How to increase downforce using CFD | Against the wind: Part 1

Tom
By Tom Suddard
May 21, 2022 | Aerodynamics, Morlind Engineering | Posted in Features | From the May 2022 issue | Never miss an article

Image Courtesy Morlind Engineering

A phrase commonly tossed around at the track: “They’ve got tons of aero.” And while that may be useful shorthand, it’s a bit of a misnomer as every car has an equal number of aerodynamics: one. 

As in, every car affects and is affected by air as it travels. 

The laws of physics apply to every car equally, but when racers say “aero,” they’re talking about using the study of aerodynamic principles to make a car faster around the track. 

The typical result of that aerodynamic study is bolting on additional parts, with two improvements being the most common: a rear wing and a front splitter. 

Why Do Race Cars Need Downforce?

Before we start making aero parts for our V8-powered Nissan 350Z, we need to define the problem. And our problem is a common one at the track: Cars are just wings with wheels. 

Think we’re crazy? Picture an airplane wing. It’s flat on the bottom and curved on the top, with a leading and a trailing edge–just like a car. Air is split at the leading edge, travels across the wing, then joins back up with its other air molecule buddies at the trailing edge. Air moving over the curved top has to move farther, and therefore faster, than air taking the direct route across the flat bottom, generating lift.

If you’re an aerodynamic engineer, we’re sorry. We know that’s an oversimplification. And we know that modern airfoils aren’t actually as simple as what we’ve described. But the basic principle of a wing is correct, even in our simple example. 

So yes, cars are inherently wing-shaped, and if you drove one fast enough, it would take off just like your last Delta flight. Our 350Z, for example, would literally fly into the air at 600 mph. 

At the end of the day, most street cars resemble wings. What do wings do? Create lift. To improve our lap times, we need to mitigate that lift–and hopefully increase downforce.

What’s wrong with lift? Well, how well do you think an airplane’s tires work at 30,000 feet? Lift removes weight from the car’s contact patch, removing available traction in the process. This is bad, especially if you’re on track. 

Of course, OEMs know this, and modern cars are all put through wind tunnels. So why do they build cars with lift? Because they’re not worried about lift, they’re worried about drag. Drag is the amount of force it takes to push a car through the air, and it’s a primary driver of fuel economy in modern cars. 

Nissan likely cared way more about reducing drag than it did about increasing downforce when it designed our 350Z. More lift was just an unfortunate side effect of creating a streamlined shape.

Lift vs. Drag and Aerodynamic Efficiency

Now that we’ve introduced lift and drag, we should talk about how intimately related they are. Lift creates drag, and you’ll see many aerodynamic devices marketed with their lift-to-drag ratio in big, bold numbers. 

This ratio is a measure of aerodynamic efficiency, and more efficient aerodynamic devices essentially do the same aerodynamic work while wasting less horsepower to overcome drag. A modern Boeing 737 has a very high lift-to-drag ratio, while a paper airplane with flat wings has a very low lift-to-drag ratio.  

Oh, and lift can go the opposite direction, too. Fly an airplane upside down, and its wings are actually making downforce. That’s all a race car wing is, which is why you’ll see them marketed with lift-to-drag ratios, too. 

What About Downforce?

We’re 550 words into this story about downforce, and all we’ve really talked about is lift. Why? Because the same tools that counteract lift can be made more aggressive until they make net downforce on the car. 

Downforce is awesome for race cars: It puts more force on each tire, giving more traction, without adding much mass to the car. A car with 1000 pounds of downforce has the traction of a car that’s 1000 pounds heavier, but it only has to use that traction to accelerate the additional mass of the aerodynamic devices added. Downforce is the closest thing to free lunch in motorsports, especially if it’s accomplished through aerodynamic elements with a high lift-to-drag ratio. 

What Can We Do About All This?

Pretty complicated problem, right? We’ve established the basics: Cars are just big wings that make lift and racers add aerodynamic devices like wings, splitters, spoilers, dive planes, wheel wickers, diffusers, flat bottoms and more to counteract lift and make downforce.

But which aerodynamic devices should we pick, and how exactly will each affect our car? Keep in mind that every single aerodynamic change affects every other part of the car, too, a fact that’s well illustrated by the design of those ugly headlights on the front of every Nissan Leaf. Those headlights meaningfully change the airflow around the Leaf’s mirrors, reducing drag and wind noise in the derpy electric car. 

Photograph Courtesy Nissan

We need to figure out how to match that level of aerodynamic sophistication in our home garage and plan a comprehensive package of improvements to make our 350Z faster. 

Sometimes this is easy–a rule book might restrict what can be done to the point where there’s only one real option for aerodynamic improvement–or sometimes you can look at what other people racing your chassis are running and crib from their notes. 

