# Alletto - 2022 - Comparison of overset mesh with morphing mesh

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 February 18, 2022, 16:22 #21 New Member   Alvi Ahmmed Join Date: Nov 2020 Posts: 7 Rep Power: 5 Thank you very much. I can get the natural frequency(fn) from St=fnD/U equation right? And am I free to choose U and D for a fixed Re=100?

 February 18, 2022, 17:14 #22 Senior Member   Michael Alletto Join Date: Jun 2018 Location: Bremen Posts: 615 Rep Power: 15 You can choose the parameters as you want as long all dimensional less parameters are the same. The frequency in the st number is the shedding frequency not the natural frequency

 February 19, 2022, 01:54 #23 New Member   Alvi Ahmmed Join Date: Nov 2020 Posts: 7 Rep Power: 5 How can I find the value of natural frequency (fn) ? From the FFT plot of the lift coff? Regards.

 February 19, 2022, 02:05 #24 Senior Member   Michael Alletto Join Date: Jun 2018 Location: Bremen Posts: 615 Rep Power: 15 See https://en.m.wikipedia.org/wiki/Natural_frequency For the natural frequency of a spring mass system. But I think I mentioned this also in the paper. Read it carefully. What is done in the python scripts is also explained in the paper. A fft of the lift signal is performed and the maximum amplitude is used to calculate the St number. Read the paper a few times if once is not enough. I usually need three or four times to fully understand a paper Tianyz likes this.

 February 19, 2022, 02:08 #25 New Member   Alvi Ahmmed Join Date: Nov 2020 Posts: 7 Rep Power: 5 Thanks very much for your guidence

 March 22, 2022, 01:44 How to set the density in the VIV case #26 New Member   Mehdi Badri Join Date: Apr 2013 Posts: 7 Rep Power: 13 Hi, According to the definition of non-dimensional mass, , on page 18 of your paper, the density of the cylinder, , is calculated as . In your paper (and its corresponding git repo), you have used , whereas it seems that if we substitute , and , the density should be . Could you please clarify how you calculated it (and why my understanding is perhaps incorrect)?

 March 22, 2022, 03:16 #27 Senior Member   Michael Alletto Join Date: Jun 2018 Location: Bremen Posts: 615 Rep Power: 15 Hello. I just found a typo in how the fluid density is calculated. The non dimensional mass m* is defined as follows in the paper: Unfortunately I forgot the pi. It should have been With the second formula we obtain the value of the fluid density specified in the paper Mehdi80 likes this.

March 22, 2022, 19:58
Thank you
#28
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Quote:
 Originally Posted by mAlletto Hello. I just found a typo in how the fluid density is calculated. The non dimensional mass m* is defined as follows in the paper: Unfortunately I forgot the pi. It should have been With the second formula we obtain the value of the fluid density specified in the paper

That is a nice explanation and helpful note!

Thanks

March 22, 2022, 23:21
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zink
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It was missing in the formula but the calculation of the density is the correct in the tables and paragraphs.

Quote:
 Originally Posted by Mehdi80 That is a nice explanation and helpful note! Thanks

 April 5, 2022, 11:12 citation #30 New Member   Wael Elorfi Join Date: Feb 2022 Location: Egypt Posts: 2 Rep Power: 0 Is there a bibtex file available for citing this paper?

 April 5, 2022, 11:34 #31 Senior Member   Michael Alletto Join Date: Jun 2018 Location: Bremen Posts: 615 Rep Power: 15 I usually copy the bibtex entry from Google scholar welorfi likes this.

 April 5, 2022, 13:40 citation #32 New Member   Wael Elorfi Join Date: Feb 2022 Location: Egypt Posts: 2 Rep Power: 0 Got it. Thanks a lot! @article{alletto2022comparison, title={Comparison of overset mesh with morphing mesh: Flow over a forced oscillating and freely oscillating 2D cylinder}, author={Alletto, Michael}, journal={OpenFOAM{\textregistered} Journal}, volume={2}, pages={13--30}, year={2022} }

 April 11, 2022, 05:15 Doi #33 Senior Member   Håkan Nilsson Join Date: Mar 2009 Location: Gothenburg, Sweden Posts: 204 Rep Power: 18 This paper has now been updated with the ISSN of the journal and its own DOI: https://doi.org/10.51560/ofj.v2.47 welorfi likes this.

