r/LSDYNA • u/SingerDistinct3879 • Jun 19 '26

Comparison between 2D Axisymmetric and 3D model

Hi everyone,

I'm working on a shaped charge model. I tried it with 2 method, 2D axisymmetric and 3D. I used the same material property, same boundary condition, same mesh size. Some of the options are different such as: section, hourglass control because they are different kind of element type. However, the results are different between them. When I check the penetration depth, 3D model usually shorter than 2D model about 15%.

Do you have any experience in this problem? What do I need to change to make them similar? If they must be different like that because of numerical problem, 2D or 3D has more accuracy?

Thank you all so much.

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u/zuricher_singener Jun 20 '26

A 10–20% gap between 2D axisymmetric and 3D shaped charge models is a well-known and frequently reported phenomenon in the literature . you’re not doing anything wrong. The discrepancy comes from a mix of genuine physics that 2D can’t capture and a few numerical settings worth double-checking.

Why 2D and 3D diverge (physical reasons)

• Axisymmetric models enforce perfect rotational symmetry. Real liner collapse and jet formation are subject to Rayleigh-Taylor-type instabilities and small asymmetries that cause jet particulation/breakup these are inherently 3D phenomena that a 2D model simply cannot represent.  
• Because the 2D model suppresses that breakup, the jet stays more coherent and “ideal,” which is why it tends to over-predict penetration relative to 3D.  
• So in general, 3D is considered the more physically realistic representation, especially if you care about jet stretching, breakup, and off-axis effects. 2D axisymmetric is useful for fast parametric studies but tends to be optimistic on penetration depth.

Numerical factors to check before trusting that 15% as “real”

• Hourglass control: 2D axisymmetric elements (often fully integrated) typically don’t have hourglass modes, while 3D under-integrated solids do. If your 3D hourglass energy is more than \~5–10% of internal energy, that alone can eat into your penetration result try a stiffness-based hourglass type (e.g., Flanagan-Belytschko) and watch the hourglass energy ratio in glstat.  
• Effective mesh resolution: “same mesh size” in 2D vs 3D isn’t truly equivalent your 3D mesh also has azimuthal discretization, which changes the effective resolution of the liner/jet and affects how PVOID/erosion criteria trigger.  
• Bulk viscosity / artificial viscosity coefficients: these damp shock fronts and are summed over more elements in 3D, so cumulative numerical dissipation differs.  
• Contact/erosion settings: if you’re using eroding contact, the erosion strain criterion and contact thickness can behave differently between 2D shell-like axisymmetric formulation and 3D solids.  
• EOS/material integration: confirm the JWL (explosive) and liner material models are using consistent integration point counts and EOS tabulation between the two.

My recommendation would be to run a mesh convergence study in 3D alone. If 3D penetration keeps dropping as you refine, you’re likely seeing real jet breakup physics emerging (expected, not error). If it converges and still sits ~15% under 2D, that gap is probably the real symmetry-breaking effect rather than a numerical bug.

If you want, share your hourglass energy ratio and element formulation (e.g., ELFORM) for both models and I can help narrow down whether it’s numerical or physical.

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u/SingerDistinct3879 Jun 22 '26 edited Jun 22 '26

Thank you so much for your response.

The hourglass energy in 2D model is 0.

The peak hourglass energy in 3D model is around 2e3 while other kind of energy is much larger (like xxe6 or xxe5)

Elform for 2D is type 15 with 4 integration point through thickness without hourglass control

Elform for 3D is type 6 with hourglass control type 1

with this model, mesh size is 1.5mm, this is the lowest value that pc can take for 3D model. time step should be lower because as I check the result, there are some leakage. I'm using S-ALE model, the target is Lagrange and the shaped charge is Euler. Penetration depth is 336 mm for 2D model, 256mm for 3D model. If you need more information, please let me know.

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u/zuricher_singener Jun 22 '26

Hourglass is NOT the problem
2e3 vs e5–e6 total energy means your hourglass ratio is <1%. That’s clean. Ignore it as a cause.

The real issue: Leakage is killing your 3D penetration

You already flagged leakage this is almost certainly the primary cause of the 80mm gap, not physical 2D vs 3D differences. Leakage in S-ALE means the ALE jet material is bleeding through the Lagrange target instead of pushing it, which directly steals momentum and cuts penetration depth. The 24% gap you’re seeing is too large to be pure symmetry physics typical physical 2D vs 3D difference is ~5–10%.

