar omegaH_;      // Heave motion frequency (rad/s)



public:

    //- Runtime type information
    TypeName("oscillatingDisplacement");


    // Constructors

        //- Construct from patch and internal field
        oscillatingDisplacementPointPatchVectorField
        (
            const pointPatch&,
            const DimensionedField<vector, pointMesh>&
        );

        //- Construct from patch, internal field and dictionary
        oscillatingDisplacementPointPatchVectorField
        (
            const pointPatch&,
            const DimensionedField<vector, pointMesh>&,
            const dictionary&
        );

        //- Construct by mapping given patchField<vector> onto a new patch
        oscillatingDisplacementPointPatchVectorField
        (
            const oscillatingDisplacementPointPatchVectorField&,
            const pointPatch&,
            const DimensionedField<vector, pointMesh>&,
            const pointPatchFieldMapper&
        );

        //- Construct and return a clone
        virtual autoPtr<pointPatchField<vector> > clone() const
        {
            return autoPtr<pointPatchField<vector> >
            (
                new oscillatingDisplacementPointPatchVectorField
                (
                    *this
                )
            );
        }

        //- Construct as copy setting internal field reference
        oscillatingDisplacementPointPatchVectorField
        (
            const oscillatingDisplacementPointPatchVectorField&,
            const DimensionedField<vector, pointMesh>&
        );

        //- Construct and return a clone setting internal field reference
        virtual autoPtr<pointPatchField<vector> > clone
        (
            const DimensionedField<vector, pointMesh>& iF
        ) const
        {
            return autoPtr<pointPatchField<vector> >
            (
                new oscillatingDisplacementPointPatchVectorField
                (
                    *this,
                    iF
                )
            );
        }


    // Member functions

        // Evaluation functions

            //- Update the coefficients associated with the patch field
            virtual void updateCoeffs();


        //- Write
        virtual void write(Ostream&) const;
};


// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

} // End namespace Foam

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

#endif

// ************************************************************************* //
Any help would be greatly appreciated. Also do you have any suggestions about where I can read about moving meshes in general. I have no knowledge of the inner workings of this topic. I'm blindly following the tutorials.

Thanks ,
Pruthvi.
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Old   May 1, 2015, 08:10
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You have to change each instance in .C and .H from oscillatingDisplacement to twoDoscillatingDisplacement. Check the Github, hpfully the changes will appear for you because I am not familiar with it

Computational methods for fluid dynamics has a small section about dynamic meshes. Also review Jasak's papers about dynamicmesh handling in OpenFOAM.

About the diffusion coefficient effect there is a report from Charmers about it
http://www.tfd.chalmers.se/~hani/kur...AM-rapport.pdf

Please note since OF2.3.0 there are major changes in the solid body motion library check
http://www.openfoam.org/version2.3.0/mesh-motion.php
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Old   May 1, 2015, 09:12
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I noticed extra few points needs to be modified in your code. I will search for my old code and upload it for you.
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Old   May 1, 2015, 10:17
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Here is the link
https://github.com/HIKassem/twoDOscillatingDisplacement

it is my first time to use github, so it could be not working or I messed up something.
Bw,
Hassan
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Old   May 10, 2015, 07:45
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Hi Hassan,

Your code worked fine. I was able to run a couple of simulations. Nice implementation. I ran into some SigFpe error. Now debugging. I will let you know if I can debug.

Nice picking up github .

Thanks,
Pruthvi.
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Old   June 19, 2015, 21:42
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Hello Mr.Hassan,

I modified the twoDOscillaitngMotion.C file so that I can introduce a phase difference of pi/2 between the pitching and plunging motions. The simulation keeps blowing up. I used checkMesh and found that the quality was degrading at the cells generated by layer addition. To rectify I switched off layer addition and tried again. This has eleminated most of the mesh check errors but the courant number is still blowing up.

All the files are here ----> https://github.com/pruthvi1991/solve...r/Kinsey_Dumas

Here is a checkMesh log when the Courant number blows up but the mesh quality is OK

Code:
Time = 9

Mesh stats
    points:           74594
    internal points:  0
    faces:            146827
    internal faces:   73328
    cells:            36510
    faces per cell:   6.029991783
    boundary patches: 6
    point zones:      0
    face zones:       0
    cell zones:       0

Overall number of cells of each type:
    hexahedra:     35543
    prisms:        11
    wedges:        0
    pyramids:      0
    tet wedges:    0
    tetrahedra:    0
    polyhedra:     956
    Breakdown of polyhedra by number of faces:
        faces   number of cells
            7   806
            8   150

Checking topology...
    Boundary definition OK.
    Cell to face addressing OK.
    Point usage OK.
    Upper triangular ordering OK.
    Face vertices OK.
    Number of regions: 1 (OK).

