Can't simulate airflow around a sketchup model

I have followed this video guide on how to simulate airflow in a sectioned object

But I get stuck at the 3:15 part where it says to interesect both volumes. My problem is that when I import my sketchup model it imports as a face, nothing else. There is no volume, no edges, nothing imported but a face.

Here is an image showing the object that I am trying to evaluate and the problem described:

I can't get past that point and I can't find another way to simulate the airflow around that object. All I want is to see how the air behaves with shapes like that one.

I have also followed the export/import instructions that were provided here

Is there any way that I can turn that into a volume or any other way to simulate the airflow around an object like that one, something like the sphynx simulation

I have also uploaded the sketchup files of the original model in case it is needed, they have been compressed and can be downloaded here:

Use Volumes Stitch Tool

Assuming your SketchUp faces bound a watertight volume (which is rare), i.e., each edge is connected to exactly 2 faces, you can use the Volumes->Stitch tool (Topology Only = True) on the Geometry Tool Palette. Then you can use the Booleans->Subtract (Topology Only = True) tool to subtract that volume from a larger bounding box ensuring that none of the SketchUp geometry faces intersect or are coincident with your bounding box.

You have to use the Topology Only option for each tool because you don't have native analytic geometry. SketchUp is a faceted modeler. Faceted geometry can not take part in standard geometry operations.

Thank you for the quick reply

I have used the stich tool with topology only set to true and it created a new volume. Then I positioned the bounding box so that the object would not be intersecting or touching the box. Then I set topology only to true in the subtract boolean and selected both the bounding box and the new volume that was previously created.

Now I only have the bounding box and still no volume created from the sketchup model. I gave a quick look at the sketchup model and I can't see any edge that isn't connected to exactly 2 faces, I even looked inside the model for any edge that could be connected to a 3rd face that wasn't visible and there was none. As far as I can see the model is a watertight volume.

I even tried with a more simple sketchup model, a uniform cube, and I still faced the same problems. I don't know what else I can do.

Shade the faces

If your original SketchUp model had only a single face, which it appears to do, then when you subtract it from the bounding box then it will not have any edges visible (there are none). If you shade all faces and then make one of the bounding box faces transparent you will see the SketchUp model faces inside.

However, it would be better if you split the original SketchUp faces to form feature edges visible in Caedium. You can do this in SketchUp by naming groups of facets and/or in Caedium select File->Preferences, then in the Properties Panel set Import->Faceted->Facet Feature Angle = 30, or something similar until you get edges showing up after import.

I did

What you suggested in the first sentence was the first thing I tried, I thought that maybe I couldn't see anything because there were no edges, so I faced everything but one of the bounding boxe's faces and there was nothing inside.

I also changed the facet feature angle and it only started showing edges below +/-5. I set it to 1, which showed all the edges and I repeated what you said in your first comment, still made no difference. I still cannot turn that into a volume that the program can work with, it either crashes, or gives an error about something not being native geometry.

I never thought this would be so complicated.

Works for me

I can not reproduce the disappearing geometry.

I just managed to create a complete simulation according to:

  1. File->Preferences, Import->Faceted->Facet Feature Angle = 15
  2. Import your dae file, you should see a series of circular edges representing your geometry
  3. Volumes->Stitch, Topology Only = true, select all faces to create 'volume'
  4. Volumes->Box, Lengths = [1 1 1], to create 'volume_1'
  5. Transforms->Translate, Translate = [-0.5 -0.5 -0.5] applied to 'volume_1' to center it around your imported geometry
  6. Booleans->Subtract, Topology Only = true, select 'volume_1', then 'volume' (order is important) to create 'volume_2'
  7. Faces->Join applied to skinny (sliver) faces to produce larger faces, while preserving most of the key feature edges
  8. Setup the substance, etc. as in the tutorials

I suggest for a more accurate simulation you use the Accuracy tool on some of the high curvature faces.

