Incorrect results

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  • Last Post 11 May 2019
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Ghost3507 posted this 10 May 2019

My structure is a bit complicated to describe but I will simplify. Assume we a rectangular beam with 20x5x5(LxWxH)mm and I applied an acceleration load of 14715 mm/s^2 in the +ve Z direction and I supported the body from the Height edges (4 edges) what is expected is that the beam will bend about the lateral plane where the face with tension and the face with compression will move/deform toward the +ve Z direction. The problem is the deformation animation is in the -ve Z direction. 

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peteroznewman posted this 10 May 2019

If you have a simply supported beam along the X axis, and apply an acceleration load in the +Z direction, the center of the beam will deform in the -Z direction.

The way to think about this is the ends of the beam are being accelerated in the +Z direction, while the mass at the center of the beam experiences an inertia force in the -Z direction, bending the center in that direction.

Note that Standard Earth Gravity has the opposite sign convention. If you assign the Standard Earth Gravity to the +Z direction, think of that as the direction that a mass will fall if unsupported.  So the center of a simply supported beam along the X axis will deform in the +Z direction. I know, it's confusing.

What is even more confusing is that in Fluent, accelerations work the same way as Standard Earth Gravity does and opposite to the way accelerations work in Structural.

Ghost3507 posted this 10 May 2019

To be clear I am dealing with a ladder chassis. Another situation is when the car decelerates the car the rear end bend toward the front end transversal bending to the side beams. so I apply a rotational acceleration about +ve Z  and support at the from wheels mounts. what I get is a rotation (in the animation) about the -ve Z axis.

Ghost3507 posted this 10 May 2019

No I currently don't use earth gravity

peteroznewman posted this 11 May 2019

It's still not clear what you put in your model, what the results are and what you expected.

Please capture a screen image using the Snipping Tool in Windows 10. Once you save a screen image to a file, you can insert the image in your reply.

Ghost3507 posted this 11 May 2019

The load direction should be about -ve Z but I inverted to show the expected deformation.

This is what I expected as the car decelerates.

This is the result when I set the load in +ve Z direction.

peteroznewman posted this 11 May 2019

What you received is what I expect. By rotationally accelerating a beam about a point at the left end, the mass at the right end is going to lag behind, pulled by the inertia forces.

Imagine you have a very flexible steel ruler, the long direction along the x axis and the thin dimension aligned with Y so the ruler lies in the XZ plane. At the left end, the ruler is fastened to shaft with a crank handle. A large mass is attached at the right end. Suddenly turn the crank about +Z and imagine what happens. The mass will lag behind and the ruler will bend the way the frame did in the received image.

Ghost3507 posted this 11 May 2019

Thank You so much explaining to me this concept.

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