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christofoo · 09-22-2013, 01:50 PM

OpenFOAM has been mentioned once or twice in this forum before, but it hasn't been given a fair shake. It is a rather general-purpose open-source numerical solver, with capabilities ranging from chemical reactions (think ICE cumbustion simulation) to 3d wind-tunnel CFD.

I think the short story is that it works and it's free; it's just a much steeper learning curve than commercial CFD packages that have GUI's.

Here is a screen-shot. I ran a tutorial which imported a 3d model of this motorbike and ran a (computationally simplistic - probably inaccurate - in this case) wind-tunnel CFD. Then I opened it in the paraviewer and put in a transverse slice and stream-tracer (a virtual smoke-wand).

That's about all I've been able to do so far.

My goal is to be able to write as simple a tutorial as possible describing running a car and mods through an OpenFOAM virtual wind-tunnel CFD.

Given how far away I am from that goal, and how low a priority this is for me, I hesitate to post now - I may not get there for another 3-12 months - but maybe this will stimulate some interest.

Learning curve, prerequisite, initial thoughts:

I guestimate a single study will require in the neighborhood of 2 million floating-point-operations for an automotive scale wind-tunnel, so I'm guessing an Intel I5-3570K clocked at 4.5 GHz, for reference, will take around 4 hours. My laptop with an I5-3210M might be more like 15 hours, as long as the heat doesn't slow it down. However the study I'm basing this on lacked some of the CPU specifics, so consider it a crude guess.
Linux required: those not already running Linux need to setup a new partition (google gparted livecd), and install Linux, preferably a supported Ubuntu version such as 12.04 LTS or 13.04. See the OpenFOAM download page: Download OpenFOAM®. I'm a Debian guy, and it wasn't a big deal to build OpenFOAM and the thirdparty package from the source packages on Wheezy, but it was a disaster on Squeeze (the previous release). Actually, I did find a Windows build using Cygwin, but I noticed it was out-of-date. Why bother? In my opinion the Linux hurdle is small compared to the others.
The learning curve is quite steep, I'm guessing in the 100-hour neighborhood. Consider OpenFOAM a command-line utility with text-file controls. I need to learn how to a) import a 3d model from STL, b) mesh the model, c) setup appropriate pre-processing and processing, d) analyze results using Paraview.
You need a good STL model of your car and a CAD program to create mods in the first place. I'm using Solidworks, which I have access to, but I don't see why Sketchup wouldn't work (or Geomagic Design or FreeCAD). I still haven't found a satisfactory Civic model for free. If you want to buy a model they often run around $75, although one website I know of will net 5 different models for $125, so maybe there's an opportunity to pool for car models at $25 each.
Even after all this, CFD will give the best results if you have a reference case that you can compare in detail to the CFD results. I think in principle that if you increase the resolution and computational detail (and simulation time) to some unreasonable quantity then the CFD results should end up being accurate. But in practice, where simulation time is not an infinite resource, there needs to be a real-world reference case to ensure the CFD isn't overly truncated.

OpenFOAM in literature

I like to read master's theses; I often learn from the background reading as well as the subject matter. This one features a comparison of OpenFOAM to a commercial CFD package - Fluent. OpenFOAM: A tool for predicting automotive relevant flow fields. In summary: OpenFOAM's predictions are realistic, but not as realistic as Fluent in terms of detatchment points, and also OpenFOAM is slower than Fluent. Also, OpenFOAM has a steep learning curve and requires investment of a lot of time to operate it properly.
Some of the Coventry University 2011 research has been posted on Ecomodder previously; I noticed they used OpenFOAM in some of their CFD studies. Aerodynamic Drag Reduction and its Effects on Electric Vehicles

