Various questions and doubts about simulating Drag Coefficient of Cylinder
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Hello all,
I have been attempting to calculate Cd for a cylinder in sea water using a 2D case. Diameter = 5mI'm using SimpleFoam solver with Realisable k-e as the turbulence model. In the documentation I am using for comparison/validation, the value of Cd listed for a smooth cylinder is 0.65 in steady-state flow situation. The value I calculated for Cd were 0.159 and 0.113 for time steps of 1e-2 and 1e-3, respectively. Both of which are far lower than they should be. My questions: 1. My mesh is limited to 100k nodes, is this too coarse to generate an accurate result? (see attached) 2. Why is the result seemingly independent of the fluid density? (it is never required as an input) 3. Why is there lift measured for the smaller of the two time steps? My requests: 1. General advice on calculation of drag coefficients for high Re 2. Any helpful advice would be greatly appreciated Attached are pictures of my mesh, and residuals & results for each run. Attachment 50422 Attachment 50420Attachment 50421 Attachment 50423 Attachment 50424 I hope someone can help - I feel like this is a simple problem but I am really struggling with it. Thank you very much! |
For clarity, these were formed using SimFlow - a GUI for OpenFOAM. I hope this is an appropriate place to post this as there is currently no SimFlow specific sub.
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Case Summary
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Here is the case summary:
Attachment 50425 |
Aref // lref
Bonus question (for extra points): What is the use of Aref/lref in 2D simulation?
I am at the end of my tether with this. Please demystify these issues and I will be eternally grateful. |
Extremely helpful, thank you.
How can one measure a value for Δz in a 2D simulation? As I understand it, OpenFOAM doesn't truly model in 2D? Is Δz therefore the cell size in z direction? Secondly, I can't make sense of why Cd should be so dependent on Area? I understand that it is intrinsically connected to the Force but cannot seem get round this circular relationship. Linear relationship between value of Cd and variance in Area? Lastly, changing "lref" seems to have no effect on the solution at all? Sorry to be a burden, thanks for all the help so far. |
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// lift, drag and moment Quote:
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Brilliant, thank you.
That is what I had initially thought, I think my confusion stemmed from a tutorial video produced by SimFlow themselves: https://www.youtube.com/watch?v=wASYUxQ9LMc at ~10:00 they set the values of lref and Aref. For the latter he sets it to be equal to dcyl^2 . Any idea why this may have been done? It is a similar case, in that it is 2D and of a cylinder in crossflow. All the best, Angus |
As it stands...
4 Attachment(s)
Recap / Overview:
Attempting to calculate drag coefficients for a smooth cylinder in crossflow using 2D steady state, incompressible case. The expected value from literature (http://www.germanlloyd.org/pdf/DNV-OS-J101_2014-05.pdf) is Cd = 0.65 for steady flow.Diameter = D = 5m Current state: After refining the mesh as much as possible within the 100k node limit and resetting the discussed parameters (Δt, lref, Aref, relaxation) the case converged in 5,118 iterations. The calculated value was found to be Cd = 2.66712 which is far larger than the expected. (Somewhat humorously, it is nearly equidistant from the expected solution as my initial calculation) Case Details: Mesh: See attached. Solver: SimpleFoam Turbulence Model: Realizable k-ε Model with full defaults on constants. Discretisation: Steady-state. Using LUST scheme for U, k and ε convection. Boundary Conditions: Inlet: Velocity inlet, of type "Surface Normal Fixed Value" (=U=1m/s) Initial Conditions: U=1m/s Monitor Reference Values: lref = D = 5 Questions: {{ Expected: Cd=0.65 // Calculated: Cd=2.66712 }}
Pictures: Mesh: Attachment 50474 Attachment 50475 Residuals & Case Summary: Attachment 50476 Attachment 50477 Any guidance at this point (specific or otherwise) would be greatly appreciated. Kind regards, Angus |
Hi,
my time is very limited but I uploaded my project work onto my web space (unfortunately it is in German). Nonetheless, maybe it will help you. You will find it here: http://www.holzmann-cfd.de/index.php...karman-strasse I am wondering why your projected area is equal to 5 - and if you say it is the diameter, the unit would be [m] and not [mē]. I can not belive that you extrude the mesh in z - direction to 1m. |
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Thank you very much for such details explanation. I want to calculate Cd1, Cd2, Cl1 and Cl2 for tandem cylinders (two cylinders are placed in a row). So, what change i have to do, Please suggest. Thanks |
not getting CD calue
hi guys
forces { type forceCoeffs; libs ("libforces.so"); writeControl timeStep; writeInterval 1; patches ("inner"); rho rhoInf; log true; rhoInf 1000; liftDir (0 1 0); dragDir (1 0 0); CofR (0 0 0); pitchAxis (1 0 0); magUInf 0.6739; lRef 0.2136; Aref 0.01958; } this is my forcecoeffs setup but m not getting proper cd value can any one plz help me on that where i m going wrong? Using simplefoam. |
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