General Mechanical

Falling under the force of gravity into a transient

• javat33489
Subscriber

Hello. I want to simulate the movement of one part under the force of gravity down, but it flies away and does not see the contact in the other. Flat task. In contact, I tried everything, set the radius, changed the type of contact. The part as if does not see the contact and flies through.

What could be the problem? Thank you.

• Rahul Kumbhar
Ansys Employee

Under the connection, insert the contact tool and check its initial status. This contact status should be close.

• Dave Looman
Ansys Employee

Maybe do an initial step where you hold the bodies in place to establish the initial contact.

• mjmiddle
Ansys Employee

The time step needs to be small enough so that contacts are within the pinball radius during a substep. Otherwise it can go from far outside contact pinball above to far outside pinball below in one time step and is never within the pinball radius to detect contact. You can increase the contact pinball, but usually you need to make time stepping low enough. There is a contact setting "predict for impact" to help with this, and you can set time stepping in the analysis settings. Also, be aware that transient usually needs some damping, either global or local. If contacts impact, they can rebound with even more energy due to the contact stiffness behavior and then the next near-contact detection can be moving faster and fly through. This behavior is noticable when viewing the time history. Lowering contact stiffness can recduce that behavior but the analysis really needs damping to prevent it.

• peteroznewman
Subscriber

Hello Javat,

If the part is in free fall, meaning that the only force acting on the part is gravity, the correct way to model this is to calculate the impact velocity from the distance of the fall, and assemble the parts so that the falling part is positioned at the point of contact. The falling part is given the Initial Condition of a downward velocity equal to the impact velocity. That means that at time=0, the contact is already closed and ready to begin decelerating the part.

Calculating the impact velocity from the drop height is a simple physics equation, or you can use an online calculator.

You still have to turn on Auto Time Stepping and make the Initial Time Step a small number like 1e-4 s, the Minimum Time Step 1e-6 s and the Maximum Time Step 1e-3 s.