[Nektar-users] Something wrong with the time-dependent reference fields (sponge-layer)?

18 Sep 2019

      Hi all,

I find when I use the <REFFLOWTIME> tag, the simulation will diverge even there is no time 't' in the reference fields.

        <FUNCTION NAME="referencefields">
             <E VAR="rho"  VALUE="rhoInf" />
             <E VAR="rhou" VALUE="rhoInf*uInf*(1-0.02*cos(k*y)*sqrt(2)/2)" />
             <E VAR="rhov" VALUE="rhoInf*vInf" />
             <E VAR="E"    VALUE="pInf/(Gamma-1)+0.5*rhoInf*uInf*(1-0.02*cos(k*y)*sqrt(2)/2)*uInf*(1-0.02*cos(k*y)*sqrt(2)/2)+0.5*rhoInf*vInf^2" />
        </FUNCTION>
        <FORCING>
            <FORCE TYPE="Absorption">
                <COEFF> SpongeCoefficient </COEFF>
                <REFFLOWTIME> referencefields </REFFLOWTIME>         
        </FORCE>
    </FORCING>

But if I use the <REFFLOW> tag, everything is fine (the reference fields is the same). So I think maybe there is something wrong with the time-dependent reference fields.

Thanks.

Shujie

At 2019-09-13 18:52:55, "蒋树杰" <anjsj@126.com> wrote:

Hi all,

First, I want to say thanks to everyone for helping me so much!

Now I want to use a time-depentdent sponge-layer. Is there anyone using this function now? The simeluation will diverge very soon. In the simualtion, It is an empty domain and gust moves with flow. The boundary condition is the same with the reference fields. I use the Compressible Flow Solver with Euler euqations. 

 My reference like this:

<FUNCTION NAME="referencefields">
            <E VAR="rho"  VALUE="rhoInf" />
            <E VAR="rhou" VALUE="rhoInf*uInf*(1-0.02*cos(k*x+k*y-k*uInf*t)*sqrt(2)/2)" />
            <E VAR="rhov" VALUE="rhoInf*uInf*(0.02*cos(k*x+k*y-k*uInf*t)*sqrt(2)/2)" />
            <E VAR="E"    VALUE="pInf/(Gamma-1)+0.5*rhoInf*uInf*(1-0.02*cos(k*x+k*y-k*uInf*t)*sqrt(2)/2)*uInf*(1-0.02*cos(k*x+k*y-k*uInf*t)*sqrt(2)/2)+0.5*rhoInf*uInf*(0.02*cos(k*x+k*y-k*uInf*t)*sqrt(2)/2)*uInf*(0.02*cos(k*x+k*y-k*uInf*t)*sqrt(2)/2)" />
</FUNCTION>

Of course with the <REFFLOWTIME>  tag:

<FORCING>
        <FORCE TYPE="Absorption">
              <COEFF> SpongeCoefficient </COEFF>
              <REFFLOWTIME> referencefields </REFFLOWTIME>
        </FORCE>
</FORCING>

The reference field is the function of x, y and t (time-dependent). Is there any thing wrong with my reference fields? 

I test the reference fields without x,y and t. It works well. It looks like:

<FUNCTION NAME="referencefields">
            <E VAR="rho"  VALUE="rhoInf" />
            <E VAR="rhou" VALUE="rhoInf*uInf" />
            <E VAR="rhov" VALUE="rhoInf*vInf" />
            <E VAR="E"    VALUE="pInf/(Gamma-1)+0.5*rhoInf*uInf^2+0.5*rhoInf*vInf^2" />
</FUNCTION>

So I think maybe my sponge-layer coffecient is fine. 

If anyone has some idea please let me. I can send my case file and mesh file to you.

Besides, I am using the Compressible Flow Solver to do some areoacoustic research. I found there is some reflection on the boundary when I use far-field boundary conditon. 
At first I think the far-field boundary condition is a non-reflection boundary conditions when relate with solving the Euler euqaitons.(in the paper: A guide to the implementation of boundary conditons in compact high-order methods for compressible aerodynamics). I do not know if there is any misunderstanding in that paper.

So I want to use the sponge-layer to reduce the reflection.

Kind regards,
Shujie
University of Southampton