Flip Normals cannot be found in GUI

I want to model an internal pipe flow. I used a cylinder form primitives. As it is shown in the figure attached, the normals direction are towards the outside region which is not what I need.
In MFiX tutorial of modeling 3D cylindrical FB, it is mentioned that we need to check “Flip STL Normals” in the “mesher sub-pane.” I do not see this icon. This icon is not also located in Geometry pane.
What should I do?
BTW, Do we need to do anything with the normal direction of the mass inflow and the pressure outflow?

Hello, I note that the figure is a closed cylinder and not a pipe. This 3D figure has a “solid” interior (it has neither a solid nor a fluid phase). In that case, you need to have “surface” and not “solid” for the geometry, which is that you need.

I recommend using two cylinders of radio r1> r2, get their “difference” and you will get a pipe of radius r1 and with a thickness [r1, r2]. I also recommend for more advanced figures to create your .stl figure with an external program e.g. tinkercad, and then import it into mfix, the process is the same.

You asked about flip normals, and this can be seen in the geometry section

I hope I’ve been helpful,
Best Regards,

Thank you for your response… But I have seen the tutorial that has used the cylinder to model a fluidized bed… Why cannot I modify the boundary condition and the initial condition to make it a pipe?
And also, the pane that you sent its picture, I do not have the “Flip Normal” botton!!!???

What version of MFiX?

In the filter list, select the reverse sense, you can use that to flip normals as well.


I used exactly the method that you recommended to build a “pipe”. However, I am not sure how I can set boundary conditions on the surfaces. In my case, I need to make the inner side of the “pipe” inlet, and the gas would go radially out through the solid part of the “pipe” and leave from the outer side. I have no idea how to select and define the regions to be the inlet or outlet. Could you give me some advice on how to do it? Thank you very much! Here is my attempt to define the configuration. chem_3d_jelly_roll.mfx (16.1 KB)


If you use the keyword browser ( shot-2021-04-24_20-54-33 icon) and search for “flip” you get this:

but clicking on “Locate control” fails. There seem to be two issues here:

  1. The “Flip normals” button does not appear until you have created an STL geometry:

if you haven’t added an STL geometry, there is no “Flip normals” button to locate, so the error message makes sense, even if it is unhelpful.

But, I see that even if the button is there, the “Locate control” fails to find it. This is a bug, I will fix this for the next release.

Thanks for reporting, user feedback like this helps us greatly in improving the usability of MFiX.

And if you don’t know about the keyword browser, try it out! MFiX is complex, and this is an attempt to make it easier to navigate around.

– Charles

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We do not recommend using the flip_stl_normals keyword. This is a legacy keywords used in older versions of MFiX. With recent versions of MFiX, normals should be flipped in the geometry pane.