These are some of the frequently asked questions about cheetahSTC and its features. If you have a question that isn't answered here, please write to us. Here are the categories:
CheetahSTC general questions
Is cheetahSTC an automatic hex meshing program?
| It's a highly automated program, not an automatic program. You direct the flow of the Cell3 and the program automates all of the repetitive tasks. You still need to design the mesh the same way you design a part in CAD.
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Is full automatic hex mesh generation possible?
| You could say that the chances of this are as high as an automatic CAD model generation program, but yes not only is it possible it's soon to become a reality when the CAD modeling process becomes the Cell3 generation process.
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Can I build a Finite Element Mesh with cheetahSTC?
| Yes you can build a hex mesh for any part model or geometry.
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Does cheetahSTC create tetrahedral and wedge elements?
| No, it can only create a full 100% hex mesh.
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What about really complex cases where it's nearly impossible to create a hex mesh?
| There is no such thing as an impossible mesh solution for cheetahSTC. Some case as very difficult and these are classified as Complex cases. If you get completely stuck then contact us and we can provide generic complex Cell3 solution. If you use the“From Cell3” function for the tough cases and the “Extrude” for the fast regions you can generally figure out any tough case.
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What sort of hardware do I need, is there any special requirements?
| Actually surprisingly no, Rhino3D and cheetahSTC will run on basically any windows machine with XP and above. However if you wish to build large models and view them at a reasonable level then we would recommend an OpenGL graphics card and a minimum 2GB of system RAM.
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Will cheetahSTC be available for other CAD packages?
| Yes and well no. Future versions of cheetahSTC will be in a standalone program as well as other CAD programs as the need and number of requests arise.
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Is cheetahSTC parametric like solid modeling CAD?
| Good question, yes it is iso-parametric. Even though Rhino3D is not a solid modeling program the Cell3 is completely solid modeling based. It is actually the only true solid modeling definition since every location inside the Cell3 has a unique address. A solids modeling CAD program actually doesn't create a solid; instead it creates a BREP (boundary representation) of trimmed NURBS. These BREPS form a closed surface however they have gaps, holes and multiple patches that are an extreme pain to mesh since you need to do a CAD cleanup step first.
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What about CAD cleanup, is this easy with cheetahSTC and Rhino3D?
| Yes it's so easy because you just don't do it at all. This was a hidden workload step that we wanted to reduce at all costs so we just eliminated the need for it. The Cell3 are associated and projected to the CAD boundary surfaces, even if the CAD model has holes in it, doesn't matter.
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What is a Cell2, Cell1 and Cell0?
| These are the Cell3 faces, edges and vertices. The number represents the dimension.
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How is it that I can't see the actual mesh, where is it?
| You can see the mesh with the‘preview mesh' function. The mesh is there in the definition of the Cell3. In a typical meshing procedure the elements are considered atomic individual elements that are independent of each other and hence can individually distort and cause problems. This means you need to see each and every individual element and run checks on it to manually fix it. This is not the case in the Cell3, since the elements are children of the Cell3 they can never be more distorted than the parent and hence there is both no need to adjust them or even look at them. What is seen on the surface of the Cell3 is what the mesh is like internally. Individual element correction may have been an acceptable practice 10 years ago when a large mesh was anything over 1 million elements, however cheetahSTC can easily generate and manipulate billions of elements. It is inconceivable to even contemplate manually fixing such large meshes on an element by element basis.
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How can I see the mesh with the demo version if I can't export the mesh, and how do I know what the quality is like?
| You can view the mesh with the "Preview Mesh" function found in the Display menu. The quality of the mesh output is dependent on your inputs and skill, cheetahSTC assists with the formation process and the smoothing functions allow you to improve the mesh however the final mesh quality is under your control. We shall be bringing out a mesh quality metric soon.
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Can I delete the internal Cell2 faces?
| Yes you can. The Cell2 are actually icons of the surface. If you delete them you can retrieve them again. If you use‘extrude' a lot to build your mesh it's a good idea to delete the internal Cell2 so that the model doesn't slow the graphics. If you delete the Cell3 icon the member Cell2 components are also deleted.
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I pressed retrieve Cell0 but it says they are already there but I can't see them.
| Yes they are there, probably on another layer that is hidden.
