1. INTRODUCTION TO MODELING Introduction To Modeling:- A model is an abstraction of reality if is not the reality that it is intended to represent, nor is it a perfect representation of that reality. Whenever we construct a model, we incorporate some aspect of reality in the model, and we have other out model tend to be rather simple. Nature on the hand, is very complex, and our understanding of her is limited. Hence, models of physical devices, system, and phenomena tend to leave out for more aspect of reality than they include. Engineers are often they represent to construct a model. They never know entirely what they leave behind. A millennium ago, engineers designed full-scale system by intuition and built them directly. Some things worked, others didn't, as designs became larger & more complex the consequence of failure becomes greater and costly little by little, engineer came to rely model to help predict the behavior of potential design and avoid large scale failures. Types Of Models:- ? Iconic Model ? Analog Models ? Symbolic Models Iconic Models:- These models are physical representation of the real world: model ships, model bridge, model airplanes. Iconic models generally are easier and faster to build than full scale system, and they can be great deal less expensive. Analog Models:- An analog model makes use of one thing to represent another. For example equation for the behavior of mechanical systems have analogies in electrical systems, F = ma, versus V = I .R If one were to construct an analogous electrical system, one could we it to study the behavior the corresponding mechanical system. The advantages of this is that the electrical system is much easier and less expensive to construct. Symbolic Models:- Symbolic models, as their name implies, use symbols to represent physical quantities, and express the relationships between these quantities in mechanical forms. There is no correspondence between symbolic model and the reality that it is intended to represent other than that which is in the head of the modeler. Symbolic models enable the engineer to try even thousand of potential designs, sometimes in only minutes, in an effort to evacuate alternatives and find better designs. Modeling a changing a single input variable to the computer model.

Concept Of Symbolic Model:- In engineering design, symbolic models enable us logically to combine disparate thoughts and facts to draw rational conclusion. Typically we are interested in the behavior or performance of a system as a function of its design. In principle, we could guess. But particularly for rather complex systems, such as a ship, an airplane, as i.e. engine, our guess could be rather incorrect. Through the use of symbolic models, we often are able to reduce our uncertainty in such estimate, symbolic models, enable us to guess at variables for which our range of uncertainty may be much lower than the uncertainty that we hold in our guess. The combination of our estimate of disparate variables is made possible through the use of mathematical models based largely on the known laws of nature. We expect three basic things from models : accuracy, resolution, and casual relation ship. The accuracy of the model is its ability to represent the real world, much like the accuracy of a thermometer is its ability to measure temperature. Resolution is the ability of the model to distinguish between alternative cases, of much as the same as & a thermometer as well as a model, is a function of its sensitivity, not of its accuracy. A model can often give us confidence that one of two design alternatives it the better, even though the model can predict the performance of either alternative only with considerable uncertainty. In order to assure that models have good resolution, they must be designed for resolution. For example in the case of determining which of two object is the warmer, we know that good resolution is obtained by using the same thermometer to measure the temperature of the both objects, whatever errors the thermometer may have, we expect that they will about the same for both measurements. Of course, an even better approach might be to measure the temperature difference between objects. The same is true in the case of modeling. Model can be construct specifically to measure the performance difference between design alternative.

2. Model Generation What Is Model Generation? The ultimate purpose of a finite element analysis is to re-create mathematically the behavior of actual engineering system. In other words , the analysis must be an accurate mathematical model of a physical prototype. In the broadest sense, this model comprises all the nodes, elements, material properties, real constants, boundary conditions, and other features that are used to represent the physical system. In ANSYS terminology, the term model generation usually takes on the narrower meaning of generating the nodes and elements that represent the spatial volume and connectivity of the actual system. Thus, model generation in this discussion will mean the process of defining the geometric configuration of the model's nodes and elements. The ANSYS program offers you the following approaches to model generation: ? Creating a solid model within ANSYS. ? Using direct generation. ? Importing a model created in a computer-aided design (CAD) system.

