LS-DYNA中的能量平衡

LSDYNA中的能量平衡time........................... 4.99735E-03time step...................... 4.45000E-06kinetic energy................. 3.80904E+09internal energy................ 5.15581E+09spring and damper energy....... 1.00000E-20hourglass energy .............. 1.34343E+08system damping energy.......... 0.00000E+00sliding interface energy....... 1.72983E+07external work.................. 4.54865E+09eroded kinetic energy.......... 0.00000E+00eroded internal energy......... 0.00000E+00total energy................... 9.11649E+09total energy / initial energy.. 1.09716E+00energy ratio w/o eroded energy. 1.09716E+00global x velocity.............. -6.63878E+01global y velocity.............. 3.44465E+02global z velocity.............. -1.86129E+04time per zone cycle.(nanosec).. 11286GLSTAT(参见*database_glstat)文件中报告的总能量是下面几种能量的和：内能 internal energy动能 kinetic energy接触(滑移)能 contact(sliding) energy沙漏能 houglass energy系统阻尼能 system damping energy刚性墙能量 rigidwall energyGLSTAT中报告的弹簧阻尼能”Spring and damper energy”是离散单元(discrete elements)、安全带单元(seatbelt elements)内能及和铰链刚度相关的内能(*constrained_joint_stiffness…)之和。

而内能”Internal Energy”包含弹簧阻尼能”Spring and damper energy”和所有其它单元的内能。

因此弹簧阻尼能”Spring and damper energy”是内能”Internal energy”的子集。

由SMP5434a版输出到glstat文件中的铰链内能”joint internal energy”跟*constrained_joing_stiffness不相关。

它似乎与*constrained_joint_revolute(_spherical,etc)的罚值刚度相关连。

这是SMP 5434a之前版本都存在的缺失的能量项，对MPP 5434a也一样。

这种现象在用拉格朗日乘子(Lagrange Multiplier)方程时不会出现。

与*constrained_joint_stiffness相关的能量出现在jntforc文件中，也包含在glstat文件中的弹簧和阻尼能和内能中。

回想弹簧阻尼能”spring and damper energy”，不管是从铰链刚度还是从离散单元而来，总是包含在内能里面。

在MATSUM文件中能量值是按一个part一个part的输出的(参见*database_matsum)。

沙漏能Hourglass energy仅当在卡片*control_energy中设置HGEN项为2时才计算和输出。

同样，刚性墙能和阻尼能仅当上面的卡片中RWEN和RYLEN分别设置为2时才会计算和输出。

刚性阻尼能集中到内能里面。

质量阻尼能以单独的行”system damping energy”出现。

由于壳的体积粘性(bulk viscosity)而产生的能量耗散(energy dissipated)在版本970.4748之前是不计算的。

在后续子版本中，设置TYPE=-2来在能量平衡中包含它。

最理想的情况下能量平衡：总能量total energy ＝初始总能量＋外力功external work。

换句话说，如果能量比率energy ratio(指的是glstat中的total energy/initial energy，实际上是total energy/(initial energy + external work)) 等于1.0。

注意，质量缩放而增加质量可能会导致能量比率增加。

注意在LSprepost的History>Global energies中不包含删掉的单元(eroded elements)的能量贡献，然而GLSTAT文件中的能量包含了它们。

