The world is very lucky to have NASA, a publicly funded body with a requirement to openly share all of its research. As we discovered in our <Link href="/blog/best-aerodynamic-resources">guide to the best aerodynamic resources</Link>, the NASA report database is a goldmine of freely available data for aerodynamicists and engineers. It also contains a huge archive of photographs and publications chronicling their history.

That’s where we came across <ExternalLink href="https://ntrs.nasa.gov/citations/20205009237">'The World’s Largest Wind Tunnels'</ExternalLink> book by Kenneth W. Mort. This publication covers the history of the 40 x 80ft closed loop and 80 x 120ft open loop tunnels at NASA Ames in California, known as the National Full Scale Aerodynamics Complex (NFAC). It provides the perfect starting point to delve into the world of full size wind tunnel testing.

<YouTubeEmbed
 videoId="m4rysu8LlLM"
 title="Electric Aviation - NASA interview on Maxwell X57"
/>

---

## Why full size?

A lot of wind tunnel testing is done at model scale for obvious reasons. The test facility can be much smaller, fans less powerful and it is much easier to produce models and test parts. If you want to take an Airbus A380 and put it into a wind tunnel, its 80m wingspan is only the start of the problems that need to be solved.

If that is the case, why do engineers and aerodynamicists still want to try to test in very large wind tunnels? NASA’s 80 x 120ft tunnel (roughly 24m by 36m) will take an actual example of full size fighter aircraft. The key word here is actual.

<ImageWithCaption
 src="/assets/images/fighter-jet-wind-tunnel-testing.jpeg"
 alt="An F/A-18 Hornet fighter jet undergoing high angle of attack testing in a wind tunnel. CREDIT: www.nasa.gov"
 width="600"
 height="579"
 positionCenter={true}
/>

The main reason for wanting to investigate a production aircraft is to establish its real-world performance. A typical problem when using wind tunnel models is that they can be too good a representation of the vehicle. For example, on a real aircraft, rivet heads, panel gaps, antenna and actuators all create drag and prevent aerodynamic surfaces working in the same way that they would do on a scale model. This is particularly vital for low-speed scenarios like take-off and landing where lift is at a premium.

The same applies for ground vehicles. A scale model Formula 1 car is very clean and stiff, so wind tunnel measurements are very accurate. A real Formula 1 car though, has gaps in the bodywork and lightweight carbon structures that deflect, giving quite different results on some occasions. Investigating these details led Williams, Toyota and Sauber to build their own full size wind tunnels in the early 2000s, which are unfortunately now banned by the regulations.

- <ExternalLink href="http://www.icas.org/ICAS_ARCHIVE/ICAS2002/PAPERS/3103.PDF">
 Check out this paper which summarises a wind tunnel test of the Dash 8 Q400
 airliner in the NASA tunnel
 </ExternalLink>

---

## Tunnel interactions

As we saw in our <Link href="/videos/scale-model-wind-tunnel-testing/qqYiz1eo-F4">scale model wind tunnel testing video</Link>, a large wind tunnel section also helps achieve more accurate results with scale model testing. Keeping the section large with a small model allows you to minimise blockage effects and interactions with the floors and walls. A lot of work carried out in the NASA tunnels centres around low speed/high angles of attack scenarios and rotorcraft where there are high downwashing effects which are very sensitive to these things.

A large air volume and working section is particularly useful for quite bluff bodies that displace a lot of air. This was demonstrated in the NASA wind tunnel during a <ExternalLink href="https://www.llnl.gov/news/lawrence-livermore-national-lab-navistar-work-increase-semi-truck-fuel-efficiency-full-scale">fuel efficiency test of a road going truck and trailer rig</ExternalLink>. The NASA 120 x 80ft tunnel is actually very well suited for this type of testing because it can only achieve 100mph in its large configuration.

<ImageWithCaption
 src="/assets/images/truck-fuel-efficiency-test.jpeg"
 alt="A full semi-trailer under test in the 80 x 120ft section of the NASA wind tunnel. CREDIT www.llnl.gov"
 width="700"
 height="483"
 positionCenter={true}
/>

These tests are desirable to help bridge the gap between CFD and the real world. CFD can model a vehicle without the constraints of a working section, but correlating the results is often difficult. Testing a truck on the road is too uncontrolled and instrumentation limited to do this effectively.

- <Link href="/videos/cutting-truck-bus-emissions-the-vecto-airdrag-tool/s2QyP6n4Zxs">
 Find out how to apply the Vecto Air Drag tool to truck and bus aerodynamics
 </Link>

---

## Large scale engineering and lots of power

NASA’s 80 x 120ft tunnel is the largest in the world and is an engineering marvel in its own right. The basis for the building complex was a closed loop 40 x 80ft tunnel conceived in the 1940s. It was converted to add the 80 x 120ft open loop section in the 1980s, powered by the same fans.

<ImageWithCaption
 src="/assets/images/external-view-of-nfac.jpg"
 alt="An aerial view of the complete NFAC. CREDIT: www.nasa.gov"
 width="700"
 height="516"
 positionCenter={true}
/>

The six fans are 40ft in diameter and each is rated at 22,500hp which generate 115mph flow in the 80 x 120ft tunnel and 345mph in the 40 x 80ft closed loop.

<ImageWithCaption
 src="/assets/images/main-nfac-fans.jpeg"
 alt="The bank of fans powering the NFAC. CREDIT: www.nasa.gov"
 width="700"
 height="560"
 positionCenter={true}
/>

Whilst the headline working section numbers for the NASA tunnels are large, the nature of a wind tunnel means that other aspects of the air circuit are much larger. Upstream of the 40 x 80ft section, before the contraction, the tunnel is 132 x 173ft (40 x 52m).

<ImageWithCaption
 src="/assets/images/1947-nasa-wind-tunnel-working-section.jpeg"
 alt="View of the 40 x 80ft working section from 1947. CREDIT: www.nasa.gov"
 width="700"
 height="467"
 positionCenter={true}
/>

The engineering challenges don’t stop once the wind tunnel itself has been built. Such large test items each come with their own logistical problems during installation and removal. The tunnel working section has a hinged roof to allow access.

All-in-all, the facility is as much a wonder of engineering as the different vehicles that have been tested in it over the last eighty years.

<YouTubeEmbed
 videoId="4PabZAx-4Yw"
 title="Testing a Boeing aircraft tail in the world's largest wind tunnel"
/>

---

##### Interesting links:

 

<ExternalLink href="https://airshaper.com/videos/wind-tunnel-test-section/L1AYo9Mk1EI">
 The wind tunnel test section explained
</ExternalLink>

The largest full size wind tunnel in the world - NASA

## Introduction

We are designing a versatile all-electric amphibious aircraft to transport people and goods. The amphibious aircraft concept we are developing is a flying boat type and it uses a hull-based fuselage to meet the hydrodynamic requirements for the aircraft to take-off and land on the water bodies. Operation from land airports is achieved through the retractable landing gear system onboard the aircraft.

<YouTubeEmbed
 videoId="ZPjjg4_UDlA"
 imgSize="hqdefault"
 title="1/5th scale model testing"
/>

We want people to travel emission-free from city centre to city centre at a fraction of the time. Small and flexible sea terminals will facilitate the onboarding and offboarding of our passengers, which is especially useful in mountainous areas along the coast.

