1、LSDYNA 中 的能量平衡 time. 4.99735E-03 time step 4.45000E-06 kinetic energy. 3.80904E+09 internal energy 5.15581E+09 spring and damper energy. 1.00000E-20 hourglass energy 1.34343E+08 system damping energy 0.00000E+00 sliding interface energy. 1.72983E+07 external work 4.54865E+09 eroded kinetic energy 0.
2、00000E+00 eroded internal energy. 0.00000E+00 total energy. 9.11649E+09 total energy / initial energy 1.09716E+00 energy ratio w/o eroded energy. 1.09716E+00 global x velocity -6.63878E+01 global y velocity 3.44465E+02 global z velocity -1.86129E+04 time per zone cycle.(nanosec) 11286 GLSTAT( 参见*dat
3、abase_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 energy GLSTAT 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_j
4、oint_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。
5、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 Ho
6、urglass 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
7、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 HistoryGlobal 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 ASCIIglstat 中 的”
8、Eroded Kinetic Energy” 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
9、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 Mi
10、sc 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 & IGNOR
11、E=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
12、 reported in GLSTAT (see *database_glstat) is the sum of 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 e
13、nergy 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
14、 5434a is independent of the constrained_joint_stiffness. It would appear to be associated with the penalty stiffness of *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
15、 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 element
16、s,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 energy are computed and written only if RWEN and
17、 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 TYP
18、E=-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 wor
19、k) 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 whereas the GLSTAT energies do include those contributions. Note that these eroded contributions
20、 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 del
21、eted or 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 en
22、ergy 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 remain in the simulation. Likewise
23、for kinetic energy. The matsum files 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 kine
24、tic 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 doesnt include such contributions as contact energy or hourglass energy. Negative internal energy in shells: To combat this spurious effect, - turn off shell
25、thinning (ISTUPD) - invoke bulk viscosity for shells (set TYPE = -2 in *control_bulk_viscosity) - use *damping_part_stiffness for parts exhibiting neg. IE in matsum Try a small value first, e.g., .01. If RYLEN=2 in *control_energy, then the energy due to stiffness damping is calculated and included
26、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
27、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
28、_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 to purchase these notes. Abrupt increases in n
29、egative 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 Manua
30、l (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 Im speaking of negative energy from normal contact forces and normal penetrations. When a penetrated node slides from its
31、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
32、 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 de
33、nsity 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 negati
34、ve 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 defi
35、nition 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). Furthermor
36、e, 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.