Cheers,
Chris

09-22-2013, 01:50 PM	#1 (permalink)
christofoo Master EcoModder Join Date: Mar 2012 Location: Salt Lake City Posts: 292 00C - '00 Toyota Corolla 90 day: 43.54 mpg (US) Thanks: 147 Thanked 190 Times in 73 Posts	Review of OpenFOAM: free CFD OpenFOAM has been mentioned once or twice in this forum before, but it hasn't been given a fair shake. It is a rather general-purpose open-source numerical solver, with capabilities ranging from chemical reactions (think ICE cumbustion simulation) to 3d wind-tunnel CFD. I think the short story is that it works and it's free; it's just a much steeper learning curve than commercial CFD packages that have GUI's. Here is a screen-shot. I ran a tutorial which imported a 3d model of this motorbike and ran a (computationally simplistic - probably inaccurate - in this case) wind-tunnel CFD. Then I opened it in the paraviewer and put in a transverse slice and stream-tracer (a virtual smoke-wand). That's about all I've been able to do so far. My goal is to be able to write as simple a tutorial as possible describing running a car and mods through an OpenFOAM virtual wind-tunnel CFD. Given how far away I am from that goal, and how low a priority this is for me, I hesitate to post now - I may not get there for another 3-12 months - but maybe this will stimulate some interest. Learning curve, prerequisite, initial thoughts: I guestimate a single study will require in the neighborhood of 2 million floating-point-operations for an automotive scale wind-tunnel, so I'm guessing an Intel I5-3570K clocked at 4.5 GHz, for reference, will take around 4 hours. My laptop with an I5-3210M might be more like 15 hours, as long as the heat doesn't slow it down. However the study I'm basing this on lacked some of the CPU specifics, so consider it a crude guess. Linux required: those not already running Linux need to setup a new partition (google gparted livecd), and install Linux, preferably a supported Ubuntu version such as 12.04 LTS or 13.04. See the OpenFOAM download page: Download OpenFOAM®. I'm a Debian guy, and it wasn't a big deal to build OpenFOAM and the thirdparty package from the source packages on Wheezy, but it was a disaster on Squeeze (the previous release). Actually, I did find a Windows build using Cygwin, but I noticed it was out-of-date. Why bother? In my opinion the Linux hurdle is small compared to the others. The learning curve is quite steep, I'm guessing in the 100-hour neighborhood. Consider OpenFOAM a command-line utility with text-file controls. I need to learn how to a) import a 3d model from STL, b) mesh the model, c) setup appropriate pre-processing and processing, d) analyze results using Paraview. You need a good STL model of your car and a CAD program to create mods in the first place. I'm using Solidworks, which I have access to, but I don't see why Sketchup wouldn't work (or Geomagic Design or FreeCAD). I still haven't found a satisfactory Civic model for free. If you want to buy a model they often run around $75, although one website I know of will net 5 different models for $125, so maybe there's an opportunity to pool for car models at $25 each. Even after all this, CFD will give the best results if you have a reference case that you can compare in detail to the CFD results. I think in principle that if you increase the resolution and computational detail (and simulation time) to some unreasonable quantity then the CFD results should end up being accurate. But in practice, where simulation time is not an infinite resource, there needs to be a real-world reference case to ensure the CFD isn't overly truncated. OpenFOAM in literature I like to read master's theses; I often learn from the background reading as well as the subject matter. This one features a comparison of OpenFOAM to a commercial CFD package - Fluent. OpenFOAM: A tool for predicting automotive relevant flow fields. In summary: OpenFOAM's predictions are realistic, but not as realistic as Fluent in terms of detatchment points, and also OpenFOAM is slower than Fluent. Also, OpenFOAM has a steep learning curve and requires investment of a lot of time to operate it properly. Some of the Coventry University 2011 research has been posted on Ecomodder previously; I noticed they used OpenFOAM in some of their CFD studies. Aerodynamic Drag Reduction and its Effects on Electric Vehicles Cheers, Chris Attached Thumbnails __________________ ECOrolla89 (modding sixth gen Corolla sedan on a budget) ECOfamily Civic and its Aero Hitch Box I owe some posts about coast-down. My sweet blog: http://cannonblast.net/christofoo Last edited by christofoo; 09-22-2013 at 01:59 PM..