## Efficiency is key for electric aviation

For electric aviation to be technically & economically viable, improving efficiency is key, especially when it comes ot aerodynamics. A low drag coefficient and a higher lift-to-drag ratio have a direct impact on the aircraft performance and eventually on the range and endurance of the plane. Some of the ways we have tried to achieve this include:

- the use of natural laminar flow airfoils
- optimised high lift devices
- a streamlined hull design without compromising on the hydrodynamic requirements
- the use of a push-pull electric propulsion setup to further reduce drag.

Overall, we are currently involved in multidisciplinary optimisations and expect to freeze the design in a year from now.

## Hydrodynamics

Hydrodynamic design is crucial for amphibious operations. For the past 12 months, we have been conducting intense hydrodynamic testing of various hull designs for the plane. In order to evaluate the hull designs for the plane in terms of stability and hydrodynamic resistance, we have performed multiple open water tests and also testing in a towing tank in collaboration with SINTEF Norway.

<YouTubeEmbed
 videoId="Et8UBTX7xDo"
 imgSize="hqdefault"
 title="Byfly Towing Tank test at Sintef Norway (video credit: El-Fly)"
/>

How simulations have helped, which parameters are important (lift and drag, roll pitch yaw) Which insights you had, which improvements you'll make based on the results.

## Analysis & optimization through simulations

With the online CFD simulation platform AirShaper, we were able to verify the feasibility of particular flight conditions. Getting the angle of attack of the wing right is crucial towards for economical cruise conditions. Using the lift & drag values from the simulations, for different angles of attack, allows us to optimize further for efficiency. Furthermore, by analysing surface pressure & surface friction, 3D streamlines, flow structures in the wake and other simulation results, we’re able to spot areas for further improvements to the aerodynamics of the plane.

<YouTubeEmbed videoId="62QS4M6M_9Q" title="Full scale model CFD in AirShaper" />

## Experimental testing

We are currently in the R&D phase involving experimental testing. Our goal is to first launch the commercial service in Norway by 2030, where it is highly suitable for the challenging terrain. We will operate from city centre to city centre, and then expand operations to other parts of the world.

Interesting links:

<ExternalLink href="https://el-fly.no/">El-Fly</ExternalLink>

Electric Aviation - The Byfly Seaplane

Tim Barber runs a prep shop and race team, <ExternalLink href="http://www.tfbperformance.com/go-racing/">TFB Performance</ExternalLink>, out of Sonoma Raceway in California. TFB Performance provides vehicle preparation, track support and driver coaching to its customers who race in Trans-Am, endurance prototypes, Spec Miata, SRO World Challenge and many more.

Customers who run racecars with aerodynamic packages typically rely on Tim’s team to optimise aero performance. The convenience and affordability of <ExternalLink href="https://www.rapidscan3d.com/software/polyworks/modeler">RapidScan</ExternalLink> and <Link href="/videos/how-to-run-an-aerodynamic-simulation/O_DNDxCtI_I">AirShaper</Link> allowed Tim to not only access aero data, but analyse the effect of set-up changes quickly and accurately, without investing in wind tunnels or complicated CFD software. Making aerodynamics accessible to a much broader range of racer than ever before.

---

## Establishing an aeromap

The amount of downforce of a racecar varies significantly with ride height and rake. So it is vital to fully understand this relationship if you want to have any chance of establishing an effective set-up over the weekend.

The best way to achieve this is to create an <Link href="/videos/how-aero-maps-can-improve-race-car-performance/u-GyTRblNmc">aeromap</Link>. This maps aerodynamic properties such as downforce and drag as a function of car set-up such as ride height and rake. These plots help to understand the sensitivities between aero coefficients and car set-up, which is an important tool to utilise to adjust the car.

- <Link href="/videos/how-aero-maps-can-improve-race-car-performance/u-GyTRblNmc">
 How aero maps can improve racecar performance
 </Link>

<ImageWithCaption
 src="/assets/images/trans-am-ta2-mustang-surface-pressure.png"
 alt="Surface pressure plot of a Trans-Am TA2 Mustang"
 width="800"
 height="469"
 positionCenter={true}
/>

OEM’s obtain these sensitivities through wind tunnel testing, but this is far too expensive for most small teams and typically requires aerodynamic specialists. Furthermore, the rules in most race categories prevent the use of wind tunnels.

Track testing is the next best option. However, varying weather and track conditions along with other cars on track, make it difficult to run representative comparison tests. Track testing also requires a lot of time from the drivers and as most of our clients have full-time jobs, this is simply not possible.

Consequently, our small team cannot afford the luxury of aerodynamic data, but our customers still rely on our expertise to optimise the aero performance of their cars. Typically, this means spending a lot of time analysing onboard data and combining it with driver feedback. However, the short race weekend formats and lack of testing makes it incredibly difficult to gain an accurate picture of how set-up changes affect the aerodynamics.

To help solve this problem, we decided to partner with RapidScan and AirShaper for our Trans-Am TA2 program. This is a road race stock car that has simple aero adjustments available. However, getting the set-up right is essential as qualifying and the 100-mile race are run on the same set of tires. So an unbalanced car will unnecessarily wear the tires that already degrade heavily throughout the race.

<ImageWithCaption
 src="/assets/images/trans-am-ta2-mustang.jpg"
 alt="Trans-Am TA2 Mustang"
 width="700"
 height="444"
 positionCenter={true}
/>

---

## 3D scanning

The first stage was to create a 3D model that could be imported into AirShaper, which involved 3D scanning one of our TA2 cars. This required no preparation from our side, and within a day Richard Feijoo, 3D solutions specialist from RapidScan, scanned the entire car from top to bottom. This scan was then converted into a 3D surface model, ready for simulating in AirShaper.

<ImageWithCaption
 src="/assets/images/raw-3d-scan-ta2-mustang.png"
 alt="The raw 3D scan of the Trans-Am TA2 Mustang"
 width="700"
 height="383"
 positionCenter={true}
/>

---

## Simulating in AirShaper

Within minutes, I had imported the 3D model into AirShaper and adjusted the set-up parameters of the simulation. I modelled a range of different car configurations with two main objectives. One was to understand how much ride height affected straight line drag, and the other was to quantify how much rake affected downforce.

Being able to quantify the aerodynamic performance from a given ride height or wing change is invaluable. This was the first time that we were able to achieve this with reliable data rather than using educated guesses. The results from AirShaper also benefitted our drivers, as we could explain what to expect instead of them having to rely on feeling the changes themselves.

<ImageWithCaption
 src="/assets/images/trans-am-ta2-increased-rake.png"
 alt="Velocity slice illustrating the aerodynamic effect of increased rake on the Trans-Am TA2 racecar"
 width="700"
 height="543"
 positionCenter={true}
/>

To understand the results in more detail, I was able to connect with AirShaper online where a specialist walked me through the data and the various models they created for our car. The PDF report provided, was also really useful. It presented the results in an understandable way and included a wide variety of colour maps illustrating surface pressure, surface friction, velocity streamlines, pressure clouds and even noise analysis. Within a few simulations we had quantitive values of the aero effect of set-up changes, without the need for wind tunnel or track tests.

---

## Beyond lap time

Drivers and teams typically prioritize speed and lap time first, but the AirShaper data can also be used to improve cooling and reliability. For example, in Trans-Am TA2, there are a number of different options for radiator and brake duct openings. By using the simulation results, we were able to adjust these openings to the optimum position that allowed enough air flow for cooling, whilst maintaining maximum downforce and minimising drag.

