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[waves2Foam] What should lref and Aref be for surface piercing cylinder in waves??

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Old   February 12, 2016, 09:40
Default What should lref and Aref be for surface piercing cylinder in waves??
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ArielJ
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Hi everyone,

I'm running a multiphase case with regular waves (using waveFoam) passing a vertical surface piercing cylinder and I'm trying to calculate the forces and force coefficients. I have them set up and they are being calculated and in the right place, but the values look very very wrong.

1) First of all, the values for the drag and lift coefficients are oscillating... does this suggest that they are not converging? I expected to see one value after the wave has reached the cylinder. The values not only are changing, but massively increase.

2) Is it even possible to use the forces and forceCoeffs function objects as they are with a multiphase solver?? I'm wondering if I have defined the phases incorrectly? Or if I need to separate the U and p values somehow into U.water and U.air, etc?

For instance, I am confused in my transportProperties file because I have the transport model set to Newtonian, so I commented out all other information (BirdCarreauCoeffs, CrossPowerLawCoeffs), as shown below:

Code:
phases (water air);

water
{
    transportModel  Newtonian;
    nu              nu [ 0 2 -1 0 0 0 0 ] 1e-06;
    rho             rho [ 1 -3 0 0 0 0 0 ] 1028;
    /*
    CrossPowerLawCoeffs
    {
        nu0             nu0 [ 0 2 -1 0 0 0 0 ] 1e-06;
        nuInf           nuInf [ 0 2 -1 0 0 0 0 ] 1e-06;
        m               m [ 0 0 1 0 0 0 0 ] 1;
        n               n [ 0 0 0 0 0 0 0 ] 0;
    }

    BirdCarreauCoeffs
    {
        nu0             nu0 [ 0 2 -1 0 0 0 0 ] 0.0142515;
        nuInf           nuInf [ 0 2 -1 0 0 0 0 ] 1e-06;
        k               k [ 0 0 1 0 0 0 0 ] 99.6;
        n               n [ 0 0 0 0 0 0 0 ] 0.1003;
    }
    */
}

air
{
    transportModel  Newtonian;
    nu              nu [ 0 2 -1 0 0 0 0 ] 1.568e-05; //1.48e-05;  --> current measurement for kinematic viscosity found at engineeringtoolbox.com
    rho             rho [ 1 -3 0 0 0 0 0 ] 1;
    /*
    CrossPowerLawCoeffs
    {
        nu0             nu0 [ 0 2 -1 0 0 0 0 ] 1e-06;
        nuInf           nuInf [ 0 2 -1 0 0 0 0 ] 1e-06;
        m               m [ 0 0 1 0 0 0 0 ] 1;
        n               n [ 0 0 0 0 0 0 0 ] 0;
    }

    BirdCarreauCoeffs
    {
        nu0             nu0 [ 0 2 -1 0 0 0 0 ] 0.0142515;
        nuInf           nuInf [ 0 2 -1 0 0 0 0 ] 1e-06;
        k               k [ 0 0 1 0 0 0 0 ] 99.6;
        n               n [ 0 0 0 0 0 0 0 ] 0.1003;
    }
    */
}

sigma           sigma [ 1 0 -2 0 0 0 0 ] 0.00;

My forces file is:

Code:
forces_object
{
    type            forces;  
    functionObjectLibs ( "libforces.so" );

    enabled         true;
    outputControl   timeStep;
    outputInterval  1;

    pName           p;
    UName           U;	
    rhoName         rhoInf; 	// Incompressible solver
    log             on;

    
    rhoInf      1.0;    	// Fluid density
    patches     ( cylinder); 

    CofR        (0 0 0);
    pitchAxis   (0 0 1);	// Gives moment coefficient

}
And forceCoeffs file:

