Wrong position\scaling problem for example "insert stream"

Submitted by davidf on Sat, 10/12/2013 - 14:30

Dear all,

I ran the example named "insert stream" , everything went fine. Then I used the supplied "postscript" script to
convert the dump (.stream) files into VTK format - that worked fine as well.

Then, I loaded the .stl and the .vtk files into Paraview, however as you can see in the attached picture
The particles are displaced from the geometry
and there seems to be some scaling issue - the geometry looks too big for the bounding box of the particles.

I'm just starting to learn to use Paraview and LIGGGHTS simulations, so any help and guidance with the initial steps
will be very much appreciated.

Thanks !
David

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cstoltz | Mon, 10/14/2013 - 12:54

The STL file is scaled by 0.005 in the input deck. If you're just loading the base STL file into Paraview w/o scaling, it will be 200x too large as compared to the particles. You'll need to scale it appropriately.

Regards,
Chris

davidf | Mon, 10/14/2013 - 19:10

Chris, thank you !

I managed to scale it down now. However, I still have a few more questions.
How did you know it should be scaled ?

And also it will be very helpful for me to begin my learning process with some working examples which I also know
what they at least should look like in Paraview.
For example, in the Benchmark#5 file there are is particularly interesting example for me which is:
(1) Bin Flow
and in the LIGGGHTS Features section in the website:
(2) The first example (video) with complex walls (pouring of particles)

Where can I find the associated input and .stl files ? Are they available ?

Best,
David

cstoltz | Mon, 10/14/2013 - 21:05

Do a search for 'Tutorial' on the LIGGGHTS page. There is a document I posted in July that contains a lot of example problems and the CAD to go with them.

Regards,
Chris

davidf | Tue, 10/15/2013 - 17:12

Chris,

I've found the tutorials and the files.
Thank you for your time !

David

davidf | Sun, 12/01/2013 - 20:43

Hi Chris,

What do you mean the base STL file is scaled by 0.005 in the input deck ? What is the input deck ? Why is the STL scaled ?
As I understand, the STL files are generated with CAD software (i.e SolidWorks) and the specific dimensions of the system
are contained in these files. I have an STL file with dimensions 500cmx500cmx1000cm. Now, I want
to simulate in LIGGGHTS the exact same geomtery with same dimensions. Should I additionaly scale it after importing it with mesh/gran ?? In fact, all the tutorials I see use the scale option and it seems like all the STL files come 200X larger... why is this so ? can't an STL file come just in the right dimensions as one needs ?

This is a bit confusing, because I imported my STL file and only after using the scale command it fitted the bounding box of the simulation, although the bounding box (domain) was defined to be 1000cmx1000cmx1200cm.

I think I miss a basic issue and perhaps a trivial... sorry for the 'noob' questions..

Thanks,
David

cstoltz | Mon, 12/02/2013 - 12:07

David,

The input deck is your input file, the set of commands that tells LIGGGHTS what to do. The scaling is used to reconcile different units systems. For example, I always use SI units in my LIGGGHTS simulations, which means that all distance units are in meters. When I create CAD geometry in SolidWorks, I work in units of mm. As a result, when I import geometry into a LIGGGHTS simulation, I need to scale it by 0.001 to get the units to m.

STL files don't retain any unit of dimensions. If you're working in a CAD platform and create a box 1m x 1m x 1m, but save it in units of mm, it will output a box with dimensions of 1000 x 1000 x 1000. When you try to import this into a LIGGGHTS simulation with SI units, it will assume it is 1000m x 1000m x 1000m, unless you scale it properly.

Regards,
Chris

davidf | Mon, 12/02/2013 - 12:25

Now everything makes sense!

Thanks