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feamesh [2017/09/15 16:49]
jgravett Adds Patch Boundary Bug Picture
feamesh [2017/10/11 14:51] (current)
jgravett Adds Unsupported Use Case
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 ====== Structural Modeling ====== ====== Structural Modeling ======
  
-OpenVSP allows a user to model aerospace structures for a geometry and generate a finite element mesh model. This model can then be exported to an external FEA software package for post-processing and further analysis. To begin, a geometry must first be created or loaded. A structure can then be defined for the geometry using the FEA Mesh GUI. The GUI is accessed by clicking "FEA Mesh..."​ in the Analysis pull down menu.     ​+OpenVSP allows a user to model aerospace structures for a geometry and generate a finite element mesh model. This model can then be exported to an external FEA software package for post-processing and further analysis. To begin, a geometry must first be created or loaded. A structure can then be defined for the geometry using the FEA Mesh GUI. The GUI is accessed by clicking "FEA Mesh..."​ in the Analysis pull down menu. Access to structural modeling and mesh generation is also available through the API.      ​
  
 ==== VSP Terminology ==== ==== VSP Terminology ====
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 {{ :​featerminology.png?​nolink&​300|}} {{ :​featerminology.png?​nolink&​300|}}
  
-A structure is defined as a collection of FEA parts associated with a single geometry. A geometry is able to have multiple structures.+A structure is defined as a collection ​or assembly ​of FEA parts. Structures are associated with a single ​parent ​geometry ​from which the outer mold line (OML) serves as the primary FEA part. A geometry is able to have multiple structures ​associated with it.
  
 === FEA Part === === FEA Part ===
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 === Zero-Depth ===  === Zero-Depth === 
  
-A zero-depth structural entity is composed of a series of beam elements. ​can be defined along either a sub-surface edge or the intersection curve of two surfaces. Stringers, longerons, and other stiffeners are represented as zero-depth entities. ​+A zero-depth structural entity is composed of a series of beam elements. ​It can be defined along either a sub-surface edge or the intersection curve of two surfaces. Stringers, longerons, and other stiffeners are represented as zero-depth entities. ​
  
 === Key-Point ===  === Key-Point === 
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 ==== FEA Mesh Screen ==== ==== FEA Mesh Screen ====
  
-Visible on all tabs of the FEA Mesh GUI are the status window, current structure identification, ​and "Mesh and Export"​ button. The status window is mainly used to display the progress of FEA mesh generation, which is run as a separate process. Error messages and warning ​will also be displayed in the status window. The current structure is listed at the bottom left of the GUI, and shows which structure is highlighted in the structure browser on the Structure tab. This is the structure that will be used to create the FEA mesh when "Mesh and Export"​ is selected. ​   +Visible on all tabs of the FEA Mesh GUI are the status window, current structure identification,​ "Mesh and Export" button, and "Reset Display" button. The status window is mainly used to display the progress of FEA mesh generation, which is run as a separate process. Error messages and warnings ​will also be displayed in the status window. The current structure is listed at the bottom left of the GUI, and shows which structure is highlighted in the structure browser on the Structure tab. This is the structure that will be used to create the FEA mesh when "Mesh and Export"​ is selected. ​If a mesh has been generated, "Reset Display"​ will hide the mesh and show the parent geometry for the selected structure, along with its FEA parts.  ​
  
 === Structure Tab === === Structure Tab ===
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 {{ :​feaunittable.png?​nolink&​600 |}} {{ :​feaunittable.png?​nolink&​600 |}}
  
-The structure browser lists all of the structures for the vehicle, along with their name, parent geometry, and surface index. Below the browser, all available geometries a structure may be added to are listed. All geometry types, with the exception of Blank and Hinge, are allowed as a structure'​s parent. The available surface indexes for each geometry are also listed. More than one structure may be added to a geometry. The "Add Structure"​ button will create a new structure on the geometry and surface indicated in the above choices. "​Delete Structure"​ will, as expected, delete the structure that is currently selected in the browser ​(warning: cannot be undone). The name of the currently selected structure may be edited below the add and delete buttons. When a structure is selected, the degenerate geometry preview can be displayed under "​Orientation"​. This feature is particularly useful for wing structural layups, where a top-down 2D view is shown. The display options can also be changed in the Geom Browser under "​Surface"​. ​  +The structure browser lists all of the structures for the vehicle, along with their name, parent geometry, and surface index. Below the browser, all available geometries a structure may be added to are listed. All geometry types, with the exception of Blank and Hinge, are allowed as a structure'​s parent. The available surface indexes for each geometry are also listed. More than one structure may be added to a geometry. The "Add Structure"​ button will create a new structure on the geometry and surface indicated in the above choices. "​Delete Structure"​ will, as expected, delete the structure that is currently selected in the browser. The name of the currently selected structure may be edited below the add and delete buttons. When a structure is selected, the degenerate geometry preview can be displayed under "​Orientation"​. This feature is particularly useful for wing structural layups, where a top-down 2D view is shown. The display options can also be changed in the Geom Browser under "​Surface"​. ​  
  