Typical Steps Involved in Model Generation Within ANSYS: A common modeling sensation might follow this general outline (detailed information on italicized subject can be found else where in this guide ): ? Begin by planning your approach. Determine your objectives, decide what basic form your model will take, choose appropriate element types, and consider how you will establish an appropriate mesh density. You will typically do this general planning before your initiate your ANSYS session. ? Enter the processor (PREP7) to initiate your model building session. Most often, you will build your model using solid modeling procedure. ? Establish a working plane. ? Generate basic geometric features using geometric primitives and Boolean operators. ? Activate the appropriate coordinate system. ? Generate other solid model futures from the bottom up. That is, create key points, and then define lines, areas, volumes as needed. ? Use more Boolean operators or number controls to join separate solid model region together as appropiate. ? Crete tables of element attributes (element types, real constants, material properties and element coordinate systems). ? Set element attribute pointers. ? Set meshing controls to establish your desired mesh density if desired. This step is not always required because default element sizes exists when you enter the program (see chapter 7). ( if you want the program to refine the mesh automatically, exit the pre-processor at this point, and activate adoptive machine ). ? Create nodes and elements by meshing your solid model. ? After you have generated nodes and elements, add features such as surface to surface contact elements, coupled degree of freedom, and constraints equations. ? Save your model data to Jobname. DB. ? Exit the pre processor.

Comparing Solid Modeling And Direct Generation : You can use two different methods to generate your model: solid modeling and direct generation. With solid modeling, you describe the geometric boundaries of your model, establish controls over the size and desire shape of your elements, and then instruct the ANSYS program to generate all the nodes and element automatically, by contrast, with the direct generation methods, you determine the location of every node and the size, shape, and connectivity of every element prior to defining these entities in your ANSYS model. Although some automatic data generation is possible, the direct generation method is essential a hands on, "manual" method that requires you to keep track of all your node numbers as you develop your finite element mesh. This detailed book keeping can become tedious for large models, contributing to the potential for modeling errors. Solid modeling is usually more powerful and versatile than generation, and is commonly the preferred method for generating your model. In spite of the many advantages of solid modeling, you might occasionally encounter circumstances where direct generation will be more useful. You can easily switch back and fourth between direct generation and solid modeling using the different techniques as appropriate to define different parts of your model. Detailed discussions of solid modeling and direct generation can be found in Chapter 5 and Chapter 9, respectively. To help you judge which method might be more suitable for a given situation, the relative advantages and disadvantages of the two approaches are summarized here. Solid Modeling:- On the plus side, solid modeling ? Is generally more appropriate for large or complex models, especially 3-D models of solid volumes. ? Allows you to work with a relatively small number of data items. ? Allows geometric operation (such as dragging and rotation) that cannot be done with nodes and elements. ? Supports the use of "primitive" areas and volume (such as polygonal areas and cylindrical volumes) and Boolean operations (intersections, subtractions, etc.) for "top town" construction of your model. ? Facilitates your use of the ANSYS program's design optimization features. ? Is required for adapting meshing. ? Is required in order to do area mesh refinement after loads have been applied (solid model loads are also required). ? Readily allows modification to geometry. ? Facilities changes to element distribution, you are not bound to one analysis model. However, solid modeling ? Can sometimes require large amounts of CPU time. ? Can (for small models) sometimes be more cumbersome, requiring more data entire than direct generation. ? Can sometimes "fail" ( the program will not be able to generate the finite element mesh) under certain circumstances. Direct Generation:- On the plus side, direct generation ? Is convenient for small or simple models. ? Provides you with complete control over the geometry and numbering of every node and every element. However, direct generation ? is usually too time consuming for all but the simplest models; the volume of data you must work with can become overwhelming. ? Cannot be used with adaptive meshing. ? Makes design optimization less convenient. ? Makes it difficult to modify the mesh (tools such as area mesh refinement, smartsizing, etc. cannot be used). ? Can become tedious, requiring you to pay more attention to every detail of your mesh, tedium can sometimes cause you to become more prone to committing errors.

