[Nek5000-users] Small random perturbations

[nek5000-users at lists.mcs.anl.gov]

Hi Ali,


Two questions:


1)  How are you forcing your flow?  (Do you have periodic boundary conditions in x ?  Are you using constant flow rate?

(p54 = -1; p55=1 would be recommended for periodic bcs

with ffx=0 in the .usr file).


2)  If your Reynolds number is Re = Ubar H / nu , where

H=channel half height and


    Re > (2/3) * 5772


then you should find that you have sustained unsteadiness.


That is, you should have linear instabilities that will grow and saturate to some truly nonlinear (unsteady) state.


[ The 2/3 is because Re=5772 is the transition point for 2D plane channel flow based on U_max, when the background profile is

U=(1-y*y)*Umax   and H=1. In this case, Ubar = 2/3 * Umax .]


In general, you won't have turbulence in 2D.


hth,


Paul


________________________________
From: Nek5000-users <nek5000-users-bounces at lists.mcs.anl.gov> on behalf of nek5000-users at lists.mcs.anl.gov <nek5000-users at lists.mcs.anl.gov>
Sent: Friday, March 9, 2018 11:21:46 AM
To: nek5000-users at lists.mcs.anl.gov
Subject: [Nek5000-users] Small random perturbations



Dear Dr.Fischer,

Actually, I am doing 2-D simulation, at least for now, so no z-dimension is up there. Am I wrong to do DNS in 2-D? Also, I just started doing so, and I think my files are not useful at this point.
Let me try more, maybe I am missing something.

Thank you for your awesome free code and your continuous support.

Ali

https://lists.mcs.anl.gov/pipermail/nek5000-users/2018-March/005133.html

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