Skews and frames are used to define local directions.
These directions can be used to apply:
Boundary conditions
Concentrated load
Fixed velocity
To define:
Rigid link orientation
Rigid body added inertia frame
General spring reference frame
Beam type spring initial reference frame
Nodal time history output frame
Two reference definitions are available in Radioss:
Skew reference
It is a projection reference to define the local quantities with respect
to the global reference. In fact, the origin of the skew remains at the
initial position during the motion even though a moving skew is defined.
In this case, a simple projection matrix is used to compute the
kinematic quantities in the reference.
In Figure 1, imposed velocity is applied in Y direction. In
/IMPVEL, skew is used. Then the imposed velocity
is computed in the Y axis of global coordinate system and then projected
onto the Y’ axis of local skew reference. Figure 1. Skew Example
Frame reference
It is a mobile or fixed reference. The quantities are computed with
respect to the origin of the frame which may be in motion or not
depending on the kind of reference frame. For a moving reference frame,
the position and the orientation of the reference vary in time during
the motion. The origin of the frame defined by a node position is tied
to the node.
Frame measures relative motion whereas skew measures global motion and
projects it to skew. Only few options use frame like imposed velocity,
TH/NODE and others use skew.
In Figure 2, rotational velocity is applied around Z axis. In
/LOAD/CENTRI, frame is used. Then rotational
velocity is around Z’ axis of frame reference not in Z axis of global
coordinate system anymore. Figure 2. Frame Example
Sections (/SECT)
A section is a cut in the structure, where forces, moments or energy will be computed and stored
in output files (using /TH/SECTIO). In Radioss/SECT, /SECT/PARAL and
/SECT/CIRCLE can be used to define a section.
Generally, to define the section, the following points are required:
A cutting plane (which defined either by 3 points or by parallelogram or by
a circular disk)
A reference point to compute forces
A direction of the section
Figure 3. Definition of a Section for an Oriented Solid
In /SECT, the cutting plane is defined by a group of elements and its
orientation by a group of nodes. Figure 4. Definition of a Section for a Shell Mesh
Then, a point is defined for the center of rotation of the section and a reference frame is
attached to this point to compute the internal efforts. Figure 5. Center of Rotation and its Associated Frame for a Section
The resultant of all forces applied to the elements and its application point are computed
by:(1)(2) Figure 6. Resultant of Force and Moment for a Node I with the Rotation Point
O
In /SECT/PARAL, the cutting parallelogram is defined by point MM1 and MM2.
Force or moment will be computed on the group of nodes which intersected by
parallelogram.
In /SECT/CIRCLE, the cutting disk is defined by point M, Radius and the normal
vector. Force or moment will be computed on the group of nodes which intersected by
a circular disk (Figure 7).
Using /SECT/PARAL and /SECT/CIRCLE the group of nodes will
be automatically selected. It is easier if you remesh the part (not like in
/SECT, you also need to update the group of nodes by
preprocessor again). Figure 7. Definition of a Section using Parallelogram and Circle Disk
