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  #1  
Old 09-06-2007, 08:20 AM
cjs cjs is offline
formula18
 
Join Date: Apr 2006
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Rhino for FEM input

Dear All,

Is it possible to defie a curve in Rhino just by typing in the cordinates of the curve?

For example; I have the coordinates for some frame sections of a hull that I want to visualize but it is almoast impossible to find the coordinates just by using the mouse pointer.

Another question; I am gonna model a RoRo vessel to use it as input for ansys workbench?, is rhino a good tool for detailed structural 3d modeling or should I stick to my SolidEdge?
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  #2  
Old 09-06-2007, 11:21 AM
Steve Baer Steve Baer is offline
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Hi cjs,
You can always input point information in Rhino through the mouse or by typing in specific coordinates. If you want your curve to pass through the input points, either create a polyline or use the InterpCrv command.

As far as detailed structural modeling, you can do this in Rhino and I have done this in the past but Rhino is not quite as "set up" for this type of work as other programs (I don't really know SolidEdge so I can't comment).

Thanks,
-Steve
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  #3  
Old 09-06-2007, 03:36 PM
cjs cjs is offline
formula18
 
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Hi Steve,

Thank You very much for Your answer..

The problem is just that I cant find the button where I can type in specific coordinates.. which one?, I use Rhino 4.0

Thanks!
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  #4  
Old 09-07-2007, 07:19 PM
Steve Baer Steve Baer is offline
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Hi cjs,
There is no button. Rhino has a command window. If you just start typing, the keyboard input will show up on the command window. Look up "command prompt" in the Rhino help file.

Thanks,
-Steve
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  #5  
Old 09-07-2007, 07:46 PM
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marshmat marshmat is offline
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CGS,
For inputting your curves by offsets, do as Steve says: type INTERPCRV (by the time you get to the "t" Rhino will know what you want) and enter the list of co-ordinate, eg:
INTERPCRV
0,0,0 <enter>
0,1,1 <enter>
0,2,4 <enter> etc.
produces a curve smoothly interpolated between the points (x,y,z) and passing exactly through all of them.
SolidEdge is optimized for mechanical design. I've never had much luck trying to coax free-form shapes out of it, especially not freeform compound curvature. Rhino will do a much better job of the hull, then if you can find a way to get the finalized hull geometry into SE (not an easy task as SolidEdge has compatibility issues with darn near every non-native format) and do the detailed structure there, you'll find the solid model will be a lot more compatible with ANSYS in the end. Translating the geometry from Rhino to SE is the tricky part.
While a Rhino model can be used to create a mesh for FEA modelling, most FEA preprocessors require closed, solid geometry- meaning your Rhino model has to be made up entirely of closed solids, a very tricky thing to maintain when dealing with a NURBS model. Solid modeller geometry is closed by default and so translates to analysis meshing a lot more smoothly.
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  #6  
Old 09-10-2007, 02:38 AM
cjs cjs is offline
formula18
 
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Thank You for Your answers.
As I told You I am going to model a roro vessel. The structure of the vessel is somewhat complex so the element have to be surface elements and maybe beams..

Surfaces is not realy solids but the solid edge modeler seams to be more appropriate for complex structures?

I have problems trying to save surface models in solid edge in different formates than .par if there is no solids in the model, any recomendations?

thanks!
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  #7  
Old 09-10-2007, 05:50 PM
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marshmat marshmat is offline
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There's a significant difference in the way the computer sees the model in each program. Understanding that difference might help you in your situation.

Rhino works with NURBS surfaces. An object in NURBS is defined by a three-dimensional grid of control points, a few numerical parameters (such as knot vectors) and an evaluation rule (an equation that takes all those sets of numbers, and calculates the position of the surface). When you draw a "solid" in Rhino, what you are really doing is creating the outer faces of that solid, and "joining" or linking them in such a way that, mathematically, the whole set of faces can be treated as if it were the surface of a solid. Mathematically it is not a solid object, it is a collection of surfaces. Rhino's thought process might be described (oversimplified) as: "Take list of co-ordinates of points, knot vector, etc. Plug into evaluation rule. Output the calculated surface, or rather, whatever points or curves on it are required for the operation at hand." (Correct me if this is too oversimplified, Steve.)

SolidEdge is a parametric solid modeller. It sees an object (a single *.par file) in terms of the union or difference of simpler 3D geometric shapes, which are themselves defined in terms of extrusions or lofts of curves/lines in 3D space. Its thought process is more along the lines of: "Take base point. Add initial protrusion. Subtract hole at (x,y,z) compared to user-specified reference. Subtract fillet. Add protrusion relative to another user-specified reference." etc. What it's doing is defining a successive series of 3D entities, each of which depends on previously defined entities- thus if you change one, all will adapt. (Conversely, if you break one, a whole bunch will f%&# up.) But this is how it thinks- every object is some combination of 3D solids with dimensions somehow related to each other, compared to Rhino in which each object's control points exist independently in an absolute/world 3D space.

SE does have some limited surface modelling ability, but not nearly as powerful as Rhino's. On the other hand, Rhino's solid modelling ability is really just a trick of joined-up surfaces. The two could complement each other rather nicely if SE were capable of more reliable import/export; it is among the worst parametric modellers for reading and writing non-native file formats (SE sucks almost as bad at writing IGES as it does at reading it).
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  #8  
Old 09-10-2007, 07:59 PM
MikeJohns MikeJohns is offline
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Is this a student project or a serious endeavor? A word of caution.....

If you want to do global analysis on a vessel you are better off with one of the dedicated packages for this otherwise you are going to get lost in the detail and chew up a lot of computing time for dubious results. Always a major problem with detailed FEA of complex structures is adequately specifying both loads and restraints, a sea going vessel is a complex dynamics problem. All those nice static stress results tend to be so much junk when the vessel is rolling pitching and yawing in its first heavy sea. So be careful ! The other way is to design to the statics and add a FOS for the dynamics. If you're already there you may know this anyway.


I commonly create solids in rhino for FEA and find it very intuitive. To build up complex shapes I make all the sub components slightly oversize and overlap them then use the Solids Union command. This certainly works for me faster than a solids modeler and avoids the issue of "void inclusions" when trying to Union exact surfaces. Solidworks reads these files in Rhino format with no problem for Cosmosworks, or I IGES them to other packages.
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