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
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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?