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