This Week in CFD

Software Releases

  • Stanford will release v2.0 of their SU2 open source CFD code on 8 January. Two other events are planned in conjunction with that release.
    • A presentation at the AIAA Aerospace Sciences Meeting on 7 January at 4pm.
    • A workshop on 15 January.
  • Siemens PLM Software announced the release of Parasolid V25.0, the latest version of their solid modeling kernel. Technical details of the curve, surface, and optimization improvements can be found at their Introducing V25.0 page.
  • Exa released PowerDELTA 1.7 for mesh preparation. [I had always thought the beauty of Exa’s CFD software was that it didn’t need a mesh.]
  • 3-D modeler Rhino 5 was released including supposedly 3,500 enhancements.
  • Beta CAE released ANSA v13.2.4.
  • CEI released TSV-Pre/Post Ver. 6 for FEA pre- and postprocessing.
SU2 computed streaklines behind a cylinder at Re=100.

SU2 computed streaklines behind a cylinder at Re=100.

CAD Interoperability Today

That’s the title of an excellent summary article by Design World’s Evan Yares about the current state of the art when it comes to exchanging CAD data from application to application. And because CAD data is the starting point for meshing and meshing is the starting point for CFD, it’s important for us to understand what the issues are.

And there are plenty of issues. While interoperability “is getting better all the time,” Yares writes “interoperability problems are par for the course” and will continue to be so “for the foreseable future.” This is another verse of a song I know very well – mesh generation also sucks but is getting better all the time.

You might want to read Yares’ article first: CAD Interoperability Today. [Actually, you should read it. Do so now.]

Be Tolerant of Tolerances

Yares offers a point-counterpoint argument about whether tolerancing (setting a small numerical value below which geometries are considered identical or adjacent) is the key culprit in the lack of CAD interoperability because different systems (e.g. the CAD system and the mesh generator) use different tolerances and/or implement them differently. On the other hand, he cites a CAD tech guru who believes that tolerance issues are the easiest to handle.

I can tell you from personal experience that tolerancing is a huge problem. From the standpoint of meshing, not only do we have to handle tolerances on the CAD level but we also have to tolerance the mesh. Consider a submarine that’s over 300 feet long that includes details like the blunt trailing edge of the propulsor blades. Subs operate at very high Reynolds numbers so when we generate the mesh we have to a) place the outer boundaries “far” away from the hull (increasing the scale of the coordinate system) and b) generate a mesh to resolve viscous boundary layers. The result is a grid with coordinate values and length scales that range over 8 or 9 orders of magnitude – from the smallest boundary layer cell to the outer boundary that’s N hull lengths away.

A Cylinder’s a Cylinder Until It’s a Cone

A subtle source of interoperability problems that’s buried “under the hood” is the issue of how a CAD system represents geometry internally. Yares cites the example of ACIS representing a cylinder as a cone with the implication that other CAD systems might represent a cylinder as something else – a cylinder or a B-Spline.

How does this manifest itself in practice? Here’s an example from our Pointwise software. We start with the ACIS file itself that some application exported and which contains representations specific to ACIS and the originating CAD system. We use CAD readers from CT Core Technologies and the first thing they do is import the ACIS file and store all the geometry in their internal representations. Finally, we have to query the Core Tech kernel, extract the data, and store it in our own kernel which likely uses yet another representation for the geometry. Just like kids sitting in a circle playing the game “telephone”, each hand-off of CAD data from one representation to another is a potential source of error.

Files, Standards, and the People Who Write Them Poorly

Yares touches only briefly on a third error source which is the rather unglamorous upkeep of various CAD importers. For example, Pointwise imports CAD geometry from IGES, STEP, Pro/E, CATIA, NX, SolidWorks, ACIS, Parasolid, STL, VRML – I’m certain I forgot something. Every time the CAD company releases a new version there’s a lag while our partners at CT Core Tech introduce the corresponding reader.

The problem with standard formats like IGES and STEP isn’t necessarily the upkeep. The problem is dealing with systems that write those files with blatant disregard for the standard. I could bore you to death [if I haven’t already] with tales of the absolute crap we’ve seen inside IGES files. What I like to tell people is there’s nothing wrong with IGES files, but there’s a lot wrong with many of the people who write IGES files.

So Now What?

You should go back to the top of this article, click on the link to Yares’ article, and read it. As he writes, if you stick with it to the end you’re a CAD nerd. Which I suppose is better than being a mesh nerd.

Later we can talk about technologies like shrink wrapping and defeaturing and why we have our own geometry kernel instead of licensing one from a 3rd party.

Applications

CFD simulation of flow over a Star Wars X-Wing fighter done with Project Falcon in Autodesk Inventor.

CFD simulation of flow over a Star Wars X-Wing fighter done with Project Falcon in Autodesk Inventor.

  • Autodesk’s Project Falcon is now available within Autodesk Inventor for doing things like CFD on an X-Wing.
  • CIMdata published a report on CAD Selection Considerations: Dealing with Sophisticated Freeform Shapes. (registration required)
  • The Team for Advanced Flow Simulation and Modeling at Rice University is applying CFD to spacecraft parachutes. [It’s getting to the point that I get irked if a magazine article isn’t immediately available online. The link is to a poor quality OCR scan of the original article from Mechanical Engineering magazine. ME Magazine hasn’t posted the article yet so this site is the best link I could find. It’s a good article and I recommend that you read it. Also check out the T*AFSM website.]
  • NASA has advanced the application of CFD to rotorcraft design.
CFD simulation of a V-22 rotor showing "turbulent worms."

CFD simulation of a V-22 rotor showing “turbulent worms.”

Business News

Events

CFD Needs More Pink

My brain has the tendency to find connections where none exist. But some coincidences can’t be ignored. Two CFD-related articles with a common element came to my attention last night, one right after the other.

First, F%^$ Yeah Fluid Dynamics brings us this pink image illustrating how a jet of fluid can bounce off another fluid surface.

Follow the bouncing jet of fluid.

Follow the bouncing jet of fluid.

Then what should appear in my RSS feed but a blog post by Mentor Graphic’s Nazita Saye titled Shrink It and Pink It in which she observes that pink is the new black, at least as far as women’s products are concerned. At least that doesn’t extend to CFD where engineers are engineers and a product’s capabilities win out over color. [I’m really tempted to do a parody of marketing CFD to women.]

Pointwise even has its own pink-related eccentricities. The default color used to draw CAD geometries is pink, a choice that goes back to the late 1980s and the reason for which is lost to the sands of time. But whether the CAD is pink or gray or whatever doesn’t make any difference, right?

A wrench in any other color is just as butch, right?

A wrench in any other color is just as butch, right?

Back in the day our Gridgen software offered the user a palette of precisely 7 colors (one was pink) with which to draw surfaces in the grid and geometry. Ten bonus points go to the first person who can tell me the significance of the number 7 in this regard.

According to color theorists, the color pink represents unconditional love and nurturing. Don’t you think CFD needs more of that? Or I have I been at this business WAY too long?

P.S. I apologize if I’ve already posted some of the links above. The last several weeks have been a blur.
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3 Responses to This Week in CFD

  1. Tim Tautges says:

    Always an entertaining read, John. Is the 7 due to roygbiv?

    – tim

  2. John Chawner says:

    Thanks, Tim. But no, Roy G. Biv is not the reason behind the 7 colors. I’ll give you and everyone else a hint: it’s more related to meshing than coloring.

  3. John Chawner says:

    And the answer is…

    With 7 colors you can apply a different color to each of the 6 faces of a structured hex block with the 7th color used for the database (aka CAD geometry).

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