Hi Stan,

I tried your suggestions, but came up with the same results (a laminar flow profile, without any "noise"). I tried first to just add the NSTEPS parameter, and then modified the WHITNOISE and UPDATEFREQ parameter as shown in your setup. It seems that the issue is somewhere else. I noticed that you specify your body force in the "w" variables. Is the Z direction the streamwise direction in your mesh?

Also, as a separate but related question: Are the setup files for the turbulent channel flow example shown here (Examples (nektar.info)) available somewhere? I have been unable to find the "TurbChFl_3DH1D.xml" file anywhere.

Marc Goldbach

PhD Student, Department of Mechanical Engineering

On Fri, Oct 8, 2021 at 3:00 PM Marc Goldbach <mgoldbac@uncc.edu> wrote:

Thanks Stan! I will try to add NSTEPS.
On Fri, Oct 8, 2021 at 2:17 PM Stanislaw Gepner <sgepner@meil.pw.edu.pl> wrote:
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Hi Marc,

I have the following set up for a transitional flow and it works for me:

    <FUNCTION NAME="BodyForce">
      <E VAR="u" VALUE="0" />
      <E VAR="v" VALUE="0" />
      <E VAR="w" VALUE="0.5*Kinvis" />
    </FUNCTION>

    </CONDITIONS>

    <FORCING>
        <FORCE TYPE="Body">
            <BODYFORCE> BodyForce </BODYFORCE>
        </FORCE>
        <FORCE TYPE="Noise">
          <WHITENOISE> 0.005 </WHITENOISE>
          <UPDATEFREQ> 100 </UPDATEFREQ>
          <NSTEPS> 1000 </NSTEPS>
        </FORCE>
    </FORCING>

Compared to what you have, I have the NSTEPS to set the number of time steps over which forcing is applied (not sure what is the default). The value you put as White noise is (I think) variance. So putting to much does not necessarily yield the result I want.

Cheers!
Stan

On 10/8/21 3:26 PM, Marc Goldbach wrote:

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Hi All,

What is the proper way to specify a noise force and a body force at the same time?

I am new to Nektar++, and am attempting to run a simulation of turbulent channel flow using a body force to drive the bulk flow, and a noise force to generate some initial "turbulent-like" fluctuations of the velocity field. However, when I setup the case based on the details found in the user guide (Forcing (nektar.info) ), the resulting flow field doesn't show any "noise component". Is there some error in my settings for this case?:

<NEKTAR>

<CONDITIONS>

...

<FUNCTION NAME="BodyForce">

<E VAR="u" VALUE="-(Re*Kinvis/H)^2/H" />

<E VAR="v" VALUE="0" />

<E VAR="w" VALUE="0" />

</FUNCTION>

</CONDITIONS>

<FORCING>

<FORCE TYPE="Body">

<BODYFORCE> BodyForce </BODYFORCE>

</FORCE>

<FORCE TYPE="Noise">

<WHITENOISE> 0.5 </WHITENOISE>

<UPDATEFREQ> 10 </UPDATEFREQ>

</FORCE>

</FORCING>

</NEKTAR>

Regards,

Marc Goldbach

PhD Student, Department of Mechanical Engineering

University of North Carolina at Charlotte
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Wydział Mechaniczny Energetyki i Lotnictwa
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Marc Goldbach
PhD Student, Department of Mechanical Engineering