【讨论】计算时间步长突然减小，然后程序终止，怎么解决？

worker

VELMAX:  defines the maximum velocity in Eulerian and Lagrangian meshes.  
（定义欧拉和拉个朗日网格最大速度）也就是材料速度而不是材料声速。

chenyn

但我理解是定义Eulerian and Lagrangian meshes的maximum velocity  
这一点好像可以在OUT文件中验证！

melody_liuy

The explicit dynamics procedure of MSC.Dytran uses relatively small time steps
dictated by the shortest natural period of the mesh: the analysis cost is in direct
proportion to the size of the mesh. There are two types of problems where the
cost-effectiveness of the analysis can be increased:
• If a mesh consists of a few, very small (or stiff) elements, the smallest (or
stiffest) element determines the time step for all elements of the mesh.
• If a few severely distorted elements are obtained by the analysis, the most
distorted element determines the time step for all elements of the mesh. This
may even end up with a too small stable time step.
Speedup of those problems can be achieved by using mass scaling (PARAM,SCALEMAS).
Mass scaling is based on adding numerical mass to an element so that its time step
never becomes less than the minimum allowable time step defined by you. Note that
mass scaling can be risky in areas where either inertia effects are relevant or contact
with other parts is expected to occur.
Problems Involving a Few Small Elements
It is common practice that meshing of real-life problems may involve some relatively
small elements: elements frequently localized in a kind of transition region and meant
to connect large structural parts to each other. Those elements determine the time step
of the whole calculation although they might be present in the model to a very limited
extent. Speedup can be realized by using mass scaling. Some guidelines:
• Make a run for one cycle and retrieve the time step of all elements by requesting
ELDLTH.
• By using a postprocessing program, see which elements are determining the
time step and filter out the elements whose time steps exceed a user-defined
minimum (DTMIN).
See what the impact would be of specifying this new time step (DTMIN). Select
the value of DTMIN such that hardly any elements would be scaled in the area
of interest (for example, as much as possible outside the impact region in a
crash simulation).
Problems Involving a Few Severely Distorted Elements
There are conceivable application areas where elements are distorted to such a high
extent that a few of them determine the time step for all elements of the mesh. For
example, crushing of a subfloor structure frequently involves failure modes associated
with the occurrence of severely distorted elements. Modeling this kind of crushing
behavior without including a failure mechanism might end up with a stable time step
that is too small. Since those elements are often present in a relatively small region,
the mass scaling method might be a good means to artificially speed up the calculation
without losing the capability to model the global crushing behavior. Note that to
prevent severely distorted elements, it is recommended that a proper failure
mechanism be included, instead of coping with the distorted elements by making use
of the mass scaling method. Some guidelines:
• Since you do not know in advance which elements will become too distorted,
you should first run the analysis as far as possible (without defining
PARAM,SCALEMAS). You should request the time step of all elements (ELDLTH).
• If the problem ends up with a too small stable time step, the analysis finishes
prematurely. See which elements are so severely distorted and decide what a
reasonable minimum time step (DTMIN) might be without affecting elements
in the area of interest. See the guidelines of the previous section.
• Rerun the analysis specifying PARAM,SCALEMAS if the region of highly distorted
elements is relatively small compared to the whole model.
• If there is too much mass added to the grid points of those elements, the model
might show significantly different inertia effects, and subsequently, different
global structural response. In order to avoid this, no more mass is added if the
numerically added mass exceeds a certain percentage (MXPERC).
• To limit the amount of overhead time spent on checking against its mass scaling
criterion, the checking is done every specified number of STEPS.

日月居士

按照我的计算经验，应该是你的边界定义有问题。请检查flow定义，全欧拉计算错误80％以上是由于边界条件定义不准确。

www_frank

设置个最小时间步试试?