Checking patch topology for multiply connected surfaces...
                   Patch    Faces   Points                  Surface topology
            topAndBottom      160      324  ok (non-closed singly connected)
                   inlet      104      210  ok (non-closed singly connected)
                  outlet      104      210  ok (non-closed singly connected)
                    wing      111      222  ok (non-closed singly connected)
                   front    36510    37297  ok (non-closed singly connected)
                    back    36510    37297  ok (non-closed singly connected)

Checking geometry...
    Overall domain bounding box (-35 -35 0) (40 35 0.5)
    Mesh (non-empty, non-wedge) directions (1 1 0)
    Mesh (non-empty) directions (1 1 0)
    All edges aligned with or perpendicular to non-empty directions.
    Boundary openness (6.627104753e-19 4.888447076e-22 -1.263269968e-14) OK.
    Max cell openness = 2.199932693e-16 OK.
    Max aspect ratio = 3.839901044 OK.
    Minimum face area = 4.396716366e-05. Maximum face area = 1.945327376.  Face area magnitudes OK.
    Min volume = 2.198358183e-05. Max volume = 0.9726636881.  Total volume = 2624.959185.  Cell volumes OK.
    Mesh non-orthogonality Max: 64.3969137 average: 9.591723067
    Non-orthogonality check OK.
    Face pyramids OK.
    Max skewness = 3.007630562 OK.
    Coupled point location match (average 0) OK.

Mesh OK.

End
Here is another one at max rotation amplitude (Gathered from mesh motion only, did not solve for fluid)

Code:
Create time

Create polyMesh for time = 25

Time = 25

Mesh stats
    points:           74594
    internal points:  0
    faces:            146827
    internal faces:   73328
    cells:            36510
    faces per cell:   6.029991783
    boundary patches: 6
    point zones:      0
    face zones:       0
    cell zones:       0

Overall number of cells of each type:
    hexahedra:     35543
    prisms:        11
    wedges:        0
    pyramids:      0
    tet wedges:    0
    tetrahedra:    0
    polyhedra:     956
    Breakdown of polyhedra by number of faces:
        faces   number of cells
            7   806
            8   150

Checking topology...
    Boundary definition OK.
    Cell to face addressing OK.
    Point usage OK.
    Upper triangular ordering OK.
    Face vertices OK.
    Number of regions: 1 (OK).

Checking patch topology for multiply connected surfaces...
                   Patch    Faces   Points                  Surface topology
            topAndBottom      160      324  ok (non-closed singly connected)
                   inlet      104      210  ok (non-closed singly connected)
                  outlet      104      210  ok (non-closed singly connected)
                    wing      111      222  ok (non-closed singly connected)
                   front    36510    37297  ok (non-closed singly connected)
                    back    36510    37297  ok (non-closed singly connected)

Checking geometry...
    Overall domain bounding box (-35 -35 0) (40 35 0.5)
    Mesh (non-empty, non-wedge) directions (1 1 0)
    Mesh (non-empty) directions (1 1 0)
    All edges aligned with or perpendicular to non-empty directions.
    Boundary openness (6.61854997e-19 9.776894152e-22 6.381205446e-15) OK.
    Max cell openness = 2.354408509e-16 OK.
    Max aspect ratio = 3.293216054 OK.
    Minimum face area = 5.64186484e-05. Maximum face area = 1.952868252.  Face area magnitudes OK.
    Min volume = 2.82093242e-05. Max volume = 0.9764341259.  Total volume = 2624.959185.  Cell volumes OK.
    Mesh non-orthogonality Max: 52.72379189 average: 8.614508389
    Non-orthogonality check OK.
    Face pyramids OK.
 ***Max skewness = 4.529272567, 1 highly skew faces detected which may impair the quality of the results
  <<Writing 1 skew faces to set skewFaces
    Coupled point location match (average 0) OK.

Failed 1 mesh checks.

En
Do you have any suggestions about fixing this problem? The mesh skewness is from the trailing edge element I suspect. I'm going to try an unstructured mesh to see what happens.

Can this problem be solved by trying a different diffusivity? I tried quadratic already.

Last edited by pruthvi1991; June 20, 2015 at 18:32.
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Old   June 20, 2015, 06:42
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Hassan Kassem
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I have few questions,

Could you please tell me which case on github you are using?
Are you using physical solver of moveDynamicMesh?
is the solver blow-up even when you use small amplitude values?

Bw,
Hassan
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