Furthermore, the regulations specify that the overall surface of our TA2 car has to meet body templates, but we are allowed to design and manufacture our own airbox and ducting. Again, using the data from RapidScan and AirShaper we simplified our build process and can now quantitatively say that we have the best aerodynamic option available. I'm looking forward to running some more cars through this process so that we can optimise the aerodynamic performance of those too!

<ImageWithCaption
 src="/assets/images/trans-am-ta2-isosurfaces-2.png"
 alt="3D pressure surfaces on the Trans-Am TA2 reference model"
 width="700"
 height="213"
 positionCenter={true}
/>

---

##### Interesting links:

 

<ExternalLink href="https://www.racecar-engineering.com/tech-explained/what-is-an-aero-map/">
 What is an aeromap?
</ExternalLink>

 

<ExternalLink href="https://www.rapidscan3d.com">RapidScan</ExternalLink>

Helping teams understand the aero of a Trans-Am TA2 Mustang

In competitive cycling, aerodynamic wheels have become a standard. Every pro rider, along with many amateurs now ride them on a daily basis. However, cyclists are often concerned about the effects of these wheels on the riding experience and the theoretical aerodynamic benefits never seem to quite match reality.

To help address this issue, <ExternalLink href="https://www.sorcycles.com/pages/about-us">Sōr cycles</ExternalLink> have developed an innovative new wheel that allows anyone to ride in any condition. To find out more about this ground-breaking technology, we interviewed the co-founders of Sōr cycles; Nicole Van Beurden (ex-triathlete) and Phil White (co-founder of Cervelo).

---

## The importance of aerodynamics in cycling

Aerodynamics has a huge effect on cycling performance and is therefore an important consideration in the design of a bike as well as the positioning and clothing of the rider. Check out our previous blog posts and videos, where we have covered <Link href="/videos/aerodynamic-clothing/A0Gx-lBzSFA">aerodynamic clothing</Link> for cyclists, the features of an <Link href="/videos/designing-an-aero-bike/DqlDpUFcScc">aero bike</Link> and discussed how teams in the <Link href="/blog/cycling-aerodynamics-team-dsm">UCI World Tour deal with cycling aerodynamics</Link>.

<ImageWithCaption
 src="/assets/images/trek_madone_bike_frame.png"
 alt="The TREK Madone SLR is a good example of an aerodynamic bike frame. CREDIT: www.magliarosso.co.uk"
 width="600"
 height="450"
 positionCenter={true}
/>

In general, one of the largest factors of resistance that a cyclist feels is <Link href="/videos/what-is-aerodynamic-drag/DbIIw-WryHY">aerodynamic drag</Link>. On flat surfaces, up to 90% of the total resistance a cyclist feels is due to aerodynamic drag. If you can minimise this, the cyclist can go faster with the same power output; gaining time over their competitors.

Over the years, there have been many innovations to improve the aerodynamics of cycling and these have mostly focused on reducing the frontal cross-sectional area. This is often achieved by making the frame thinner but using taller tubes to maintain stiffness. To further improve aerodynamics, manufacturers have also introduced fairings to cover the wheels, commonly known as “disc wheels” to reduce the wake produced by the wheel.

<ImageWithCaption
 src="/assets/images/aero_bike_wheel_covers.jpg"
 alt="An extreme example of an aerodynamic wheel cover. CREDIT: www.triathlonvibe.com"
 width="600"
 height="451"
 positionCenter={true}
/>

While these aerodynamic advancements have helped reduce cross-sectional area, it has influenced bikes to become more unstable in crosswinds. The nature of thin bicycle wheels means a small tyre contact path which results in less rolling resistance and aerodynamic drag, but also instability. Innovations such as disc wheels can further increase this instability.

The majority of the work carried out by the cycling industry to reduce drag has concentrated on relatively low yaw angles (i.e. not much sidewind). The yaw angle is the angle of the wind relative to the direction of travel. "We felt that the real-world side winds were higher than the theoretical models and that typical rider speeds were lower than the speeds used in testing," explains Nicole Van Beurden, ex-pro long-distance triathlete and co-founder of Sōr cycles.

To prove this, Sōr cycles conducted some real world testing, with the results in the video below. It was found that the yaw angle is a lot higher in real life than in the theoretical scenarios bike manufacturers have been designing their aero for.

<YouTubeEmbed
 videoId="--uvHp_UaqE"
 title="Measuring yaw angle in real world conditions"
/>

"At these high yaw angles, cyclists often feel instability and are vulnerable to high crosswind gusts," says Van Beurden. "They can also experience further instability from flow separating from the disc wheel causing buffeting. This phenomenon is like a wing stalling, the result is instability, turbulence and drag. These instabilities make it especially difficult for smaller and lighter cyclists, most often women, to take advantage of these disc wheels."

---

## The Sōr wheel

To solve this problem, Phil and Nicole developed a unique new wheel, the Sōr wheel, and Sōr cycles was born. "We have designed a wheel to work at the yaw angle that the rider is feeling," highlights Van Beurden. "The Sōr wheel design immediately adapts to the conditions the rider feels at that second."

In other words, the Sōr wheel is a full wheel for race bicycles that is capable of deforming. When subjected to a headwind it remains a simple narrow wheel, but under side winds the wheel deforms with the leeward side bulging out to form an aerodynamic shape. This encourages the airflow to remain attached for longer, helping to reduce drag and eliminate buffeting.

Should the conditions change a second later, the shape of the wheel automatically adapts to a new optimal shape. This avoids the instability and buffeting problems that is common with aero wheels because the wheel is constantly changing to suit the conditions.

<ImageWithCaption
 src="/assets/images/sor-wheel-morphing.jpg"
 alt="The Sōr wheel changes shape according to the conditions to achieve optimal cycling performance. CREDIT: www.sorcycles.com"
 width="700"
 height="428"
 positionCenter={true}
/>

What is truly innovative about this design is that the wheel’s aerodynamic morphing is completely passive. "We took inspiration from the way the wing of a bird or bat can instantaneously and passively optimize itself to wind conditions," explains White. "There are no control systems, no actively moving parts, no linkages, no sensors, no power, just a thin balloon like material that stretches freely but snaps back into shape."

"This intelligent material can dynamically adjust to even the changes in airflow caused by the riders legs," continues White. "It is also so smooth that the rider will not feel any vibrations from the material deforming. Furthermore, the design has a central hidden structure which carries most of the load without effecting the aerodynamics and keeps the wheel extremely lightweight. By separating out these duties, each component is optimized for its role and the overall wheel weight can be much lower while also being more aerodynamic."

<VideoEmbed
 videos={[
 {
 src: 'https://airshaper.storage.googleapis.com/website/video/slo_mo_lswt.webm',
 type: 'video/webm;codecs="vp8, opus"',
 },
 {
 src: 'https://airshaper.storage.googleapis.com/website/video/slo_mo_lswt.mp4',
 type: 'video/mp4',
 },
 ]}
/>

---

## The aerodynamics behind the Sōr wheel

The principle behind the Sōr wheel is relatively simple. At 0 degrees angle of attack, the wheel is in its default position and is thin to keep the frontal surface area low. At angles of attack higher than 0 degrees (combined sidewind and forward velocity), the leeward side of the wheel features low pressure and will “bulge out” creating a convex shape. The windward side feels a higher pressure and becomes concave.

