RE: [LS-DYNA2] Issue with unit system in LS-DYNA.

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l...@schwer.net

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Sep 22, 2021, 9:45:31 AM9/22/21
to Yash Chordiya, LS-DYNA2

Your materials are 1000 more dense when specifying mass as tonne than kilogram, so the wave speed is 1000 time faster. Hence the stable time step will be 1000 times smaller.                      --len

 

From: ls-d...@googlegroups.com <ls-d...@googlegroups.com> On Behalf Of Yash Chordiya
Sent: Wednesday, September 22, 2021 12:28 AM
To: LS-DYNA2 <ls-d...@googlegroups.com>
Subject: [LS-DYNA2] Issue with unit system in LS-DYNA.

 

Dear all,

I was trying to simulate a blast analysis on reinforced concrete wall. When I used tonne, mm, MPa unit system the computer or run time is 64 hrs for simulation time of 0.04 s. Wherein, if I use the SI units i.e., kg, m, Pa unit system the analysis is taking hardly 15 mins. My doubt is if the number of elements and nodes generated are same, size of the keyword file is same then why this drastic variation in run time!

 

Thank you.

 

Regards,

Yash

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James M. Kennedy

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Sep 22, 2021, 10:57:01 AM9/22/21
to Yash Chordiya, LS-DYNA2

Dear Yash,

 

Please make sure your units are consistent.

 

https://www.dynasupport.com/howtos/general/consistent-units

 

----------------------------

MASS

LENGTH

TIME

FORCE

STRESS

ENERGY

DENSITY

YOUNG's

35MPH    
56.33KMPH

GRAVITY

kg  

M

s

N

Pa

J

7.83e+03

2.07e+11

15.65

9.806

kg

Cm

s

1.0e-02 N

 

 

7.83e-03

2.07e+09

1.56e+03

9.806e+02

kg

Cm

ms

1.0e+04 N

 

 

7.83e-03

2.07e+03

1.56

9.806e-04

kg

Cm

us

1..0e+10 N

 

 

7.83e-03

2.07e-03

1.56e-03

9.806e-10

kg

Mm

ms

kN

GPa

kN-mm

7.83e-06

2.07e+02

15.65

9.806e-03

g

Cm

s

dyne

dyne/cm²

erg

7.83e+00

2.07e+12

1.56e+03

9.806e+02

g

Cm

us

1.0e+07 N

Mbar

1.0e+07 Ncm

7.83e+00

2.07e+00

1.56e-03

9.806e-10

g

Mm

s

1.0e-06 N

Pa

 

7.83e-03

2.07e+11

1.56e+04

9.806e+03

g

Mm

ms

N

MPa

N-mm

7.83e-03

2.07e+05

15.65

9.806e-03

ton

Mm

s

N

MPa

N-mm

7.83e-09

2.07e+05

1.56e+04

9.806e+03

lbf-s²/in

In

s

lbf

psi

lbf-in

7.33e-04

3.00e+07

6.16e+02

386

slug

Ft

s

lbf

psf

lbf-ft

1.52e+01

4.32e+09

51.33

32.17

 

 

 

 

 

 

 

 

 

 

kgf-s²/mm

Mm

s

kgf

kgf/mm²

kgf-mm

7.98e-10

2.11e+04

1.56e+04

9.806e+03

kg

Mm

s

mN

1.0e+03 Pa

 

7.83e-06

2.07e+08

 

9.806e+03

g

Cm

ms

1.0e+1 N

1.0e+05 Pa

 

7.83e+00

2.07e+06

 

9.806e-04

 

----------------------------

 

Sincerely,

James M. Kennedy

KBS2 Inc.

September 22, 2021

sabbir siddique

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Sep 22, 2021, 11:26:05 PM9/22/21
to LS-DYNA2
The reason for more computing time for small element size is obvious. Time step depends on element size.
del t = l/c

where c, (the wave travel velocity)depends on material properties like Elastic modulus, density and poison's ratio which is constant for a material.

So, time step size shall be large for large element size that results in less computing time.

But material properties need to be consistent for all units.

- Sabbir

Yash Chordiya

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Oct 7, 2021, 5:55:40 AM10/7/21
to LS-DYNA2
Thank you all, for your suggestions and comments, the issue has been resolved. I had made a minor mistake, in one input entry I gave the stress in Pa instead of MPa.
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