We didn’t see either of these options as particularly useful. NASA’s TT2 rules barely limit aerodynamic improvements, and we didn’t find any 350Zs with well-developed and -documented aero packages in the wild.

Plus, we figured this was an opportunity to learn about a fascinating new world–the world of computational fluid dynamics, better known by its abbreviation, CFD. CFD is essentially a virtual wind tunnel where a computer simulates each air molecule as it interacts with your car, your aerodynamic devices, the road and the other air molecules. 

Teams that have taken advantage of modern CFD include every NASCAR effort, every F1 team, every IndyCar team and even Grassroots Motorsports. Yeah, you read that right. Here’s how we put our 350Z into the virtual wind tunnel, what we learned along the way, and how you can use this tool to make your own car faster.

Wind Tunnels vs. CFD

Stop screaming. We hear you. You’re saying, “Go to a wind tunnel! That’s what all the other race cars do when they have questions about aerodynamics!”

Well, not quite. Sure, that was the answer a decade ago. And wind tunnels still have a place in modern motorsports. But more and more, the professional teams are actually using CFD. Why? Multiple reasons. First, wind tunnels are expensive. It takes a rolling road wind tunnel to best the data that CFD computers can spit out, and parking your car on that supersized fan/treadmill combination costs thousands of dollars per hour. 

The factories have access to all sorts of aero research, including both scale and full-size wind tunnels. An example of that aero knowledge: how the Nissan Leaf’s headlights direct air around the mirrors. Photography Credits: Courtesy GM (model), Courtesy Porsche (GT3)

Second, wind tunnels require the car’s physical presence. And while that’s not that big of a deal, they also require every aerodynamic configuration to be represented. If you want to test three wing designs and three splitter designs, you’d better buy and bring a functional copy of each–as well as a checkbook to pay for the time you spend in the tunnel changing parts on the car. 

CFD, in comparison, doesn’t require taking a car anywhere. It doesn’t require any parts. In fact, testing a different wing is as simple as drawing it onto the 3D model, meaning it allows teams to try more things in more conditions. CFD also allows far better visualization and data capture than a wind tunnel, as instrumenting and measuring each test isn’t even an item on the to-do list. The computer handles all of that automatically. 

Finding a Virtual Wind Tunnel

Okay, so CFD is cool, but how do mere mortals like us access it without picking the lock on the Red Bull Racing server room? That question led us to Morlind Engineering, a small firm based just feet from the Panoz race shop next door to Road Atlanta. It specializes in CFD work for aerodynamic companies and for racers just like us, and owner Rob Lindsey agreed to open the hood on each step of the process as the folks there analyzed our 350Z’s aerodynamic performance. 

Of course, it does take money to hire Morlind. A CFD analysis package starts at about $4500, though prices vary based on how much you’d like to test and whether you already have a CAD model of your car that can be placed in the tunnel. We think it’s a fair price to pay for what we learned, but let’s not get ahead of ourselves. It’s time for step one: scanning. 

How to 3D Scan Your Race Car at Home

The first step of the CFD process? Create a CAD model of the car. That starts with a three-dimensional scan of the car. Morlind offers two ways to do this: by bringing the car to the shop, or by following its step-by-step instructions at home. Since we wanted to keep working on our 350Z in our own garage while the CFD work was completed, we chose to scan our car at home. 

What’s it take to scan a car? Morlind has perfected a method based on photogrammetry, which uses hundreds of photos and a fancy computer algorithm to assemble a scan of the car without any specialized equipment. All you need is a digital camera with a fixed-focal-length lens; we used a 10-year-old DSLR. 

Before shooting photos, though, we needed to chalk the car. Computers struggle with reflective surfaces, and sprayable chalk dulls them while easily washing off. It also creates a unique pattern all over the car. By spraying different colors on different sides of our 350Z, we ensured that every square inch would be unique, and the computer wouldn’t have any issues combining the photos into a model of the car. 

We don’t have the budget for wind tunnel time, but we can still perform aero research via a CFD analysis. The first steps: Cover our LS-swapped Nissan 350Z in chalk and take a lot of photos. Photography Credits: Chris Tropea

Chalk applied, it was time for photos. Morlind’s advice: Shoot 500 to 1000 photos, each perpendicular to the panel being photographed and with even lighting. We circled the car on a ladder, stool and creeper, gathering hundreds of photos of every panel. Then we uploaded our photos, sat back and waited.

How to Create a 3D CAD Model From Your 3D Scan

With our part done, it was time for Morlind owner Rob Lindsey to get to work. It only took a few hours for our car’s 3D scan to appear on his screen, but, as he explained, that was just the tip of the iceberg. 