The flexibility of the material allows the wheel to absorb gusts of wind. While the Sōr disc can keep the airflow attached up to around 28 degrees of apparent wind, compared to approximately 15 degrees for the leading competitor with a rigid disc.

<ImageWithCaption
 src="/assets/images/drag-graph-of-aero-wheels-2.png"
 alt="The Sōr wheel keeps airflow attached up to approximately 28 degrees and delivers around 90 Watts of thrust. CREDIT: www.sorcycles.com"
 width="600"
 height="427"
 positionCenter={true}
/>

The material of the Sōr wheel not only changes shape leeward versus windward side, but it also changes shape across a single rotation. So, the shape at the same side of the wheel can be different in front and behind the hub. This allows the Sōr wheel to turn into an airfoil and even result in thrust at certain angles. Similar to a sail on a boat, the difference in pressure created by the concave windward side and the convex leeward side generates thrust.

<ImageWithCaption
 src="/assets/images/sail-physics.png"
 alt="A diagram illustrating the physics behind a sail on a boat. CREDIT: www.2.bp.blogspot.com"
 width="400"
 height="600"
 positionCenter={true}
/>

---

## The other benefits of the Sōr wheel

The benefits of this wheel are not just purely aerodynamic, the confidence this technology provides to the rider can also contribute to optimising performance. "By having more stability, people peddle more efficiently," reveals Van Beurden. "If a normal disc wheel is destabilising you, you use your core to compensate for this. This represents a loss in terms of power going to the pedals."

Overall, the Sōr wheel provides a paradigm shift on how aerodynamic devices on a bike wheel can work. Not only does it reduce the buffeting and instability, but also provides measurable advantages over traditional disc wheels in crosswinds. This will help cyclists of all abilities ride faster for longer in a wide range of conditions.

---

Interesting links:

 

<ExternalLink href="https://www.sorcycles.com/">Sōr cycles</ExternalLink>

 

<Link href="/videos/aerodynamic-clothing/A0Gx-lBzSFA">
 Aerodynamic clothing for cyclists
</Link>

 

<Link href="/videos/designing-an-aero-bike/DqlDpUFcScc">
 Designing and Aero Bike to win the Ironman
</Link>

 

<Link href="/blog/cycling-aerodynamics-team-dsm">
 Cylcing aerodynamics - Interview with Team DSM
</Link>

The technology behind the Sōr wheel

The impressive processing power now available to home computers is quickly changing the way people apply complicated engineering principles. The types of CFD simulation and analysis that companies such as Airshaper now provide used to be the domain of high-end software, only available to large engineering concerns. Now, cloud computing means that the only thing you need to conduct CFD simulations is access to a web portal.

At the same time, there is a vast amount of aerodynamic information and research on the internet. Knowing where to start and what sources to trust can be overwhelming. So, to help you on your aerodynamic journey, we have put together this guide which lists our favourite aerodynamic resources, research and tools that we think you will find the most useful.

---

##### [1) Fluid Mechanics 101](#fluidmechanics101)

##### [2) Advanced Fluid Mechanics course from MIT](#advancedfluidmechanicscourse)

##### [3) OpenFOAM tutorials](#openfoamtutorials)

##### [4) NASA Technical Reports Server (NTRS)](#nasatechnicalreports)

##### [5) Technical paper websites](#technicalpapers)

##### [6) OpenFOAM](#openfoam)

##### [7) XFOIL and XFLR5](#xfoilandxflr5)

##### [8) Airfoiltools](#airfoiltools)

##### [9) AirShaper](#airshaper)

##### [10) KYLE.ENGINEERS](#kyleengineers)

##### [11) Official Formula 1 website](#formula1website)

##### [12) Aerodynamic books](#books)

---

## Education

It is essential to thoroughly understand the fundamental principles behind aerodynamics before diving into your own CFD studies. Massive online open courses (MOOCs) are widely available and, whilst some are quite basic, others are very comprehensive. YouTube also has a huge variety of technical content created by some engaging and knowledgeable engineers.

### 1) Fluid Mechanics 101 <a name="fluidmechanics101"></a>

<ImageWithCaption
 src="/assets/images/fluid-mechanics-101-home-3.png"
 alt="Fluid Mechanics 101 YouTube channel"
 width="700"
 height="492"
 positionCenter={true}
/>

##### What is it?

- A very popular YouTube channel that focuses on explaining the principles of fluid mechanics as well as computational and numerical methods

##### Why do we like it?

- An accessible set of videos that cover all the theory you need to understand when running CFD simulations
- All content is scientifically validated and referenced correctly
- Provides formulae, methods and data which saves you having to dig through textbooks and research papers

##### Where can you find it?

- You can watch the videos on the <ExternalLink href="https://www.youtube.com/channel/UCcqQi9LT0ETkRoUu8eYaEkg">Fluid Mechanics 101 Youtube channel</ExternalLink>
- For more detailed lectures and courses check out the <ExternalLink href="https://www.fluidmechanics101.com/index.html">Fluid Mechanics 101 website</ExternalLink>

---

### 2) Advanced Fluid Mechanics course from MIT <a name="advancedfluidmechanicscourse"></a>

<ImageWithCaption
 src="/assets/images/advanced-fluid-mechanics-mit-course-2.png"
 alt="Advanced Fluid Mechanics course from MIT"
 width="700"
 height="462"
 positionCenter={true}
/>

##### What is it?

- A four-part course that is free to use as long as you are happy not to receive any certification and have time limited access
- It covers the continuum viewpoint, fluid statics, inviscid flow, Bernoulli, control volume theorems and applications

##### Why do we like it?

- This set of courses is aimed at a relatively high level and requires a healthy time commitment, but does cover a lot of ground
- If you choose to be a verified learner, you can complete the course at your own pace
- Good mixture of teaching videos and interactive concept questions to help you practice what you have learnt

##### Where can you find it?

- You can register and access the <ExternalLink href="https://learning.edx.org/course/course-v1:MITx+2.25.1x+1T2022/home">Advanced Fluid Mechanics course here</ExternalLink>

---

### 3) OpenFOAM Tutorials <a name="openfoamtutorials"></a>

<ImageWithCaption
 src="/assets/images/openfoam-tutorial-collections-2.png"
 alt="OpenFOAM tutorials"
 width="700"
 height="479"
 positionCenter={true}
/>

##### What is it?

- A collaborative database of teaching materials that make up the official OpenFOAM user tutorials

##### Why do we like it?

- Every resource has been compiled by aerodynamic experts from all over the world
- Tutorials are categorised into specific collections to suit different levels
- A wide variety of resources including videos, presentations and articles

##### Where can you find it?

- All the tutorials can be found on the <ExternalLink href="https://wiki.openfoam.com/index.php?title=Tutorials">OpenFOAM Tutorials homepage</ExternalLink>
- You can also find AirShaper's contribution on the <ExternalLink href="https://wiki.openfoam.com/Adaptive_Mesh_Refinement_by_Air_Shaper">Adaptive Mesh Refinement page</ExternalLink>
- Before founding AirShaper, I also created a variety of <Link href="/courses">OpenFOAM training courses</Link> to teach companies how to utilise OpenFOAM in their design process. These are somewhat outdated, but may still be useful

---

## Research

Once you have completed your learning, it’s time to take on some projects! As with any new undertaking, it's always worth exploring previous studies. This not only gives you a head start, but can also help you avoid making mistakes.

