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Drag and lift coefficients of a oscillating cylinder

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Old   March 7, 2017, 13:47
Post Drag and lift coefficients of a oscillating cylinder
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Hello my friends,

I am simulating the flow around a oscillating cylinder (forced oscillation). For that, I used a UDF I found here at cfd-online. No problem regarding that, I can even see the oscillation in the solution's final animation.

However, I am facing a problem with the drag and lift monitors. Both return oscillating values (as expected), but those are in the figures of 10E4 or more, and show no major frequency of oscillation. In the literature, these Cd and CL values hardly go greater than 2.5 (when lock-in occurs, which my case is far from).

So far, I tried changing from the k-epsilon to the SST model; checked the mesh scale and the reference values; tested out different parameters of remeshing and smoothing (dynamic mesh); and even took the forces from Fluent and made some calculations by my own (which gave me results near those ridiculous values ). Now, I have no clue of what's causing this error, so here am I recurring to you guys - do you have any idea of what may be happening?

For your information, I am considering a 0.2m cylinder diameter with a inlet-velocity of 9E-4m/s, which gives a Reynolds=200. The oscillation has a 0.03m amplitude and frequency=0.16Hz. The fluid is water and the timestep is 0.02sec.

I wonder if this velocity is "too small" for Fluent maybe?

Hope you guys can save me in this one hahah
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Old   March 8, 2017, 04:46
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Quote:
Originally Posted by dbrin View Post
I wonder if this velocity is "too small" for Fluent maybe?
You are grasping at straws here. If that was the case you should blame your computer for not having quadruple precision .

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Originally Posted by dbrin View Post
For your information, I am considering a 0.2m cylinder diameter with a inlet-velocity of 9E-4m/s, which gives a Reynolds=200. The oscillation has a 0.03m amplitude and frequency=0.16Hz. The fluid is water and the timestep is 0.02sec.
A 200 mm diameter oscillating +/- 30 mm in water... That's not really an oscillating cylinder but a torpedo or small boat maybe? I would expect "large" forces. Are you sure you didn't use density of air or something instead of water to calculate the coefficients?
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Old   March 8, 2017, 18:04
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Thanks for the help, LuckyTran.
What do you mean exactly by "quadruple precision"? Because in the Fluent Launcher I only see a "Double Precision" in the options, which I marked. Would there be a third or fourth precision? [Probably I am grasping at straws in fact, but I tried many options and none of them seemed to help anything. I am getting tired of trying hahah].
Also, do you think it is in fact expected high force values? Maybe if I changed the oscillation's parameters I would get lower numbers, what do you think?
Once again, thank you for helping, at least I can discuss this matter with someone
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Old   March 8, 2017, 18:27
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There is no third or fourth precision. That is exactly my point. The reason there is no third or fourth precision is not Fluent's fault, but simply that your machine does not have it. If you want to blame really small numbers, then you need to call IEEE and have them come up with a triple/quadruple standard.

Do you even get the correct drag coefficient for a stationary cylinder?
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Old   March 8, 2017, 20:45
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Ok, I got it. I'll try a smaller diameter, bigger velocity combination, keeping Re=200. Maybe it will work finally.
Yes, it worked just fine for a stationary cylinder.
Thank you sir, I'll let you know any news.
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