******************* 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, I am a student at Warsaw University of Technology and I will be using Nektar++ for my masters thesis. I have a question about pressure boundary conditions for velocity correction scheme. To ask this question i had to write down some formulas that is why it was more conviniant to attache it to this message as a pdf document. Best Regards, Krzysztof Michałowski
Hi Krzysztof, Nektar++ doesn't enforce incompressibility exactly in the Velocity Correction Scheme. It's chosen like this in the splitting error. You can check that with any run you already have and the postprocessing utility FieldConvert to get the divergence of your field. FieldConvert -m divergence mesh.xml field.fld field_div.fld Hopefully if the constant in front is not very big it should be of the order of TimeStep^ORDERSCHEME. With regards to the pressure BCs, the components of the velocity that appear in the equation are extrapolated as well, so expect another O(TimeStep^ORDER) from there again. I am not sure whether this solves your concerns, Best, ________________________________ From: nektar-users-bounces@imperial.ac.uk <nektar-users-bounces@imperial.ac.uk> on behalf of Michałowski Krzysztof (STUD) <krzysztof.michalowski.stud@pw.edu.pl> Sent: 14 October 2025 12:32 To: nektar-users <nektar-users@imperial.ac.uk> Subject: [Nektar-users] Pressure boundary conditions for velocity correction scheme This email from krzysztof.michalowski.stud@pw.edu.pl 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, I am a student at Warsaw University of Technology and I will be using Nektar++ for my masters thesis. I have a question about pressure boundary conditions for velocity correction scheme. To ask this question i had to write down some formulas that is why it was more conviniant to attache it to this message as a pdf document. Best Regards, Krzysztof Michałowski
Hi Victor, Thank you for the answer. I do not think Krzysztof question is about enforcing incompresibilty per se, but rather about, what seems to be an inconsistency in the initialization of the time stepping with regard to the pressure boundary condition. Namely, higher order IMEX method is not self starting and needs to be initiated by a multistage method. It seems that during this couple of steps there might be an inconsistency with regard to which we ask this question. Krzysztof has been looking into this, and his observation are outlined in the pdf. He suggests that pressure boundary condition (eq. 4 of the pdf) and the discrete in time (8) variant are consistent for a developed time stepping IMEX, but not for the initialization step. So the question is, does it matter, or is it that we do not see the equivalence? Best regards, Stan *Faculty of Power and Aeronautical Engineering* Nowowiejska 24 Str. 00-665 Warsaw, Poland phone +48 (22) 234 75 23 <https://www.pw.edu.pl/engpw> On 14.10.2025 15:16, Ballester Ribo, Victor wrote:
Hi Krzysztof,
Nektar++ doesn't enforce incompressibility exactly in the Velocity Correction Scheme. It's chosen like this in the splitting error. You can check that with any run you already have and the postprocessing utility FieldConvert to get the divergence of your field.
FieldConvert -m divergence mesh.xml field.fld field_div.fld
Hopefully if the constant in front is not very big it should be of the order of TimeStep^ORDERSCHEME.
With regards to the pressure BCs, the components of the velocity that appear in the equation are extrapolated as well, so expect another O(TimeStep^ORDER) from there again.
I am not sure whether this solves your concerns, Best,
------------------------------------------------------------------------ *From:* nektar-users-bounces@imperial.ac.uk <nektar-users-bounces@imperial.ac.uk> on behalf of Michałowski Krzysztof (STUD) <krzysztof.michalowski.stud@pw.edu.pl> *Sent:* 14 October 2025 12:32 *To:* nektar-users <nektar-users@imperial.ac.uk> *Subject:* [Nektar-users] Pressure boundary conditions for velocity correction scheme
This email from krzysztof.michalowski.stud@pw.edu.pl 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,
I am a student at Warsaw University of Technology and I will be using Nektar++ for my masters thesis. I have a question about pressure boundary conditions for velocity correction scheme. To ask this question i had to write down some formulas that is why it was more conviniant to attache it to this message as a pdf document.