Code:
forceCoeffs_object
{
	type 	forceCoeffs;
	functionObjectLibs ("libforces.so");
	patches (cylinder);

	pName 	p;
	Uname 	U;
	rhoName rhoInf;				// Reference Density 
	rhoInf  1.0;				// Density/1000
	
	log 	true;				// Dump to file -- change to false to not save a file 

	CofR 	(0.0 0 0);			// Centre of rotation 
	liftDir (0 1 0);			// Direction of lift coefficient  ---> y-direction (transverse to flow)   ?????
	dragDir (1 0 0);			// Direction of drag coefficient  ---> drag acts in opposite direction to flow (-x) 
	pitchAxis (0 0 1);			// Pitch moment axis  --> changed from pitchAxis (0 0 1)
	magUInf 0.4622;				// free stream velocity magnitude 
	lRef 6.0;         			// reference length (cylinder diameter)
	Aref 1.488;         			// reference area: lref * zmesh (zmesh is approx. pi*D/76)

        outputControl   timeStep;
        outputInterval  1;
}
I would really appreciate any help on this, I know it's a basic application but I am still very new to Openfoam and cfd and am struggling to find an explanation to help me understand what is going on.
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Old   February 17, 2016, 09:47
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Hi, I'm posting again here because I'm still having the same issue! I am calculating hugely incorrect drag coefficients (I am expecting typical values for a cylinder in waves and am getting a fluctuating drag coefficient with an average at -2,6222!

Some of my files are posted in the previous post so I won't repost. Can anyone please please offer advice on getting the drag coefficient?? I am so confused about why the values are so crazy! Please help!
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Old   February 22, 2016, 12:44
Default What should lref and Aref be for surface piercing cylinder in waves??
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Hi, I know that this is a really common thread topic, but after looking at these threads and sites (among many others):

http://www.cfd-online.com/Forums/ope...lref-aref.html

http://www.sourceflux.de/blog/lamina...ound-cylinder/

I know that lref is the reference length and Aref is the reference area but I am so confused on how to set these correctly. I am getting non-convergent drag values on the order of 10^6 and I've been trying to understand where I'm going wrong!

Can someone please explain to me how to correctly set lref and Aref?? I've been reading so many different things I am thoroughly confused on how to set it for my case. My cylinder diameter is 6 m, the cylinder extends down 10 m below the surface and up 10 m above the surface. I am using waveFoam.

Please help!!
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Old   February 22, 2016, 14:21
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Hi everyone,

I've put up a similar posting here [Moderator note: it's the post above] but posting this here too because I'm running waveFoam.

First of all, I am trying to get drag and lift coefficients. I was getting oscillating values so I now have a case running for much longer to see if it converges. However, I am also getting values for the forces and pressures that I suspect are at least two orders or magnitude higher than I am expecting.

Can anyone please help me understand what the units of p_rgh and drag force are and what exactly is being calculated? I know this is a stupid question but I can't for the life of me see where the numbers are coming from even though the solution "looks" right.

note: I have added together the viscous and pressure components of the in-line force to calculate drag). I am very very confused why my values seem so much higher than I expect (but would likely explain the very high drag values??)

Please help! I'm so confused... I have been reading everything I can find and I can find NOTHING that explains this in a way that I can follow.

Last edited by wyldckat; August 25, 2018 at 10:19. Reason: moved posts to the same thread
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Old   February 23, 2016, 10:17
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Niels Gjoel Jacobsen
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Hi Ariel,


Try to compare with the force from the Morrison equation.

The following reference could also be a good place to get confidence in the results:

Code:
@article{ paulsenBredmoseBingham2014,
Author = {Paulsen, B. T. and Bredmose, H. and Bingham, H. B.},
Title = {{An efficient domain decomposition strategy for wave loads on surface piercing circular cylinders}},
Journal = {{Coastal Engineering}},
Year = {{2014}},
Volume = {{86}},
Pages = {{57-76}},
}
Kind regards,

Niels
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Last edited by wyldckat; August 25, 2018 at 10:19. Reason: placed previous answer on another post here as well
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Old   February 25, 2016, 09:31
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Hi everyone,

I am trying to run free surface waves flowing past a surface piercing vertical cylinder. I am using waveFoam for the simulation. I've been trying to fix this for weeks now and getting very frustrated and not able to make any progress! I am attempting to run a case using linear wave theory around a cylinder that has a diameter of 6m and the water depth is 25 m. I have the case running and everything "looks" correct. The velocity values seem to be correct.

