Hi YiLin, The spatial derivative in Nektar++ is calculated using the Lagrange polynomial on quadrature points. You can refer to section 2.4.2 of this book Karniadakis G.E., Sherwin S.J. (2005). Spectral/hp element methods for computational fluid dynamics. Oxford University Press. Cheers Ankang ________________________________ From: nektar-users-bounces@imperial.ac.uk <nektar-users-bounces@imperial.ac.uk> on behalf of Paul Liu <liuyilin.liu@gmail.com> Sent: 17 February 2022 17:32 To: nektar-users <nektar-users@imperial.ac.uk> Subject: [Nektar-users] Acoustic Solver Question This email from liuyilin.liu@gmail.com 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 Everyone, One thing I am noticing with NEKTAR++ is that you specify the base flow function in the configuration/xml file. However, both LEE and APE equations require the estimation of the spatial gradients of the base flow. I was wondering what method of approximation is used within NEKTAR++ as the configuration files does not ask for an analytical expression of the spatial gradients. Thank you so much YiLin Liu 403-397-3457 liuyilin.liu@gmail.com<mailto:liuyilin.liu@gmail.com> MASc Student at CML<https://cml.mech.ubc.ca/> UBC