Restricting the direction of displacement

This section shows you how to influence and restrict the displacement direction of design nodes.

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DVCON_BEAD

In SIMULIA Tosca Structure.bead the displacement direction of the design nodes (optimization displacement vector) is normally determined as the surface normal (see Update of optimization displacement vectors (VECTOR)). A restriction of the displacement direction in bead optimization may be necessary due to two reasons:

  1. Standardly, the displacement direction in bead optimization is determined in the initial model and is not modified during optimization (OPT_PARAM, VECTOR = FIRST). In order to define own displacement directions the original displacement directions may be restricted.
  2. Some areas in the design domain should not be modified during optimization, e.g. the boundary of the design domain. These areas may be restricted using the CHECK_DOF or CHECK_BC parameters.
Important:

The displacement boundary condition must be unique. In contrast to FE boundary conditions of several load cases, the total of all supports for all load cases are considered as supports in the optimization. A prescribed node displacement as an optimization boundary condition is also not permitted.

Loading Node Fixations via the Interface from the FE Program

The full or partial fixation of nodes is the most common and most important type of restriction, it is practically used in every optimization model. The most efficient method is to define the displacement restrictions in the FE preprocessing as an extra load case in the transition model and then to load it via the FE interface in the optimization preprocessor.

In order to do this, the interface must first be activated with OPTIONS, READ_BC=.... In this way all node fixations for the optimization model can be defined in advance in the FE preprocessor. The fixation is always based on the FE displacement coordinate system of the node.

The parameter

CHECK_BC = ALL

activates the node fixations of the node group (ND_GROUP parameter) which are loaded through the FE interface. Fixations that reference nodes not contained in the node groups are not activated.

To prevent loaded fixations from being activated enter: 

CHECK_BC = NO

Definition of the Displacement Direction by Command

The CHECK_DOF parameter can be used to restrict other displacement directions if necessary as an addition or at a later stage. When entering node fixations by command and in contrast to load the node fixations via the FE interface, it is necessary to first compile all nodes that should be assigned a certain attribute to a node group. The coordinate system must also be defined or loaded.

With the parameter

CHECK_DOF = cs_name,[FIX|FREE],[FIX|FREE],[FIX|FREE]

all the displacements of all nodes in the node group selected with ND_GROUP are fixed (FIX) or free (FREE) relative to the specified coordinate directions of the coordinate system, cs_name. Either FREE or FIX is allowed for each coordinate direction.

Note:

The main difference between CHECK_BC and CHECK_DOF is: CHECK_BC is read in through the FE interface whereas CHECK_DOF is defined in the optimization preprocessor. Generally, with CHECK_BC each node has its own fixation in its own displacement coordinate system, whereas all nodes of the node group are all fixed in the same coordinate system with CHECK_DOF.