# Relative displacement calculation

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• Last Post 17 February 2020
masud407 posted this 04 February 2020

I was trying to find the relative displacement of the center of the nucleus with respect to the bottom surface along the direction of the force due to applied vibration. My idea was getting the displacement along the force for both center of the nucleus and the bottom surface and then subtracting them to find the relative displacement. However, I am not getting any relative displacement here. Can anyone please explain? I have attached the wbpz.file.

Attached Files

Aniket posted this 06 February 2020

Ansys employees are not allowed to download the files from the community.

https://catalog.ansys.com/product/5b3bc6857a2f9a5c90d32eaf/relative-displacem

-Aniket

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masud407 posted this 07 February 2020

masud407 posted this 10 February 2020

Hello Peter,

Thanks a lot for your suggestion. I have managed to install it. However, I was supposed to get a relative displacement at the center after applying force/vibration at the bottom surface. Unfortunately, I am getting zero relative displacement. Is this happening because there is no fixed support? I have attached the file above.

peteroznewman posted this 11 February 2020

I'm looking at your model, I don't like the geometry condition where the element thickness goes to zero.  It would be better if you made a small thickness rather than have zero thickness.  Also, you seem to have a cut boundary face as if for a symmetry condition, but I don't see any symmetry boundary condition.

The mass of the bodies, m = 4.56e-12 kg

The force on the bodies, f = 6.7e-18 N is the amplitude of a sinusoid with about a 1.9 Hz frequency.

If you add a Modal analysis and use a fixed support on the blue face that the translational joint holds to apply the sinusoidal force to the bodies, you will find the body has a natural frequency of 22.6 kHz. That seems very high for biology. Maybe good for bone.

Using a simple single DOF calculator, you would only expect 7.2e-14 mm of relative motion between the mass and the base.

Is this what you intended to simulate?

Aniket posted this 11 February 2020

I have deleted the post with the extension attachment. Users should download the extensions only after agreeing to clickwrap agreement. Kindly do not share it with the users directly!

Best Regards!

Aniket

masud407 posted this 11 February 2020

Thanks Peter.

Yes, I am expecting relative displacement in nanometer sclae. I will try with a non-zero thickness, however, I didn't get the comments on "symmetry boundary condition". Would you please describe a bit?

I actually applied 30 Hz frequency which was inside the sin (2*pi*frequency). How did you get this 1.9 Hz?

Where did you add that single DOF calculator?

masud407 posted this 13 February 2020

Hello Peter/Aniket,

Can you please tell me what node 1,2 and 3 refers here in the relative displacement tab?

masud407 posted this 13 February 2020

Hello Peter,

As per your suggestion, I have changed the following things:

1) increased the thickness between the nucleus and bottom surface.

2) Applied symmetric boundary conditions.

Still for 30 Hz or 100 Hz of vibration, I am not getting any relative displacement. How did you manage to get that small relative displacement? Any suggestions?

Attached Files

peteroznewman posted this 14 February 2020

Hi Aniket,

It seemed okay because they are available for everyone, but I didn't think of the clickwrap issue.

Best Regards,
Peter

masud407 posted this 14 February 2020

Hello Aniket,

Thanks.

peteroznewman posted this 14 February 2020

Hello masud407,

There are two analysis systems in the attached file. Which one should I look at?  I looked at system B.

Here is the graph of the Joint Force load.

See that the period of one cycle is 0.575 seconds. This means the frequency = 1/Period = 1.74 Hz.

Here are the Joint Force Details.  Note that the Angular Measure is Degrees.

You only have 628*time but you need 10800*time in the sin function to get 30 Hz (30*360).

peteroznewman posted this 14 February 2020

Hello masud407,

Add a Modal analysis to the Model cell of the A system block.  Under the Modal section of the outline, pick the bottom face (where the joint is scoped to) and apply a Fixed Support.  Solve.

The first natural frequency is 24.5 kHz.  The mass of half of the blob is 4.4e-12 kg.

When you have a particular set of values for a mass and a spring rate, it is easy to compute a natural frequency of that mass-spring system.  In fact, if you have any two quantities, you can calculate the other one. A mass-spring is called a one DOF system and there are well know equations that dictate its oscillatory motion.  A specific amplitude and frequency of force applied to the mass will result in an amplitude of displacement that can be simply calculated. That is what I did above.

masud407 posted this 15 February 2020

Hello Peter,

Thanks for your suggestion. I changed the frequency (used sin(10800*time) in the B part.However, it looked like a straight-line and showed no relative displacement. Am I missing any boundary conditions?

I have done the modal analysis and found the exact results that you mentioned. However, I didn't use the process (single-DOF) that you mentioned to find the relative displacement yet.Rather, I used the ANSYS app for the relative displacement.

Attached Files

peteroznewman posted this 15 February 2020

I also don't see meaningful displacements, even when I use 100 G accelerations. I don't know why. Maybe it has something to do with working in the micron size? Someone from ANSYS would have to dig in here.  Try a simple model in a millimeter size part, where you know what the answer should be and see if you get meaningful displacements.

masud407 posted this 16 February 2020

Hello Peter,

I checked with different geometry (large scale in mm) and used the same conditions as my previous one. It showed the relative displacement clearly.

For my previous geometry, am I making anything wrong with boundary conditions or ANSYS is unable to read the smaller relative motion?

peteroznewman posted this 17 February 2020

I think your model was good.

Maybe the way ANSYS is handling micron sized geometry is causing it to lose all the significant digits in the solution and produce garbage output. The way to prove that is with a really simple model where you know the analytical solution. Build a mm scale model and show the solution with the correct result while a micron scale model has a solution that is garbage.