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My Cell3 keep distorting around the leading and trailing edges of an aerofoil.
| This is a very common problem. There are 3 ways to fix this. The first is to adjust the weights so that you maintain the densification, the second is to use the stream surfaces and the third is to modify the Cell3 configuration. This link shows an ideal Cell3 solution. Copy and keep this as a template to apply to all aerofoil trailing edges.
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What types of files does cheetahSTC import?
| cheetahSTC will read any file format that Rhino3D can import. The most common and robust is STEP then IGES. The other formats are .vda, .dwg, .dxf, .dgn, .sldprt, .sldasm, .fbx, .3ds, .lwo, .stl, .obj, .raw, .asc, .csv, .xyz, .raw, .gf, .gft, .gdf, .m, .pts, .ply, .slc, .zpr, .x, .skp, .pat, .vml, .wrl There are also third party translators for Catia, UG, Parasolid, HPGL, Inventor, NC, ProE, Solidedge and Solidworks.
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Are there any Solidworks plugin plans?
| Depending on demand we may implement a future Solidworks plugin.
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And what formats does cheetahSTC export?
| CheetahSTC saves the Cell3 inside Rhino3D, so it only exports the actual mesh. You need cheetahSTC to read a Cell3 file. The export file formats are: Fluent, Gambit, Gmsh, Nastran, OpenFOAM 1.5+, Plot3D, StarCD V3, and VTK. If you can't find your format here, just ask and we shall add it.
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I am having problems trying to get a smooth result.
| Try adjusting the Factors and Weights. Then if that doesn't work use the stream projection internal surfaces. If that doesn't work then you will need to fix the Cell3 connectivity.
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What is densification?
| It's a process unique to cheetahSTC. In an element based mesh a smoother will typically even out all your meshes. So you have to get it sort of smooth then add extra boundary layer elements at the final step. Then if you need to smooth again you need to repeat a lot of steps. The smoother in cheetahSTC allows you to increase the number of elements in a region to both create boundary layers and assist in creating a smoother mesh.Densification is also used to reduce the total element count for large models like aircraft and still have a fine mesh on the boundary layers.
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How do I deal with thin tapers and knife edges, won't there be a wedge elements in that zone?
| This is a bit tricky sometimes. In terms of CFD the amount of energy and momentum in the fluid is directly related to the ratio of captured volume to surface area ratio, in regions where the space it disappearing like under a car tire for instance, you can ignore the sharp end of the wedge and finish the mesh off with a step.In a Stressed part, such as a knife edge, you may need to know what is happening right down to the edge of the wedge. In this case you will need to mesh right to that edge with fine elements. However when you get right to the sharp part remember that nothing is entirely sharp in nature, not even a surgeons scalpel in the microscopic sense. If you still want to analysis the sharp edge you need to decide on a finite step edge thickness and leave at least 2 orthogonal elements across that edge.
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What about defeaturing, what functions does cheetahSTC have to handle this?
| "defeaturing" why do you need to do that for? The whole purpose of doing a simulation is to simulate the actual geometry, why add another level of approximation. cheetahSTC has a different methodology to other meshing and simulation systems, we treat difficult regions with Complex Cell3 and fine details can be added in the later stages of the Cell3 creation process. If you have a small feature that doesn't play an important role then just ignore it, there is no need to increase your CAD cleanup workload.
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Why is the Cell3 file so compact, how can you fit 1 billion elements into a file that's only 60MB's?
| The answer to this question is a rather complex one. It's a combination of mathematical data compression and the STC. Since every element in the Cell3 has a valid STC each and every element is related to each other in a mathematical definition. The mesh is not stored as individual and randomly related elements but as a global mathematical function. This is continuum mechanics.
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Can I convert my existing meshes over to Cell3?
| Indeed you can. If demand is high enough we shall include a function to automate this process, but remember, no tets allowed. In the mean time if you saved your block edges then just import these as either STEP or IGES and run‘Form Cell3'.
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What is a hex dominant mesh and how is this different to cheetahSTC meshes?
| Hex dominant is a way of trying to pretend that one has a Hex mesh. The difficult part to meshing is fitting the elements to the boundary; Hex dominant meshes place the rubbish elements right where they absolutely shouldn't be placed. This is where all the action occurs in a CFD simulation, in the boundary layer. cheetahSTC only creates hex elements with quality orthogonal elements in the boundary layers.
