Learning Forum

[aliBaratian]

hi there

how can i define contacts between two beam elements in transient structural? the scope gets yellow when i do so!

thanks

[peteroznewman]

You define contact the same way you would in Static Structural, set the geometry filter to Edge and pick one beam as the Contact side, and the other beam as the Target side of the contact pair.

[aliBaratian]

what is your ANSYS version? when i do so there appears a question mark not a green check mark ! in fact i want the upper come down and touch the lower when it reaches it. please look at the pic attached. thanks 

[peteroznewman]

Ali,

You missed the feature by one release...

in 18.0, if you want contact between beam-shaped objects, you will have to represent those beam-shaped objects with either solid or shell elements.

Regards,

Peter

[aliBaratian]

thanks a lot.

 

[aliBaratian]

Thanks for your nice answer. Now i have managed to define the contacts but as in pictures when the touch the solution blows up! I decrease the time step and increase mesh but the problem exists! I look forward to your reply, regards

[peteroznewman]

Hello Ali,

The Transient Structural model shows you suddenly applying a 1000 N load on the end of a cantilever. Is that what you intended to simulate?

You also suddenly turn gravity on at time = 0. That is not physically possible.

I can understand a Static Structural model with gravity and a 1000 N load hanging on the end of a cantilever beam, but I can't understand what you are trying to simulate in Transient Structural.

If you do in fact want to drop a 1000 N load on the end, then you should turn this into a Static Structural pre-stress analysis, where the gravity load is solved for the two crossed beams without the 1000 N load on the end, and then in Transient Structural, the end load is applied.

Does the 1000 N represent a 102 kg mass being hung on the end of the cantilever?  The dynamics of hanging a 102 kg mass on the end of the beam is very different from applying a 1000 N force to the end of the beam because F = ma and the end of the beam weighs very little, so there will be a large acceleration, while if you hang a 102 kg mass on the end of the beam, the acceleration will be much smaller. In either case, you want time steps on the order of 1 millisecond in order to compute the dynamics.

Regards,

Peter

[aliBaratian]

hi peter, you see my system is large. first i am trying to model simple ones. one part of the system is as in the picture below.

1- i want the rope to be able to slide fictionally in the polyethylene arc tube if necessary. so i need the contact which till now i have not managed to set it up correctly !

2-i do not know how to model the rope so i have used beam for it ! unfortunately it has bending strength unlike rope.

thanks in advance.

[peteroznewman]

Hi Ali,

1. Frictional sliding of a rope in a tube is computationally very difficult. It's good to first capture some of the problem in a simpler model.

2. A spring has no bending stiffness, but you can't apply a force to the end of a spring, you have to attach it to ground or another body.  But this gets back to my point about the 1000 N force. What mass is associated with that force?  What is physically creating that force?

Regards,

Peter

[aliBaratian]

 in fact this is a part of a bigger support for a lighter than air balloon tethered to the ground. but the problem is the contact between these two concentric beams, i really appreciate your help. 

 

[peteroznewman]

Hi Ali,

What do you want to simulate in a support consisting of a rope through a polyethylene tube?

I assume the tube is a straight tube before the load is applied. 

How is the tube connected to the bigger support?  If the center of the tube is connected to the bigger support, you could ignore the sliding of the rope in the tube. You could assume a symmetric problem, and cut the tube to half its length. You could put a spring between the end of the tube and the ground. Then you could put a displacement BC on the cut center face of the tube and displace it upward while the spring on the end of the tube pulls the tube into a bowed shape.

The rope in this symmetric condition is just going to lie on the tube ID and not slide at all. So the spring on the end of the tube going to a point on the ground is a good simulation of the rope in the tube.

Regards,

Peter