July 7, 2022, 12:42
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zink
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Hi,

Can anyone explain the m in this equation? Is it only the mass of the cylinder?
Does this include the added mass as well?

Quote:
 Originally Posted by mAlletto U* = Uoo/(fn D) , fn = ω/(2 π) = (k / m)0.5/(2 π) So if you want to achieve a given U* for a given diameter of the cylinder with a given mass and inflow velocity you have to set k accordingly. Hope this helps

 July 8, 2022, 06:45 #35 Senior Member   Michael Alletto Join Date: Jun 2018 Location: Bremen Posts: 615 Rep Power: 15 It's only the mass of the cylinder

July 8, 2022, 13:46
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zink
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I need some clarification regarding your paper
Q-1:

You have mentioned that
density of the cylinder is set to 14817 kg/mˆ3. It should be called as the density of the fluid not the cylinder?

Q-2:
In your calculations, in the case of Ur 4, you have used the k = m*(2*pi*f)ˆ2 that gives k value of 148.28.

I want to ask about the f used in the calculation. We know that the reduced natural frequency of the system is given as Fr = I/Ur and Ur= Unif/f*D , here f is the frequency in the flow medium . It could be air, water or any medium used in the simulation. In the present case, it is the frequency in the fluid having density 14817 kg.m3.

f= 0.0656/(4*0.0016) = 10.25

As you are using cylinder mass m=0.03575 ,then the k should be calculated as

k=163.108

Please clarify? I just want to learn that how f is related with k when you are using medium other than air. Should it include the added mass or not?

Quote:
 Originally Posted by mAlletto It's only the mass of the cylinder

 July 9, 2022, 03:35 #37 Senior Member   Michael Alletto Join Date: Jun 2018 Location: Bremen Posts: 615 Rep Power: 15 Q1: It is the density of the cylinder. If you fix the mass and the dimensions of the cylinder the density can be calculated Q2: The added mass is not considered. It is actually not known in advance since it is solution dependent. What did you mean with added mass exactly. Can you specify it more precisely? The purpose of setting the constants in the paper in the way I did it, is to match all relevant non-dimensional constants. This are U*, m* and Re. Actually you can choose all combination of dimensional quantities as you want as long as the non-dimensional quantities remain the same. If the non-dimensional quantities are the same you can compare the non-dimensional solutions. Have a look at the pi-theorem https://en.wikipedia.org/wiki/Buckingham_%CF%80_theorem Best Michael

April 12, 2023, 11:12
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Quote:
 Originally Posted by hani Comments and questions are welcomed on the article Alletto, Comparison of overset mesh with morphing mesh: Flow over a forced oscillating and freely oscillating 2D cylinder, OpenFOAM Journal, January 2022, https://doi.org/10.51560/ofj.v2.47.
Hi Michael,

Many thanks for your contribution, one quick question is about overset mesh, for interpolation fringe, it's better to walk away from the cylinder boundary to avoid a huge gradient, is it right? When using overset mesh for your case, have you ever faced the issue at the interface between the overset component and background component? I mean the velocity and pressure field at this interface are not consistent, I think that's due to the issue about mass conservation, can we improve this phenomenon by adjusting the interpolation scheme?

 April 12, 2023, 15:07 #39 Senior Member   Michael Alletto Join Date: Jun 2018 Location: Bremen Posts: 615 Rep Power: 15 Hello, yes better avoid regions of strong gradients where to place the overset boundary. Best Michael

May 13, 2023, 13:41
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Quote:
 Originally Posted by mAlletto Is someone is interested the case files of the paper can be found here https://gitlab.com/mAlletto/openfoam...ases4PaperSend
Hi Michael,

Thank you for your sharing, which helps me a lot.
I noticed you used cubic for div(phi, U) term instead of other second-order discretization schemes. I tried linearUpwind, QUICK, and cubic, and the results changed a lot, actually for the overset mesh, linearUpind predicts the highest frequency of lift coefficient, but the cubic returns the highest amplitude and reasonable frequency. Can you please explain why, is there any useful reference to support it?

Best,
Tian