Patran does not check for consistently oriented surfaces in two-dimensional entities such as patches and quadrilateral elements. Properly oriented surfaces are required for correct modeling of 2-D XY and 2-D axisymmetric thermal radiation models. Since Patran does not ensure consistent surface orientation, the user must assume this responsibility. Failure to do so will result in erroneous models.

The computation of viewfactors requires knowledge of the normal to the surfaces for which the viewfactors are being computed. In addition, we must also know from which side of the surface the radiation is emanating. In 3-D models this is not a problem since Patran consistently uses only one orientation of the parametric axes, called the right hand orientation. However, for 2-D models, Patran allows either clockwise or counterclockwise or a combination of both orientations for surfaces. Note that this orientation is with respect to the global coordinate system, and not the local element coordinate system. Viewfactor uses information gained from an assumed orientation of the surfaces to determine from which side of a surface the radiation emanates. Surfaces in two and three dimensions are shown in Figure 3‑7 with different surface normal orientations.

Viewfactor assumes that all coordinate systems are oriented in a right hand (also known as counterclockwise) arrangement (i.e., if you place your right hand so that the fingers point from the first coordinate axis to the second coordinate axis, then the raised thumb points up out of the paper in the positive direction of the third coordinate axis).

To apply this test to cylindrical coordinate systems, the fingers of the right hand should be pointed in the direction of increasing polar angle. The term “counterclockwise” comes from the fact that for right-handed systems viewed from the front, the direction of rotation from the first axis (e.g., x-axis), to the second axis (e.g., y-axis), is counterclockwise. Sketches illustrating these concepts are shown in Figure 3‑8.

InViewfactorthe direction of the surface normal for 3-D surfaces is determined by the right hand rule as you point your fingers in the direction determined by the order in which the surface corner nodes are given. Thus, if the normal to a surface is pointing toward you, the order of the nodes describing that surface will appear counterclockwise. For 3-D models, Patran automatically takes care of this and you do not need to be concerned with it. The relationship of 3-D surface orientation and node ordering is illustrated in Figure 3‑9.

For 2-D (either Cartesian or axisymmetric) models, you must take care to correctly orient the surfaces. The term “surface” here generically means a boundary of the model. For objects modeled in 2-D space surfaces are represented as lines. The orientation of these lines in Viewfactor is determined by the order in which their beginning and ending points are given. You imagine yourself walking in the plane of the model, feet on the plane and on the side determined by the right-hand orientation. Then as you walk from the beginning point to the ending point of the line, the principle or positive normal direction is that which the right arm points when extended horizontally. The relationship of node order to normal direction and this method of determining normal direction for 2-D surfaces (lines) is illustrated in Figure 3‑10.

Patran allows you to arbitrarily mix left handed and right handed oriented systems for 2-D entities such as patches and two-dimensional elements. This is not allowed when using Viewfactor. Since facilities to automatically manage orientation of 2-D entities are not currently available, you must take care of this task. Failure to do so will result in erroneous 2-D models. The following three subsections contain information to help manage this task.

There are several ways to show the orientation of patches and two-dimensional elements in Patran.

Keep in mind that these tests for determining orientation are for a view from above or in front of the plane containing the model. “Above” or “in front of” means with respect to a right-handed coordinate system for the plane.

The parametrization of a geometric entity is not displayed as default. The parametrization of geometric entities can be displayed by clicking on display of the top level menu bar and then selecting geometric and turning on the parametric direction button.

Several patches with their C1 parametric directions are shown in Figure 3‑11, along with an indication of right-handed (desired) or left-handed (undesired) orientation. Basically, if viewed from above the plane containing the model (above with respect to a right-handed system), the C1 parametric direction should point in the counterclockwise direction around the perimeter of the patch for properly oriented patches.

The next test may be used to determine a patch’s orientation. They are not recommended because they do not conform to the engineering and mathematical standards for surface orientation. These tests are based on the order of the corner grid IDs for the patch and on the order of the edges of the patch. Patran specifies the corner grid and edge order of right hand oriented patches to be clockwise, whereas most users will be familiar with the usual counterclockwise orientation.

The patch corner grid ordering may be shown by clicking on geometry in Patran and selecting Action: show, Object: surface, Method: attribute and selecting the surface. When the spreadsheet comes up, click on the vertices button to see the corner grids and their order. Look at the graphics window to see if the grids in this order go clockwise around the patch. If they do, then this is a properly oriented patch.

The ordering of corner grids and edges is shown in Figure 3‑12 for various right-handed (properly) and left-handed (improperly) oriented paths.

The orientation of two-dimensional elements (i.e., quadrilaterals and triangles), may be determined by using the element verification menu. Since elements are typically much more numerous than patches, take care to properly orient all patches before beginning to generate elements to ensure that there are no improperly oriented elements.

The commands listed above for determining the element orientation are described fully in the Patran Reference Manual. Note, however, that the ordering for nodes on a properly oriented element is counterclockwise as is customary in engineering analysis, and that this is opposite of the ordering of corner grids on a properly oriented patch.

Improperly oriented patches may be reversed with the modify action on the geometry menu.

The patch may also be deleted and a new properly oriented patch created in its place, or the patch may be overwritten with a properly oriented patch with the same patch ID. Please see the Patran Reference Manual for information on the various patch menus.

Elements may be reversed with the AUTOREVERSE option of the NORMALS submenu of the VERIFY action on finite elements form. These forms are all described in the Patran Reference Manual. You must exercise caution when reversing elements since Patran may not correctly transform the boundary conditions associated with an element edge when the element is reversed. Other data associated with the element may not be transformed correctly either.

In most cases, if you have left-handed elements, it is recommended that the Patran finite element entities be deleted. Then the orientation problems should be corrected at the patch level before any finite element entities are generated.

It is important that elements in two-dimensional models (Cartesian and axisymmetric) are correctly oriented. Be aware of the problem and plan carefully to avoid it.

Then, as each patch is made, its orientation should be checked and verified to be correct. Improperly oriented patches should be reversed. This may be done with the modify action of geometry menu.

Carefully check to make sure all patches are properly oriented before beginning to generate the finite elements, finite element properties, and LBCs.