[boffin5]

Like Odysseus, when finally in sight of Ithica, once more gets swept out to sea, I am having a new problem with my chtMultiRegion case,
after having assured myself that it was under control. Although it runs, I checked the streamlines to characterize the flow through
the duct, and found that it is all wrong.

This is maddening, since my test case, consisting of a block with a hole containing a radiator, works properly. In it, the
streamlines run nicely through the flow path, as seen in attachment 1. But when I substitute in a fuselage with a radiator fairing,
leaving everything else the same, the streamlines have a mind of their own, and make no sense, as can be seen in
attachment 2. I know there are those who will say 'you have a bad surface geometry', but when I use the same fuselage body in a simpleFoam case,
the streamlines look much better, as shown in attachment 3.

I suspect the problem has something to do with coupling between the fluid (airflow) and solid (the radiator) regions; one place where
this is controlled is in constant/fluid/fvOptions and constant/solid/fvOptions. In these, there are two options for interpolationMethod,
and I have tried both of them, but the streamlines are still bad:

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

fluidTosolid
{
    type            constantHeatTransfer;

    interpolationMethod cellVolumeWeight;
    //interpolationMethod mapNearest;
    
    nbrRegionName   solid;
    master          false;

    nbrModel        solidTofluid;
    fields          (h);
    semiImplicit    no;
}


porosityBlockage
{
    type            interRegionExplicitPorositySource;

    interRegionExplicitPorositySourceCoeffs
    {
        //interpolationMethod cellVolumeWeight;
        interpolationMethod mapNearest;
        nbrRegionName   solid;

        type            DarcyForchheimer;
        
        // D 100;  // Very little blockage
        // D 200;  // Some blockage but steady flow
        // D 500;  // Slight waviness in the far wake
        // D 1000; // Fully shedding behavior

        d   (20 20 20);    // (120 100 100)
        f   (20 20 20);    // (120 100 100)

        coordinateSystem
        {
            type    cartesian;
            origin  (0 0 0);
            coordinateRotation
            {
                /*type    axesRotation;
                e1      (0.5 0 0.866025);
                e2      (0 1 0);*/
                
                type    axesRotation;
                e1      (0 1 0);
                e2      (0 0 1);
            }
        }
    }
}

limitT
{
    type       limitTemperature;
    active    yes;
    
    selectionMode all;
    min        10;
    max        700;
    
}

// ************************************************************************* //

Also, I have tried different values for the DarcyForchheimer coefficients, but there was no difference.

What baffles me is that the test case works, but the real world case does not, when the setup is identical. And, the geometry seems to be okay,
as it works properly in a simpleFoam case.

Once again, I am hoping that the community, with people far smarter than me, can help me resolve this. I have put a zip file of the case in
my Dropbox account; here is the link: https://www.dropbox.com/scl/fi/9fzjx...9dlvtxpt6&dl=0