[andersn]

Hello everyone
I have successfully make a DNS of channel flow with the solver given here:
http://www.cfd-online.com/Forums/ope...s-2-1-0-a.html
The result is good.
But when I use the solver（ico—DNS）to simulate a duct flow(all the boundarys I set as wall except the inlet and outlet which I set as periodic boundary).And the value of dp/dx is 2 times of the value of channel flow case.
I have changed the scheme of ddt to Euler/CN/backward.All the results are not good compared to results of Gavrilakis(1992,Direct numerical simulation of turbulent flow in a square duct).
I also have refined my mesh but still failed getting a good result.

Can you give me some advice?
Thank you very much!

I have attached my case here.

regards!

andersn

[Z.Q. Niu]

Hi, andersn! I'm working in a duct of 1mm X 1mm, maybe there are some difference from yours . I found that the boundary condition of U in your dictionary of 0 is set as cyclic. should it be wall? i.e. fixedvalue? I have another question, How did you get the initial condition of p and U when your simulation parameters such as geometry size, and gradP changes？

[andersn]

hello Niu
To my case,the flow direction I set as periodic boundary condition.As for the initial condition of p and U,I set it according http://www.cfd-online.com/Forums/ope...s-2-1-0-a.html using the tool perturbU.what kind of your case?Are you working on duct DNS also?

I have update my meshes.

Domain size:6.4X1X1
grid:60*80*80
distance between first grid to wall:  0.002
the  stretching ratio:1.06

but the result seems worse.

Can anyone give me some suggestion?

regards!

andersn

[Z.Q. Niu]

Hello, anderson, in a duct flow , the boundary layer of wall of normal and that of wall of spanwise is superposed at the four corner, maybe the character of turbulence here don't agree the character of channel flow, not duct flow. How much is your duct flow and men velocity streamwise?

[huangxianbei]

is the flow direction the only homogenous direction in the duct flow?As in channel flow, the homogenous directions are streamwise and spanwise

[andersn]

Thank you for your suggestion.what do you mean by that"only homogenous direction".In my case,the duct flow direction is along x-axis, while the other four boundaries are wall(no-slip).

[huangxianbei]

Originally Posted by andersn

Thank you for your suggestion.what do you mean by that"only homogenous direction".In my case,the duct flow direction is along x-axis, while the other four boundaries are wall(no-slip).

That means the statistics in this direction won't vary.

[andersn]

yes,The flow direction is the only homogenous direction which I use periodic boundary condition.
I have try some numerical scheme,but the rms of velocity in the flow direction is not good either(the attached files).Is it the problem of fvScheme or PISO method?
why are DNS of channel flow and duct flow so different?
I have been confused by it for a few weeks.


regards!

andersn

[huangxianbei]

Originally Posted by andersn

yes,The flow direction is the only homogenous direction which I use periodic boundary condition.
I have try some numerical scheme,but the rms of velocity in the flow direction is not good either(the attached files).Is it the problem of fvScheme or PISO method?
why are DNS of channel flow and duct flow so different?
I have been confused by it for a few weeks.


regards!

andersn

Perhaps, I see your scheme is quite different from the channel one. Have you tried the default schemes in channel flow as in tutorial?
Are you sure your simulation reached fully developed?

[andersn]

yes.this scheme is quite different from the channel one.
But at first I use the same scheme as the channel flow.I didn't change anything except the boundary condition of spanwise which I modified to wall.And I have refined my grid also.The flow time is so long and I am sure it has reached fully developed.

I have simulated the channel flow case also. And the result is good enough.

[huangxianbei]

Originally Posted by andersn

yes.this scheme is quite different from the channel one.
But at first I use the same scheme as the channel flow.I didn't change anything except the boundary condition of spanwise which I modified to wall.And I have refined my grid also.The flow time is so long and I am sure it has reached fully developed.

I have simulated the channel flow case also. And the result is good enough.

ummm, I have no idea about this now
In my simulation, the steady state was reached after about 160000s while the deltaT=0.5s, so it's quite long

[andersn]

what kind of problem of yours? a duct flow or a channel flow?Can you share it with us.which solver did you use?Maybe the problem exist in fvSheme or polyMesh or something else.

[huangxianbei]

Originally Posted by andersn

what kind of problem of yours? a duct flow or a channel flow?Can you share it with us.which solver did you use?Maybe the problem exist in fvSheme or polyMesh or something else.

Channel flow. I use the icoFoam changed as a DNS solver from the idea of Steven.

[Z.Q. Niu]

Hello Xianbei,
There are not homogenous in the two direction in duct flow(spanwise and normal wall).

[Z.Q. Niu]

Originally Posted by andersn

yes.this scheme is quite different from the channel one.
But at first I use the same scheme as the channel flow.I didn't change anything except the boundary condition of spanwise which I modified to wall.And I have refined my grid also.The flow time is so long and I am sure it has reached fully developed.

