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May 30, 2003, 12:32 
Complex terrain geometry as using AutoCAD?

#1 
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Does anyone know how to make solid object for complex terrain or landscape using AutoCAD or any other CAD related software? So, it can be imported to PHOENICS?


June 2, 2003, 00:51 
Re: Complex terrain geometry as using AutoCAD?

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Dai,
In autocad you need to make the structure out of soild enties. They need to be constructed in a very particular way, with the corners set up in a prescibed format. This is in the help files in Autocad. I have successfully created shapes in Autocad r14 and exported them across  now I only have access to Autocad Lite (no 3D). This process is almost as painfull as building complex geometries in VR VIewer. If you are not tied to a particular software package, maybe something like 3d Studio max would be easier to build the complex data set. Hope this helps. Leon 

June 2, 2003, 12:44 
Re: Complex terrain geometry as using AutoCAD?

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Hi,
I might have some hints for you, but you should state your problem more precisely:  In what data format do you have your digital terrain model available? (Surfer grid,...)  How do you intend to generate the grid? With PIL, with your own tools or with a thirdparty software like GeoGrid? regards Volker 

June 2, 2003, 13:53 
Re: Complex terrain geometry as using AutoCAD?

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Hi, Thank you for your response.  In what data format do you have your digital terrain model available? (Surfer grid,...)
> Let's say the number of points are around 400, and has x, y, and z coordiantes. The format can be changed readily.  How do you intend to generate the grid? With PIL, with your own tools or with a thirdparty software like GeoGrid? > I need to make the data set to reperent earth surface in cartesian coordiante as a solid object, and import it to PHOENICS with cartesian coordinate not BFC. Thank you. 

June 3, 2003, 05:11 
Re: Complex terrain geometry as using AutoCAD?

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Hi,
unfortunately, I don't have much experience with the use of cartesian grids for flow around complex shaped objects. I think there is a tool DXF2VR delivered with Phoenics that might help you. You could also write your own data conversion tool that converts your surface representation into appropriate PIL statements that set the cell porosity of all cells "under" the earth's surface to zero. However, I am still not sure what you intend to simulate actually. What kind of terrain can you represent with about 20*20 horizontal grid points? Do you want to simulate wind flow over this terrain? Then you would have a problem with the ground boundary conditions if you use a cartesian grid. regards Volker 

June 5, 2003, 16:05 
Re: Complex terrain geometry as using AutoCAD?

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Thank you for your help. I am not sure why there will be a problem if catertesian grid is used for ground boundary condition in this case? Could you explain me more? Thank you again.


June 6, 2003, 10:02 
Re: Complex terrain geometry as using AutoCAD?

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Hi,
whether or not a cartesian grid might cause inaccuracies depends on what you are actually going to model and what flow features are important for you. "Complex terrain" is too general. regards Volker 

June 11, 2003, 02:06 
Re: Complex terrain geometry as using AutoCAD?

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So how would you construct a BFC grid of a terrain if you have 1. An STL file 2. an X, Y, Z, file
Thanks. 

June 11, 2003, 04:33 
Re: Complex terrain geometry as using AutoCAD?

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Hi,
I have never used STL files but to process such a file one would probabely use thirdparty grid generation software because otherwise it is not easily handled. However, Matlab has some functions that allow scattered data interpolation through Delaunay triangulations. It can actually do 2D cubic interpolation on a TIN and that's what is needed to set up a grid over such a terrain representation. This kind of scattered data interpolation in Matlab is however not yet mature, as far as I know it has problems with large triangulations. What kind of X Y Z file do you mean: A regular grid with fixed grid spacing dx and dy? Or scattered elevation data like sampled height contour lines? regards Volker 

June 13, 2003, 09:05 
Re: Complex terrain geometry as using AutoCAD?

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The conversion between scattered data and regular grid I can manage. My problem is to find an easy way to make a Phoenics BFC grid from such a regular grid. Any suggestions?
Regards 

June 15, 2003, 09:20 
Re: Complex terrain geometry as using AutoCAD?

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Hi,
unfortunately, there is no "easy" way to design such a grid unless you use commercial software like GeoGrid. If you do not want to spend lots of money into a grid generation system, you might check out MegaCads, which is a fullfeatured structured multiblock grid generation system. It is available for free (Unix/Linux only) at: http://www.megacads.dlr.de/home.html Of course you would need to write your own import/export filters. As far as I know, stateoftheart grid generation is almost as complicated as the flow solvers itself. If you really want your own handmade solution for complex terrain grids, I would recommend to use Matlab. For a simple singleblock grid you just have to interpolate your surface representation to the target ground nodes. The setting of the xy positions of those ground nodes is the complicated part because you usually want to concentrate the grid towards the centre of the domain and this grid zooming must be done smoothly. For the vertical grid spacing you use simple sigmaaltitude coordinates with a concentration of the grid lines towards the ground. Unless you use Satellite to view the grid you need to write a GUI that displays the grid and the surface representation and possibly roughness and infrastructure. This is quite a nice piece of programming in Matlab but time consuming. I have actually developed a complex terrain grid generation system in Matlab but it is not public domain. I hope this will help you a bit regards Volker 

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