You can download a rough proof of concept simulation at

Can't make one on my own

I have been non-stop for over 5 hours trying to replicate what you did but I cannot achieve the same result.

I don't understand the "Faces-Join" part, it makes everything into 2 faces without any edges, I can't create an inlet, outlet faces and walls to achieve the same result.

I have absolutely no idea how you made that and I have followed your instructions, analysed your file hundreds of times and I have searched the forums for any help but none was found, I am completely lost here.

It should be more easier than this to convert a sketchup import into a volume because this is a nightmare.

Selective use of Faces Join

You only apply Faces->Join to a select few faces, not all faces. For more details see the section "Remove slivers" in "5 Tips for CFD Flow Volume Creation".

You've completed the volume once you've performed Booleans->Subtract. Selective use of Faces->Join to is to better configure the volume ready for the meshing process.

Getting better but still not there yet

I have managed to join the sliver faces, I joined the same that you have, but now I am stuck with a big box that is missing the shape of the object inside. I have set the walls and added Air to the volume but it still doesn't work.

I have read the mesh troubleshooting guide and found that when I use the Scalar Fields->Vol it creates a big blob outside my box. When I do the same with the file you made it shows the volume inside the box as it is expected.

I analyzed my mesh and everything looks ok so I don't understand what is happening. Do I still have to reverse the resulting box subtraction? I think all I have now is something like the second image from point 3:

And I should have the inverse of that?

Get the geometry right before physics setup

There's no point in trying to setup the physics if your geometry isn't correct.

For the subtract operation you have to select the box then the SketchUp volume. The first selection is the volume that will have subsequently selected volumes removed from it. The bounding box has to totally enclose the SketchUp volume.

You only need to join maybe about 5 sliver faces, you still want to have feature edges in the high curvature regions. When complete you should have quite a few edges still visible. Click Home->Start (reset) on the simulation I shared to see what the final flow volume edges should look like.

That's what I did

I followed your instructions exactly like you said and I even redid it just now to re-confirm, and this is what I am left with after subtracting following the order that you mentioned and also after joining a few of the objects' faces.

Left is your file, right is the one I just did after the subtract and joining slivers part:

As you can see, until that part everything appears to be as it should, but past that part nothing works as it should. I add air substance to the volume, apply walls to all faces, add inlet to the left wall of the box and outlet to the right wall and then I try to run vector field U applied to the volume and this is what happens; left without U and right with U, both at the same angle and view distance:

As you can see, the vector is running outside the volume or leaking outside the volume, it is not being contained. Nothing like the one you made.

Surface mesh first

Instead of creating the volume and surface meshes in one step by requesting the velocity, first assign Scalar Fields->E Ratio (which is a surface mesh quality metric) to all your faces - watch for warnings/errors in the Properties Panel. To view the individual elements see "View Mesh".

If any of the your faces fail to surface mesh then the volume mesh will leak as you are seeing. If there are warnings then try applying Accuracy = Custom, Resolution = 40 or higher onto your SketchUp faces. This will cause the surfaces to remesh hopefully without warnings then you can proceed to assign velocity (which will create the volume mesh, again watch for warnings/errors) and run the simulation.

I did mesh as I mentioned before

And I meshed again, following your instructions and this is what happens:

When I apply E Ratio to all faces no warnings or errors show up, instead this is all that it shows:

I still added the Accuracy you suggested and this is what shows after it re-meshed at resolution 40, still no warnings or errors were shown:

It doesn't look like any of the mesh examples I saw in the tutorials and blogs posted here, the polygon/triangle grid.

Delete all previous results first

You need to delete all previous results, which will clear the mesh, before requesting E Ratio, which will regenerate just the surface mesh.

To see individual surface mesh elements see "View Mesh", specifically:
Select all the faces that you want to see the mesh on. In the Properties Panel set:

  • Style = Flat
  • Transparent = checked (on)

Also just assign Accuracy to the SketchUp faces, not the bounding box faces.

