流体错误解决

个人流体模拟错误分析
本人刚刚接触ansys半年，对学习期间出现的一些能够解决的错误总结如下，与大家共享。

其中可能会有不足的地方，请大家批评指正。

Coefficient matrix has a negative diagonal. Probably indicates divergent solution. VX Node= 194.
系数矩阵的对角线是负数，可能导致解发散。

发散时首先检查加载是否完全。

需要约束的对象没有约束，就会出现发散的情况。

Nominal fluid density must be positive. A value of -1 indicates the default value was not changed. A positive value of this property must be input.
Nominal Density = -1.
解决方法：Main Menu> Solution> FLOTRAN Set Up> Fluid Properties，将density项选为AIR-IN
Nominal fluid viscosity must be positive. A value of -1 indicates the default value was not changed. A positive value of this property must be input. Nominal Viscosity = -1.
解决方法：Main Menu> Solution> FLOTRAN Set Up> Fluid Properties，将viscosity项选为AIR-IN
*** WARNING ***
The degree of freedom solution is not available. The PLVECT command is ignored.
自由度不可用，此命令被忽略。

Main Menu> General Postproc> Read Results> Last set
*** ERROR ***
Maximum number of conjugate gradient iterations exceeded three times. The run is terminated. DOF solved was PRES.
共轭梯度的最大值的迭代数超过三次。

进程终止。

这条语句是求解时出现的。

在指定的求解器清扫数内共轭梯度求解器不满足收敛条件。

模型可能需要更大的清扫数。

解决方法：FLDATA19,TDMA,PRES,100
*** ERROR ***
Error attempting to back up unuseable results file. Check READ/WRITE permission. T_post_01.rfo.
在偿试回到结果文件时出错。

检查读写许可。

jobname.rfo
在求解时出现。

通常rfo文件很大，不知是不是因为出错的原因。

可以将它删除，好像不影响程序运行。

运行正常的文件是没有rfo文件的。

*** ERROR *** CP = 119.000 TIME= 15:24:54
Volume mesh failure - perhaps due to:
(1) Poorly shaped triangle facets: Mesh surfaces with tris, modify mesh control to get good tri mesh, then VMESH again.
(2) Complex geometry: Subdivide volume and try again.
体网格划分错误，可能是由于：
1，错误的三角面形状：用三角形划分表面，修改网格控制以得到好的三角网格，然后再次划分。

2，复杂几何体：把体分成小无块，再试一下。

*** WARNING *** CP = 119.578 TIME= 15:25:04
Mid-nodes of some elements have been modified to lie on straight edges because of distortion with the original mid-nodes. This condition is sometimes eliminated by tetrahedron element improvement when enabled. Issue ESEL, STRAIGHTENED to select such elements for listing or plotting.
警告：
由于原始中央节点的变形，一些单元的中央节点被修改从而位于直边上。

这个条件有时可以由优化四面体单元解决。

*** WARNING *** CP = 6.547 TIME= 19:13:13
Element 51 references undefined MURX or BH table for material 2.
先建立一个平面，网格划分，再建立一个平面，网格划分，第二个平面的第一个元素就会出现这个警告。

*** ERROR *** CP = 17.969 TIME= 14:59:37
One or more elements have become highly distorted. Excessive distortion of elements is usually a symptom indicating the need for corrective action elsewhere. Try ramping the load up instead of step applying the load (KBC,1). You may need to improve your mesh to obtain elements with better aspect ratios. Also consider the behavior of materials, contact pairs, and/or constraint equations. If this message appears in the first iteration of first substep, be sure to run shape checking of elements.
单元扭曲严重。

处理：KBC,0
GUI:/Solution> Analysis Type > Sol'n Controls > Transient – Full Transient Options
*** ERROR *** SUPPRESSED MESSAGE CP = 219.578 TIME= 15:54:13 Tetrahedral element has negative or zero volume. Nodes on element must be numbered according to the right hand rule.
Element ID= XXXX.
*** ERROR *** CP = 222.312 TIME= 15:54:24
Element has negative or zero volume in morphing process. Reduce the time step size or
change the remeshing frequency controls parameter to make remeshing happen earlier, and restart the solution from the last available result set.
Number of Elements= XXXX.
单元体积为负。

一般在FSI瞬态分析时发生在流体单元中。

原因是大变形时单元受到挤压变形，导致体积为负。

解决：减小时间步长。

或者由程序定义时间步长：
flda,time,step,-1
时间步长会小到在单一的任何一个时间步长内，流场中任意一点的运动距离都不会大于一个单元的长度。

这防止了流体的大变形。

固体时间步长和FSI时间步长都可以随便设置了。

时间步长可以不一样。

但最好小一点。