Wind tunnel? What wind tunnel?
We successfully scanned our LS-swapped 350Z using chalk dust and an old camera so the wizards at Morlind Engineering could meticulously model its aero properties, eventually producing a perfectly accurate, fluid-tight CAD model that was ready for CFD–industry-speak for computational fluid dynamics.
The next step? It’s time to put our virtual car into Morlind’s virtual …
Sure, it has a price tag, but I think it's pretty wild that this sort of service is available to pretty much anyone.
I'm not looking to improve the aero setup on any car, but it's good to know that I don't have to reserve time in a physical wind tunnel if I change my mind.
In reply to Colin Wood :
best part it's free. we do the test and build the parts. the parts are a normal price.
the OEMs print small scale models out of SLA (process) and wind tunnel those, it scales well to production size vehicles with simple math rather than print a whole 1:1 car.
I've always wanted to do CFD testing but the software you can get for it is either free and insanely complex to even run, or very incredibly expensive and basically out of reach of most people who are doing this for hobby.
It's also "free" for those of us who look at what others have successfully done and then copy their modifications as closely as possible. Sometimes "monkey see, monkey do" does work beneficially.
Seriously, this article was very informative and shows what can be done without actually doing anything at all, so to speak. (no wind tunnel)
Hey, I'm in charge of the Bob Seger references.
That testing would cost more than my vehicle is worth
Something that was surprising to me was I recently watched a video about drag coefficients that made it seem like it is not a super exact science. The choice of wind tunnels can apparently have a fairly significant effect on the results from what that video said. We aren't talking totally different, but enough that differences like in this article of .30 vs .32 kind of just happen. Then again with what they say in the article maybe part of that is just that different vehicles of the same model can vary a fair bit based on tiny differences in alignment of panels and things.
fidelity101 said:the OEMs print small scale models out of SLA (process) and wind tunnel those, it scales well to production size vehicles with simple math rather than print a whole 1:1 car.
Many years ago I worked for a company that designs and builds wind tunnels. Testing is mainly done on scale models simply due to economics, since building a wind tunnel large enough to hold a full size car (or airplane or whatever) would cost a lot more money. NASA has a couple tunnels at their Ames facility in California that are large enough for full size testing; those are the only ones I can think of offhand, but there may be others.
In reply to TiviamTonal :
Airflow is hard to pin down exactly. A "fancy" tunnel will invest more in conditioning the air in the test section -- insuring there's non-turbulent, temperature-stabilized flow parallel with the test section. A simpler (and less expensive to build and run) tunnel may not. Sometimes you don't need an exact value, you just need to be able to compare against a baseline with some repeatability.
In reply to stuart in mn :
Building wind tunnels...that would have been fun. It's not just the size that drives cost, though...it's power! I spent some time at NASA Ames, and more at the Air Force's complex in the southeast -- we did our high-speed testing primarily at night, because the compressors moving the air would pull enough power to run a small city. Not so bad for cars, since the speeds are relatively low, so moving all that air doesn't brown out the surrounding neighborhoods.
Anyone use airshaper? https://airshaper.com/
Superfastmatt on youtube uses it alot and runs are significantly cheaper than what is quoted in the article.
Warlock said:In reply to stuart in mn :
Building wind tunnels...that would have been fun. It's not just the size that drives cost, though...it's power! I spent some time at NASA Ames, and more at the Air Force's complex in the southeast (Marietta?) -- we did our high-speed testing primarily at night, because the compressors moving the air would pull enough power to run a small city. Not so bad for cars, since the speeds are relatively low, so moving all that air doesn't brown out the surrounding neighborhoods.
The tunnel I test at (one of them anyway) does have a deal with the local energy supplier to limit testing during peak hours in the summer. We are testing cars at nearly 200mph, at full scale.
Speed is the biggest scaling issue, requiring power at a speed cubed rate (double the speed --> 8* the power). The slow tunnel (130mph) uses 2200HP worth of fans. The fast tunnel uses 5000HP worth of fans. That just the air side, rolling wheels and system cooling are also significant parts of the cost.
The comment about comparative versus absolute data holds for all testing. All wind tunnels are measuring sticks with their own scales that may be very close to each other but still have their eccentricities. Comparing between wind tunnels is a challenge at best.