注意它们的贡献可以通过ASCII>glstat中的”Eroded Kinetic Energy”& “Eroded Internal Energy”来绘制。

侵蚀能量(Eroded energy)是与删掉的单元相关的内能和删掉的节点相关的动能。

典型来说，如果没有单元删掉”energy ratio w/o eroded energy”等于1，如果有单元被删掉则小于1。

删掉的单元与”total energy/initial energy”比率没有关系。

总能量比率增加要归于其它原因，比如增加质量。

重述一下，将一个单元删掉时，文件glstat中的内能和动能不会反映能量的丢失。

取而代之的是能量的丢失记录在glstat文件的”eroded internal energy” & “eroded kinetic energy”中。

如果用内能减去”eroded internal energy”将得到分析中还存在的单元的内能。

对动能也一样。

matsum文件中的内能和动能只包含余下(noneroded)的单元的贡献。

注意，如果在*control_contact卡中将ENMASS设置为2，则与删掉的单元的相关的节点不会删掉，”eroded kinetic energy”是0。

在LSprepost中History>Global 只是动能和内能的简单相加，因此不包含接触能和沙漏能等的贡献。

－－－－－－－－－－壳的负内能：为了克服这种不真实的效应－－关掉考虑壳的减薄(ISTUPD in *control_shell)－－调用壳的体积粘性(set TYPE=-2 在*control_bulk_viscosity卡中)－－对在matsum文件中显示为负的内能的parts使用*damping_part_stiffness;先试着用一个小的值，比如0.01如果在*control_energy中设置RYLEN=2，因为刚性阻尼而能会计算且包含在内能中。

－－－－－－－－－－正的接触能：当在接触定义中考虑了摩擦时将得到正的接触能。

摩擦将导致正的接触能。

如果没有设置接触阻尼和接触摩擦系数，你将会看到净接触能为零或者一个很小的值(净接触能＝从边和主边能量和)。

所说的小是根据判断－在没有接触摩擦系数时，接触能为峰值内能的10%内可以被认为是可接受的。

－－－－－－－－－－负的接触能：突然增加的负接触能可能是由于未检测到的初始穿透造成的。

在定义初始几何时考虑壳的厚度偏置通常是最有效的减小负接触能的步骤。

查阅LS-DYNA理论手册的23.8.3&23.8.4节可得到更多接触能的信息。

负接触能有时候因为parts之间的相对滑动而产生。

这跟摩擦没有关系，这里说的负接触能从法向接触力和法向穿透产生。

当一个穿透的节点从它原来的主面滑动到临近的没有连接的主面时，如果穿透突然检测到，则产生负的接触能。

如果内能为负接触能的镜像，例如glstat文件中内能曲线梯度与负接触能曲线梯度值相等，问题可能是非常局部化的，对整体求解正确性冲击较小。

你可以在LS-prepost中分离出有问题的区域，通绘制壳单元部件内能云图(Fcomp > Misc > Internal energy)。

实际上，显示的是内能密度，比如内能/体积。

内能密度云图中的热点通常表示着负的接触能集中于那里。

如果有多于一个的接触定义，sleout文件(*database_sleout)将报告每一个接触对的接触能量，因此缩小了研究负接触能集中处的范围。

克服负接触能的一般的建议如下：－消除初始穿透(initial penetration)。

(在message文件中查找”warning”)－检查和排除冗余的接触条件。

不应该在相同的两个parts之间定义多于一个的接触。

－减小时间步缩放系数－设置接触控制参数到缺省值，SOFT=1 & IGNORE=1除外(接触定义选项卡C)－对带有尖的边的接触面，设置SOFT=2(仅用于segment-to-segment接触)。

而且，在版本970中推荐设置SBOPT(之前的EDGE)为4对于部件之间有相对滑移的SOFT=2的接触。

为了改进edge-to-edge SOFT=2接触行为，设置DEPTH=5。

请注意SOFT=2接触增加了额外的计算开消，尤其是当SBOPT或者DEPTH不是缺省值时，因此应该仅在其它接触选项(SOFT=0或者SOFT=1)不能解决问题时。

