- Nektar-users - imperial.ac.uk

Restarting a VIV case in VCSMapping solver
by Chengjiao Ren 13 Feb '23

13 Feb '23

******************* This email originates from outside Imperial. Do not click on links and attachments unless you recognise the sender. If you trust the sender, add them to your safe senders list https://spam.ic.ac.uk/SpamConsole/Senders.aspx to disable email stamping for this address. ******************* Dear Nekar++ users, Hope the email finds you well. I would like to know if anyone has tried restarting a VIV problem in VCS Mapping solver? In my case (VIV of a circular cylinder), when restarting a simulation from a flow field (restart2 in the attached figure 1, "*.map" for mapping and "*.fld" for initial conditions), the force in the motion direction has large fluctuations at the initial few steps. These large fluctuations cause subsequent inconsistent evolutions for all signals compared with the original ones. This issue is found for a forced vibration case when the domain has an initial acceleration, as per figure 2 below. Noted that if we compare the Fy evolution (restart 2 in figure 1) with that of the restart 1 (only define the "u,v,w,p" in "*.fld" file for initial conditions), they are quite similar. I therefore suppose if the p field due to coordinate transform is not considered in "*.map", resulting in significant changes on forces when the domain has non-zero acceleration. Does anyone have any suggestions on restarting a VIV case? Since the initial difference doesn't decay during the simulation, it cannot be ignored currently. Thanks in advance. Chengjiao Figure 1, a free vibration case where the cylinder has 1-dof in y-direction Figure 2, a forced vibration case where the cylinder is forced to vibrate in x-direction. Dr Chengjiao Ren Research Associate School of Engineering * Perth WA 6009 Australia +61 413077330 * chengjiao.ren(a)uwa.edu.au<mailto:chengjiao.ren@uwa.edu.au>

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2D geometry but rotates about the z_axis
by Ali Ahmed Abdulrasool - علي احمد عبد الرس ول 11 Feb '23

11 Feb '23

******************* This email originates from outside Imperial. Do not click on links and attachments unless you recognise the sender. If you trust the sender, add them to your safe senders list https://spam.ic.ac.uk/SpamConsole/Senders.aspx to disable email stamping for this address. ******************* Hello Stan, I'm grateful for your response. I'm sorry, but I'm new at using Nektar. For me, it doesn't matter which axis the channel spins on as long as it does it from the channel's face. I thought that Nektar only offers the option of rotation about the z axis. Is that correct? I.e., can I rotate the channel around any axis I want? Or can I just insert velocity values for the channel wall instead of using the "forcing" option and all of its requirements? I mean like this <REGION REF="0"> <D VAR="u" VALUE="-2.0*y"/> <D VAR="v" VALUE="2.0*(x-0.4)"/> <D VAR="T" VALUE="1600"/> <N VAR="p" USERDEFINEDTYPE="H" VALUE="0"/> </REGION> In addition, according to your suggestion, can making a homogeneous (Fourier) expansion result in a 3D mesh with a high computational cost? I want to know whether there is a difference in computation time between quasi-3D and normal 3D so that I can just create a 3D model using Gmesh and convert it to Nektar. Thank you in advance, Ali -- *University of Kerbala* website : https://uokerbala.edu.iq/en/ <https://uokerbala.edu.iq/en/> Location : https://goo.gl/maps/pNQ9LkdMbi72 <https://goo.gl/maps/pNQ9LkdMbi72> *This email and any files transmitted with it are confidential and intended solely for the use of the individual or entity to whom they are addressed. If you have received this email in error please notify the system manager. *

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Special treatment of WallViscous boundary
by Chengeng Qian 08 Feb '23

08 Feb '23

******************* This email originates from outside Imperial. Do not click on links and attachments unless you recognise the sender. If you trust the sender, add them to your safe senders list https://spam.ic.ac.uk/SpamConsole/Senders.aspx to disable email stamping for this address. ******************* Hello everyone, I tried to run CompressibleFlowSolver to resolve some non-dimensional problems. I found that I must set parameter Twall if I wanted to use WallViscous boundary conditions. Twall is referenced in NavierStokesCFE::SpecialBndTreat <https://doc.nektar.info/doxygen/latest/class_nektar_1_1_navier_stokes_c_f_e…>, which is used to calculate diffusion flux on boundary. Are there some references which can show me more details of this special treatment of WallViscous boundary? Any comment is appreciated. Best, Chengeng

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2D geometry but rotates about the z_axis
by Ali Ahmed Abdulrasool - علي احمد عبد الرس ول 07 Feb '23

