# How to implement a sensitivity analysis for certain items

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 October 13, 2014, 21:50 How to implement a sensitivity analysis for certain items #1 New Member   Mason Join Date: Sep 2014 Posts: 16 Rep Power: 2 Hi, I have read lots of the Sensitivity Analysis of some items, e.g. mesh size, inflation layer parameters,,, in this forum. But I'm not very sure how to implement this analysis. In my view maybe like below. Please give your comments. For the first layer thickness(correlated with Yplus) of inflation layer, if we want to do a sensitivity analysis we need to keep the other mesh parameters same and just change first layer thickness(maybe 1e-2mm, 5e-3mm, 1e-3mm,,,) to finish several simulation and see if results(especially the variable you concern, maybe Drag Force) varies obviously.If no big difference, then the analysis is finished and we think that results are independent of first layer thickness. We can use this thickness(maybe 5e-3mm) in future sensitivity analysis of other parameters. If varies obviously, then we need to do further analysis of first layer thickness with other values(maybe 5e-2mm,,5e-4mm). All in all, we need to find a region of first layer thickness within which variable we concern varies little(Form a flat region as below picture), then we can take this region as final value in further simulation. Sensitivity analysis.png Is my understanding about sensitivity analysis right or proper? Thank you so much.

 October 13, 2014, 23:24 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 10,824 Rep Power: 85 I should write an FAQ to explain this idea more thoroughly. The main problem with your description is that the whole concept assumes that the output parameter converges as the input parameter is refined. So coarse inputs (such as a coarse mesh) should give large changes as the input parameter is refined, fine inputs (such as a fine mesh) should give small changes as the input parameter is refined, and eventually the changes in input parameter result in changes in output parameter which is too small to see. Further refinement in the input parameter does not affect the output. That is the ideal situation, some things which can cause headaches include: * The sensitivities are coupled, so mesh refinement requires time step refinement and might also change you convergence criteria. * You cannot refine the mesh too much as the simulation run time or memory requirements becomes too large * Effects such as numerical round-off means you cannot refine the mesh or time step too far either This is a very simple outline of it. This reference goes into more detail: http://journaltool.asme.org/Template...umAccuracy.pdf and the text book "Computational Fluid Dynamics" by Roache is the seminal textbook on CFD accuracy, so is definitely worth a read if you want a thorough understanding of it. Mason liu likes this.

October 14, 2014, 01:39
#3
New Member

Mason
Join Date: Sep 2014
Posts: 16
Rep Power: 2
Quote:
 Originally Posted by ghorrocks I should write an FAQ to explain this idea more thoroughly.
Thank you. This would be really great for us.

Quote:
 Originally Posted by ghorrocks That is the ideal situation, some things which can cause headaches include:
OK, you have given me a good clarification of sensitivity analysis, yes this is the ideal situation.

Quote:
 Originally Posted by ghorrocks This is a very simple outline of it. This reference goes into more detail: http://journaltool.asme.org/Templates/JFENumAccuracy.pdf and the text book "Computational Fluid Dynamics" by Roache is the seminal textbook on CFD accuracy, so is definitely worth a read if you want a thorough understanding of it.
Thank you, I'm trying to find this book's pdf edition for convenient read.

October 14, 2014, 02:12
#4
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Mason
Join Date: Sep 2014
Posts: 16
Rep Power: 2
Quote:
 Originally Posted by ghorrocks I should write an FAQ to explain this idea more thoroughly.
Hi, Glenn, thank you. Another question.

Is it acceptable that last cell thickness in inflation layer is very small than nearby tetra element size? Like below picture, I know that mesh in (B) is obviously good, but sometimes it's really hard to realize this expansion ratio. And my question is that 'mesh in (A) is acceptable???' Thanks a lot.

mesh.jpg

 October 14, 2014, 04:57 #5 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 10,824 Rep Power: 85 Case B is preferable. But whether A is acceptable will depend on what you are modelling. Different models have different sensitivities to mesh quality. The best thing to do here is to do a sensitivity study on this parameter (the ratio of the element size from the tets to the first prism layer) and see if it makes a difference in your case.

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