Best Regards, Krzysztof Michałowski
_______________________________________________ Nektar-users mailing list Nektar-users@imperial.ac.uk https://mailman.ic.ac.uk/mailman/listinfo/nektar-users
Hi, I understand now your question Stan and Krzysztof. I am no longer capable of helping you. Hopefully another member more on the developing side of the team can help you. Sorry, thanks and best, ________________________________ From: nektar-users-bounces@imperial.ac.uk <nektar-users-bounces@imperial.ac.uk> on behalf of Stanisław Gepner <stanislaw.gepner@pw.edu.pl> Sent: 15 October 2025 07:33 To: nektar-users <nektar-users@imperial.ac.uk> Subject: Re: [Nektar-users] Pressure boundary conditions for velocity correction scheme Hi Victor, Thank you for the answer. I do not think Krzysztof question is about enforcing incompresibilty per se, but rather about, what seems to be an inconsistency in the initialization of the time stepping with regard to the pressure boundary condition. Namely, higher order IMEX method is not self starting and needs to be initiated by a multistage method. It seems that during this couple of steps there might be an inconsistency with regard to which we ask this question. Krzysztof has been looking into this, and his observation are outlined in the pdf. He suggests that pressure boundary condition (eq. 4 of the pdf) and the discrete in time (8) variant are consistent for a developed time stepping IMEX, but not for the initialization step. So the question is, does it matter, or is it that we do not see the equivalence? Best regards, Stan Faculty of Power and Aeronautical Engineering Nowowiejska 24 Str. 00-665 Warsaw, Poland phone +48 (22) 234 75 23 [cid:part1.ajnevNkk.hE1T5Kh3@pw.edu.pl]<https://www.pw.edu.pl/engpw> On 14.10.2025 15:16, Ballester Ribo, Victor wrote: Hi Krzysztof, Nektar++ doesn't enforce incompressibility exactly in the Velocity Correction Scheme. It's chosen like this in the splitting error. You can check that with any run you already have and the postprocessing utility FieldConvert to get the divergence of your field. FieldConvert -m divergence mesh.xml field.fld field_div.fld Hopefully if the constant in front is not very big it should be of the order of TimeStep^ORDERSCHEME. With regards to the pressure BCs, the components of the velocity that appear in the equation are extrapolated as well, so expect another O(TimeStep^ORDER) from there again. I am not sure whether this solves your concerns, Best, ________________________________ From: nektar-users-bounces@imperial.ac.uk<mailto:nektar-users-bounces@imperial.ac.uk> <nektar-users-bounces@imperial.ac.uk><mailto:nektar-users-bounces@imperial.ac.uk> on behalf of Michałowski Krzysztof (STUD) <krzysztof.michalowski.stud@pw.edu.pl><mailto:krzysztof.michalowski.stud@pw.edu.pl> Sent: 14 October 2025 12:32 To: nektar-users <nektar-users@imperial.ac.uk><mailto:nektar-users@imperial.ac.uk> Subject: [Nektar-users] Pressure boundary conditions for velocity correction scheme This email from krzysztof.michalowski.stud@pw.edu.pl<mailto:krzysztof.michalowski.stud@pw.edu.pl> 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, I am a student at Warsaw University of Technology and I will be using Nektar++ for my masters thesis. I have a question about pressure boundary conditions for velocity correction scheme. To ask this question i had to write down some formulas that is why it was more conviniant to attache it to this message as a pdf document. Best Regards, Krzysztof Michałowski _______________________________________________ Nektar-users mailing list Nektar-users@imperial.ac.uk<mailto:Nektar-users@imperial.ac.uk> https://mailman.ic.ac.uk/mailman/listinfo/nektar-users
