Explicit Grid Boundary Conditions

Introduction

If needed, velocity boundary conditions can be explicitly assigned to numbered nodes by using <fix> elements within the single <DisplacementBCs> element. This approach can only be used in XML input files and is rarely used, except perhaps in separate software or scripts that create an XML input file automatically. The more common way to create boundary conditions on nodes is to use shape commands.

Explicit Velocity Conditions

An explicit velocity is assigned to one node with a <fix> element:

<fix node='(node)' dof='(dir)' style='(style)' time='(time)' id='(id)'>(velocity)</fix>

where the text of the <fix> tag (i.e., (velocity)) is the value of the fixed velocity with standard units of mm/sec. When using an arbitrary function (see Style=6 below) the text of the tag should be the a user-defined function of t (in ms) that evaluates to velocity in mm/s. The attributes of the <fix> element are:

• dof determines if the x direction ((dir)=1), the y direction ((dir)=2), the z direction ((dir)=3), the R direction when axisymetric ((dir)=1), or the Z direction when axisymmetric ((dir)=2) will have the specified velocity. You can also set velocity in a direction that is not along an axis. For details see help on skewed velocity conditions, which modifies the dof attribute and adds angle and angle2 attibutes to define the skew angle.
• style Determines the time dependence of the boundary condition. The possible styles are defined above. If this attribute is omitted, it is assumed to be 1.
• time A time parameter used for various styles described in the next section. If this attribute is omitted, it is assumed to be 0.0.
• id This attribute sets an id for the node that can be used to sum only forces on certain nodes when global archiving reaction forces. It must be negative.

(style) specifies the style of the applied velocity. Each velocity depends on two parameters specified by arguments (value) and (time). If either argument is not supplied, they are set to zero. The styles are described below. The standard units are mm/s for (value) and ms for (time) (but the units may change depending on the (style) setting). (function) - if the (style) is function (or 6), this attribute in XML files specifies a user defined function. In scripted files, the user defined function should replace the (value) argument and must be entered as quoted text. The function should evaluate to the desired velocity in mm/s. (id) - this attribute in XML files specifies and ID for the boundary condition. In scripted files, IDs are set using the BoundaryID command.