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The mesh becomes distorted around aerofoils how can I fix this?
| This is a rather common problem with all smoothers around aerofoils. There are 3 ways to fix this, the first is to split it into 2 halves, the top and bottom section. Smooth them independently. The second is to use a stream surface. Select the set of Cell3 that are moving the most, retrieve the Cell2, then smooth these only with the smoother with this setting "DecoupleCell3=Yes", or use the Equalizer on the Cell2, then after these are where you want them create a new layer (in the layer dialog) place the smoothed Cell2 in there and lock that layer. The third way to fix this problem is use a better Cell3 solution. Go to the gallery and look for Aerofoils and wings.
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The mesh around a 3 sided intersection doesn't look right, how can I fix this?
| Unless your surrounding Cell3 are not completely distorted then here are a few things you can try. The factors for all the Cell3 coming into the 3 junction, (we call this a Valence 3), should all be equal. If you have a long or high aspect ratio Cell3 connected then add a split to make them all equal (factors). The last resort is a manual override, retrieve the Cell0 and use the Move Cell0 function to set the position.Another way is to use the Equalize function and set the relaxation to 0.1. Run it for about 12 iterations.
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How can I improve the mesh quality?
| Try modifying the factors, this can have a rather significant effect and its very quick and easy to do. If you have a region where the mesh is opening up, use the weights to bring it in. If you have a region that is crunching into the boundary surface then the aspect ratio is too high, add a split to the Cell3. There is a lot you can do with the available tools but if your base Cell3 is not well designed you will need to fix that by redoing those sections.
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The smoother sometimes takes a very long time, how can I speed it up?
| The smoother is actually extremely fast, its the projection step that takes all the time. You can speed this up by setting ProjectFreq to a number equal to one half the number of iterations, and by going to the Rhino Options dialog and opening the Menu "Mesh", select Jagged&faster. Another way to speed up the Smoothing process is to smooth as you build. This is not so important for small models but it becomes a necessity in larger models where the progressive steps you make are dependent on the smoothed result. Smooth sections then place the Cell3 into separate layers and lock them.
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I pressed smooth and nothing moves.
| Two things are happening here, either there is a locked layer or the Cell3 smooth solution has reached equilibrium. Try changing the factors and weights to improve the quality.
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Why do I need to update my license if I bought a perpetual license?
| Glad you asked that question. cheetahSTC is protected via a USB hardware encryption key. If you were to lose this key or have it stolen then you would need to purchase a new license, which hardly seems fair. So we update your current key every 3 months via the request license and install update functions.
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What happens if I lose my key or if it's stolen?
| We will send you a replacement key for a small fee and issue you with a new license. Go to here and fill in the form to request a new key. The lost key will be disabled.
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Is the license able to run on a network?
| Our licenses are encoded onto a convenient USB key. You can chooseto have one license per key, or multiple licenses on the same key. Onelicense per key gives you the freedom to work on various computers bytaking the key with you, whilst a multi-licensed key is good fornetwork access to the available licenses. We allow multipleinstallations of cheetahSTC, but you will only be able to run up to asmany concurrent sessions as you have licenses.
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Is cheetahSTC able to run on 64bit?
| Yes.cheetahSTC is a plugin to Rhino3D and the current version 4.0 is 32bit. The next release of Rhino3D 5.0 is 64bit, therefore cheetahSTC will also be 64bit native.In the mean time if you are having issues with massive models sizes then we can recommend this solution. Install Vista 64 with at least 8GB of Ram. This will enable Rhino3D and cheetahSTC full access to 3.2 GB of Ram. It is solid and will enable you to create models with 12 thousand Cell3 and 1 Billion elements.
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What graphics card do I need, will I need a Quadro of FireGL card?
| We recomend using the newer game cards that have OpenGL support. We have tested the Nvidia GTX275 with 896MB ram and it works very well. A Quadro card is a nice option if you want to spend that much on hardware. But its not nessesary.
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Does Cheetah support the Wacom tablets like the 21UX Cintiq
| Yes it does and it works very well indeed.
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