Hi Luqi, Thank you for your question. I assume that you are using the Incompressible Navier-Stokes solver, in that case the only parameter required is Kinvis which can take arbitrary values. Whether it is non-dimensionalised or dimensionalised depends on how the simulation is set up. In theory, the Reynolds number is defined by some velocity scale, U, length scale, D, and Kinvis; Re = UD/Kinvis. In practice, we usually "pre-set" U = 1 (i.e., U = 1 as a boundary condition for the inlet or laminar centerline velocity for pipe flows), and D = 1 (chord of airfoil, or diameter of a pipe). Therefore, the user guide simply shows Kinvis = 1 / Re. Then, the timescale usually defined as t = D/U is simply 1. Kinvis can take any value as long as the simulation matches the desired Re defined. I hope that this helps, and I am happy to clarify further. Cheers, Chi Hin ________________________________ From: nektar-users-bounces@imperial.ac.uk <nektar-users-bounces@imperial.ac.uk> on behalf of Luqi Zhang <luqizhang2@gmail.com> Sent: 08 April 2025 10:34 To: nektar-users <nektar-users@imperial.ac.uk> Subject: [Nektar-users] Questions about the units of the parameters set in Nektar++. This email from luqizhang2@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. Dear Nektar++ Team, I apologize for disturbing you again. I'm currently having trouble setting the kinvis value and noticed that the Nektar++ User-guide sets it to <kinvis=1/Re>. Does this mean that when defining parameters in <PARAMETERS>, they need to be dimensionless , such as timestep, kinvis. If so, how do I define the reference velocity and length in Session.xml for doing dimensionless? Or are they just default values? Moreover, should u,v,p in <BOUNDARYCONDITIONS> and <InitialConditions> be dimensionless, as well? Thank you very much for your time and support. I look forward to your response. Best regards, Luqi Zhang