Reply To: Successful explicit analysis

[Reno Genest]

Hello,

From this graph, we can tell that you have some kinetic energy and zero internal energy at time=0. Your wooden object probably has some initial velocity and comes into contact with the steel tank right away because we see the internal energy go up and the kinetic energy go down right away after time=0. Usually, we would like to have a bit of a delay (have a gap between the wooden object and the tank) before internal energy starts increasing. The gap between the wooden object and the tank would ensure that you don't have initial penetration in your contact; initial penetration can cause contact issues that can be seen in the sliding energy.

If you only have an initial velocity in your model (no prescribed velocity or displacement, no forces, no pressure, etc.), the total energy in your simulation should be constant. You could plot the total energy to see what it looks like. But, from the kinetic and internal energies, we see that the sum of the 2 energies is larger at the end of the simulation compared with time=0. This is not correct if you only have an initial velocity and no external work done by prescribed motion or other loads. So, you might want to verify your model.

Also, note that in LS-DYNA, conservation of momemtum is enforced, but  conservation of energy is not. So, the run will keep running even though energy is not conserved. It is the responsibility of the user to verify that energy is conserved by post-processing the GLSTAT database after a run.

I would also plot the sliding energy (contact energy) and hourglass energy. If you don't have friction, sliding energy should be zero or very low. A rule of thumb is to have sliding energy and hourglass energy lower than 10% of internal energy.

A typical energy graph with only initial velocity would look like this:

 

And the energy ratio should be equal to 1:

You will find more information on energies in LS-DYNA here:

https://lsdyna.ansys.com/total-energy/

https://www.dynasupport.com/tutorial/ls-dyna-users-guide/energy-data

 

Let me know how it goes.

 

Reno.