3. STEPS IN PLANNING The Importance Of Planning:- As you begin your model generation, you will ( consciously or unconsciously) make a number of decision that determined how you will mathematically simulate the physical system: what are the objectives of your analysis? Will you model all, or just a portion, of the physical system? How much detail will you include in your model? What kinds of elements will you use ? how dense should your finite element mesh be? In general, you will attempt to balance computational expense (CPU time, etc) against precision of results as you answer these questions. The decisions you make in the planning stage of your analysis will largely govern the success of failure of your analysis efforts. Choose A Model Type ( 2-D, 3-D, Etc.):- Your finite element model may be categorized as being 2- diamentioal or 3-dimentional, and as being composed of points elements, line elements, area element, or solid element. Of course, you can intermix different kinds of elements are required (taking care to maintain the appropriate compatibility among degree of freedom). For, example, you might model a stiffened shell structure using 3-D shell elements to represent the skin and 3-D beam elements to represent the ribs. You choice of model dimensionally and element type will often determined which method of model generation will be most practical for your problem. LINE models can represent 2-D or 3-D beam or pipe structures, as well as 2-D models of 3-D axisymmetric shell structure. Solid modeling usually does not offer much benefit for generating line models, they are more often created by direct generation methods. 2-D SOLID analysis models are used for thin planar structure (plane stress)' "infinitely long" structure having a constant cross section )[plane strain), or axisymmetric solid structures. Although menu 2-D analysis models are relatively easy to create by direct generation methods, they are usually easier to create with solid modeling. 3-D SHELL models are used for thin structures in 3-D space. Although soma 3-D shell analysis models are relatively easy to create by direct generation methods, they are usually easier to create with solid modeling. 3-D SOLID analysis models are used for thick structures in 3-D space that have neither a constant cross section nor an axis of symmetry. Creating a 3-D solid analysis model by direct generation usually requires considerable efforts. Solid modeling will nearly always make the job easier. Choose Between Linear & Higher Order Elements:- The ANSYS program's element library two basic types of area and volume element: linear (with or without extra shapes), and quadratic. These basic element types are represented schematically in Figure let's examine some of the considerations involved in choosing between these two basic element types: Figure : Basic area and volume types available in the ANSYS program (a) Linear Isoparametric (b) Linear Isoparametric with extra shapes (c) Quadratic 4.CO-ORDINATE SYSTEMS Types Of Coordinate System:- The ANSYS program has several types of coordinate systems, each used for a different reason: ? Global and local coordinate systems are used to locate geometry items (nodes, keypoints, etc.) in space. ? The display coordinate system determines the system in which geometry items are listed or displayed. ? The nodal coordinate system defines the degree of freedom directions at each node and the element results data. ? The element coordinate system determines the orientation of material properties and element results data. ? The results coordinate system is used to transform nodal or element results data to a particular coordinate system for listings, displays, or general post processing operations (POST1). The working planes, which is separate form the coordinate systems discussed in this chapter, is used to locate geometric primitives during the modeling process. See Chapter 4 for mare information about the working plane. Global and Local Coordinate Systems :- Global and local coordinate system are used to locate geometry items. By default, when you define a node or a keypoint, its coordinates are interpreted in the global Cartesian system. For some models. However, it may be more convenient to define the coordinates in a system other than global Cartesian. The ANSYS program allows you to input the geometry in any of three predefined (global) coordinate system, or in any number of user defined (local) coordinate systems. Global Coordinate Systems:- A global coordinate system can be thought of as an absolute a reference frame. The ANSYS program provides three predefined global systems: Cartesian, cylindrical, and shellac. All three of these systems are right-handed and, by definition, share the same origin. They are identified y their coordinate system (C.S.) numbers: 0 for Cartesian, 1 for cylindrical, and 2 for spherical, see figure,