I have simulated the channel flow case also. And the result is good enough.

Dear andersn,
Did you initialize your U filed with perturbU or other ways when simulating channel flow?

Best Regards!
Z.Q. Niu

[huangxianbei]

Originally Posted by andersn

what kind of problem of yours? a duct flow or a channel flow?Can you share it with us.which solver did you use?Maybe the problem exist in fvSheme or polyMesh or something else.

ddtSchemes
{
    default         backward;
}

gradSchemes
{
    default         Gauss linear;
    grad(p)         Gauss linear;
    grad(U)         Gauss linear;
    grad(UMean)     Gauss linear;//in order to calculate vt
}

divSchemes
{
    default         none;
    div(phi,U)      Gauss linear;
    div(phi,k)      Gauss limitedLinear 1;
    div(phi,B)      Gauss limitedLinear 1;
    div(B)          Gauss linear;
    div(phi,nuTilda) Gauss limitedLinear 1;
    div((nuEff*dev(T(grad(U))))) Gauss linear;

}

laplacianSchemes
{
    default         none;
    laplacian(nu,U) Gauss linear corrected;
    laplacian((1|A(U)),p) Gauss linear corrected;
    laplacian(DkEff,k) Gauss linear corrected;
    laplacian(DBEff,B) Gauss linear corrected;
    laplacian(DnuTildaEff,nuTilda) Gauss linear corrected;
}

interpolationSchemes
{
    default         linear;
    interpolate(U)  linear;
}

snGradSchemes
{
    default         corrected;
}

fluxRequired
{
    default         no;
    p               ;
}

convertToMeters 1;

vertices
(
    (0 0 0)
    (12.56 0 0)
    (0 1 0)
    (12.56 1 0)
    (0 2 0)
    (12.56 2 0)
    (0 0 6.28)
    (12.56 0 6.28)
    (0 1 6.28)
    (12.56 1 6.28)
    (0 2 6.28)
    (12.56 2 6.28)
);

blocks
(
    hex (0 1 3 2 6 7 9 8)   (128 64 128) simpleGrading (1 10 1)
    hex (2 3 5 4 8 9 11 10) (128 64 128) simpleGrading (1 0.1 1)
);

edges
(
);

boundary
(
    bottomWall
    {
        type            wall;
        faces           ((0 1 7 6));
    }
    topWall
    {
        type            wall;
        faces           ((4 10 11 5));
    }

    sides1_half0
    {
        type            cyclic;
        neighbourPatch  sides1_half1;
        faces           ((0 2 3 1));
    }
    sides1_half1
    {
        type            cyclic;
        neighbourPatch  sides1_half0;
        faces           ((6 7 9 8));
    }

    sides2_half0
    {
        type            cyclic;
        neighbourPatch  sides2_half1;
        faces           ((2 4 5 3));
    }
    sides2_half1
    {
        type            cyclic;
        neighbourPatch  sides2_half0;
        faces           ((8 9 11 10));
    }

    inout1_half0
    {
        type            cyclic;
        neighbourPatch  inout1_half1;
        faces           ((1 3 9 7));
    }
    inout1_half1
    {
        type            cyclic;
        neighbourPatch  inout1_half0;
        faces           ((0 6 8 2));
    }

    inout2_half0
    {
        type            cyclic;
        neighbourPatch  inout2_half1;
        faces           ((3 5 11 9));
    }
    inout2_half1
    {
        type            cyclic;
        neighbourPatch  inout2_half0;
        faces           ((2 8 10 4));
    }
);

mergePatchPairs
(
);

Sorry for forgotting. here is the blockmesh and fvscheme

[huangxianbei]

I have just got to know that the fvm used in DNS is not as accurate as spectral method. So if a fvm is used in DNS ,the mesh should be much finer. I got this from a doctor, and I want to confirm the message, if you know something about the accuracy of the two methods, please show to me

[mgg]

Originally Posted by huangxianbei

I have just got to know that the fvm used in DNS is not as accurate as spectral method. So if a fvm is used in DNS ,the mesh should be much finer. I got this from a doctor, and I want to confirm the message, if you know something about the accuracy of the two methods, please show to me

http://www.mcs.anl.gov/~fischer/nek5...00_dec2010.pdf

Is that helpful? I am using OpenFOAM for DNS pipe flow. I am also interested in this topic.

[itchy]

Hi,

why everybody use ico-DNS instead of pisoFoam, pimpleFoam or icoFoam for DNS in channel-flow??
These solvers should give the same results and their are still implemented in OF.

kind regards
Florian

[mgg]

Originally Posted by itchy

Hi,

why everybody use ico-DNS instead of pisoFoam, pimpleFoam or icoFoam for DNS in channel-flow??
These solvers should give the same results and their are still implemented in OF.

kind regards
Florian

I do pipe flow with pimplefoam, as it has fvOptions, which I can make pressure compensation for cyclic bc.