 === Part Tab === === Part Tab ===
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 {{ :​feashellcap.png?​nolink&​205|}} {{ :​feashellcap.png?​nolink&​205|}}
  
-The Part tab provides organization and control of FEA parts for the selected structure. The browser at the top of the tab lists all FEA parts for the structure, and includes their name, type, structural entity type, and property assignment. To the left of the browser controls are available for reordering FEA parts. The ordering of parts in the browser is important in the case of tagging conflict, where priority is given to the part higher up in the browser. For example, if two FEA sub-surfaces overlap, the overlapping region is tagged according to the FEA part earlier in the list. Another feature of the browser is that multiple FEA parts can be selected at once. The options under "​General"​ can then be set for the group of selected parts (or individual part). To add an FEA part to a structure, the FEA part type must first be indicated. The available FEA part types are slice, rib, spar, fixed point, dome, rib array, slice array, line, rectangle, ellipse, control surf, and line array. For more details on these part types, see the "FEA Part Types" section of this wiki page. One or more FEA parts can be deleted by selecting "​Delete Part". A part can be renamed with the input in the middle of the tab. If a single FEA part is selected, the "Edit Part" button will open the FEA Part Edit GUI. This GUI, which can also be accessed by double clicking an FEA part in the browser, offers controls and options unique to each FEA part type. +The Part tab provides organization and control of FEA parts for the selected structure. The browser at the top of the tab lists all FEA parts for the structure, and includes their name, type, structural entity type, and property assignment. To the left of the browsercontrols are available for reordering FEA parts. The ordering of parts in the browser is important in the case of tagging conflict, where priority is given to the part higher up in the browser. For example, if two FEA sub-surfaces overlap, the overlapping region is tagged according to the FEA part earlier in the list. Another feature of the browser is that multiple FEA parts can be selected at once. The options under "​General"​ can then be set for the group of selected parts (or individual part). To add an FEA part to a structure, the FEA part type must first be indicated. The available FEA part types are slice, rib, spar, fixed point, dome, rib array, slice array, line, rectangle, ellipse, control surf, and line array. For more details on these part types, see the "FEA Part Types" section of this wiki page. One or more FEA part can be deleted by selecting "​Delete Part". A part can be renamed with the input in the middle of the tab. If a single FEA part is selected, the "Edit Part" button will open the FEA Part Edit GUI. This GUI, which can also be accessed by double clicking an FEA part in the browser, offers controls and options unique to each FEA part type. 
  
 === Material Tab === === Material Tab ===
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 === Mesh Tab === === Mesh Tab ===
  
-The Mesh tab is available for controlling aspects of the FEA mesh solver. The options available are similar to those seen on the Global tab to the CFD Mesh GUI. Further insight into these settings can be found here: [[http://​www.openvsp.org/​wiki/​doku.php?​id=cfdmesh|CFD Mesh Wiki]]. Below the mesh control settings, the files exported during FEA mesh generation are listed. The first two, *.stl and *.msh, are general mesh formats that do not include any materials or properties. The mass output file is a *.txt file that lists the mass of each FEA part, divided between ​the shell and beam FEA elements. The remaining two export files are *.dat files written in Nastran and Calculix/​Abaqus FEA mesh format. These files may be imported ​to an external FEA mesh program for further processing and analysis.+The Mesh tab is available for controlling aspects of the FEA mesh solver. The settings are saved with the individual structure, so a structure must be selected when making use of this tab. The options available are similar to those seen on the Global tab of the CFD Mesh GUI. Further insight into these settings can be found here: [[http://​www.openvsp.org/​wiki/​doku.php?​id=cfdmesh|CFD Mesh Wiki]]. Below the mesh control settings, the files exported during FEA mesh generation are listed. The first two, *.stl and *.msh, are general mesh formats that do not include any materials or properties. The mass output file is a *.txt file that lists the mass of each FEA part, divided between shell and beam FEA elements. The remaining two export files are *.dat files written in Nastran and Calculix/​Abaqus FEA mesh format. These files may be imported ​into an external FEA mesh program for further processing and analysis.
  
