Resultados de los análisis descriptivos de la variable Revitalización urbana

analysis in a model that contains PARTs by either using the SETREE Bulk Data entry or the DTI,SETREE Bulk Data entry. See“User Interface for Multilevel Superelements”for a description of these entries and how they are used.

If the model consists of main bulk data superelements only, the program checks to see that all exterior points belong to the residual structure before performing a single-level analysis. If a model is defined using the main bulk data only and exterior points exist that do not belong to the residual structure, then the program automatically performs a multilevel analysis. The SETREE and DTI,SETREE entries can also be used, in this case, to control the processing order and connectivity of the superelements.

7.2 Comparison of Single- and Multilevel Analysis

Single-Level Analysis

Up to this point, this guide has described only the case of single-level superelement analysis (that is, all exterior points of any superelements belong to the residual structure). In this case, the reduced matrices of each superelement connect only to points that belong to the residual structure, and each superelement can be processed independently of all others. However, all superelements must be processed before the residual structure can be processed.

Advantages

Single-level processing is the simplest way to define superelements and is recommended for your initial use of superelements. As you become an experienced user of superelements, you can try using multilevel analysis for efficiency (or for another reason, such as multi-step dynamic reduction).

In dynamic analysis, single-level processing is ideal for lightly coupled structures. That is, if there is very little dynamic coupling between components (for example, the interface connection is rigid), then a single-level solution is fine.

During the initial phases of a project, when any part of a structure can be changed, restarts with model changes may be cheaper than using multilevel superelements.

Single-level analysis is the default if PARTs are used.

Sample of a Single-Level Processing Tree

The aircraft model shown inFigure 7-1is divided into six superelements. The interface points between each superelement are placed in the residual structure. Because each superelement

connects only to points that belong to the residual structure, we obtain a single-level processing tree, as shown inFigure 7-2.

Figure 7-1. Aircraft Model

Figure 7-2. Single Processing Level Tree

Because all the interface points between superelements belong to the residual structure, this single-level tree has a large residual structure, compared to that of a multilevel tree.

We should point out that superelements 5 and 6 are disjoint superelements. A disjoint superelement consists of more than one unique piece, with no direct connection between the pieces (at least inside the current superelement). Superelement 6 consists of the two wing tips of the plane, and superelement 5 consists of the two inboard wing pieces. (We know that in a real aircraft the wing structure is continuous through the fuselage, but for purposes of visualization the model is broken up in this manner.)

The following figures demonstrate the idea of disjoint superelements. In each figure, an x is used to indicate the exterior points (in this case residual structure points to which superelement 6 attaches). In this simplified representation, only a few exterior points are shown on each edge.

In a real aircraft model, there might be hundreds of grid points on these boundaries.

Figure 7-3. Wing Tip Superelement

Figure 7-3shows the model of the outboard wing tip superelement (SEID = 6). This model contains the interior points, exterior points, and elements for superelement 6. As NX Nastran processes superelement 6, it first creates a set of G-sized matrices (including both the interior and exterior points) and then applies (MPC- and SPC-type) constraints. These steps are then followed by a reduction process to A-sized matrices representing the mass, damping, loading, and stiffness of superelement 6, as seen by the residual structure. Graphically, we can idealize these matrices as follows (the x’s represent the residual points to which superelements connect):

Figure 7-4. Reduction Process, SEID=6

Superelement 5 would be treated in a similar manner. Figure 7-4 shows the physical model for superelement 5.

Figure 7-5. Wing Root Superelement

Once again, NX Nastran generates a set of G-sized matrices and reduces them to the exterior DOFs. This process is idealized in the following representation of the reduced matrices for superelement 5. Once again, in these figures, the x’s represent the residual structure points to which superelement 5 connects.

Figure 7-6. Reduction Process, SEID=5

All other superelements are processed, and at this point we can show an idealized representation of the residual structure for the single-level tree version of the airplane model.

Figure 7-7. Reduction Process, Total Model

Although this model no longer resembles the physical airplane model with which we started, it provides the same answers as the full finite element model for static analysis.

In this idealized single-level representation, the x’s represent the residual structure points and the circled numbers represent the reduced matrices for the superelements. The lines indicate the residual structure points to which each superelement connects. Because this is a single-level model, connection (attachment) between superelements does not occur until the residual structure, when all of the pieces are assembled. In a single-level tree, any points where a connection between superelements occurs is interior to the residual structure.

The advantages of single-level analysis are most obvious during the initial phases of a project, when any part of a design can change. In a single-level analysis, only the superelements containing the changes need to be reprocessed; the rest of the original superelement matrices are assembled with these new matrices at the residual structure, to create the new solution.

Although only small parts of the model needed to be processed, correct answers are available for the entire model, including deformed plots and stress contours in your postprocessor.