### 4) NASA Technical Reports Server (NTRS) <a name="nasatechnicalreports"></a>

<ImageWithCaption
 src="/assets/images/nasa-technical-reports-2.png"
 alt="NASA technical reports"
 width="700"
 height="377"
 positionCenter={true}
/>

##### What is it?

- A searchable database of a vast library of research papers and technical reports
- It dates back to the beginnings of the NACA, forerunner of NASA, in the early 20th century

##### Why do we like it?

- These papers cover everything from airfoil and duct design to hypersonic research
- Even the Apollo mission reports are available

##### Where can you find it?

- You can search through the database via the <ExternalLink href="https://ntrs.nasa.gov/search">NTRS - NASA Technical Reports Server homepage</ExternalLink>

---

### 5) Technical paper websites <a name="technicalpapers"></a>

<ImageWithCaption
 src="/assets/images/technical-paper-libraries.png"
 alt="Technical paper libraries"
 width="700"
 height="546"
 positionCenter={true}
/>

##### What is it?

- Online databases containing abstracts for technical papers

##### Why do we like it?

- There are thousands of papers on a whole range of engineering subjects available
- Similar to the NASA reports, they summarise fascinating and useful research
- They tend to be for members only, but can be bought individually or are often free in academia

##### Where can you find it?

- Many industries have their own society websites which include a library of technical papers, such as the <ExternalLink href="https://www.sae.org/publications/technical-papers">Society of Automotive Engineers</ExternalLink> and the <ExternalLink href="https://www.aerosociety.com/news-expertise/national-aerospace-library/e-library/">Royal Aeronautical Society</ExternalLink>

---

## Tools

There are many commercially available software products out there, but the licence costs mean that only large companeis can afford them. Fortunately, there are many free tools that can do a similar job.

### 6) OpenFOAM <a name="openfoam"></a>

<ImageWithCaption
 src="/assets/images/openfoam-paraview.png"
 alt="OpenFOAM ParaView. CREDIT: commons.wikimedia.org"
 width="700"
 height="424"
 positionCenter={true}
/>

##### What is it?

- OpenFOAM is an open-source library for running CFD simulations

##### Why do we like it?

- It is extremely powerful and forms the basis of many commercial CFD products and consultancies
- It’s widely used as the maths engine behind bespoke front-ends in industry, so learning about it is a useful skill for your CV. Discover other ways to boost your CV by reading our blog on <Link href="/blog/building-your-career-in-aerodynamics">building a career in aerodynamics</Link>

##### Where can you find it?

- You can download and install it from the <ExternalLink href="https://www.openfoam.com/">OpenFOAM homepage</ExternalLink>
- Check out our tutorial video on <Link href="/videos/cfd-results-how-to-analyse-openfoam-data-with-paraview-25-minute-tutorial/kczZPc4M-ms">how to analyse OpenFOAM data with ParaView</Link>

---

### 7) XFOIL and XFLR5 <a name="xfoilandxflr5"></a>

<ImageWithCaption
 src="/assets/images/xflr5-example.png"
 alt="An example of XFLR5. CREDIT: www.sourceforge.net"
 width="700"
 height="548"
 positionCenter={true}
/>

##### What is it?

- These tools are quite similar and allow you to perform a preliminary aerodynamic analysis on basic shapes like airfoils and sails

##### Why do we like it?

- A complicated CFD simulation of a full design is usually the end goal of a project but it can be very time consuming and difficult to set-up. These tools are free and easy to use and are effective ways of getting into the right ballpark

##### Where can you find it?

- You can download and install these tools from the <ExternalLink href="https://web.mit.edu/drela/Public/web/xfoil/">XFOIL homepage</ExternalLink> and the <ExternalLink href="http://www.xflr5.tech/xflr5.htm">XFLR5 homepage</ExternalLink>

---

### 8) Airfoiltools <a name="airfoiltools"></a>

<ImageWithCaption
 src="/assets/images/airfoiltools-4.png"
 alt="Airfoil plotter on Airfoiltools"
 width="700"
 height="470"
 positionCenter={true}
/>

##### What is it?

- A comprehensive database containing lift, drag and moment performance characteristics for different airfoils
- A lot of the data comes from the NACA reports previously discussed

##### Why do we like it?

- It makes a lot of sense to start from a known <Link href="/videos/airfoil-design-for-a-drone/kAXN3MlQxxc">airfoil design</Link> when trying to create a wing or blade, so a database like this eliminates a vast range of unsuitable designs, saving you a lot of time

##### Where can you find it?

- You can browse through different airfoils on the <ExternalLink href="http://airfoiltools.com/">Airfoiltools website</ExternalLink>

---

### 9) AirShaper <a name="airshaper"></a>

<ImageWithCaption
 src="/assets/images/airshaper-simulation.png"
 alt="AirShaper"
 width="700"
 height="391"
 positionCenter={true}
/>

##### What is it?

- AirShaper is a cloud-based virtual wind tunnel software that runs CFD simulations on 3D models

##### Why do we like it?

- It's very accessible as no license is required, you just need internet access
- You can upload your own 3D models easily and these don't even have to be watertight
- Simulations run quickly and afterwards, you receieve a PDF report containing all the results and graphs

##### Where can you find it?

- Log in to run your own CFD simulations on the <ExternalLink href="https://airshaper.com/">AirShaper homepage</ExternalLink>
- Find out how to use AirShaper to <ExternalLink href="https://airshaper.com/videos/how-to-run-an-aerodynamic-simulation/O_DNDxCtI_I">run aerodynamic simulations</ExternalLink>
- For further information on aerodynamic theory, concepts and applications, check out <ExternalLink href="https://airshaper.com/videos/page/1">AirShaper's video database</ExternalLink>

---

## General interest

There are also plenty of online resources that explain interesting aerodynamic concepts from industry that you can then apply to your own projects. There’s nothing better than gaining a new insight into a topic without it feeling like you are actually learning!

### 10) KYLE.ENGINEERS <a name="kyleengineers"></a>

<ImageWithCaption
 src="/assets/images/kyleengineers.png"
 alt="KYLE.ENGINEERS YouTube channel"
 width="700"
 height="418"
 positionCenter={true}
/>

##### What is it?

- A YouTube channel which focuses on all things related to motorsport engineering

##### Why do we like it?

- This channel has been created by a former Formula 1 aerodynamicist, so there is a lot of aerodynamic content
- Good use of annotations which makes it easier to understand the complex aerodynamic concepts used in motorsport

##### Where can you find it?

- You can watch the videos on the <ExternalLink href="https://www.youtube.com/c/KYLEDRIVES">Kyle.Engineers Youtube channel</ExternalLink>

---

### 11) Official Formula 1 website <a name="formula1website"></a>

<ImageWithCaption
 src="/assets/images/f1-technical-website.png"
 alt="Technical page on F1.com"
 width="700"
 height="634"
 positionCenter={true}
/>

##### What is it?

- The Technical section of the official Formula 1 website

##### Why do we like it?

- As it's the official F1 website it has good access to photography, information and contributors
- Useful illustrations explain complex and often hidden aerodynamic tricks that race teams use to maximise performance

##### Where can you find it?

- You can find it on the <ExternalLink href="https://www.formula1.com/en/latest/tags.technical.4lP4Pth1kICO2iCQqGAIIU.html">Technical page of the official Formula 1 website</ExternalLink>

---

### 12) Aerodynamic books <a name="books"></a>

There are thousands of technical books that cover aerodynamics and although these require purchasing, this doesn't mean that they should be ignored. Below is a selection of our favourites and remember that many are available as eBooks and can be bought second hand from the likes of eBay and Adebooks.