Local Coordinate Systems:- In many cases, it may necessary to establish your own coordinate system, whose origin is offset from the global origin, or whose orientation differs from that of the predefined global systems. (See figure for an example of a coordinate system defined by rotations) such user defined coordinate systems, known as local coordinate systems, can be creates in the following ways: ? Define the local system in terms global Cartesian coordinates. Command: LOCAL GUI: Utility Menu>WorkPlane>Local Coordinate Systems>Create Local CS>At Specified Loc ? Define the local system in terms of existing nodes. Command: CS GUI: Utility Menu>WorkPlane>Local Coordinate Systems>Create Local CS>By 3 Nodes ? Define the local system in terms of existing keypoints. Command: CSKP GUI: Utility Menu>WorkPlane>Local Coordinate Systems>Create Local CS>By 3 Keypoints ? Define the local system to be centered at the origin the presently defined working plane. Command: CSWLPA GUI: Utility Menu>WorkPlane>Local Coordinate Systems>Create Local CS>At WP Origin ? Define the local system in terms of active coordinate system with the CLOCAL command (see the late section titled "The Active Coordinate System"). (There is no GUI equivalent for the CLOCAL command.) When a local coordinate system is defined, it becomes the active coordinate system. As you create a local system, you assign it a C.S. identification number (which must be phase of your ANSYS session. To delete a local system, use one of the following methods. Command: CSDELE GUI: Utility Menu>WorkPlane>Local Coordinate Systems>Create Local CS To view the status of all global and local coordinate systems, use one of the following method: Command: CSLIST GUI: Utility Menu>List>Local Coordinate System Your local coordinate systems can be Cartesian, Cylindrical, or Spherical, similar in form of the three predefined global system. Note that you may define local cylindrical and spherical coordinate systems in either circular or elliptical configuration. Additionally, you can define a toroidal local coordinate system, as illustrated in figure. Working Plane :- Although your cursor appears as a point on your screen, it actually represents a line through space, normal to the screen. In order to be able to pick a point with your cursor, you first need to define an imaginary plane that, when intersected by the normal line of your cursor, will yield a unique point in space. This imaginary plane is called a working plane. Another way to think of the intersection between your cursor and your working plane is to picture your cursor as a point that moves around on your working plane. The working plane, then, acts a "tablet" on which you write with your cursor.( the working plane need not be parallel to your display screen).

A working plane is an infinite plane with an origin, a 2-D coordinate system, a snap increment (discussed below) and display grid. You can define only one working plane at a time. (creating a new working plane eliminates your existing working plane). The working plane is separate from the coordinate system; for e.g. the working plane can have different point of origin and rotation than the active coordinate system. See the section called "Working plane Tracking". In this chapter for a discussion of how the force the active coordinate system to track the working plane.

5. SOLID MODELING An Overview Of Solid Modeling Operations:- The purpose of using a solid model is to relieve of the time consuming task of building a complicated finite element model by direct generation. Lets talk a brief look at some of the solid modeling and meshing operations that you can use to speed up the creation of your final analysis model. Building Your Model From The Bottom Up:- Key points, the points that defines the vertices of your model are the "lowest order" solid model, entities. If in building your solid model, you first create your key points and than used those key points to define the "higher order" solid model entities (that is lines, areas & volumes, you are said to be building your model "from the bottom up". Keep in mind that models built from the bottom up are defined within the currently active coordinate system.

Figure:- "Bottom Up" Construction

Building Your Model From Top Down:- The ANSYS program also gives you the ability to assemble your model using geometric primitives, which are fully defined lines, areas & volumes. As you create a primitive the program automatically creates all the "lower" entities associated with it. If your modeling efforts begins with the "higher primitive entities", you are said to building you model "from the top down". You can freely combine bottom up and top down modeling techniques, as appropriate, in any model. Remember that geometric primitives are built with in the working plane. While bottom up techniques are defined against the active coordinate system. If you are mixing techniques, you may wish to consider using the CSYS, WP or CSYS, 4 command to force the coordinate system to follow the working plane.

Figure: "Top down" constructions

Using Boolean Operators:- You can "sculpt" your solid model using intersection, subtractions, and other Boolean operations. Boolean allows you to work directly with higher solid model entities to create complex shapes. (Both bottom up and top down creations can be used in Boolean operations).

Figure: Create complex shapes with Boolean operations

Dragging & Rotating:- Boolean operators although convenient, can be computationally expensive. Some times a model con be constructed more efficiently by dragging or rotating.