Why can’t the 3D scan go directly to the virtual wind tunnel? Because it’s just a computer-generated mesh in the shape of our 350Z, and it isn’t a surface the computer sees as airtight. To run any CFD analysis, Rob had to turn this scan into a CAD model of the car, complete with spinning wheels and an engine bay. 

Think of it this way: The 3D scan generated a file similar to what you’d see if you draped fabric over your car and took its picture. Now it was Rob’s job to build the underlying car represented by that fabric. 

Along the way, Rob would also model the bumper openings, engine bay, drivetrain, underside, wheels and more, with the goal of simulating real life as closely as possible. Morlind’s CFD model would simulate airflow over each of these things, and that would take time. 

Morlind Engineering used our photos to create a full CAD model of our 350Z. Now we can work on improving the car’s aero profile. Image Courtesy Morlind Engineering

There’s no shortcut for this part of the process, which also represents most of the cost for this CFD work. Morlind usually bills 60 to 100 hours to model a race car. Rob spent approximately 80 hours modeling our 350Z from scratch in Siemens NX, using the 3D scan to guide him as he created every individual plane of the body by hand, down to the rolled fenders we’d done just a few weeks earlier. 

The result was amazing: a perfectly accurate 3D model of our 350Z in the computer, complete with the LS swap, radiator, suspension control arms and more. Now it was time for the real magic to begin. We’ll put our car in Morlind’s virtual wind tunnel in the next installment of this series.

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Comments
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Colin Wood
Colin Wood Associate Editor
3/24/22 1:56 p.m.

It's pretty wild how much computing power is required to generate the results.

Sure, I could probably plug some numbers into my computer, but I'd probably be waiting weeks before getting any results.

Still really cool, though.

tuna55
tuna55 MegaDork
3/24/22 2:06 p.m.

That's a neat explanation of how the car gets modeled. Are there DIY solutions? I've done CFD multiple times for internal flow, and each time been quite accurate. I've never had a reason to do external flow, but I suspect it is not much different. The model is the hard part.

VolvoHeretic
VolvoHeretic GRM+ Memberand Reader
3/26/22 12:48 a.m.

Nice article. Too bad I don't have $4000+. 

Tom Suddard
Tom Suddard GRM+ Memberand Director of Marketing & Digital Assets
3/26/22 7:12 a.m.

In reply to tuna55 :

Thanks, I'm glad you liked it. Sadly, there aren't any solutions to modeling that don't require time. You could do every bit of modeling yourself in free CAD software (here's my primer on CAD for first-timers), but the expertise and free time to do it is something I just didn't have. 

tuna55
tuna55 MegaDork
3/27/22 8:12 a.m.
Tom Suddard said:

In reply to tuna55 :

Thanks, I'm glad you liked it. Sadly, there aren't any solutions to modeling that don't require time. You could do every bit of modeling yourself in free CAD software (here's my primer on CAD for first-timers), but the expertise and free time to do it is something I just didn't have. 

I have tons of cad experience, but I think the main concern is the accuracy, especially on the little bits like panel gap and trim and such. Without scanning, just starting from scratch, I would have a lot of doubts unless you could confirm with a coast down or something. 

Tom Suddard
Tom Suddard GRM+ Memberand Director of Marketing & Digital Assets
4/6/22 2:14 p.m.

In reply to tuna55 :

Yeah, there's the rub. I've been working for more than a year on a story about how to quickly/accurately/cheaply scan parts at home, but it's proving incredibly difficult to get the resolution I'd hoped for. If/when I figure it out, though, I'll make sure to write about it. 

tuna55
tuna55 MegaDork
4/6/22 2:34 p.m.
Tom Suddard said:

In reply to tuna55 :

Yeah, there's the rub. I've been working for more than a year on a story about how to quickly/accurately/cheaply scan parts at home, but it's proving incredibly difficult to get the resolution I'd hoped for. If/when I figure it out, though, I'll make sure to write about it. 

I'm not at the tip top state of the art in that area, but I believe it (scanning accurately into 3D CAD) to be beyond the DIY, and likely more expensive than just renting a day at a friendly wind tunnel.

 

At a previous employer, we paid a scanning company to scan the job site into CAD. It was much bigger than a car, but much less detailed. I think the resolution was several inches, whereas for an accurate CFD of a car you would need two powers of ten better than that. It cost tens of thousands of dollars for the scan. One day on site and weeks of post processing. This was a while ago, but it's pretty far off of what I imagine the DIY racer needs.

stafford1500
stafford1500 GRM+ Memberand Dork
4/6/22 4:17 p.m.