#### 12.1) <ExternalLink href="https://link.springer.com/book/10.1007/978-3-642-56026-2">Computational Methods for Fluid Dynamics</ExternalLink>

<ImageWithCaption
 src="/assets/images/computational-methods-for-fluid-dynamics.jpg"
 alt="Computational Methods for Fluid Dynamics book"
 width="400"
 height="400"
 positionCenter={true}
/>

#### 12.2) <ExternalLink href="https://www.wiley.com/en-gb/Low+Speed+Wind+Tunnel+Testing%2C+3rd+Edition-p-9780471557746">Low-Speed Wind Tunnel Testing</ExternalLink>

<ImageWithCaption
 src="/assets/images/low-speed-wind-tunnel-testing.png"
 alt="Low-Speed Wind Tunnel Testing book"
 width="262"
 height="371"
 positionCenter={true}
/>

#### 12.3) <ExternalLink href="https://www.cambridge.org/core/books/lowspeed-aerodynamics/077FAF851C4582F1B7593809752C44AE">Low-Speed Aerodynamics</ExternalLink>

<ImageWithCaption
 src="/assets/images/low-speed-aerodynamics.jpeg"
 alt="Low-Speed Aerodynamics book"
 width="262"
 height="376"
 positionCenter={true}
/>

#### 12.4) <ExternalLink href="https://www.amazon.co.uk/Race-Car-Aerodynamics-Designing-Technical/dp/0837601428">Race Car Aerodynamics</ExternalLink>

<ImageWithCaption
 src="/assets/images/race-car-aerodynamics.jpeg"
 alt="Race Car Aerodynamics book"
 width="262"
 height="347"
 positionCenter={true}
/>

---

Our guide to the best Aerodynamic resources

## Back to the Future

The case for streamlined cars isn't new - for decades, mankind has searched for higher top speeds, especially in racing. And because aerodynamics are by far the most dominant force to overcome at high speeds (even at highway speeds, over half the energy goes to pushing air away...), reducing the aerodynamic drag coefficient has been key.

With 1.000+ HP Bugatti Veyron's and Chiron's, it's easy to think speeds above 400 km/h are a very recent achievement. But already in 1938, Mercedes reached 432.7 km/h with their W125 - on a public road!

<ImageWithCaption
 src="/assets/images/mercedes_w125.jpg"
 alt="The record breaking Mercedes W125 - Drag coefficient Cd of 0.235 (Image credit: mercedes-benz.com)"
 width="810"
 height="540"
 positionCenter={true}
/>

So how did the Mercedes W125 beat the Veyron decades ago? You guessed it - aerodynamics. The aerodynamic power required to push the air away scales with the cube of velocity, whereas mechanical losses (like drivetrain and rolling resistance) only scale linearly with velocity. So at these speeds, aerodynamics easily account for over 90% of all energy needs.

The Bugatti Chiron has a drag coefficient of around 0.36. That's not very impressive, but then again, sports cars are usually aiming for good downforce & style. An F1 car, for example, has a terrible drag coefficient of around 1.00, as it has enough power to overcome the drag penalty in favor of more downforce.

But when top speed is your goal, the case is different. The W125 had 'just' 616 HP (still a massive amount by 1938 standards) and still was able to reach over 400 km/h/. How? Via a super streamlined bodywork which featured a drag coefficient of just 0.235 - that's over a third more streamlined than the Chiron!

## Fuel economy

Of course, road cars need to carry people around, preferably in a practical, safe and affordable way, so the constraints are quite different. Still, as mentioned above, even at regular highway speeds, aerodynamics are by far the most important source of energy loss.

So it's no surprise OEMs have been trying to cut those losses, even long before the breakthrough of electric cars. After all, aerodynamic efficiency has a direct impact on fuel consumption. Ford, for example, experimented with different shapes for their "Probe" concept car series. The Probe V had a drag coefficient of just 0.137!

<ImageWithCaption
 src="/assets/images/ford_probe_y.jpg"
 alt="The Ford Probe Y - Drag coefficient Cd of 0.137 (Image credit: hotcars.com)"
 width="810"
 height="540"
 positionCenter={true}
/>

## It's all about the looks

So why don't all vehicles look like this? Well, apart from practicality, I would argue the looks are the biggest problem. You could convince me to like such a shape, as I love aerodynamics, but a car for the masses simply needs to look stunning.

Recent attempts have been made to make cars as aerodynamic as possible while still maintaining good looks, like the Lightyear One (featuring a Cd value "below 0.20") and the Mercedes EQXX (featuring a Cd value of 0.17!).

<ImageWithCaption
 src="/assets/images/mercedes_eqxx.jpg"
 alt="The Mercedes VISION EQXX concept - Drag coefficient Cd of 0.17 (Image credit: mercedes-benz.com)"
 width="810"
 height="455"
 positionCenter={true}
/>

Still, although it looks nice, I think this design is "compromised". The long, low tail, for example, is very specific. Too specific to appeal to the masses, at least in the beginning.

That's why I'm so impressed with cars like the Tesla Model S Plaid and the Lucid Air, which both claim a Cd value of around 0.20. Yes, that's higher than the VISION EQXX, but only slightly. And in my opinion, the Tesla and Lucid both still look like normal cars - no compromise in terms of design and therefore much more appealing towards the masses.

## The EV push

With the breakthrough of electric cars, a lot of focus is now on the range of these vehicles. Every kWh/km of energy consumption that can be saved, makes the car cheaper (fewer battery cells), lighter (less battery weight) and more efficient (less weight means lower rolling resistance). With range anxiety holding back a lot of potential buyers, aerodynamics again play a key role. The more streamlined the car, the bigger the range for the same battery.

It's not a coincidence that Tesla has a massive focus on aerodynamic efficiency - not only for their cars, but also for the Tesla Semi truck. And they are not alone: newcomers like Lucid and Rivian also work hard to make surfaces as clean and streamlined as possible.

## The Future

So what could we achieve in the future in terms of no-compromise design? Well, a lot depends on what you define as "no-compromise design". Some designs just take time to get used to.

#### GAC ENO. 146

If we stick to open wheels, then the GAC ENO. 146 concept, presented in 2019, presents a nice example of what is possible. A drag coefficient of just 0.146 is extremely low, even compared to the EQXX, and the car still looks fairly normal in terms of proportions / tail length.

<ImageWithCaption
 src="/assets/images/gac_eno.jpg"
 alt="The GAC ENO. 146 concept - Drag coefficient Cd of 0.146 (Image credit: evspecifications.com)"
 width="810"
 height="377"
 positionCenter={true}
/>

#### Pimp my Tesla Model S Plaid

Another option is to further refine existing designs and perhaps add just a little bit of compromise. Vincent Keromnes, domain leader dynamic benchmarking at our partner <ExternalLink href="https://portal.a2mac1.com/">A2MAC1</ExternalLink>, took a Tesla Model S Plaid (yup, they 3D scanned the whole car down to the last detail) and then covered the rear wheels (like on the Lightyear One).

