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Page 7: 2Dmesh

  • fig1
  • Figure 1 - 2Dmesh Sketchboard
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Purpose: The 2Dmesh module runs in a separate window called the sketchboard that is opened by pushing the Sketch button at the top of the inital panel. This sketchboard can be used to generate meshes using a 2-step process:

1. Create a closed boundary - The 2Dmesh module functions almost like a small 2D CAD program as it has several geomery manipulation functions which also can be found in ordinary CAD applications. It is possible to create any kind of 2D geometry made up of arcs and lines. The main requirement is that the end points of the edges have to be coincident to form a valid mesh area.

2. Mesh the enclosed area - Once a closed boundary has been established, it is possible to create structured and free meshes based on several different techiques. All 4 methods are described below in the Mesh section.

Sketchboard geometry can be saved as an LS-PrePost command file with the Save button. This file can then be read into LS-PrePost as a normal command file or with the Load button. With the Load button it is also possible to import line, arc, and polyline entities from a .dxf file. Curves can alternatively be imported in to the sketchboard from the Curves Interface on Page 7 which means that it is possible to import IGES, VDA, and XYZ curve files. The selected curves in the Curves Interface will be imported and projected on to the 2Dmesh sketchboard. A best fit plane is calculated from the selected curves which means that the curves do not have to lie in the global X-Y plane. Any elements created on geometry imported in this way will be transformed back to the best fit plane in 3D.


Controls:

point

Point - It can be useful to create points as reference positions when creating new geometry. It is not necessary to create points in this panel before creating lines or arcs as they will automatically be created in the line and arc functions. With the Pick radio button selected, points can be created by left-clicking in the sketchboard. If KeyIn is selected, a single point with coordinates (X,Y) is created when the Create or Apply buttons are clicked. A point can be created with a % between 2 points when Between is selected. A value between 0 and 100 should be typed in the text field. After two points have been picked, a new point will be created between them. The Curve function creates a specified number of points on a curve (line or arc). The number in the point on curve field includes the end points.


line

Line - With the Pick radio button selected, a line can be created by clicking two points on the sketchboard. If KeyIn is selected, the endpoints of the line can be entered manually by clicking the Create or Apply button after entering each (X,Y) pair. If Point LA is selected, a line can be created by manually entering the starting point coordinates, (x1,y1), as well as the line length, L, and angle, A. The zero angle is defined as pointing to the right. Positive angles are defined in the counter-clockwise direction.

  • fig3
  • Figure 3 - Point to Arc

Point2Arc - The Point2Arc function creates a line from a curve tangent to an arc. The picked point has to be outside the arc otherwise an error message will be displayed. Two different tangential lines from a point to an arc will always be possible to create. The one which is closest to the cursor position where the arc is clicked will be selected.

 

  • fig4
  • Figure 4 - Arc to Arc

Arc2Arc - The Arc2Arc function creates a line which is tangent to two arcs. Four possible lines can be created if the arcs are separate. The line which is closest to the cursor position where the the two arcs are clicked will be selected.

 

  • fig2
  • Figure 2 - Line Offset

Offset - Both lines and arcs can be Offset by a value. A curve will be offset to the side where the cursor is when the mouse is clicked. A preview of the new offset curve is shown when the mouse pointer is close to a curve as shown in Figure 2 (The blue line is a preview of where the new offset line will be created).



arc

Arc - Every arc angle must be less than 180 degrees in 2Dmesh. With the Pick radio button selected, the points defining the arc are created by clicking on the sketchboard. If KeyIn is selected, the points can be entered manually. There are two different ways of creating arcs:

  • arc2

Arc by 3 Points - The center point is the first point to be selected. If the Radius checkbox is activated and a value has been entered, the second point will be automatically placed this distance from the first point along the vector from Point 1 to Point 2. If the Angle checkbox is activated and a value has been entered Point 3 will be automatically created as well.

  • arc3

Arc by 2 Points and Radius - When using this option, the Radius checkbox is activated by default, and a value must be entered. The arc will always be created to have a counter-clockwise curvature. The given radius must be larger than half the distance between the two points otherwise it is impossible to create an arc with the given input, and an error message will be displayed.


circle

Circle - The circle function can be seen as a macro to the Arc function as the result will be a circular geometry consisting of four arcs. The inputs are the center point (Point 1) and a point on the periphery. With the Pick radio button selected, the points are created by clicking on the sketchboard. If KeyIn is selected, the points can be entered manually. If the Radius checkbox is activated and a value has been entered, this will be used for the radius of the circle. Otherwise the radius will be calculated based on Point 1 and Point 2.


trim/extend

Trim/Extend - Lines and arcs can be trimmed and/or extended to other lines and arcs in this interface with the option to Break a curve into two parts if the curve is trimmed (made shorter). It is the curve end which is closest to the mouse cursor which will be trimmed/extended. A cross marker is displayed on the curve end that will be modified as shown in Figure 5.