 === Display Tab === === Display Tab ===
  
-The Display tab is used to control visualization of the generated FEA mesh. Therefore, it has no functionality until the "Mesh and Export"​ button has been selected. At the top of the tab under "​Display",​ toggle buttons are available for controlling the display of all sets checked under "​Display Element Sets". "Draw Mesh" shows the borders of each triangular shell element. "Color Elements" ​is applies a different color to each element set. "Draw Nodes" displays each individual FEA node. "Draw Element Orientation Vector"​ draws the normal vector for beam elements. For shell elements, this button draws a vector pointing in the direction of increasing U. Since the FEA mesh is unstructured,​ this orientation vector is useful when laying out composite laminate layers. Under "​Display Element Sets", FEA parts are grouped by their shell and beam elements. Their visibility is controlled by the check boxes next to each set or by using the "Show All" and "Hide All" buttons. ​+The Display tab is used to control visualization of the generated FEA mesh. Therefore, it has no functionality until the "Mesh and Export"​ button has been selected. At the top of the tab under "​Display",​ toggle buttons are available for controlling the display of all sets checked under "​Display Element Sets". "Draw Mesh" shows the borders of each triangular shell element. "Color Elements"​ applies a different color to each element set. "Draw Nodes" displays each individual FEA node. "Draw Element Orientation Vector"​ draws the normal vector for beam elements. For shell elements, this button draws a vector pointing in the direction of increasing U. Since the FEA mesh is unstructured,​ this orientation vector is useful when laying out composite laminate layers. Under "​Display Element Sets", FEA parts are grouped by their shell and beam elements. Their visibility is controlled by the check boxes next to each set or by using the "Show All" and "Hide All" buttons. ​
  
 {{ :​feadisplay.png?​nolink&​850 |}} {{ :​feadisplay.png?​nolink&​850 |}}
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 === Skin === === Skin ===
  
-Skin is defined as the outer mold line (OMLof the structure'​s parent geometry. It is automatically added to a structure upon its creation, and can only be removed by deleting the structure. The skin is composed of shell FEA elements, which the property for can be specified. The only additional option available for skin in the FEA Part Edit GUI is to delete the skin triangles after the FEA mesh has been generated. ​+Skin is defined as the OML of the structure'​s parent geometry. It is automatically added to a structure upon its creation, and can only be removed by deleting the structure. The skin is composed of shell FEA elements, which the property for can be specified. The only additional option available for skin in the FEA Part Edit GUI is to delete the skin triangles after the FEA mesh has been generated. ​
  
 === Slice === === Slice ===
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 === Rib === === Rib ===
  
-Ribs are a derivation of slice parameterized specifically for wing geometries. Ribs are defined from the leading edge to the trailing edge of the wing, parallel to the local XZ plane. In the FEA Part Edit GUI, "Edge Normal"​ can be used to specify the rib as perpendicular to the wing's leading edge, trailing edge, or any spars that have been added to the structure. ​+Ribs are a derivation of slice parameterized specifically for wing geometries. Ribs are defined from the leading edge to the trailing edge of the wing, parallel to the local XZ plane. In the FEA Part Edit GUI, "Edge Normal"​ can be used to specify the rib as perpendicular to the wing's leading edge, trailing edge, or any spars that have been added to the structure. ​Ribs can be constrained to a particular start and end wing section, from which they will automatically be trimmed to. Therefore, it is possible to place one rib at a wing cross-section and have another rotated rib trimmed to the cross section. Ribs are automatically sized without calculating surface intersections by assuming a linear trapezoidal wing. If modeling a rib on a complex wing geometry, the "Trim to Bounding Box" toggle can be used.     
  
