Learning Forum

[広典 小松]

Hello

I am looking to simulate the free-fall of a ball, aiming to create an air domain to replicate real-world experiments. Currently, I am using ALE (Arbitrary Lagrangian-Eulerian) to generate the air domain. I am using the MAT_NULL keyword to represent air properties, but I wanted to confirm if it is appropriate for simulating air. If there are alternative materials (MAT) that you recommend for this purpose, please let me know.Additionally, I would like to inquire about the keywords required for modeling the contact between the air domain and the ball. Is CONSTRAINED_LAGRANGE_IN_SOLID the appropriate keyword for this purpose, or are there alternative approaches you would suggest?

I appreciate your guidance on these matters.

[Ian Do]

Hello 小松,

ALE can be considered as an independent hydrocode solver, or it can also "couple" with our LS-DYNA solver to provide for fluid-structure interaction (FSI). In that case, the ALE materials can interact with the LAG parts/mats.

There are 2 ALE solver versions:

OLD solver version = unstructured ALE mesh allowed ==> uses *CONSTRAINED_LAGRANGE_IN_SOLID (CLIS) for providing the interaction between the ALE mats (fluids) and the LAG structure.

NEW solver version = assumes ALE mesh is structured (regular mesh indices in XYZ dirs), so it is called STRUCTURED-ALE (or SALE) ==> this uses *ALE_STRUCTURED_MESH_FSI (for coupling) which is a simplified but much easier to use version of CLIS. I would use the newer SALE. Both will require a lot of time investment in learning how to use. 

To your question:

Air may be defined via *MAT_NULL + *EOS_IDEAL_GAS.

Pls consult the ANSYS Learning Hub for online seminar on ALE before using it. It will save you lots of time.

 

Ian Do

[広典 小松]

Thanks for the reply.

Currently, I would like to simulate free-falling objects on a sand model using DEM in the air region of ALE.
I look at reference materials and set it up, but the time required for analysis is enormous.
Is it possible to use DEM and ALE together?

I will attach the Keyword with which I set ALE as an image. I'm using *EOS_LINEAR_POLYNOMIAL , is this a mistake?

 

Also, do you think that the LS-PrePost space is a space where nothing exists, not even a vacuum?

[Ian Do]

Hello,

Your description is not clear enough. Plese send a free body diagram and explain clearly what you want to model.

Your question " I'm using *EOS_LINEAR_POLYNOMIAL , is this a mistake?" 

That is not enough info for me to answer. You are using it to model what? Under what conditions ...? Please be clear & precise.

Regards,

Ian

[広典 小松]

Sorry for the lack of explanation.

First of all, as shown on the left, we want the object to fall freely on the sand. In fact, we have successfully simulated setting gravity and free-falling.
The figure on the right is a model of the air region, and the air region was created at a height that fits the falling object according to the floor in the figure on the right.

If I try to continue the analysis of the free-fall simulation with this additional air region, it will be displayed as taking more than 20,000 days.

　　　　　

 

Below is an image of the keyword related to "air". I don't include keyword settings related to gravity, sand, or falling objects, so please say it if you need to.

 

This is the setting of the air and each part in *CONSTRAINED_LAGRANGE_IN_SOLID.

This is the CONTROL_ALE setting

[広典 小松]

In addition, the units are mm,ms,kg,Gpa

[peteroznewman]

I suggest moving the object mesh down so the tip is just about to touch the top of the sand. Assign the impact velocity to the object.  Delete the Air mesh from the model. The sand will spray out as the object is brought to rest. This model will solve in a reasonable amount of time.  Once you have that result, you can copy the model and add a much smaller air volume that encloses the path of the sand particles that spread the furthest and highest.

[広典 小松]

Thank you for your advice.
The only problem is that if you place the object just before touching the top of the sand.
When I create sand, I make it with a gap of about 5 mm from the container. This is because if you don't open the gap, gravity will scatter the sand on its own.

Therefore, it takes about 0.1 seconds for the sand to settle in the container, as shown in the image.

To solve this problem, I tried to analyze it from a slightly off-the-ground surface, using INITIAL_VELOCITY_GENERATION and INITIAL_VELOCITY_GENERATION_START_TIME.

If you have any other good solutions, please let me know.

Do you have any problems with keywords related to "air"? I would like you to tell me this as well

[Ian Do]

Hello,

Which solver do you want to use to model the air? There are CESE, ALE, ICFD available for simulating fluids (gas in this case). Which to use depends on your objectives.

I agree with Peter's clever suggestion of minimizing air space. But first you must define your objectives. Why is the air important in this case? There are 3 interactions, not all are important:

[1] impactor-sand ==> Pure LAG simulation - most important - you already did this.

[2] impactor-air ==> most likely negligible (too short a drop, not enough time to develop aero force?)

[3] sand-air ==> most likely negligible (?) If not, please explain why this is critical? Sand-air coupling may not be very accurate (not sure available in the new SALE solver yet, I do not think so).

Regards,

Ian Do

 

[広典 小松]

My main goal is to see how much force is generated on an object when it collides with sand.
Until now, ANSYS Workbensh's explicit dynamic analysis used the explicit method kinetic material "SAND" for analysis.
However, in order to investigate the collision with sand, we thought that the discontinuity of sand must be taken into account, so we have come to this day.
In addition to investigating the forces generated by collisions between objects and sand, we also want to reproduce the scattering of sand.
When investigating these two, I would like to create an air region because I think that it cannot be said that there is no influence of air at all.

For these purposes, which is best for you, CESE, ALE or ICFD?
And the Sand-air coupling is still meaningless?
I appreciate your guidance on these matters.

[Ian Do]

[1] I think very likely " forces generated by collisions between objects and sand" will not be affected much by the air because the dropping distance is too short to develop any kind of drag, ~ 1m.

[2] Sand to air interaction is also likely not going to be very significant at all at this speed. The old ALE solver may be able to couple DEM-ALE. But this was experimental and we never had it validated against real data (simply, there is no such data available). And the new SALE solver does not couple with DEM.  

Your best approach is simplest = LAG-DEM impact. 

 

jFYI - I am currently out on vacation and will not be back until 1/8/24. I may not be able to check email and this while I am away.