07 Feb '23

******************* This email originates from outside Imperial. Do not click on links and attachments unless you recognise the sender. If you trust the sender, add them to your safe senders list https://spam.ic.ac.uk/SpamConsole/Senders.aspx to disable email stamping for this address. ******************* Hello Nektar community, I want to simulate a two-pass square channel with rotation effect using the Nektar++ solver. First, I tested 2D geometry, and it worked. However, when I tried to add rotation, it didn't work. As far as I'm aware, Nektar's rotation is constrained on the z axis, therefore I built my channel around that. I created "GEOMETRY DIM="2" SPACE="3">" so that a 2D mesh could be obtained in 3D space and rotated about the z-axis. Nevertheless, I keep getting the message "Fatal: Level 0 assertion violation output coord 2 is not defined." Who can advise me on what to do? I truly appreciate it. Thanks, Ali A. Abdulrasool -- *University of Kerbala* website : https://uokerbala.edu.iq/en/ <https://uokerbala.edu.iq/en/> Location : https://goo.gl/maps/pNQ9LkdMbi72 <https://goo.gl/maps/pNQ9LkdMbi72> *This email and any files transmitted with it are confidential and intended solely for the use of the individual or entity to whom they are addressed. If you have received this email in error please notify the system manager. *

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FieldConvert as a FILTER in the Session File
by İlteber Özdemir 05 Feb '23

05 Feb '23

******************* This email originates from outside Imperial. Do not click on links and attachments unless you recognise the sender. If you trust the sender, add them to your safe senders list https://spam.ic.ac.uk/SpamConsole/Senders.aspx to disable email stamping for this address. ******************* Hello all, I am using the FieldConvert module as a FILTER in my session file. I noticed that not all of the modules are working. So far I found removefield and fieldfromstring to be not working. I paste below how I set the FILTER. For removefield it gives "Field does not exist" error, and for fieldfromstring the code exits. I think both of the problems are related to the field variables. For removefield just add removefield:fieldname="u" .On the other hand, vorticity and gradient modules work. Has somebody noticed a similar problem, or have I made a mistake while setting the filter? Thank you in advance for your help. <FILTER TYPE="FieldConvert"> <PARAM NAME="OutputFile">timeSteps/fcFiles/out.chk</PARAM> <PARAM NAME="OutputFrequency">1.0e+01</PARAM> <PARAM NAME="Modules"> vorticity gradient fieldfromstring:fieldname="dist":fieldstr="sqrt(x^2+y^2)" </PARAM> </FILTER> Kind regards, Ilteber -- İlteber R. Özdemir

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Help with using DIRK Time Integration Methods
by Andrew Barnes 31 Jan '23

31 Jan '23

******************* This email originates from outside Imperial. Do not click on links and attachments unless you recognise the sender. If you trust the sender, add them to your safe senders list https://spam.ic.ac.uk/SpamConsole/Senders.aspx to disable email stamping for this address. ******************* Hello everyone, I'm trying to use the DIRK Time Integration methods in the CompressibleFlowSolver but keep getting an error when trying to specify the order and variant. If I specify only <I PROPERTY="TimeIntegrationMethod" VALUE="DIRK" /> in SolverInfo it will work perfectly, however I am unsure which order this is using. When I try to define VALUE="DIRKOrder3" or in a separate TIMEINTEGRATIONSCHEME section as below instead, it will produce the error bad_function_call. <TIMEINTEGRATIONSCHEME> <METHOD> DIRK </METHOD> <ORDER> 3 </ORDER> </TIMEINTEGRATIONSCHEME> Could anyone clarify: 1. Which order is used for the default "DIRK" model? 2. What would be the correct way to specify the order of DIRK model? Thank you in advance, Andrew

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Help with spatially-varying sourcing function.
by Edward Threlfall 28 Jan '23

28 Jan '23

******************* This email originates from outside Imperial. Do not click on links and attachments unless you recognise the sender. If you trust the sender, add them to your safe senders list https://spam.ic.ac.uk/SpamConsole/Senders.aspx to disable email stamping for this address. ******************* Dear community, I've set up a convection problem in the incompressible Navier-Stokes solver. I have an equation for the time-evolution of the temperature to which I would like to add a right-hand-side describing sources of local heating. Can I do this in Nektar++? I have tried implementing it via the bodyforce, however this seems to apply a spatially-uniform source and I want something like the Gaussian profile below. I am trying: <FUNCTION NAME="BodyForce"> <E VAR="u" VALUE="Gr*T" EVARS="u v T p" /> <E VAR="v" VALUE="0" EVARS="u v T p" /> <E VAR="T" VALUE="(1e10)*exp(-((x+0.25)^2+y^2)/(0.05^2))" EVARS="u v T p x y" /> </FUNCTION> then <FORCING> <FORCE TYPE="Body"> <BODYFORCE> BodyForce </BODYFORCE> </FORCE> </FORCING> Can anyone help, please? Thanks! Ed.

