October 31, 2022 at 7:28 pmmichail.stockfeltSubscriber
If you make a body with an internal pressure, you have to tie it down somehow or you will get rigid body motion. Usually by binding an orifice - tying up the pipe that usually connects to the vessel.
But this can become a problem if you for some reason get any imbalance in the model - accidentally missed some tiny inner surface (of the possibly complex shape) that multiplied with a large pressure results in your vessel tearing at the pipe sideways, or if you actually have no pipe (a balloon for example), or for whatever reason.
I once tried to model a pair of scissors to see what the results were when it cut a wire, and I had some problems balancing the forces. Ended up adding a rod that went out to nowhere just to bind that and get the clipping parts as unaffected by the constraints as possible.
Inertia Relief does not seem to be the answer, I can't get it to work because it seems to assume an acceleration field.
Is there any other trick to nullify resultant forces like that?
November 4, 2022 at 1:42 pmDaniel ShawAnsys Employee
Symmetry is commonly used to model pressure vessels. Symmetry boundary conditions provide numerical stability.
If the issue is numerical stability caused by small unbalanced nodal forces, you can try activating "weak springs" in the Mechanical UI. Mechanical will then add spring elements with small stiffnesses to the model at DOFs without sufficient restraint. The weak springs will carry the imbalanced nodal forces and prevent rigid body motion. You can use a reaction probe to check the amount of force developed in the weak springs. If it is very small, then they provided stability without affecting the overall model behavior. If it is large then, the model does not have sufficient restraint and you need to manually restrain additional DOFs.
November 5, 2022 at 5:55 pmmichail.stockfeltSubscriber
These vessels are not symmetic, and the pressures involved are… not low… so the resultant unbalanced nodal forces tend to be quite wild. I need “strong springs”… Will definitely try that reaction probe thing, thanks!
My solution so far is to add features where I can put restraints, I just would prefer not to because they inevitably add quirks.
Another solution would be to use a wossname explicit transient analysis and set the density to “Neutron Star” and let it just fly, I suppose.
November 6, 2022 at 12:04 amDaniel ShawAnsys Employee
You can also try running a Mechanical implicit time transient analysis. The inertia and damping forces might provide some stability. It seems like the model is underestrained. You may need additional restraints.
November 6, 2022 at 6:16 pmmichail.stockfeltSubscriber
- You must be logged in to reply to this topic.
Simulation World 2022
Check out more than 70 different sessions now available on demand. Get inspired as you hear from visionary companies, leading researchers and educators from around the globe on a variety of topics from life-saving improvements in healthcare, to bold new realities of space travel. Take a leap of certainty and check out a session today here.
Earth Rescue – An Ansys Online Series
The climate crisis is here. But so is the human ingenuity to fight it. Earth Rescue reveals what visionary companies are doing today to engineer radical new ideas in the fight against climate change. Click here to watch the first episode.
Subscribe to the Ansys Blog to get great new content about the power of simulation delivered right to your email on a weekly basis. With content from Ansys experts, partners and customers you will learn about product development advances, thought leadership and trends and tips to better use Ansys tools. Sign up here.
- How to calculate the residual stress on a coating by Vickers indentation?
- An Unknown error occurred during solution. Check the Solver Output…..
- Saving & sharing of Working project files in .wbpz format
- Solver Pivot Warning in Beam Element Model
- Understanding Force Convergence Solution Output
- Colors and Mesh Display
- whether have the difference between using contact and target bodies
- The solver engine was unable to converge on a solution for the nonlinear problem as constrained.
- Massive amount of memory (RAM) required for solve
- What is the difference between bonded contact region and fixed joint
© 2022 Copyright ANSYS, Inc. All rights reserved.