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++.
 

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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