Value of δ in HOutflow condition
******************* 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 am using the HOutflow condition at the outlet of my flow domain with the incompressible Navier-Stokes solver. According to the user guide, this is enforcing a boundary condition defined by Dong, Karniadakis and Chryssostomidis in equation 3 of their paper: A robust and accurate outflow boundary condition for incompressible flow simulations on severely-truncated unbounded domains. This boundary condition is also stated in equations 11.27 and 11.28 of the user guide. Both the paper and the user guide state that δ is "chosen to be sufficiently small" without explaining what constitutes sufficiently small. Would anyone happen to know what value of δ is used by Nektar++ or where I could find it? Kind regards, Isaac
Hi Issac, Delta defines the sharpness of step-like function defined by the tanh. If it is too small, the profile would resemble a sharp jump and may cause numerical issues or misses the points. If it is too large, it would flatten the step and thus spread the effect. In nektar, I think we use 1/20 (=0.05) for delta. See the member data m_delta in nektar++/ solvers/IncNavierStokesSolver/EquationSystems/Extrapolate.h Hope this helps Cheers, Mohsen From: nektar-users-bounces@imperial.ac.uk <nektar-users-bounces@imperial.ac.uk> on behalf of Isaac Rosin <isaac.rosin1@ucalgary.ca> Date: Friday, 21 March 2025 at 22:40 To: nektar-users@imperial.ac.uk <nektar-users@imperial.ac.uk> Subject: [Nektar-users] Value of δ in HOutflow condition ⚠ External sender. Take care when opening links or attachments. Do not provide your login details. This email from isaac.rosin1@ucalgary.ca 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 am using the HOutflow condition at the outlet of my flow domain with the incompressible Navier-Stokes solver. According to the user guide, this is enforcing a boundary condition defined by Dong, Karniadakis and Chryssostomidis in equation 3 of their paper: A robust and accurate outflow boundary condition for incompressible flow simulations on severely-truncated unbounded domains. This boundary condition is also stated in equations 11.27 and 11.28 of the user guide. Both the paper and the user guide state that δ is "chosen to be sufficiently small" without explaining what constitutes sufficiently small. Would anyone happen to know what value of δ is used by Nektar++ or where I could find it? Kind regards, Isaac
Hi Mohsen, Thank you for your response, this is very helpful. I just checked the source file you referred to and can confirm the value of δ is 1/20. All the best, Isaac ________________________________ From: Mohsen Lahooti <Mohsen.Lahooti@newcastle.ac.uk> Sent: March 31, 2025 7:53 AM To: Isaac Rosin <isaac.rosin1@ucalgary.ca>; nektar-users@imperial.ac.uk <nektar-users@imperial.ac.uk> Subject: Re: Value of δ in HOutflow condition [△EXTERNAL] Hi Issac, Delta defines the sharpness of step-like function defined by the tanh. If it is too small, the profile would resemble a sharp jump and may cause numerical issues or misses the points. If it is too large, it would flatten the step and thus spread the effect. In nektar, I think we use 1/20 (=0.05) for delta. See the member data m_delta in nektar++/ solvers/IncNavierStokesSolver/EquationSystems/Extrapolate.h Hope this helps Cheers, Mohsen From: nektar-users-bounces@imperial.ac.uk <nektar-users-bounces@imperial.ac.uk> on behalf of Isaac Rosin <isaac.rosin1@ucalgary.ca> Date: Friday, 21 March 2025 at 22:40 To: nektar-users@imperial.ac.uk <nektar-users@imperial.ac.uk> Subject: [Nektar-users] Value of δ in HOutflow condition ⚠ External sender. Take care when opening links or attachments. Do not provide your login details. This email from isaac.rosin1@ucalgary.ca 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 am using the HOutflow condition at the outlet of my flow domain with the incompressible Navier-Stokes solver. According to the user guide, this is enforcing a boundary condition defined by Dong, Karniadakis and Chryssostomidis in equation 3 of their paper: A robust and accurate outflow boundary condition for incompressible flow simulations on severely-truncated unbounded domains. This boundary condition is also stated in equations 11.27 and 11.28 of the user guide. Both the paper and the user guide state that δ is "chosen to be sufficiently small" without explaining what constitutes sufficiently small. Would anyone happen to know what value of δ is used by Nektar++ or where I could find it? Kind regards, Isaac
participants (2)
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Isaac Rosin
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Mohsen Lahooti