Learning Forum

[Koperkovy]

Hello everyone,

my last topic I created had very unspecific question, so I decided to delete it and create a new one.

Currently I am running a model under sinusoidal load and I was interested in geting results when the steady state is archived. Is it possible to set time for ex. after 1s to start calculating results or I need to go through every single time from beginning till the end? In attachment is examplary acceleration to describe the problem.

 

Thank you very much!

[peteroznewman]

Below is the image you attached. In future posts, please use the Insert Image button.

Is the graph above an input to the system or an output from the system?

It looks like the underlying signal has higher frequencies than are represented by the time step shown in the graph. 

The trend line looks like the steady state acceleration will be zero. If the graph is an input, what does the model look like with a zero acceleration?

[Koperkovy]

The graph above is the acceleration output of the system.

So I started the simulation from 9th second to get steady state, but I see distortion in the beginning. Do you know why?

And changing friction coefficient does not change the system output?

Edit

 

[peteroznewman]

You have a system that vibrates and the vibration is dying out due to damping and friction. How do the inputs to the system change between step 1 and step 2, which begins at 9 seconds?

[Koperkovy]

System inputs stay the same all the time. Here are the results with numerical damping set to 0.

I wanted to make force rise more gradually, so I also increased number of segments of input force to 400.

Acceleration

Velocity

Displacement

 

[peteroznewman]

Numerical Damping is not to be used to damp the mechanical system. It may not be needed at all, leave it Program Controlled.

Please read the ANSYS Help system on Damping.  Go to Mechanical APDL > Structural Analysis Guide > Chapter 1.2

Read the link to Global Alpha and Beta Damping (Rayleigh Damping) which in Mechanical are the Stiffness and Mass Coefficients shown below. Don't just use the numbers I show, those are only examples. What do you know about the damping in the real system you are simulating? Do you have any experimental data?

In addition to that damping, you can have friction in the contacts.

[Koperkovy]

Thank you a lot! 

Could you also explain what is going in first 0.125 sec? Is it structure going from transient state to steady state?

Also changing friction coefficieint shouldn't change from sine wave into sth. more disorted like in first 0.125s?

Acceleration. 

Velocity

[peteroznewman]

Yes, you have a start-up transient that dies out by 0.25 and then you have steady-state periodic motion.

No, changing friction coefficient isn't automatically going to introduce high frequency oscillations.

[Koperkovy]

Yes, you have a start-up transient that dies out by 0.25 and then you have steady-state periodic motion.

No, changing friction coefficient isn't automatically going to introduce high frequency oscillations.

I feel like I messed up something but partial results were promising. Unfortunately solver could not finish calculating. My goal was to represent friction. In real experiment I used vibrometer to capture that phenomena.

[peteroznewman]

I am still unclear on what you want to demonstrate in simulation.

[Koperkovy]

I figured it out, I made wrong assumption of point that I am taking results from.

 

Thank you once again for your time