It is not completely crazy to use 3d models made by others and found online. You will very likely need to modify/fix them for use as a CFD model, but it get you so much closer for not much effort. More detail can be generated using the photogrametry method Tom discussed, using free-ware and some compute time. All scanners produce is a set of surface points. The trick as he noted is a seasoned CAD driver to convert the "POINT CLOUD" into a set of useful surfaces. All CFD winds up using the surfaces to generate the bounding conditions. Gaps typical of production cars add lots of computational requirement, so bridge those gaps where you can, especially for a first draft.

On the tunnel versus CFD comparison: CFD gives lots of on AND off surface data and can be a very good tool for direction. If you need absolute pressures/loads/moments you will need to spend a good bit of time setting your model up. The compute time and the follow-on post-process willl likely be days for all this data for a single configuration in most DIY applications. Tunnel testing on the other hand only provide gross loads/moments of the entire assembly. Local pressures can be collected, but not to the resolution of the CFD. Test time for these results is minutes, vs days. The prep time is not insignificant and preparing a car for a tunnel test should be considered the same as preparing it for a track test - aero loads can be destructive quickly... Tunnel testing can be had for anywhere from ~$250 per hour up to the $1 per second rate. CFD testing is generally costed out as $ per CPU per computation time ($/CPU hour) and is not a small number in typical usage. It can be done on stout home machines, just budget days of computation time for even simple models.

As with any testing, a single test case does not provide much information. You MUST test multiple configurations to generate direction for good/bad, better/worse, etc. The definition of those is also not an explicit formula, knowledge of the end use AND/OR the direction you want to go is a requirement.

The real challenge of DIY CFD is the shear amount of control inputs you CAN use/tune to make the model operate. Getting a model to run is not the hard part. Getting a model to run and produce useful results is the trick.

I spend time doing both CFD and tunnel testing so I come at this from the point of view that CFD provides great directional and relational information, but tunnel testing can generate results much faster per part change.

Warlock
Warlock New Reader
4/20/22 2:50 p.m.
stafford1500 said:

...CFD provides great directional and relational information, but tunnel testing can generate results much faster per part change.

And that's the basis of a long-running gag dating back to when CFD was in its infancy:  you spent weeks building your model, an entire night running the code, and you had a handful of runs around a half-dozen airspeeds for one attitude and one configuration.  Meanwhile, the primitive wind tunnel guys next door finished an entire test and were building up the next one. :)

For those lamenting the cost and availability of CFD and wind tunnel time, there's still lots of utility in really old-school free-air testing.  An afternoon on track or an old runway, a couple of video cameras, and a well-laid pattern of cloth tufts or oil droplets can get you inside your working envelope of aero shapes and positions, and even help generate some calculated results to check against if you have the opportunity to build a computer model later...and if not, at least you have proof you're in the right ballpark.

stonebreaker
stonebreaker New Reader
4/21/22 2:34 p.m.
Warlock said:
stafford1500 said:

...CFD provides great directional and relational information, but tunnel testing can generate results much faster per part change.

And that's the basis of a long-running gag dating back to when CFD was in its infancy:  you spent weeks building your model, an entire night running the code, and you had a handful of runs around a half-dozen airspeeds for one attitude and one configuration.  Meanwhile, the primitive wind tunnel guys next door finished an entire test and were building up the next one. :)

For those lamenting the cost and availability of CFD and wind tunnel time, there's still lots of utility in really old-school free-air testing.  An afternoon on track or an old runway, a couple of video cameras, and a well-laid pattern of cloth tufts or oil droplets can get you inside your working envelope of aero shapes and positions, and even help generate some calculated results to check against if you have the opportunity to build a computer model later...and if not, at least you have proof you're in the right ballpark.

That, and maybe some of those suspension movement sensors I've seen advertised in GRM.  Use the suspension sensors to calculate the downforce generated by the different aero configurations.

leegrx7
leegrx7 New Reader
5/20/22 8:50 a.m.

Great info and well presented (i actually easilly followed the logic Thru the article) but the true bottom line on the detail of aero improvements is a really complex problem . Resounding success CAN BE  a cliff edge VERY CLOSE to serious problems/failure. The current Mercedes  F1 design is the prime example of the “cliff edge” of aerodynamic success. Their wind tunnel, their ultra aero expertise and their almost limitless pile of $$ has  pushed  their new F1 aero design over the line of success into a porpoise (sp?) mode that makes the race car almost un-drivable  at speed .The up and down oscillation frequencies  are litterally “hammering” the driver. I expect The team will eventually solve the problem but what a mess in the interim!

RodrigoVEng
RodrigoVEng
5/21/22 11:36 p.m.

In reply to Tom Suddard

Hi, nice work.

Have you tried 3d scanners like creality cr scan or any similar?

 

 

 

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