September 2015 - Nektar-users

Stability of channel flow with Nektar++
by Stanisław Gepner 05 Nov '15

05 Nov '15

Dear Sirs, First of all thank you for developing this great software and making it available. I am planing to investigate stability of an incompressible channel flow using Nektar. I have studied the manual and the source (the version from the git repo) and repeated some of the examples from the IncNavierStokesSolver folders Tests and Examples. The channel flow stability test (Tests/ChanStability.xml) uses a base flow (.bse) and an initial guess (.rst) and converges to an expected result. Now, as practice I tried to play around with the settings, first by changing the Driver to Arpack (this works as well). Since I might not know the initial approximation I tried to get rid of the initial guess, and replaceed it with some random initial vector (I have put zeros, non zeros, the awgn()). This time the problem did not coverage at all (I increased the numbers of iterations, tried to modify the number and size of the time steps, change the dimension of the Krylov space). Is this an expected behaviour? And since this is a test, the problem is set up to be small and quick and would only converge with the proper initial vector? Should I expect a similar problem for a larger 3D case? My second question concerns the base flow. I tried to recover the base flow using the stationary solver (both classical and adaptive) but I failed. Could you point me at an example of how to set this up? Also I noticed (or I think I did) that the Example/ChanStability-Base.xml does not produce a correct flow field. Am I right? I would appreciate if you would comment on my questions. Best Regards, Stan Gepner

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Adding Body force for the Channel flow
by Jian Yu 06 Oct '15

06 Oct '15

Dear all, I am now trying to compute the channel flow using the Compressible Flow Solver. I have found the body force with derivatives non-trivial to add. Once the flow in a channel has reached a fully developed state, inertial forces in the channel, which are related to the mean favourable streamwise pressure gradient (pressure drop in a duct), are balanced with the wall friction forces. In this case, the body force f would be in the following form f=-tw/h, where tw is the wall friction containing velocity derivatives, h is the channel scale. Can anybody tell me how to perform this flow in Nektar? Thank you very much! Best Jian Yu

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Need help in stability analysis of flow over cylinder
by Uddin, Md (uddinma) 29 Sep '15

29 Sep '15

Uddin, Md (uddinma) has shared OneDrive for Business?files with you. To view them, click the links below. <https://mailuc-my.sharepoint.com/personal/uddinma_mail_uc_edu/Documents/Ema…> [https://r1.res.office365.com/owa/prem/images/dc-generic_20.png]<https://mailuc-my.sharepoint.com/personal/uddinma_mail_uc_edu/Documents/Ema…> Cylinder(1).evl<https://mailuc-my.sharepoint.com/personal/uddinma_mail_uc_edu/Documents/Ema…> <https://mailuc-my.sharepoint.com/personal/uddinma_mail_uc_edu/Documents/Ema…> [https://r1.res.office365.com/owa/prem/images/dc-generic_20.png]<https://mailuc-my.sharepoint.com/personal/uddinma_mail_uc_edu/Documents/Ema…> Cylinder(1).xml<https://mailuc-my.sharepoint.com/personal/uddinma_mail_uc_edu/Documents/Ema…> <https://mailuc-my.sharepoint.com/personal/uddinma_mail_uc_edu/Documents/Ema…> [https://r1.res.office365.com/owa/prem/images/dc-generic_20.png]<https://mailuc-my.sharepoint.com/personal/uddinma_mail_uc_edu/Documents/Ema…> Cylinder-base(1).xml<https://mailuc-my.sharepoint.com/personal/uddinma_mail_uc_edu/Documents/Ema…> <https://mailuc-my.sharepoint.com/personal/uddinma_mail_uc_edu/Documents/Ema…> [https://r1.res.office365.com/owa/prem/images/dc-generic_20.png]<https://mailuc-my.sharepoint.com/personal/uddinma_mail_uc_edu/Documents/Ema…> Cylinder-Stability(1).xml<https://mailuc-my.sharepoint.com/personal/uddinma_mail_uc_edu/Documents/Ema…> Hi all, I am trying to do Biglobal floquet stability analysis of flow over a cylinder. My base flow has converged and I am getting the Strouhal numbers correctly for different Reynolds numbers. Then I tried to do the stability analysis, but I am not getting the desired results for floquet multipliers. I think there are some wrong settings in my input file that I am not able to figure out. Can anyone look at my input files and help me figuring out the possible source of error? Please check the attached files. Here are the steps I am following: 1. Solve for the 2D base flow using UnsteadyNavierStokes equations. (Input file Cylinder.xml) 2. Get the base flow (.bse files) by taking 32 time slices over one period. (Input file Cylinder-base.xml) 3. Use the base flow to set up stability simulation for a specified LZ (Input fie Cylinder-Stability.xml) The output .evl file is also attached. General data: Re=220, LZ=2.4166, Expected floquet multiplier=0.45. I am new to the solver. Any help will be greatly appreciated. Regards, Arfan Uddin

