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-   -   Airfoil moving inside a fluid - Results discussion (http://www.cfd-online.com/Forums/star-ccm/120321-airfoil-moving-inside-fluid-results-discussion.html)

 GM_XIII July 4, 2013 09:13

Airfoil moving inside a fluid - Results discussion

2 Attachment(s)
Hi all:

I am trying to simulate an airfoild moving inside a fluid in Star CCM. The airfoil must move as a chirp function that i have defined. I ran it smoothly without problems using the following Models:

Segregated Fluid Temperature, Gravity, Two-Layer All y+ Wall Treatment, Realizable K-Epsilon Two-Layer, Constant Density, Rigid Body Motion and Three Dimensional.

I set a enough number of inner iterations to keep the residuals low. The problem is when i analyse the results they dont seem reasonable. I suspect the problem is caused by calculating the Downforce refered to the Laboratory Coordinate System instead of one linked to the airfoil, as the ''strage' fluctioations occur when the acelerattion change its sign.

I upload you some images and an excell document with all the data directly from star-ccm.

Very thanks you

pd: https://www.dropbox.com/s/juffyv05vouozvv/chirp.xlsx here is the excel data

 dahicke July 4, 2013 18:19

Hi David, Have you looked at a velocity vector plot on the symmetry line to see, if there is some identifiable cause to the fluctuation? If not, try plotting pressure as color with velocity vectors on the background. Is the flow separating at that reversal? Have you refined the mesh to determine that it is not influencing the flow? Good Luck, Dave

 GM_XIII July 5, 2013 08:09

Quote:
 Originally Posted by dahicke (Post 437878) Hi David, Have you looked at a velocity vector plot on the symmetry line to see, if there is some identifiable cause to the fluctuation? If not, try plotting pressure as color with velocity vectors on the background. Is the flow separating at that reversal? Have you refined the mesh to determine that it is not influencing the flow? Good Luck, Dave
Thank you for your answer Dave. I will record that representation as soon as i can but correct me i'm wrong, if ihad reversal i shouldn'n have had a lower value of Downforce. By the way,i dint post it but in the drag this fluctuation is not that notorial.

 dahicke July 5, 2013 17:02

If I understand your setup right, you have a symmetric airfoil at some angle of attack which is vibrating at your specified chirp function driving z position. You are measuring down force (-lift) and drag over time as the system vibrates.

I am thinking that at the change in direction depending on the rate of acceleration, you may see a momentary flow separation due to the change in momentum of the air around the foil, which is unable make the abrupt change in direction. If this is happening, then down force would fluctuate because of loss of lift by the air foil.

Dave

 psk July 18, 2013 13:40

Flapping flat plate-pointDisplacement quries

Good Day Folks,

I am trying to do flapping flat plate like cantilever beam simply up & down with amplitude is -12 to +12 ( similar to this you tube video: https://www.youtube.com/watch?v=s_gtukpyHXk ) and also wing beat frequency =10 Hz.

I am go through wing motion tutorial in pimpleDyMFoam in OF 1.7. I don't know how to set pointDisplacement for my case.I am looking only translation not rotation so, i can delete fixed axis ,( ie in red color) i hope its correct. kindly secure and guide me, how i can proceed?

_______________________________________________
dimensions [0 1 0 0 0 0 0];

internalField uniform (0 0 0);

boundaryField
{
movingwing
{
type sixDoFRigidBodyDisplacement;
mass 22.9;
centreOfMass (0.4974612746 -0.01671895744 0.125);
momentOfInertia (1.958864357 3.920839234 2.057121362);
orientation
(
0.9953705935 0.09611129781 0
-0.09611129781 0.9953705935 0
0 0 1
);
velocity (0 0 0);
acceleration (0 0 0);
angularMomentum (0 0 -0.5);
torque (0 0 0);
rhoName rhoInf;
rhoInf 1;
g (0 -9.81 0);
report on;
constraints
{
maxIterations 500;

fixedLine1
{
sixDoFRigidBodyMotionConstraint fixedLine;
tolerance 1e-9;
relaxationFactor 0.7;
fixedLineCoeffs
{
refPoint (0.25 0.007 0.125);
direction (0 1 0);
}
}

fixedAxis1
{
sixDoFRigidBodyMotionConstraint fixedAxis;
tolerance 1e-06;
relaxationFactor 0.7;
fixedAxisCoeffs
{
axis ( 0 0 1 );
}
}
}

restraints
{
verticalSpring
{
sixDoFRigidBodyMotionRestraint linearSpring;

linearSpringCoeffs
{
anchor (0.25 0.007 0.125);
refAttachmentPt (0.25 0.007 0.125);
stiffness 4000;
damping 2;
restLength 0;
}
}
axialSpring
{
sixDoFRigidBodyMotionRestraint linearAxialAngularSpring;

linearAxialAngularSpringCoeffs
{
axis (0 0 1);
stiffness 700;
damping 0.5;
referenceOrientation \$orientation;
}
}
}
value uniform (0 0 0);
}

inlet
{
type fixedValue;
value uniform (0 0 0);
}

outlet
{
type fixedValue;
value uniform (0 0 0);
}

symmetry-------> front
{

symmetryPlane;

}

symmetry-------> Top
{

symmetryPlane;

}
symmetry-------> Bottom
{

symmetryPlane;

}

wallone----------> opposite to front( ie Back)
{

type fixedValue;
value uniform (0 0 0);
}

".*"
{
type fixedValue;
value uniform (0 0 0);
}
}
____________________________________

Million thanks,
david

 GM_XIII July 18, 2013 16:55

Hi David

It is great to see you are trying to run a similar case and i am also interested in running this kind of symulations on Openfoam. But in order to openfoam expert see it, you should publish a new post in the openfoam forum, when you do so please let me kniw by pm ;)

About my case, i couldnt do the graphic that dahicke advised me as i have not full access to the cluster where i run my simulations but i will have the results soon.

Best regards

 psk July 18, 2013 22:29

Greetings GM_XIII,

(But in order to openfoam expert see it, you should publish a new post in the openfoam forum, when you do so please let me kniw by pm)------->I don't know what it means,If u don't mind please be specific?

If you have any idea about my problem( see above post) its look like cantilever beam to flap up & down, how I can set for amplitude angle-12 to +12, and let frequency is 10hz.

Million Thanks,
david

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