Questions about the units of the parameters set in Nektar++.
******************* This email 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
Hello Luqi, Indeed, the simulation will be non-dimensional. If you non-dimensionalise the simulation w.r.t. the inflow velocity then you will have a velocity of 1 at the inflow, otherwise it will be a factor of your reference velocity, wherever you take it. Remember you will have to use a non-dimensional mesh. If you opt to use a dimensional one you will have to scale your viscosity by the reference length (nu=lref/Re). All the best, Guglielmo ________________________________ 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
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
participants (3)
-
Chan, Chi Hin
-
Luqi Zhang
-
Vivarelli, Guglielmo A