Hi Stan, Could you kindly share his pdf to us? I don't find it in the attachment. Thanks, Boyang ________________________________ From: nektar-users-bounces@imperial.ac.uk <nektar-users-bounces@imperial.ac.uk> on behalf of Stanisław Gepner <stanislaw.gepner@pw.edu.pl> Sent: Wednesday, October 15, 2025 7:33 To: nektar-users@imperial.ac.uk <nektar-users@imperial.ac.uk> Subject: Re: [Nektar-users] Pressure boundary conditions for velocity correction scheme Hi Victor, Thank you for the answer. I do not think Krzysztof question is about enforcing incompresibilty per se, but rather about, what seems to be an inconsistency in the initialization of the time stepping with regard to the pressure boundary condition. Namely, higher order IMEX method is not self starting and needs to be initiated by a multistage method. It seems that during this couple of steps there might be an inconsistency with regard to which we ask this question. Krzysztof has been looking into this, and his observation are outlined in the pdf. He suggests that pressure boundary condition (eq. 4 of the pdf) and the discrete in time (8) variant are consistent for a developed time stepping IMEX, but not for the initialization step. So the question is, does it matter, or is it that we do not see the equivalence? Best regards, Stan Faculty of Power and Aeronautical Engineering Nowowiejska 24 Str. 00-665 Warsaw, Poland phone +48 (22) 234 75 23 [cid:part1.ajnevNkk.hE1T5Kh3@pw.edu.pl]<https://www.pw.edu.pl/engpw> On 14.10.2025 15:16, Ballester Ribo, Victor wrote: Hi Krzysztof, Nektar++ doesn't enforce incompressibility exactly in the Velocity Correction Scheme. It's chosen like this in the splitting error. You can check that with any run you already have and the postprocessing utility FieldConvert to get the divergence of your field. FieldConvert -m divergence mesh.xml field.fld field_div.fld Hopefully if the constant in front is not very big it should be of the order of TimeStep^ORDERSCHEME. With regards to the pressure BCs, the components of the velocity that appear in the equation are extrapolated as well, so expect another O(TimeStep^ORDER) from there again. I am not sure whether this solves your concerns, Best, ________________________________ From: nektar-users-bounces@imperial.ac.uk<mailto:nektar-users-bounces@imperial.ac.uk> <nektar-users-bounces@imperial.ac.uk><mailto:nektar-users-bounces@imperial.ac.uk> on behalf of Michałowski Krzysztof (STUD) <krzysztof.michalowski.stud@pw.edu.pl><mailto:krzysztof.michalowski.stud@pw.edu.pl> Sent: 14 October 2025 12:32 To: nektar-users <nektar-users@imperial.ac.uk><mailto:nektar-users@imperial.ac.uk> Subject: [Nektar-users] Pressure boundary conditions for velocity correction scheme This email from krzysztof.michalowski.stud@pw.edu.pl<mailto:krzysztof.michalowski.stud@pw.edu.pl> 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, I am a student at Warsaw University of Technology and I will be using Nektar++ for my masters thesis. I have a question about pressure boundary conditions for velocity correction scheme. To ask this question i had to write down some formulas that is why it was more conviniant to attache it to this message as a pdf document. Best Regards, Krzysztof Michałowski _______________________________________________ Nektar-users mailing list Nektar-users@imperial.ac.uk<mailto:Nektar-users@imperial.ac.uk> https://mailman.ic.ac.uk/mailman/listinfo/nektar-users
Hi Boyang, I am not sure why, but in my case the original e-mail from Krzysztof ended up in the spam filter. I am now reposing it as an attachment to this message. Best regards, Stan *Faculty of Power and Aeronautical Engineering* Nowowiejska 24 Str. 00-665 Warsaw, Poland phone +48 (22) 234 75 23 <https://www.pw.edu.pl/engpw> On 15.10.2025 11:52, Boyang Xia wrote:
Hi Stan,
Could you kindly share his pdf to us? I don't find it in the attachment.
Thanks, Boyang ------------------------------------------------------------------------ *From:* nektar-users-bounces@imperial.ac.uk <nektar-users-bounces@imperial.ac.uk> on behalf of Stanisław Gepner <stanislaw.gepner@pw.edu.pl> *Sent:* Wednesday, October 15, 2025 7:33 *To:* nektar-users@imperial.ac.uk <nektar-users@imperial.ac.uk> *Subject:* Re: [Nektar-users] Pressure boundary conditions for velocity correction scheme
Hi Victor,
Thank you for the answer. I do not think Krzysztof question is about enforcing incompresibilty per se, but rather about, what seems to be an inconsistency in the initialization of the time stepping with regard to the pressure boundary condition. Namely, higher order IMEX method is not self starting and needs to be initiated by a multistage method. It seems that during this couple of steps there might be an inconsistency with regard to which we ask this question.