  • fig5
  • Figure 5 - Trim/Extend

When the curve to be operated on is selected with the left mouse button, the curve to be trimmed/extended to is to be selected. Appropriate error messages will be displayed if the operation for some reason cannot be completed



fillet

Fillet - A fillet with a specified Radius can be created between lines and/or arcs. Often there are several possibilities where the fillet can be added. It will be placed closest to where the two mouse clicks are made.

  • fig7
  • Figure 7
  • fig6
  • Figure 6

There are four different locations to place a fillet for two crossing lines as shown in Figure 6. There are eight different locations to place a fillet for two arcs (circles) as shown in Figure 7.



tranlate

Translate - Points and curves can be translated with the option to create several copies of the selected entities in the process. Entities (points, lines, and curves) can be selected by either picking or selecting by rectangular area if the Area radio button is toggled. When the Area toggle is active, there is an option to select entities that are partly inside the selected area or partly outside with the In and Out radio buttons. The desired translation distance must be typed into the Enter distance field. The selected entities will be translated in either X, Y, or XY direction depending on the selected option in the options menu. The entities are translated when Tran- or Tran+ is clicked.


rotate

Rotate - Points and curves can be rotated with the option to create several copies of the selected entities. Entities (points, lines, and curves) can be selected by either picking or selecting by rectangular area if the Area radio button is toggled. When the Area toggle is active, there is an option to select entities that are partly inside the selected area or partly outside with the In and Out radio buttons.

  • rotate plus
  • rotate minus

The desired rotation angle must be typed in the text field. The rotation center point is located at the coordinate (x,y). A point can be clicked, and the text field will be automatically filled in. The entities are rotated when the rotate left and rotate right are pressed.


edge

Edge - A sketchboard edge is created from one or several curves. If several curves are used for creating an edge, the curves have to share end point coordinates. Edges have two properties: number of elements and a bias value. With the bias value, the nodes on the edge can be biased to one of the end points as shown in Figure 8.

  • fig8
  • Figure 8 - Edge Biasing

A bias value of 0.5 has been set on the lower right end point. This means that the element at this end will be 0.5 times as large as the element on the opposite end. An edge is created when a number of curves have been selected and the middle mouse button is pressed or when the Apply button is clicked. The edge is displayed in green, and the end points for the edge are marked with small squares. The Auto-chain curve function is activated by default and will automatically determine if there are any other curves attached at any of the selected curve endspoints and if so will expand the selection of curves until no more adjacent curves are found or until there are multiple curves to choose between or until the angle to the next connected curve is more than the selected break angle.



mesh

Mesh - There are four different techniques to create elements on a mesh area. These are shown in Figure 9 and in clockwise order starting from the upper-left are:

  • mesh types
  • Figure 9

(1) a topologic four-edge area filled with a structured mesh of quadrilateral elements only, (2) a topologic three-edge area filled with a structured mesh of quadrilateral elements only, (3) a topologic three-edge area where the structured mesh degenerates in one corner (the elements in that corner will be triangular - the others will be quadrilateral), (4) an arbitrary shaped area with optional holes in it will be filled with a non-structured mesh with only quadrilateral elements if there are an even number of elements around the boundary. If there are an odd number of elements on the boundary, one triangular element will be added.

Meshes on different mesh areas are not merged with any adjacent mesh area. If mesh areas should be merged in the final model this should be taken care using the DupNod Interface on Page 2. To create a mesh on an area, a closed loop of edges has to be selected. A preview of the mesh is displayed when a closed loop is found, and a question dialog window asks if the elements should be kept. If Yes is clicked, the elements are created and moved from the sketchboard to the 3D graphics window. If No is clicked, no elements are created.

  • fig11
  • Figure 11
  • fig10
  • Figure 10

Mesh Transitions - Mesh transitions in the first two structured meshing techniques are automatically taken care of if the combination of elements per edge allows for a structured mesh transition. Transitions from one to two elements or from one to three elements are possible for quadratic mesh areas as shown in Figure 11.



3d

3D Primitive - Options are provided for creating 3D meshes of cones and cylinders.

Zin - Zoom in by clicking and dragging a box in the sketchboard.

Zout - Zoom out by a pre-defined scale factor.

Settings - Toggle grid on/off, control grid spacing, toggle snap-to-grid on/off.


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© 2012 Livermore Software Technology Corporation