Solved: How to simulate a 2DOF Vibration problem

JSD97 · ‎06-13-2020

Hi, how can i simulate the next excersise:

Im not an expert on Nastran-in-CAD, but i've been trying to learn about all the types of simulations and i tryed to use and understand the direct transient response and a direct frequency response because those seems to be the ones that i should use for this example. But i have some problems trying to simulate the wave. The excersise is giving us the wave speed basically, the amplitude and the lenght, so it is posible to know the frequency following the expression c=f*Wavelenght where "c" is the wave speed. Then, other thing i dont know how to do is connect the points between the springs and put the C.G. with the 1000kg mass at the distance given to each spring.

If there's a way to tell the springs to compress at certain time or in some manner give them the sinusoidal input, i don't know.

Maybe somebody can help me with this, thanks so much.

Anonymous · ‎06-14-2020

Are you sure that you want to simulate this in Nastran? It looks like a rigid body problem. So I would try to simulate this with Inventor's Dynamic Simulation.

JSD97 · ‎06-14-2020

Hi, thanks for your answer.

Amm... Even if i try to simulate this with Inventor Dynamic simulation, i dont know how to insert the sinusoidal as a movement input. I have made dynamics with springs in the past in inventor because you can insert them as a connector but i dont know how to connect them without a part, cause the excersise doest not specify anyone but it gives you the mass center, mass magnitude and the ratio of gyration (i assume this is because inertial purposes).

I'm asking for advice in Nastran because i though it would be better, knowing that is a more complex software and a specialized FEA Software. But, if there's a less complex way to do this, im open to receive any advice. Thanks

Anonymous · ‎06-18-2020

Just for fun I set up a simple model for Dynamic Simulation last night. Hopefully I calculated the simulation parameters correctly. That's what happens in the first two and a half seconds:

The car definitively needs some dampers. 😉

@JSD97 wrote:
... i dont know how to insert the sinusoidal as a movement input ...

I used an input diagram for an imposed motion of a dummy part.

You need one for each "wheel".

@JSD97 wrote:
... but i dont know how to connect them without a part ...

Of course you need moving parts in Dynamic Simulation. But you can use empty dummy parts if you just need moving points. Such a part needs some mass though, because the simulation is based on the equation f=m*a. Dynamic Simulation uses a mass of 1 kg and a moment of inertia of 10000 kg mm^2 for a dummy part without a solid body by default.

@JSD97 wrote:
... the excersise doest not specify anyone but it gives you the mass center, mass magnitude and the ratio of gyration (i assume this is because inertial purposes). ...

The part that represents the car needs the right mass and moment of inertia (calcultated from the mass and radius of gyration) in the Body Properties.

I used Inventor 2012 for the attached files to be quite sure that you can read them, because I do not know which (hopefully newer) release you have.

Sorry to everybody for this post about Dynamic Simulation in the Nastran forum. 🙄

JSD97 · ‎06-19-2020

Hi Mr. Puschner

Thank you so much for your help!

I tried myself to replicate the same you did but im only getting a strange response.

Im attaching my files, if you dont mind to, maybe you can check my model and tell me what im doing wrong.

I have some questions, for example, why should we have to use a dummy part for the pitch reason? and also for the bouncing one.

There's another thing i noticed. Even in your model, there's a time when the springs starts to rotate due the movement of the mass but i think that should not happen (because a car spring doesnt rotate, just moves vertically), so, is there a way to constraint the movement of the springs in a vertical way?, avoiding them to rotate and then deliver a weird simulation above the third second. I noticed that the model properties says that the system has Four degree of freedom (and, technically this excersise is about of 2DOF). Maybe that's because we are not constraining the spring rotation, so that's why above the third second (using your model) the mass surpass the ground limit (because the springs rotates)

Anonymous · ‎06-23-2020

@JSD97 wrote:
... I tried myself to replicate the same you did but im only getting a strange response.
Im attaching my files, if you dont mind to, maybe you can check my model and tell me what im doing wrong. ...

The springs in you model have no prestress. So the "car" is falling into the uncompressd springs. If you add 327 mm to the free length of the front spring and 178,36 mm to the free length of the rear spring, the simulation will start with a equilibrium of froces.

The other difference between your an my model is the initial position of the car. The wavelenght of the "rough road" is given with 10 m (although the sketch of the excersise picture shows a wavelength of 2.5 m). So if the "front wheel" is at the zero-crossing of the sine wave, the "rear wheel" is in a valley of the sine wave. So the "car" starts with a pitch. In your model the rear wheel is getting a pulse when the simulation starts, moving from 0 mm to -50 mm in zero time.

Anonymous · ‎06-23-2020

@JSD97 wrote:
... why should we have to use a dummy part for the pitch reason? and also for the bouncing one. ...

Because there is no joint with a rotational and a translational dof with axis of movement that are perpendicular, you need to connect a prismatic and a revolution joint with a dummy part.

The same is with the combination of a revolution joint and the spring joint. You can't connect to joints without a dummy part.

Anonymous · ‎06-23-2020

@JSD97 wrote:
... there's a time when the springs starts to rotate due the movement of the mass but i think that should not happen (because a car spring doesnt rotate, just moves vertically), so, is there a way to constraint the movement of the springs in a vertical way?, avoiding them to rotate and then deliver a weird simulation above the third second ...

That's why I limited the simulation to 2 1/2 seconds and mention that the car needs some dampers. Whithout any damping the amplitude of bouncing and pitching will rise infinitely. In real life you would lose control of the car and the car would overturn.

Of course you can limit the springs to move only translational and perpendicular to the "car". You just need more dummy parts. But that would not help. The amplitude of the spings would still rise infinitely and punch through to negative length.

There is not limitation to make the simuation more realistic. Feel free to model wheels and the rough road, use 3d contacts with realist stiffness, damping and friction, limit the movement of the springs and so on. But you still need damping.

If you add damping to both springs (at least 0,2 N s/mm) of my model and increase the simulation time to 10 s, you will see a more realistig behaviour. If you increase the simulation time to 20 s you will see how the car will settle down after 10 cycles of the rough road sine wave.

Anonymous · ‎06-23-2020

@Anonymous wrote:
... after 10 cycles of the rough road sine wave. ...

Sorry it's after 10 s and about 14 cycles. 🤔

And realist and realistig should be realistic. 😐

And if someone finds more mistakes ... feel free to keep them. 😉

JSD97 · ‎06-23-2020

@Anonymous Thank you so much for your guidance. I Now i have a better POV of the simulation and all the configurations that i must take with the springs and the joints.

Also thanks for take your time and give those explanations! You've helped me a lot. i hadn't understood why i must use those "dummy" parts and why my springs didn't hold the "car".

I will follow all your advices for this one and any other simulation since now. Thanks again!