English version:Total energy reported in GLSTAT (see *database_glstat) is the sum of …internal energykinetic energycontact (sliding) energyhourglass energysystem damping energyrigidwall 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”.The “joint internal energy” written to glstat by SMP 5434a is independent of the constrained_joint_stiffness. It would appear to be associated with the penalty stiffnessof *constrained_joint_revolute (_spherical, etc). This was a missing energy term prior to SMP rev. 5434a. It is still a missing energy term in MPP rev. 5434a. It does NOT appear when a Lagrange Multiplier formulation is used.The energy associated with *constrained_joint_stiffness appears in the jntforc file and is included in glstat in “spring and damper energy” and “internal energy”. Recall that “spring and damper energy”, whether from joint stiffness or from discrete elements,is always included in “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 damping energyare computed and written only if RWEN and RYLEN, respectively, are set to 2. Stiffness damping energy is lumped into internal energy.Mass damping energy appears as a separate line item “system damping energy”.Energy dissipated due to shell bulk viscosity was not calculated prior to revision 4748 of v. 970. In subsequent revisions, set TYPE=-2 to iclude this energy in the energy balance.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.Note that added mass may cause the energy ratio to rise. (See~/test/erode/taylor.mat3.noerode.mscale.k)The History > Global energies do not include the contributions of eroded elements whereasthe 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. Eroded energy is the energy associated with deleted elements (internal energy) and deleted nodes (kinetic energy). Typically, the “energy ratio w/o eroded energy” would be equal to 1 if no elements have been deletedor less than one if elements have been deleted. The deleted elements should have no bearing on the “total energy / initial energy” ratio. Overall energy ratio growth would be attributable to some other event, e.g., added mass.Restated, when an element erodes, the internal energy and kinetic energy in glstat do not reflect the energy loss. Instead the energy losses are recorded as “eroded internal energy” and “eroded kinetic energy” in glstat. If you subtract “eroded internal energy” from “internal energy”, you have the internal energy of elements which remainin the simulation. Likewise for kinetic energy. The matsum file’s internal energy and kinetic energy include only contributions from the remaining (noneroded) elements.An example is attached. Note that if ENMASS in *control_contact is set to 2, the nodes associated with the deleted elements are not deleted and the “eroded kinetic energy” is zero. (See ~/test/m3ball2plate.15.k)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 internal energy in shells:To combat this spurious effect,- turn off shell thinning (ISTUPD)- invoke bulk viscosity for shells (set TYPE = -2 in *control_bulk_viscosity)- use *damping_part_stiffness for parts exhibiting neg. IE in matsumTry a small value first, e.g., .01.If RYLEN=2 in *control_energy, then the energy due to stiffness damping is calculated and included in internal energy.(See negative_internal_energy_in_shells for a case study)Positive contact energy:When friction is included in a contact definition, positive contact is to be expected. Friction SHOULD result in positive contact energy.In the absence of contact damping and contact friction,one would hope to see zero (or very small) net contact energy (net = sum of slave side energy and master side energy). “Small” is a matter of judgement — 10% of peak internal energy might be considered acceptable for contact energy in the absence of contact friction. (~/test/shl2sol/sphere_to_plate.examine_contact_damping_energy.k appears to illustrate that contact damping (VDC = 0, 30, 90) produces positive sliding (or contact) energy)Negative contact energy:Refer to p. 3.14, 3.15 of “Crashworthiness Engineering Course Notes” by Paul Du Bois. Contact jane@ to purchase these notes.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 could be 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-Prepost). Actually, internal energy density is displayed, i.e., internal energy/volume. Hot spots in internal energy density 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.Some general suggestions for combating negative contact energy are as follows:- Eliminate initial penetrations (look for “Warning” in messag file).- Check for and eliminate redundant contact conditions. You should NOT have more than one contact definition treating contact between the same two parts or surfaces.- Reduce the time step scale factor.- Set contact controls back to default except set SOFT=1 and IGNORE=1 (Optional Card C). - For contact of sharp-edged surfaces, set SOFT=2 (applicable for segment-to-segment contact only). Furthermore, in v. 970, setting SBOPT (formerly EDGE) to 4 is recommended for SOFT=2 contact where relative sliding between parts occurs. For improved edge-to-edge SOFT=2 contact behavior, set DEPTH to 5. Please note that SOFT=2 contact carries some additional expense, particularly using nondefault values of SBOPT or DEPTH, and so should be used only where other contact options (SOFT=0 or SOFT=1) are inadequate.。