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Uddin, Md (uddinma) wants to share the file Cylinder-base(1).xml with you
by Uddin, Md (uddinma) 25 Sep '15

25 Sep '15

To view Cylinder-base(1).xml, sign in<https://mailuc-my.sharepoint.com/personal/uddinma_mail_uc_edu/_layouts/15/a…> or create an account.

1 0

Uddin, Md (uddinma) wants to share the file Cylinder-Stability(1).xml with you
by Uddin, Md (uddinma) 25 Sep '15

25 Sep '15

To view Cylinder-Stability(1).xml, sign in<https://mailuc-my.sharepoint.com/personal/uddinma_mail_uc_edu/_layouts/15/a…> or create an account.

1 0

Uddin, Md (uddinma) wants to share the file Cylinder(1).evl with you
by Uddin, Md (uddinma) 25 Sep '15

25 Sep '15

To view Cylinder(1).evl, sign in<https://mailuc-my.sharepoint.com/personal/uddinma_mail_uc_edu/_layouts/15/a…> or create an account.

1 0

Uddin, Md (uddinma) wants to share the file Cylinder(1).xml with you
by Uddin, Md (uddinma) 25 Sep '15

25 Sep '15

To view Cylinder(1).xml, sign in<https://mailuc-my.sharepoint.com/personal/uddinma_mail_uc_edu/_layouts/15/a…> or create an account.

1 0

How to process "filename_eig_0" like file
by Hongfu Zhang 25 Sep '15

25 Sep '15

Hi All, I want to computethe eigenmode of a period field using BiGlobal Floquet analysis. I have gotten the "filename_eig_0" like file, how to do post-processing with this kind of file? Thanks very much for any help! Best Regards! Hongfu

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Help required in figuring out biglobal floquet stability analysis of flow over cylinder
by Arfan Uddin 25 Sep '15

25 Sep '15

Hi all, I am trying to do Biglobal floquet stability analysis of flow over a cylinder. My base flow has converged and I am getting the Strouhal numbers correctly for different Reynolds numbers. Then I tried to do the stability analysis, but I am not getting the desired results for floquet multipliers. I think there are some wrong settings in my input file that I am not able to figure out. Can anyone look at my input files and help me figuring out the possible source of error? Please check the attached files. Here are the steps I am following:1. Solve for the 2D base flow using UnsteadyNavierStokes equations. (Input file Cylinder.xml)2. Get the base flow (.bse files) by taking 32 time slices over one period. (Input file Cylinder-base.xml)3. Use the base flow to set up stability simulation for a specified LZ (Input fie Cylinder-Stability.xml) The output .evl file is also attached. General data: Re=220, LZ=2.4166, Expected floquet multiplier=0.45. I am new to the solver. Any help will be greatly appreciated. Regards,Arfan Uddin

1 0

Need help in stability analysis of flow over cylinder
by Arfan Uddin 25 Sep '15

25 Sep '15

Hi all, I am trying to do Biglobal floquet stability analysis of flow over a cylinder. My base flow has converged and I am getting the Strouhal numbers correctly for different Reynolds numbers. Then I tried to do the stability analysis, but I am not getting the desired results for floquet multipliers. I think there are some wrong settings in my input file that I am not able to figure out. Can anyone look at my input files and help me figuring out the possible source of error? Please check the attached files. Here are the steps I am following:1. Solve for the 2D base flow using UnsteadyNavierStokes equations. (Input file Cylinder.xml)2. Get the base flow (.bse files) by taking 32 time slices over one period. (Input file Cylinder-base.xml)3. Use the base flow to set up stability simulation for a specified LZ (Input fie Cylinder-Stability.xml) The output .evl file is also attached. General data: Re=220, LZ=2.4166, Expected floquet multiplier=0.45. I am new to the solver. Any help will be greatly appreciated. Regards,Arfan Uddin

1 0