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mirko094
(@mirko094)
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June 1, 2020 1:18 pm  

hi, i'm a student . I have a problem with my FEM model. I have already checked the properties and materials. I performed a modal analysis where the first frequencies correspond to those of a CANTILEVER BEAM to verify that there are no errors in the model. However as soon as I go to apply the loads I get absurd displacements. I applied two loads concentrated around the end of the blade, one simulating the lift equal to 7780 N (z axis) and one simulating the centrifugal force of 178876 N (along y). As a result I get a displacement z z 8 meters long when the whole model is about 5 meters long. What could be the problem?

thank you

1591013883-1.png

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Negative Volume
(@negativevolume)
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Posts: 246
June 1, 2020 1:56 pm  

@mirko094

I know you said that you checked the properties and materials, I'm assuming that this means your unit system. That would be the first thing that I would have suggested because you may just have a mismatch in your unit system and may not actually have 8 meters of displacement. 

https://www.dynasupport.com/howtos/general/consistent-units

Is the screen clip that you are showing from a dynamic animation or is this the shape of the blade initially?

And just to be clear, you have rigidly constrained the blue end as the cantilever correct?


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mirko094
(@mirko094)
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June 1, 2020 2:05 pm  

The model is in millimeters. I entered E and G in megapascals while the density in tonnes / mm ^ 3. The screen is the final step of dynamic animation . This is the model with the constrained. I check all dof in the faces as show in the figure.

1591016701-3.png

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Negative Volume
(@negativevolume)
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June 2, 2020 3:55 pm  

@mirko094

Is the direction that you are applying the force correct? It looks like the y-force may be applied in the negative direction which would make sense that it would shoot upward +z. 

I apologize if this seems rudimentary, I'm just going off what you've told me and what I can see. 


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mirko094
(@mirko094)
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June 2, 2020 4:00 pm  

@negativevolume , no problem but thank you for the interest.
The forces are applied in the right direction, or at least I think it is. I set along z and along y and looking at the graphic arrows are in the right direction. If you want I can load the model if you have the opportunity to view it


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Negative Volume
(@negativevolume)
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June 3, 2020 4:25 pm  

@mirko094

If you have the end constrained with *Boundary_spc_set using a node set then you can output the force from that by adding *Database_spc. 

You can also define a cross_section load cell towards the base of the blade using *Database_cross_section_set and adding *Database_secforc. 

Using either of these methods, you will be able to look at the forces and determine the directionality of the loads. If you are getting -y force then I would flip the direction of your load. You can also just try doing this from the start as a test to see if it addresses the issue. 


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mirko094
(@mirko094)
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June 3, 2020 5:15 pm  

@negativevolume
sorry but it is my first model on hypermesh (optistruct) and I have not understood what I should do in detail.


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Negative Volume
(@negativevolume)
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June 3, 2020 6:19 pm  

@mirko094

As a first step, try flipping the direction of the y force.


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mirko094
(@mirko094)
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June 6, 2020 7:23 pm  

@negativevolume
I performed 2 analyzes inverting the force along y but I had almost no change in the displacement. now what do I do?


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mirko094
(@mirko094)
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Posts: 9
June 6, 2020 7:31 pm  

@negativevolume

with the force along y axis i have 7,088 meter (displacement along z) ,instead with the force along -y i have 10,745 meter(displacement along z)


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