<ImageWithCaption
 src="/assets/images/tesla_model_s_plaid_with_wheel_covers.jpg"
 alt="The Tesla Model S Plaid with wheel covers - 3D models by A2MAC1"
 width="810"
 height="411"
 positionCenter={true}
/>

Vincent then ran both cars on our platform and the results couldn't be more clear: the wake & energy loss normally caused by the rear wheels "chopping up" the air has been reduced dramatically. The impact on Cd? A 3.5% drop in drag coefficient, which is very significant in terms of aerodynamic efficiency!

#### Aptera

You can actually go even further and question the entire layout of modern cars - start over from scratch, aiming for the highest possible efficiency. That is exactly what Aptera did - they basically took a drop shape and added wheels to it. 3 that is, not 4.

<YouTubeEmbed
 videoId="VwQXGKmDGgU"
 title="Aptera - Drag coefficient Cd of 0.13"
/>

## A note on Cd values

The Cd values in the article are the official values as stated by the manufacturers. Very often, we see differences when taking these cars to the wind tunnel.
Different tyres, rims, ride height settings etc can make it harder to compare these cars.
Nevertheless, the relative differences in this article do tell a very interesting story of substantial progress.

## Conclusion

The aerodynamic efficiency of vehicles has come down dramatically, especially with the focus on range for electric vehicles. Somehow, it feels like we have not reached the limits yet and we may be seeing even more streamlined designs in the future!

Some may look like normal cars with cleverly concealed aero tricks. Others may look downright futuristic and appeal to a more specific audience. But one thing is certain: aerodynamics will play a key role in the design of our future (electric) cars!

##### Interesting links:

 

<Link href="/videos/how-to-run-an-aerodynamic-simulation/O_DNDxCtI_I">
 How to run your own simulation
</Link>

 

<ExternalLink href="https://portal.a2mac1.com/">A2MAC1</ExternalLink>

 

<ExternalLink href="https://aptera.us/">Aptera</ExternalLink>

EV Range - The Race For Aerodynamic Efficiency

## Introduction

Aerodynamics is a very complex field. Air is invisible and to see what is going on, you need to somehow add something to the flow (like smoke) or the surface of the model (tufts) or measure a derived quantity (like pressure) at different points to re-construct what is happening in terms of velocity from your experience with similar cases.

While this has worked well in the past and fantastic things have been achieved by talented engineers with years of experience, today’s race for performance, efficiency and energy saving can only be won if detailed and precise 3D datasets are available. To obtain this, there are basically two, sometimes competing approaches available: CFD (computational fluid dynamics) and wind tunnel flow measurements. Each in his own way, both approaches are similarly complex.

CFD requires to solve the complex Navier-Stokes equations that describe the flow around an object. Which in theory is fairly straight-forward but solving them for larger setups quickly becomes extremely challenging. The discretization (meshing) of the model geometry (often including ventilation openings, rotating wheels and small details like bolts and wires), the large number of computational cells typically used (50 Mio cells and more) and the effect of model surface structure that determines the point of turbulent transition and flow detachment need to be tackled, to name just a few. This results in simulations that must be run overnight on high-performance clusters, produce datasets in the order of several tens Gb per configuration and require experienced engineers to setup, run and analyze.

The picture is the same if you turn to highly advanced optical flow measurement techniques like PIV (particle image velocimetry) or traversing systems capable of mapping the flow around the model in a wind tunnel. It helps that wind tunnel tests are anyways required for validation or WLTP emissions calculations. And you can be reasonably confident that the measurement data represents reality fairly well, at least for the setup in the wind tunnel. However, an expensive setup including powerful lasers and cameras, or traversing equipment is needed. Plus, again, a set of well-trained engineers that can handle the calibration of the optics or the programming of the traversing system. Not to mention the finer points like laser safety, seeding, several hours of measurement time and days taken up by post-processing.

Bottom line for both approaches is: The achievable quality and accuracy of the data is impressive. But for an initial assessment of a design during the concept phase, a quick comparison of configurations and most optimization tasks they are both simply too expensive and time consuming. To accelerate aerodynamic development, we must provide a simplified, yet effective toolset.

## Speeding up insight

In terms of simulation, such options include cloud platforms like AirShaper that automates the process of CFD simulations. Instead of spending days on pre-processing, solving & post-processing, the process is streamlined to provide results often within hours. The compromise is that many of the small, expert software settings are hidden, in return for a more generalized / standardized approach & simulation output.

<YouTubeEmbed
 videoId="O_DNDxCtI_I"
 title="How to Runa Simulation on AirShaper"
/>

Along the same idea but for experimental wind tunnel work streamwise gmbh developed ProCap, a scan-and-paint approach that allows to analyze the flow in real-time while at the same time providing a fully 3D volumetric dataset. Rather than providing ultra-precise results in terms of spacial resolution and accuracy, this approach offers a much more flexible, time efficient way to obtain quantitative data in the form of a fully 3D flow field, perfect also to validate CFD results.

<YouTubeEmbed videoId="TFdnU6Gnz_I" title="ProCap in Wind Tunnels" />

## Case study

We want to introduce the two new concepts in some detail here but also clearly and openly illustrate the benefits and drawbacks of both techniques. To this end, both AirShaper simulations and ProCap measurements were applied to the Drivaer model, a reference car geometry widely used in the automotive industry. As both techniques provide full 3D data, the comparison of the results is straight forward using a CFD post-processor. The setup was a 1:4 scale model with fixed wheels at 25 m/s.

### simulation

In terms of simulation, one of the main choices to make is how to approach the boundary layer. This is the layer of air sticking to the surface and it features high velocity gradients normal to the wall. One way to take this into account is to use prism layers, which are very "flat" cells used to increase the resolution normal to the wall, to capture the velocity gradients and flow separation locations. But this is expensive in terms of mesh count and it is also more challenging to automate, especially on complex geometries. Therefore, wall functions are often used: these are "precalculated" velocity profiles that are then applied to the relatively large cells close to the boundary layer. It makes the whole process of meshing and solving a lot more robust, but can create slight differences in terms of boundary layer behaviour. The benefits of AirShaper, though, include the possibility to work with non-watertight models (no need to spend weeks closing small gaps & holes) and have results within hours of uploading the 3D model.

### measurement

The accuracy of a measurement is typically limited by how accurately you can re-create reality in the wind tunnel. Error sources includes the presence of walls, boundary layers on the floor, mounting points of the model, etc. When using a probe, the finite size of the probe will also affect the flow and especially close to a surface (below one probe tip diameter or 4 mm for ProCap), errors become significant. When working with ProCap, an additional source of error (quite similar to mesh resolution in CFD) must be taken into account. As the spatial resolution is a direct consequence of the number of measurement points per volume, a too short measurement time can result in poor resolution of velocity gradients. Typical measurement times for the volumetric data shown in this comparison are 30 minutes total, with only 30 minutes to factor in for setting up at the beginning. To do a comparison of different configurations one would focus on a single plane and thus be able to cut measurement further. Compared to using a traversing system, ProCap typically achieves a speed-up of 50 to 100.

<ImageWithCaption
 src="/assets/images/accelerating-erodynamic-design-picture-1.jpg"
 alt="Figure 1: Overview of the two datasets. AirShaper (CFD) left, ProCap (measurement) right."
 width="800"
 height="542"
 positionCenter={true}
/>

### results

Overall flow pattern (figure 1): the overall flow & pressure patterns provided by AirShaper and ProCAP measurements are very similar. They both provide an excellent overview and understanding of the flow topology and lead to the same conclusions in terms of wake size & structure, high & low pressure zones around the car etc. at a fraction of time and money. In some areas differences are visible as they would be in any comparison of numerical and experimental data, some of which are pointed out here (Figure 2).