Krzysztof has been looking into this, and his observation are outlined in the pdf. He suggests that pressure boundary condition (eq. 4 of the pdf) and the discrete in time (8) variant are consistent for a developed time stepping IMEX, but not for the initialization step.
So the question is, does it matter, or is it that we do not see the equivalence?
Best regards, Stan
*Faculty of Power and Aeronautical Engineering* Nowowiejska 24 Str. 00-665 Warsaw, Poland phone +48 (22) 234 75 23
<https://www.pw.edu.pl/engpw> On 14.10.2025 15:16, Ballester Ribo, Victor wrote:
Hi Krzysztof,
Nektar++ doesn't enforce incompressibility exactly in the Velocity Correction Scheme. It's chosen like this in the splitting error. You can check that with any run you already have and the postprocessing utility FieldConvert to get the divergence of your field.
FieldConvert -m divergence mesh.xml field.fld field_div.fld
Hopefully if the constant in front is not very big it should be of the order of TimeStep^ORDERSCHEME.
With regards to the pressure BCs, the components of the velocity that appear in the equation are extrapolated as well, so expect another O(TimeStep^ORDER) from there again.
I am not sure whether this solves your concerns, Best,
------------------------------------------------------------------------ *From:* nektar-users-bounces@imperial.ac.uk <mailto:nektar-users-bounces@imperial.ac.uk> <nektar-users-bounces@imperial.ac.uk> <mailto:nektar-users-bounces@imperial.ac.uk> on behalf of Michałowski Krzysztof (STUD) <krzysztof.michalowski.stud@pw.edu.pl> <mailto:krzysztof.michalowski.stud@pw.edu.pl> *Sent:* 14 October 2025 12:32 *To:* nektar-users <nektar-users@imperial.ac.uk> <mailto:nektar-users@imperial.ac.uk> *Subject:* [Nektar-users] Pressure boundary conditions for velocity correction scheme
This email from krzysztof.michalowski.stud@pw.edu.pl <mailto:krzysztof.michalowski.stud@pw.edu.pl> 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,
I am a student at Warsaw University of Technology and I will be using Nektar++ for my masters thesis. I have a question about pressure boundary conditions for velocity correction scheme. To ask this question i had to write down some formulas that is why it was more conviniant to attache it to this message as a pdf document.
Best Regards, Krzysztof Michałowski
_______________________________________________ Nektar-users mailing list Nektar-users@imperial.ac.uk <mailto:Nektar-users@imperial.ac.uk> https://mailman.ic.ac.uk/mailman/listinfo/nektar-users <https://mailman.ic.ac.uk/mailman/listinfo/nektar-users>
Hi Stan,Krzysztof, Thanks for the document. We definitely observe pressure spikes when we are solving marginally resolved flows are restart and it will be useful to see if this is the source of the problem. It would definitely be good if we can reduce this issue. In our redesign we have discussed dumping two (or more) time steps to allow for accurate restarts but this will obviously lead to larger output files. I need to go back and look at the code but I thought we were using the previous time steps for the pressure extrapolation. Certainly for the semi-Lagrangian approach we have to be careful to do this. @Boyang, Victor, Henrik: can we have a look at Krzysztof note and perhaps have a discussion about it when I am back later next week? Cheers, Spencer. Thanks for sending along the pdf. Professor Spencer Sherwin (he/him) Department of Aeronautics Imperial College London City and Guilds Building South Kensington Campus London SW7 2AZ Phone: +44 (0)20 7594 5052 http://www.imperial.ac.uk/people/s.sherwin On 15 Oct 2025, at 10:04, Stanisław Gepner <stanislaw.gepner@pw.edu.pl> wrote: Hi Boyang, I am not sure why, but in my case the original e-mail from Krzysztof ended up in the spam filter. I am now reposing it as an attachment to this message. Best regards, Stan Faculty of Power and Aeronautical Engineering Nowowiejska 24 Str. 00-665 Warsaw, Poland phone +48 (22) 234 75 23 <https://www.pw.edu.pl/engpw> [cid:bde927e2-1a3c-4cc9-bbcf-d6e851c7d418@GBRP265.PROD.OUTLOOK.COM] <https://www.pw.edu.pl/engpw> On 