One of the major challenges to scale model testing is detail. If you have a 1mm (0.040 inch) detail in a full size car that does scale down to 1:10 scale (0.1mm or 0.004 inch) and can give the same aerodynamic result if dealt with properly, not just by scaling. This is function of the Reynolds number, which is a convoluted way to reference the size and speed of the part being tested. The small scale can be adjusted for by using something thicker than air and adjusting speed to get the right Reynolds number. Water tunnels are a good example of this for small scale testing of things that normally work in the air at full scale. The higher density of the water helps compensate for the small scale.
The smallest motorsports models I have worked with were 33%. Since that time more tunnels have been built to handle full scale testing and that provides an easier job to make parts that can be tested AND RACED without the complication of having to make parts in two different processes.
The quoted prices from the article and from Airshaper are functions of time/accessability/involvment. If I recall correctly, the GRM case required the CFD model to be built from scans or parts created from scratch in CAD. That is not trivial (read costly). Airshaper allows you to upload models that are already created (taking the creation time out of the equation). Time has been doing what it does for the CFD technology as far as making it cheaper with every passing day. Add in the complexity that you can throw at a model and you wind up with more complex models that can run faster and still cost the same amount of money. Some of the complications that can make a model 'expensive' are: rotating wheels/tires, cooling flow simulation, thermal flow if you are dealing with high energy effects or high speeds, and transonic effects if you are really going fast.
Yes, a huge part of the expense was modeling the car--if we'd already had a CAD model it would have been way cheaper. I discussed this in the article.
And these runs were also with spinning wheels and cooling system/engine bay airflow, which is again quite expensive to model.
I have this AutoCAD 3D model car I drew years ago. Just what kind of model do these virtual wind tunnels want? Mine is hollow with the windows and body panels having a true thickness. I can export in .DXF or .DWG file format. Do I have to recreate this model into a one piece 3D solid or a bunch of dimensionless surfaces?
stafford1500 said:The comment about comparative versus absolute data holds for all testing. All wind tunnels are measuring sticks with their own scales that may be very close to each other but still have their eccentricities. Comparing between wind tunnels is a challenge at best.
There's a recent article in Racecar Engineering magazine about the Mustang GT3. Multimatic developed its aero at WindShear's tunnel, but the FIA homologated it at Sauber's which has a lower ceiling and came up with different data. It ended up requiring a tweaked aero package for FIA sanctioned races. That annoyed Larry Holt.
Warlock said:In reply to stuart in mn :
Building wind tunnels...that would have been fun. It's not just the size that drives cost, though...it's power! I spent some time at NASA Ames, and more at the Air Force's complex in the southeast -- we did our high-speed testing primarily at night, because the compressors moving the air would pull enough power to run a small city. Not so bad for cars, since the speeds are relatively low, so moving all that air doesn't brown out the surrounding neighborhoods.
I was involved in a couple projects for NASA Ames, I remember they had to let the local electric utility know when they were going to run the fans on the big tunnels. If I recall correctly their largest tunnel uses 250,000hp worth of motors to run the fans.
[not to get too far off topic but this is one of my favorite wind tunnel stories: my company had a test facility here in Minnesota with a number of wind tunnels of various sizes and speeds, and they often ran tests during second shift. The test facility was in kind of a remote area, and one night a couple kids drove in and parked for a session of necking. What they didn't know is they were parked right downstream of the supersonic tunnel...they got their clothes back on and tore of there pretty quickly after our technicians fired up the tunnel.]
In reply to VolvoHeretic :
Generally, the model does need to be complete solid or a collection of solids that form a water/airtight volume. The outer skin/etc being the surface of the volume. Open top cars add more complexity since you are basically modeling a really thick wall/floor panel.
The file type usually needed is iges or stl, but there are some flavors of software that can read in most any 3d model.
j_tso said:stafford1500 said:The comment about comparative versus absolute data holds for all testing. All wind tunnels are measuring sticks with their own scales that may be very close to each other but still have their eccentricities. Comparing between wind tunnels is a challenge at best.
There's a recent article in Racecar Engineering magazine about the Mustang GT3. Multimatic developed its aero at WindShear's tunnel, but the FIA homologated it at Sauber's which has a lower ceiling and came up with different data. It ended up requiring a tweaked aero package for FIA sanctioned races. That annoyed Larry Holt.
WindShear is getting approval for homologation. I just recently had a discussion about this at work...
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