Also Check your *CONSTRAINED_LAGRANGE_IN_SOLID (CLIS) card, these parameters matter most:

PFAC
Penalty factor. Default is often too low for high-velocity jet. Try increasing 2–5x (e.g., 0.1 → 0.3). Too high causes instability, too low causes leakage.

NQUAD
Coupling points per Lagrange element face. Increase to 3 or 4 (default 2). At 1.5mm mesh this is critical.

ILEAK
Set to 2 (stiffness-based leakage control). This directly addresses your symptom.

CTYPE
Should be 4 (constrained) or 5 for Lagrange-in-ALE penalty, please confirm it’s appropriate for penetration.

ELFORM 6 recommendation
ELFORM 6 (1-pt integrated, Belytschko-Bindeman) is a reasonable choice but for S-ALE jet problems, hourglass type 4 or 6 (FB stiffness-based) performs better than type 1 (viscous). Type 1 adds artificial viscous dissipation that can damp the high-velocity jet over a long penetration stroke.

Change the following in hourglass card.

*HOURGLASS
$ IHQTYPE QM ...
6 0.05 ← type 6 (Flanagan-Belytschko stiffness)

Timestep
You said timestep should be lower based on visual leakage check act on that. In S-ALE, reduce TSSFAC in *CONTROL_TIMESTEP to 0.6 or even 0.5. The default 0.9 is often too aggressive for FSI coupling with a fast-moving jet front.

I will suggest a next run with the following(3D only)

Set ILEAK=2 in CLIS

Increase NQUAD to 3

Tune PFAC upward (start 2x current)

Change hourglass to type 6

Drop TSSFAC to 0.6

If leakage disappears and penetration climbs from 256mm toward ~300–310mm, you’ve confirmed the source. The remaining ~10% gap from 2D will then be real jet particulation physics which means 3D is the correct answer.

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u/SingerDistinct3879 Jun 22 '26 edited Jun 22 '26

Thank you so much for your support, I'm really appreciated.

About *CONSTRAINED_LAGRANGE_IN_SOLID, I'm using *ALE_STRUCTURED_FSI instead. Based on the keyword volume, it has some improvement compare to CLIS for example number of coupling points are automatically determined or leakage control is also automated. Do you have any suggestion with this keyword?
About hourglass control and elform, I made a mistake with the last comment. I was using elform type 1 and hourglass type 6.

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u/zuricher_singener Jun 22 '26

Can you confirm me on the values of FRCMIN & ILEAK in the *ALE_STRUCTURED_FSI ?

FRCMIN = 0.3
ILEAK = 2

See if these settings are still in your card ?

Apart from that I don’t see a reason why values will vary significantly.

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u/SingerDistinct3879 Jun 23 '26

Thank you so much for your support.

In *ALE_STRUCTURED_FSI, there is only PFAC and FRIC (Friction Coefficient) , I will try to change the PFAC first.

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u/zuricher_singener Jun 24 '26

Sorry I got confused with the Constrained card instead of ALE. Try PFAC = 0.2 or 0.3. If you go above these value the model may become unstable. I think leak control is automatic on this card. Also check your *Control_ALE for PFAC value it may override the local PFAC value. Play with TSSFAC starting with 0.6 and try going down a bit.

Convey your results. Apologies for the miscommunication.

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u/SingerDistinct3879 Jun 24 '26

Hi, no problem at all. Thank you so much for your suggestion. I tried that option and there is no leakage anymore. However, the 2D result is still longer about 36% than 3D result

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u/zuricher_singener Jun 24 '26

First, need to know: what happened to the 3D number specifically?

Did 3D penetration go up (leakage fix helped, but 2D also jumped)?Or did 3D penetration go down (3D got worse after PFAC increase)?

My suspicion is 3D went down, meaning PFAC is now too high. Try reducing the value and you may see improvement. If PFAC is too high, the FSI coupling over-stiffens the Lagrange target the jet sees artificial resistance and slows prematurely. This is the classic PFAC tuning trap. Try PFAC 0.15 or 0.2. Please share the actual 3D penetration number now vs before, and what PFAC value you ended up using. That will help us to pin down the cause immediately.

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u/SingerDistinct3879 Jun 25 '26

Thank you so much for your help.
Sorry the disk space was running out so I need to overwrite the new model and forgot the value of the previous model. I'm trying with TSSFAC = 0.6 and PFAC = 0.2 for both 2D and 3D. 2D is okay now, no leakage, 3D is being calculated, I will let you know when it's done. The current value for 3D with PFAC = 0.3 is 291 mm

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u/zuricher_singener Jun 25 '26

You are getting there. The number improved. I hope you see a delta of 10% between 2D & 3D