Figure: Dragging an area to create a volume

Moving & Copying Solid Model Entities:- A complicated area or volume that appears repetitively in your model need only be constructed once; it can then be moved, rotated, and copied to new location on your model. You might also find it more convenient to place geometric primitives in their proper location by moving them, rather than by changing the working plane.

Figure: Copying an area Meshing:- Your ultimate objective in building a solid model is to mesh that model with nodes and elements. Once you have completed the solid model, set element attributes, and established meshing control, you can then turn the ANSYS program lose to generate the finite element mesh by taking care to meet certain requirements, you can request a "Mapped" mesh containing all quadrilateral or brick elements. Figure: Free and mapped meshes Moving & Copying Nodes & Elements:- Automatic meshing is a huge improvement over direct generation of nodes and elements, but it can some times be computationally time consuming. If your model contains repetitive features, you might find that the most efficient approach to model generation would be to model & mesh a pattern region of your model then generates copies of that meshed region. (copying a mesh in this manner will generally be faster than separately meshing repeated features).

Figure: Copying a meshed area Solid Model Loads:- In the ANSYS program, loads are normally associated with nodes and elements. However, using solid modeling, you will often find it inconvenient to define loads at nodes and elements. Fortunately you mat assign loads directly to the solid model; when you invite the solution calculations (SOLVE command), the program will automatically transfer these solid model loads to finite element model. Revising Your Model (clearing & deleting):- Before your can revise your model, you need to be aware of the hierarchy of solid model and finite element model entities. A lower order entity can not be deleted if it attached to a higher order entity. Thus a volume, can not be deleted if it has been meshed, a line cannot be deleted if it is attached to an area, and so forth. If an entity is attached to any loads, deleting or redefining that entity will delete the attached loads from the database. The hierarchy of modeling entities is as listed bellow:

Highest Elements (& Element Loads) Nodes (and Nodal Loads) Volumes (and Solid Model Body Loads) Areas (and Solid Model Surface loads) Lines (and Solid Model Line loads) Lowest Key points (And Solid Model Point Loads)

If you need to revise a solid model after it has been meshed, you must usually first delete all the nodes and elements in the portion of the model to be revised, using the x CLEAR command (menu path Main Menu > Preprocessor > Clear). Once the solid model is cleared, you can delete (from top down) and redefine solid model entities to revise your model. As an alternative to clearing, deleting, and redefining, you can some time consider modifying the key points directly, using one of these methods.

Command : KMODIF Gui : Main Menu>PreProcessor>Move/Modify>Set of KPs By using KMODIF of its GUI path, you automatically clear the redefine the attached lines, areas and volumes. After you have revised your solid model, you will need to remesh the portions that where cleared.

Figure: Revising a meshed solid model

Creating Your Solid Model From The Bottom Up:- Any solid model, whether assembled from the bottom up or from the top down, is defined in terms of keypoints, lines, areas, and volume. Figure illustrates these entities.

Figure: Basic solid model entities

Keypoints are the verticals, lines are the edges, area are the faces, and volumes are the interior of the objects. Notice. Notice that there is a hierarchy in these entities: volumes, the highest-order entities, are boundary be areas, which are boundary by lines, which in turn are boundary by keypoints. Keypoints:- When building your model from the bottom up, by defining the lowest-order solid model entities, keypoints. Keypoints are defined within the currently active coordinate system. You can then define lines, areas, and volumes connecting these keypoints. You do not always have to explicitly define all entities in ascending order to create higher-order entities: you can define areas and volumes directly in terms of the key points at their verticals. The intermediate entities will then be generated automatically as needed. For example, if you define a brick-like volume in terms of the eight keypoints at its corners, the program will automatically generated the bounding areas and lines.