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Nektar++ Workshop 2023: 13-15th June, London
by David Moxey 23 Jan '23

23 Jan '23

******************* This email originates from outside Imperial. Do not click on links and attachments unless you recognise the sender. If you trust the sender, add them to your safe senders list https://spam.ic.ac.uk/SpamConsole/Senders.aspx to disable email stamping for this address. ******************* Dear all, I hope you are all well! This is a brief email to say that we are planning to hold the next Nektar++ workshop for 2023 on 13th-15th June 2023 at King's College London. Please do therefore put the date in your diaries! We are finalising timings for the event, and will be back in touch with more details on registration over the next few weeks. We look forward to seeing you in London in June. Many thanks, Dave -- Dr David Moxey, PhD MMath FHEA Reader in Engineering david.moxey(a)kcl.ac.uk | web: davidmoxey.uk Department of Engineering, King's College London, Strand, London, WC2R 2LS.

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FIeldConvert homogeneous expansions
by Guillermo Suárez 18 Jan '23

18 Jan '23

******************* This email originates from outside Imperial. Do not click on links and attachments unless you recognise the sender. If you trust the sender, add them to your safe senders list https://spam.ic.ac.uk/SpamConsole/Senders.aspx to disable email stamping for this address. ******************* Hi, I have a 2.5D solution (2D in the x-y plane with Fourier expansion in the z direction) run with the incompressible Navier-Stokes solver and I need to replicate it N times in the homogeneous direction. If the solution has HomModesZ for an extension LZ, I need the new solution to fit N replica in an extension N*LZ with a resolution N*HomModesZ. I have tried several modules of FieldConvert, but rather than N replicas of the original solution, the result I get is a stretched version of it. In principle this should be a very simple thing to do, as the coefficients of mode k of the original solution should be copied to mode k*N of the new solution, leaving all modes that are not multiples of N empty (zeroes). The trouble is that implementing this requires an understanding of how modes are labeled and I don't manage to get the hold of it. For a simple 1D-periodic solution whose Fourier expansion modes are saved in a sequential complex array, replicating three times would be as simple as: A_original = {a0, a1, a2, a3, ..., an} A_new = {a0, 0, 0, a1, 0, 0, a2, 0, 0, a3, ..., 0, 0, an} Could someone please give me some hints as to how to implement this? Perhaps an existing module that I can take as a reference and the sort of changes that could get the intended result? I believe this would not take more than a few minutes for someone that is familiar with the code. Perhaps someone acquainted with the working can help me implement such a replication module for FieldConvert? Thank you. Cheers, Guillermo

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About Modified Arnoldi Driver
by 王舟阳 12 Jan '23

12 Jan '23

******************* This email originates from outside Imperial. Do not click on links and attachments unless you recognise the sender. If you trust the sender, add them to your safe senders list https://spam.ic.ac.uk/SpamConsole/Senders.aspx to disable email stamping for this address. ******************* Dear Nektar++ Users: I am Wang Zhouyang, and I am sorry to present some questions about the Modified Arnoldi Driver in linear stability analysis: 1. When I was doing parallel computation, I was prompted that Arpack algorithm could not implement parallel in Nektar++. Although this is true based on other user communications here, I still want this issue to be confirmed. Thank you very much. 2. As described in the user documentation, the Arpack package can specify properties such as LargestMag or LargestReal etc to output eigenvalues. For the Modified Arnoldi algorithm, what is the property of the eigenvalues utilized to write the result? (For example, nvec=4, do the four eigenvectors output after iteration have the largest four magnitudes or the top four growth rates or other criteria?) 3. In solving, is it reasonable to have a mode with a frequency of 0? I understand it as a flow phenomenon of pure exponential growth (or decay).In my humble opinion, the frequency may suggest a flow structure with oscillation, and it would be unstable (or attenuated) regarding to the growth rate. Thank you very much for your help, and I sincerely hope to get your answers. Thanks to the Nektar++ project team. Best Regards, Wang Zhouyang

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