Issue with STL geometry and pde toolbox

Hello,

I want to use the pde toolbox for solving structural modal problems. For this I just started with the examples provided with matlab software.

Firstly I used the exmaple for calculating the natural frequencies of a square plate with dimensions 10x10x1:https://www.mathworks.com/help/pde/ug/vibration-of-a-square-plate.html. This script works fine and the natural frequencies are plausible. For training purpose, I just designed a simple beam and converted it to STL file and imported it with "importGeometry" functon. However, I clampled the beam one side and caluclated the natural frequencies, but I figured out that a.) takes very long and b.) I'am getting over thousand of natural frequencies which are similiar to each other. Also the modeshapes looks like I have some modes with spikes of the surface. I guessed that there is something wrong with the geometry, so I exported a square plate with 10x10x1 as mentioned in the example above and getting the same strange behaviour using the code of the example !

I think there can be an issue with the STL geometry.. So is there any special things I have to consider when I export STL files from the CAD software ?

Thanks !

Best regards,

Tugrul

Answer 1

[Ravi Kumar]

Check the dimensions of the expoerted geometry. Some tools export by default in mm, you mght be seeing many more frequenceis if you have model is mm and propeties in SI unites.

Regards,

Ravi

[Tugrul Öztürk]

Hello Ravi,

thanks for your quick reply ! I found there is an "scale" command to scale a geometry. I will try to scale with a factor of 1E-3, since I think it is exported in mm.....

[Tugrul Öztürk]

Update: Thank you for the good hint Ravi... it was a realy stupid mistake... wasting time....

For all who have the same issue, I solved it like shown below:

....

geom = importGeometry(model,'FileName.stl');
scale(model.Geometry,1E-3); % 1E-3, when you have to scale from mm to m

or you can use

scale(geom,1E-3); 

since geom is the handle pointing to the geometry saved in the model object (here "modal-solid")

Now, the natural frequencies are plausible and the "real" work can begin :)

Best regards,

Tugrul