接触重合度 Contact overlap tolerance的相关问题
1 引言
当运行UDEC时,经常出现的一个错误信息是"contact overlap"的size太大, 这意味着块体在运动过程中块体与块体之间的接触出现了问题. 本文就这个issue作一些解释,同时介绍如何解决这个问题.
2 命令格式解释
在UDEC 7.0 之前, 设置接触重合度的命令格式为
set ovtol 0.1从7.0版本开始, 这条命令改写成
block contact tolerance overlap 0.1如果在GIIC中操作, 命令工具[Settings]->[Mech]:
默认的接触重合度Contact overlap tolerance为块体角半径的一半. 块体角半径Block rounding radius是在模型开始时设置的. 在7.0版本中命令格式为:
block tolerance corner-round-length 0.013 消除错误方法
如果不能确定接触重叠的原因，可以使用SET cscan 命令(7.0之前的版本)。这个命令让程序每n个循环更新一次角半径中心的坐标。默认值为100. 该命令会使圆角中心的位置更频繁地更新，从而使触点和重叠的计算更准确。
消除这种错误的首要方法是在SET ovtol命令中设置合适的值。如果初始的角半径长度非常小，或者模型的几何体内有角度非常小的块，这个过程是有用的。不过如果使用不慎，这种方法也是非常危险的，因为如果 ovtol设置得 过大，就会出现错误的结果。
其它一些变量的合理设置也能有效地改善块体之间的接触, 确保在运行过程中不错差错. 现总结如下:
(1) set btol
sets maximum distance a contact may open before it is deleted (default = 0.55 × rounding length).
The value of ATOL is also used to determine whether a contact is created. BTOL is used to determine whether a contact is deleted. If two blocks are found to be separated by a gap that is equal to or less than ATOL, a contact is created. Conversely, if an existing contact acquires a separation that is greater than BTOL, the contact is deleted. This logic ensures that the data structure for all potential contacts is in place before physical contact takes place. It also ensures that contact searching is only done for moving blocks; there is no time wasted on relatively inactive blocks.
(2) set delc off/on
Contacts are automatically deleted when the separation is greater than 0.55 times the rounding length (default). Use the PRINT status com-mand to find the limiting separation value being used. If off, contacts are never deleted. (The default is off.)
The SET delc off command can be used to prevent UDEC from deleting contacts that appear to be separating as a result of dilation.
The PFix Tool allows allows fluid pressure to be specified within domains (i.e., “open” spaces between blocks) within the optional range. If sett all is specified, the pressure setting applies to all domains except the outer domain. It should be noted that the specified fluid pressures do not change unless a different PFIX or PFREE is used or new contacts are formed or deleted. To avoid forming or deleting contacts, use larger rounding lengths or Set DELC off.
(3) set upcon n
updates contact locations only every n cycles. (Default is n = 1.)
(4) set dscan n
sets frequency of domain check. The default is every three cycles.
set dscan 100000
turn off scan for new contacts to speed calculation
(5) set uplen n
updates contact length only every n cycles (default = 100). The length is also updated every time the CYCLE command is issued.
(6) set add_dil off/on
If SET add_dil off (default), the effective friction is equal to jfric. If SET add_dil on, the effective friction angle is obtained by adding the dilation angle (jdil) to jfric, if the dilation is positive, or subtracting it from jfric if the dilation is negative.
If on, the joint dilation angle will be added to or subtracted from the friction angle depending on direction and magnitude of shear displacements (default is off).
4 3DEC中的接触重合度
(1) btol
controls flatness of block faces. If one of the corners of a block face is out-of-plane by btol, then the block face will be split up into triangular faces. The default is atol/10.
(2) atol
sets general tolerance for the model, used for:
(1) minimum distance between gridpoints;
(2) minimum area (atol*atol) of sub-contacts;
(3) tolerance to link gridpoints of joined blocks; and
(4) tolerance to select vertices close to the common plane.
(default = (.0012) × average model dimensions in x, y and z)
5 结束语
这个笔记强调了接触重合度的相关问题, 明白这些命令背后的工作机理,能够避免一些运行过程中出现的错误.
