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

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 February 12, 2016, 08:40 What should lref and Aref be for surface piercing cylinder in waves?? #1 Senior Member   ArielJ Join Date: Aug 2015 Posts: 127 Rep Power: 11 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.

 February 17, 2016, 08:47 #2 Senior Member   ArielJ Join Date: Aug 2015 Posts: 127 Rep Power: 11 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!

 February 22, 2016, 11:44 What should lref and Aref be for surface piercing cylinder in waves?? #3 Senior Member   ArielJ Join Date: Aug 2015 Posts: 127 Rep Power: 11 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!!

 February 22, 2016, 13:21 #4 Senior Member   ArielJ Join Date: Aug 2015 Posts: 127 Rep Power: 11 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 09:19. Reason: moved posts to the same thread

 February 23, 2016, 09:17 #5 Senior Member   Niels Gjoel Jacobsen Join Date: Mar 2009 Location: Copenhagen, Denmark Posts: 1,901 Rep Power: 37 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 __________________ Please note that I do not use the Friend-feature, so do not be offended, if I do not accept a request. Last edited by wyldckat; August 25, 2018 at 09:19. Reason: placed previous answer on another post here as well

February 25, 2016, 08:31
#6
Senior Member

ArielJ
Join Date: Aug 2015
Posts: 127
Rep Power: 11
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 = -, which gives me (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
 blockMeshDict.txt (12.1 KB, 7 views) controlDict.txt (1.8 KB, 5 views) forces_object.txt (1.3 KB, 10 views) forceCoeffs_object.txt (1.6 KB, 9 views)

March 2, 2016, 09:29
Interfoam boundary conditions for uniform flow in a wave tank
#7
Senior Member

ArielJ
Join Date: Aug 2015
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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
 system.zip (9.0 KB, 9 views) 0.zip (10.3 KB, 19 views)

 March 14, 2016, 08:22 #8 Senior Member   ArielJ Join Date: Aug 2015 Posts: 127 Rep Power: 11 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!

 March 14, 2016, 11:09 #9 Senior Member   Niels Gjoel Jacobsen Join Date: Mar 2009 Location: Copenhagen, Denmark Posts: 1,901 Rep Power: 37 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 __________________ Please note that I do not use the Friend-feature, so do not be offended, if I do not accept a request.

 March 15, 2016, 07:18 #10 Senior Member   ArielJ Join Date: Aug 2015 Posts: 127 Rep Power: 11 Ah ok I see. I've never used integrated forces actually... do you do this directly in paraview?

February 21, 2017, 16:21
#11
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Bashar
Join Date: Jul 2015
Posts: 74
Rep Power: 11
Quote:
 Originally Posted by arieljeds 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 .