Defining Keypoints:- ANSYS provides methods that you can use for defining keypoints, including: ? To define individual keypoints: Command: K GUI: Main Menu>Preprocessor>Create>Keypoints>In Active CS Main Menu>Preprocessor<Create<Keypoints>On Working Ratio. ? To define keypoints at a given location on an existing : Command: KL GUI: Main Menu>Preprocessor>Create>Keypoints>In Active CS Main Menu>Preprocessor<Create<Keypoints>On Line W/Ratio. Generating Keypoints From Existing Keypoints:- Once you have created an initial pattern of keypoints, you can generate additional keypoints using the methods listed below. (many of the Boolean commands will also create keypoints). ? To create a key points between two existing keypoints: Command: KBETW GUI: Main Menu>Preprocessor>Create>Keypoints>KP between KPs ? To generate a keypoints between two keypoints: Command : KFILL GUI: Main Menu>Preprocessor>Create>Keypoints>Fill between KPs ? To create a keypoint at the center of a circle arc defined by three locations: Command: KCENTER GUI: Main Menu>Preprocessor>Create>Keypoints>KP at Center ? To generate additional keypoints from a pattern of keypoints: Command: KGEN GUI: Main Menu>Preprocessor>Copy>Keypoints ? To generate a scaled pattern of keypoints from a given keypoints pattern, use the KSCALE command. You cannot access the KSCALE command directly in the GUI. ? To generate a reflected set of keypoints: Command: KSYMM GUI: Main Menu>Preprocessor>Reflecte>Keypoints ? To transfer a pattern of keypoints to another coordinate system: Command: KTRAN GUI: Main Menu>Preprocessor>Move/Modify>Tranfer Coord>Keypoints ? To define a default location for undefined nodes or keypoints, use the SOURCE command. You cannot access the SOURCE command directly in the GUI. ? To calculate and move a keypoint to an intersection: Command: KMOVE GUI: Main Menu>Preprocessor>Move/Modify>To Intersect ? To define a keypoint at an existing node location: Command: KNODE GUI: Main Menu>Preprocessor>Create>Keypoints>In Active CS

Viewing, Selecting, and Deleting Keypoints:- You can maintain keypoints using the methods listed below:

? To list defined keypoints: Command: K GUI: Utility Menu>list>Keypoints>Coordinates + Attributes Utility Menu>list>Keypoints>Coordinates Only Utility Menu>list>Keypoints>Hard Points Utility Menu>list>Picked Entities>Keypoints>Coordinates only Utility Menu>list>Picked Entities>Keypoints>Coords+ Attributes

? To display selected keypoints: Command : KPLOT GUI: Utility Menu>Plot>Keypoints Utility Menu>Plot>Specified Entities>Keypoints ? To select keypoints: Command: KSEL GUI: Utility Menu>Select>Entities ? To detect unmeshed keypoints Command : KDIST GUI: Main Menu>Preprocessor>Delete>Keypoints If you have issued the command /PNUM,KP,1 (menu path Utility Menu>PlotCTsis>Numbering), keypoints numbers will also attached to these higher-order entities. Using Miscellaneous Keypoints Commands:- ANSYS also provided these methods for Working with keypoints: ? To calculate the distance between two keypoints: Command: KDIST GUI: Main Menu>Preprocessor>Modeling-Ckeck Geom>KP distance ? To modify the coordinates defining a keypoint (that is, move a keypoint to a new location), use one of the methods shown below. Doing so will automatically clear my meshed region attached to the specified keypoints, and will also redefine the higher-order entities attached to that keypoints. (A keypoints can also be redefined using the K command, but only if it is still "free", that is, not yet attached to a line, nor meshed). Command: KDIST GUI: Main Menu>Preprocessor>Move/Modify>Set Of KPs Main Menu>Preprocessor>Move/Modify>Single KP

Hard Points:- Hard points are actually a special type of key points. You can use hard points to apply loads or obtain data from arbitrary points on lines and areas within your model. Hard points do not modify either the geometry or topology of your model. Most of the keypoint commands, such as FK,KLIST, and KSEL, can be applied to hard points. In addition, hard points have there own set of commands and extension in the GUI. If you issue any commands that updates the geometry of an entity, such as Boolean or simplification commands, any hard points associated with that entity are deleted. Therefore, you should add any hard points after completing the solid model. If you delete an entity that has associated hard points, those hard points are either. ? Deleted along with entity (if the hard point is not associated with any other entities. ? Detached from the deleted entity (if the hard points is associated with additional entities). You can not manipulate hard points with commands to copy, move, modify, or modify keypoints, hard points have their own suite of commands and GUI controls.