 === Spar === === Spar ===
  
-Spars are another extension of slice made for wing geometries. They are defined from the root to tip of the wing, oriented vertically along the local XZ plane. ​An option is included ​to constrain ​spar to a specific ​wing section, which can be useful ​for a multi-section ​wing. +Spars are another extension of slice made for wing geometries. They are defined from the root to tip of the wing, oriented vertically along the local YZ plane. ​Similar ​to ribs, trapezoidal ​wing section ​is used for sizing the cutting plane unless the "Trim to Bounding Box" option is selected. Start and end wing sectional constraint can be applied to spars as well
  
 === Dome === === Dome ===
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 ==== Best Practices ==== ==== Best Practices ====
  
-Although a focus of the development for structural modeling in OpenVSP has been making it as robust as possible, certain cases can cause structural ​setup or mesh generation errors. Some of these may be worked around while others do not have a solution at this time. If you believe that you have found a bug or unsupported use case that is not identified here, please send us an email or post on the Google Group here: [[https://​groups.google.com/​forum/#​!forum/​openvsp]]+Although a focus of the development for structural modeling in OpenVSP has been making it as robust as possible, certain cases can cause structural ​layout issues ​or mesh generation errors. Some of these may be worked around while others do not have a solution at this time. If you believe that you have found a bug or unsupported use case that is not identified here, please send us an email or post on the Google Group here: [[https://​groups.google.com/​forum/#​!forum/​openvsp]]
  
-  - Avoid coplanar overlapping surfaces. Since the FEA mesh solver cannot identify a single intersection curve for two coplanar surface, mesh errors are likely to result. A common example of this is attempting to place a rib directly on a flat wing root or tip cap (see the images below). One workaround for this is to use a line subsurface to assign the root or tip with rib properties. ​Another option ​is to place a rib or slice at the root or tip, but set the root or tip cap to "​None"​. ​{{ :fearibcap2.png?​nolink&​800 |}}+  ​- Save your model frequently, especially before mesh generation. Use the FEA mesh generator with coarse mesh settings to check for potential errors while building up your structural model.  
 +  ​- Avoid coplanar overlapping surfaces. Since the FEA mesh solver cannot identify a single intersection curve for two coplanar surface, mesh errors are likely to result. A common example of this is attempting to place a rib directly on a flat wing root or tip cap (see the images below). One workaround for this is to use a line subsurface to assign the root or tip with rib properties. ​{{ :​fearibcap2.png?​nolink&​800 |}} 
 +  - FEA parts directly at the edge of a surface may not meshed correctly, so it is best to offset the FEA part just slightly from the surface edge. An example of this is adding ​a rib to a wing root or tip when the cap is set to "​None"​. ​This will likely cause the rib to mesh incorrectly or be lost   
   - Avoid placing planar FEA parts at the exact same location. Doing this will cause the FEA parts to be merged. In the merge, only data for the FEA part higher in the FEA part browser will be kept.   - Avoid placing planar FEA parts at the exact same location. Doing this will cause the FEA parts to be merged. In the merge, only data for the FEA part higher in the FEA part browser will be kept.
-  ​- Mesh errors may occur when an intersection occurs directly on the edge of a surface patch. One example of this is for a slice placed at an absolute location of 7.6875 on a standard pod (see the image below). If this occurs, try adjusting the FEA part location very slightly (absolute location change of 1e-5 should fix the issue). ​ {{ :​feapatchboundaryerror.png?​nolink&​600 |}} +  - If placing a fixed point on an FEA part surface other than the skin, make sure the fixed point is located within the intersection curve of the parent part. Otherwise, the fixed point can cause the parent surface to be trimmed incorrectly
-  ​- If placing a fixed point on an FEA part surface other than the skin, make sure the fixed point is located within the intersection curve of the parent part. Otherwise, the fixed point will be lost+  - Ribs and spars on complex wing geometries may not intersect the skin correctly. If this is the case, try using sectional constrains or using the "Trim to Bounding Box" option. Slice can also be used to model a rib or spar.  
-  - Ribs and spars on complex wing geometries may not intersect the skin correctly. If this is the case, slice can always ​be used to model a rib or spar.  +  - Mesh errors often result when intersection curves overlap. This could occur if a spar is placed on a control surface edge, a slice is aligned directly with a line sub-surface,​ or a similar intersection conflict is created. 
-  - Mesh errors often result when intersection curves overlap. This could occur if a spar is placed on a control surface edge, a slice is aligned directly with a line sub-surface,​ or a similar intersection conflict is created. ​  ​+  - Currently, a planar slice that completely intersects the first and last constant U/W curve (feature line) of a closed U/W geometry is not supported in the FEA mesh generator. Similarly, a slice that completely intersects a wing leading or trailing edge feature line is not supported.
  
 ==== API and Scripting ==== ==== API and Scripting ====
feamesh.1505519374.txt.gz · Last modified: 2017/09/15 16:49 by jgravett