Energy

ls_dyna

Energy
  
Reported Energies
  
Total energy reported in GLSTAT (see *database_glstat) is the sum of the following:
  
internal energy
  
kinetic energy
  
contact (sliding) energy
  
hourglass energy
  
system damping energy
  
rigidwall energy
  
"Spring and damper energy" reported in the glstat file is the sum of internal energy of discrete elements, seatbelt elements, and energy associated with joint stiffnesses (*constrained_joint_stiffness. . . . ). "Internal Energy" includes "Spring and damper energy" as well as internal energy of all other element types. Thus "Spring and damper energy" is a subset of "Internal energy".  
  
Energy values are written on a part-by-part basis in MATSUM (see *database_matsum).  
  
Hourglass energy is computed and written only if HGEN is set to 2 in *control_energy. Likewise, rigidwall energy and system damping energy are computed and written only if RWEN and RYLEN, respectively, are set to 2.  
  
The energy balance is perfect if total energy = initial total energy + external work, or in other words if the energy ratio (referred to in glstat as "total energy / initial energy" although it actually is total energy / (initial energy + external work)) is equal to 1.0.  
  
The History > Global energies do not include the contributions of eroded elements whereas the GLSTAT energies do include those contributions. Note that these eroded contributions can be plotted as "Eroded Kinetic Energy" and "Eroded Internal Energy" via ASCII > glstat.  
  
The total energy via History > Global is simply the sum of KE and internal energies and thus doesn't include such contributions as contact energy or hourglass energy.  
  
Negative contact energy
  
Abrupt increases in negative contact energy may be caused by undetected initial penetrations. Care in defining the initial geometry so that shell offsets are properly taken into account is usually the most effective step to reducing negative contact energy. Refer to sections 23. 8. 3 and 23. 8. 4 in the LS-DYNA Theory Manual (May 1998) for more information on contact energy.  
  
Negative contact energy sometimes is generated when parts slide relative to each other. This has nothing to do with friction -- I'm speaking of negative energy from normal contact forces and normal penetrations. When a penetrated node slides from its original master segment to an adjacent though unconnected master segment and a penetration is immediately detected, negative contact energy is the result.  
  
If internal energy mirrors negative contact energy, i.e., the slope of internal energy curve in glstat is equal and opposite that of the negative contact energy curve, it's possible that the problem is very localized with low impact on the overall validity of the solution. You may be able to isolate the local problem area(s) by fringing internal energy of your shell parts (Fcomp > Misc > internal energy in LS-POST). Hot spots in internal energy usually indicate where negative contact energy is focused.  
  
If you have more than one contact defined, the sleout file (*database_sleout) will report contact energies for each contact and so the focus of the negative contact energy investigation can be narrowed.  
  
My general suggestions for combating negative contact energy are as follows:
  
Set contact controls back to default except set SOFT=1 and IGNORE=1 (Optional Card C).  
Or, set contact controls back to default except set SOFT=2. Furthermore, in v. 970, setting SBOPT (formerly EDGE) to 5 and DEPTH to 5 on Optional Card A in addition to setting SOFT to 2 will improve sliding behavior and edge-to-edge behavior (though at an increased cost).  
The specifics of your model may dictate that some other approach be used.