Box mesh

I also made the mex of 2 of the boxe's faces and notice that my box is not showing the same connections in two corners like yours; left is yours and right is mine:

In yours all the corners are symetrically connected while mine has 2 corners in both faces "missing" that same symetrical connetion. Sections in question are pointed by the black arrows.

Could this be the cause of the "leak"?

Both mesh corner connections are valid

The different mesh connections in the corners is not the cause of the leak. Both types of connections are valid.

Both meshes were identical

I also did the last steps you indicated but this time on your file and it also showed a box with collored crosses exactly like my file.

This time I did what you told me, just E-Ratio on both files, yours and mine. I also set the faces to flat and assigned accuracy 40 to the sketchup faces on both files, here is the result; left is your file, right is my file:

Yours completed with no errors or warnings while mine showed that it exceeded maximum triangle count. Regardless, it still created the polygon mesh exactly like in the other file.

Meshing stopped

Maximum triangle count warning means the meshing stopped on a face, which will cause the volume mesh to fail. As part of the message there is a link (www...), click it and it will select the face that caused the warning. Delete the Accuracy property on that face to avoid over resolving it.

Still didn't work

I removed the accuracy on the face that was causing the problem and ran the E-ratio without any issues this time. Then I tried to plot the vector field U to the volume and it still did the same as before, showed outside the box in a big blob of arrows.

Share your project file to diagnose problem

Please share your latest project file (.sym) so I can try to diagnose the problem. Please use a file sharing service such as Dropbox, rather than the previous service you used - it was loaded with questionable links and unrelated downloads.

I am sorry if the service I

I am sorry if the service I used previously gave a untrust impression, but I didn't have any other way to do it.

Here it is in drobpox download:

Volume inside out

Thanks for sharing the project file. Certainly the volume mesh looks wrong - inside out. If you assign Vector Field->Normal to all faces you can see the vectors on the bounding box point inwards, whereas they should point outwards.

My best guess at what happened is that you subtracted the bounding box from the SketchUp volume. You need to be sure when you drag and drop Booleans->Subtract you first select the bounding box volume in the Select Dialog, click OK, click Select/Deselect, select the SketchUp stitched volume in the Select Dialog, click OK, then confirm the operation by clicking Done.

Also you don't need to bother assigning Accuracy, as I suggested earlier, at least while you are producing a first pass simulation.

That was it

The problem was really with the subtract selection. From your previous instructions I was making the selection incorrectly.

Once I dropped the subtract I was selecting the box and then the sketchup in the same selection window by holding Ctrl key. I thought that this was what you meant that I should do the selection like that while hold Ctrl key down.

Now that you specified exactly how that selection is done I was able to finally achieve a working result.

Indeed the subtracting result was the inverse, no wonder the software was always plotting outside of it.

Thank you so very much for all your support, now I am able to continue evaluating your software during this trial and study. One thing is certain, support here has been excelent

Create Residual Monitor

Great to see your progress. A couple more pointers:

  1. Always create a Residual Monitor, as in all the RANS Flow tutorials, to see how the solvers are behaving. Without it you can't be sure that your results are meaningful.
  2. Use the Accuracy tool selectively on certain faces to better resolve your geometry around high curvature regions.

Having some trouble on a part

I have been trying to replicate your file by following all your steps, my problem is that when I reach the point "Faces -> Join" everything becomes one volume with 2 faces without any edges; the outside box and the inside shape become one face part of the same volume.

When I follow this step, none of the edges are being preserved and I don't understand why. I have tried just joining the box faces and then the object, all faces, both objects and every 2 connected faces, the result is always the same, the edges are not being preserved.

Thank you so much

It finally works, well at least the file you provided does. I am studying the Venturi effect and the wakes on shapes similar and equal to the one I am using. Although you have used a top to bottom air flow, I managed to change it to a lateral flow and finally observe what I was looking for, the way the air drags itself inwards and then wakes on the opposite counter-shape.

I will be using your guide for importing the other similar shapes from now on and use the file you provided as support as well.

Thank you so much for all your help.