15.10.2025 11:52, Boyang Xia wrote: Hi Stan, Could you kindly share his pdf to us? I don't find it in the attachment. Thanks, Boyang ________________________________ From: nektar-users-bounces@imperial.ac.uk<mailto:nektar-users-bounces@imperial.ac.uk> <nektar-users-bounces@imperial.ac.uk><mailto:nektar-users-bounces@imperial.ac.uk> on behalf of Stanisław Gepner <stanislaw.gepner@pw.edu.pl><mailto:stanislaw.gepner@pw.edu.pl> Sent: Wednesday, October 15, 2025 7:33 To: nektar-users@imperial.ac.uk<mailto:nektar-users@imperial.ac.uk> <nektar-users@imperial.ac.uk><mailto:nektar-users@imperial.ac.uk> Subject: Re: [Nektar-users] Pressure boundary conditions for velocity correction scheme Hi Victor, Thank you for the answer. I do not think Krzysztof question is about enforcing incompresibilty per se, but rather about, what seems to be an inconsistency in the initialization of the time stepping with regard to the pressure boundary condition. Namely, higher order IMEX method is not self starting and needs to be initiated by a multistage method. It seems that during this couple of steps there might be an inconsistency with regard to which we ask this question. Krzysztof has been looking into this, and his observation are outlined in the pdf. He suggests that pressure boundary condition (eq. 4 of the pdf) and the discrete in time (8) variant are consistent for a developed time stepping IMEX, but not for the initialization step. So the question is, does it matter, or is it that we do not see the equivalence? Best regards, Stan Faculty of Power and Aeronautical Engineering Nowowiejska 24 Str. 00-665 Warsaw, Poland phone +48 (22) 234 75 23 <https://www.pw.edu.pl/engpw> [cid:efda6af1-420f-4a6e-a496-e314eff9ef74@GBRP265.PROD.OUTLOOK.COM] <https://www.pw.edu.pl/engpw> On 14.10.2025 15:16, Ballester Ribo, Victor wrote: Hi Krzysztof, Nektar++ doesn't enforce incompressibility exactly in the Velocity Correction Scheme. It's chosen like this in the splitting error. You can check that with any run you already have and the postprocessing utility FieldConvert to get the divergence of your field. FieldConvert -m divergence mesh.xml field.fld field_div.fld Hopefully if the constant in front is not very big it should be of the order of TimeStep^ORDERSCHEME. With regards to the pressure BCs, the components of the velocity that appear in the equation are extrapolated as well, so expect another O(TimeStep^ORDER) from there again. I am not sure whether this solves your concerns, Best, ________________________________ From: nektar-users-bounces@imperial.ac.uk<mailto:nektar-users-bounces@imperial.ac.uk> <nektar-users-bounces@imperial.ac.uk><mailto:nektar-users-bounces@imperial.ac.uk> on behalf of Michałowski Krzysztof (STUD)<krzysztof.michalowski.stud@pw.edu.pl><mailto:krzysztof.michalowski.stud@pw.edu.pl> Sent: 14 October 2025 12:32 To: nektar-users <nektar-users@imperial.ac.uk><mailto:nektar-users@imperial.ac.uk> Subject: [Nektar-users] Pressure boundary conditions for velocity correction scheme This email from krzysztof.michalowski.stud@pw.edu.pl<mailto:krzysztof.michalowski.stud@pw.edu.pl> 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, I am a student at Warsaw University of Technology and I will be using Nektar++ for my masters thesis. I have a question about pressure boundary conditions for velocity correction scheme. To ask this question i had to write down some formulas that is why it was more conviniant to attache it to this message as a pdf document. Best Regards, Krzysztof Michałowski _______________________________________________ Nektar-users mailing list Nektar-users@imperial.ac.uk<mailto:Nektar-users@imperial.ac.uk> https://mailman.ic.ac.uk/mailman/listinfo/nektar-users _______________________________________________ Nektar-users mailing list Nektar-users@imperial.ac.uk<mailto:Nektar-users@imperial.ac.uk> https://mailman.ic.ac.uk/mailman/listinfo/nektar-users
participants (5)
-
Ballester Ribo, Victor
-
Boyang Xia
-
Michałowski Krzysztof (STUD)
-
Sherwin, Spencer J
-
Stanisław Gepner