I have 2 major problems:

1) I want to get an estimate of my pressures analytically, so I used p = -\rho\frac{\partial \phi}{\partial t}, which gives me p = \rho g a (cosh terms cancel when z = 0). This gives me a value of approximately 1.51x10^4 Pa. In my simulations, however, I am getting very different values. I read that p (or p_rgh) is divided by rho but this still doesn't make sense??

2) My drag coefficient is not only not settling down to any value but I am getting values with a magnitude of 10^5!!! I have been posting questions and asking and researching and I cannot work out where I'm going wrong... very frustrating!!!

I'm attaching my blockMeshDict, my controlDict file, and the input files forces_object and forceCoeffs_object.

I would really really appreciate any help or advice. I have been really struggling with this now for weeks and have not been able to get any answers.
Attached Files
File Type: txt blockMeshDict.txt (12.1 KB, 7 views)
File Type: txt controlDict.txt (1.8 KB, 4 views)
File Type: txt forces_object.txt (1.3 KB, 10 views)
File Type: txt forceCoeffs_object.txt (1.6 KB, 9 views)
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Old   March 2, 2016, 10:29
Default Interfoam boundary conditions for uniform flow in a wave tank
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ArielJ
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Hi everyone,

I'm trying to run a case using interFoam to calculate uniform flow past a cylinder. I am having issues getting the case to run properly.

For one, the flow seems to be radiating outward and being affected by the side walls, even though they should be far enough away.

I am including my system and my 0 directories to show what boundary conditions I'm using.

I'm also not sure if my setFieldsDict is set up correctly.

I really appreciate any help or advice anyone might have to offer to help me fix my case.

Thank you!!
Attached Files
File Type: zip system.zip (9.0 KB, 9 views)
File Type: zip 0.zip (10.3 KB, 18 views)
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Old   March 14, 2016, 09:22
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Hi Niels,

Thanks for getting back to me, I'll look into that paper and will compare forces.

I think that my problem might lie in how I'm setting up my forces functionObject. I know this is really basic but I'm getting so frustrated that everything appears correct, but I can't get convergence or numbers that make sense.

Does this make sense:

if lref = diameter = 6 m, for a surface piercing cylinder, Aref = pi*D*h, where h = depth = 25 m??

And for a flow moving from the negative x to positive x:

Code:
CofR (0 0 0);    // For cylinder with origin at (0,0,0)
liftDir (0 1 0); // Where the y domain is width
dragDir (1 0 0);
I am convinced that something in this file is incorrect because my simulations look correct. I am currently attempting this using only a potentialCurrent to see if I could make sense of my results.


This has been driving me crazy because I know it's something basic but I just don't understand where I'm going wrong!
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Old   March 14, 2016, 12:09
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Niels Gjoel Jacobsen
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Hi,

I have never used the force coefficient functionObject, merely the integrated forces. I like to be as close as possible to the raw data (without knowing the exact pressure field) and for me, the force coefficients are one step too far.

Kind regards,

Niels
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Old   March 15, 2016, 08:18
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Ah ok I see. I've never used integrated forces actually... do you do this directly in paraview?
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Old   February 21, 2017, 17:21
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Quote:
Originally Posted by arieljeds View Post
Hi everyone,

I'm trying to run a case using interFoam to calculate uniform flow past a cylinder. I am having issues getting the case to run properly.

For one, the flow seems to be radiating outward and being affected by the side walls, even though they should be far enough away.

I am including my system and my 0 directories to show what boundary conditions I'm using.

I'm also not sure if my setFieldsDict is set up correctly.

I really appreciate any help or advice anyone might have to offer to help me fix my case.

Thank you!!
Hi,
Did you manage to setup your case? I have similar issues in 2D simulation of flow past plate .
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