<ImageWithCaption
 src="/assets/images/accelerating-erodynamic-design-picture-2.jpg"
 alt="Figure 2: Highlighting areas with differences between the two datasets."
 width="800"
 height="344"
 positionCenter={true}
/>

Boundary layer (Figure 2, A): In the simulation data, the thickness of the boundary layer is determined by the cell size in this area. For ProCap, the resolution close to the surface is limited by the physical dimensions of the probe (4 mm) which explains that the velocity does not reach zero at the wall.

Underfloor aerodynamics (Figure 2, B): the velocity of the air underneath the car is higher in the AirShaper simulations compared to the ProCAP results. One explanation for this is the impact of scaled model testing: the ProCAP results are based on physical wind tunnel results, where surface roughness on both car & ground can influence results. Also, depending on the upstream length of the wind tunnel test section, the boundary build-up can be quite significant (reducing the air speed below the car), as the floor is static. The AirShaper simulation also has a static floor (although a moving floor is also possible), but of limited length. Another argument is that the wall functions under-predict the boundary layer thickness, especially because a perfectly smooth surface is assumed. And in terms of ProCAP measurements, the relatively coarse spatial resolution of the capturing in that area (due to the confined space and difficulties in the optical tracking with cameras placed above the model) leads to interpolation effects, reducing local gradients and peak velocities.

Shear layers (Figure 2, C): the "thickness" of the transition layer between the wake behind the car (slower moving air being dragged along) and the free stream air (air flowing on top of the wake) is smaller in simulation compared to the ProCAP measurements. One reason for this is that the AIrShaper simulation uses a turbulence model to resolve velocity variations, and perhaps the applied model underpredicts the thickness of this shear layer. In terms of ProCAP measurements, the finite dimension of the probe and resolution of the grid creates smoothing effect, which can overpredict the shear layer thickness.

Wake regions (Figure 2, D): Differences of the wake behind the wheel illustrate the sensitivity to surface roughness and small details, both difficult to reproduce correctly in CFD geometry and the wind tunnel model.

## Which tool to use?

Both the AirShaper and ProCap offer an alternative to the high-end high-cost approach in aerodynamic development able to provide 80% of the information for 20% of the budget. Can they provide valuable data and help to understand and improve the design? Absolutely. Is there a tradeoff between speed and accuracy? To some extent, yes. But the goal is not to replace the high-end stuff but rather to offer an option where otherwise no data at all would be available.

Challenging in this comparison, but also in most real-life cases, is the lack of ground truth. As so often, there remains a grey zone in terms of which method accounts for what difference. Of course, when validating & developing such tools, it's important to benchmark results versus "well known & documented" cases (like this Tesla Model Y). This helps to build a "general understanding" of the limitations of each tool. But for specific, non-standardized cases, an amount of uncertainty around result accuracy will always remain.

While AirShaper strongly accelerates the digital phase, with a fully automated CFD workflow able to analyze & compare the full flow pattern of various design concepts in just hours, ProCap provides substantial advantages during physical wind tunnel testing, correctly capturing fine details & flow separation on the fly in the wind tunnel.

Which tool? Both please :)

Interesting links:

 

<ExternalLink href="https://www.streamwise.ch/">Streamwise</ExternalLink>

 

<Link href="https://app.airshaper.com/projects/new?accuracy=regular">
 Run Your Own Simulation
</Link>

Accelerating the aerodynamic design process

The <ExternalLink href="https://catesbytunnel.com/">Catesby Tunnel</ExternalLink> is a multi-million pound project that is set to become a new global benchmark for full scale vehicle testing. The project has been in development since 2013 and this week it will be open for business for the very first time. To find out more, we spoke to Jon Paton, Group Leader at <ExternalLink href="https://www.totalsimulation.co.uk/computational-fluid-dynamics/catesby-tunnel/">TotalSim</ExternalLink> who are leading the engineering behind the project.

---

## What is the Catesby Tunnel?

The Catesby Tunnel was originally constructed as a Victorian railway tunnel on the Great Central railway main line in the UK. It was built in 1897 and was in service for 68 years until it eventually closed in 1966. It is a perfectly straight 2.7km long tunnel, which took approximately 30 million bricks to build.

<ImageWithCaption
 src="/assets/images/roadcar-catesby-tunnel.jpeg"
 alt="A roadcar testing in the refurbished Catesby Tunnel. CREDIT: www.catesbytunnel.com"
 width="810"
 height="540"
 positionCenter={true}
/>

In 2013, the Aero Research Partners Ltd initiated the design phase to convert the disused railway tunnel into a state-of-the-art vehicle testing facility. A science and technology park will also be built adjacent to the tunnel, offering CFD, 3D scanning and other services. The Aero Research Partners Ltd is made up of TotalSim, a private investor and an automotive OEM.

---

## The Catesby Tunnel in numbers

The tunnel is 2.7km (1.7 miles) in length and 8.2m wide which is unusually wide for a tunnel in the Victorian era. In fact, it's width is more than double that of conventional tunnels at that time, pushing Victorian construction to the limit. This results in a cross-sectional area of 40m².

Another impressive statistic is the gradient of the track bed, which is a constant 1:176. So for each 176m travelled, the height only changes by 1m, which is particularly useful for repeatable testing. The Victorian engineers couldn’t have done much more to make it perfect for an aerodynamic test facility.

<ImageWithCaption
 src="/assets/images/1949-catesby-tunnel.jpeg"
 alt="The Catesby Tunnel in 1949. It's large width makes it ideal for an aerodynamic test facility. CREDIT: www.warwickshirerailways.com"
 width="810"
 height="565"
 positionCenter={true}
/>

---

## How does it compare to road testing?

Real world testing such as on a test track or proving ground is an important element in vehicle development programmes. From an aerodynamics point of view, it offers many advantages over testing in a wind tunnel:

- The engine and other components are running at true operating temperatures
- The asphalt and therefore tyre behaviour is more realistic as opposed to a partial or full moving floor in a wind tunnel
- There are no flow-disturbing connection elements holding the car in place, as would be the case in a wind tunnel
- There are minimal <Link href="/videos/how-wind-tunnels-work-blockage-factor-wall-effects-scaled-model-similarity-number-moving-floor/qqYiz1eo-F4">blockage effects</Link> which can affect the quality of the airflow

However, the downside of testing in the real world is the lack of repeatability due to changing weather conditions, which is particularly challenging in the UK:

- The magnitude and orientation of the wind can change, drastically impacting aerodynamic drag
- The temperature, humidity and density of the air can change, again impacting drag and also engine performance
- Real roads are often not perfectly flat and can feature bumps

The Catesby Tunnel, by contrast, suffers from very few of these problems. It sits below thousands of tons of earth, providing it with an enormous thermal inertia. This greatly reduces variations in temperature, density and humidity, which strongly increases the repeatability of vehicle tests.

<ImageWithCaption
 src="/assets/images/catesby-tunnel-road-resurfacing.png"
 alt="Relaying the road surface of the Catesby Tunnel. CREDIT: www.semlep.com"
 width="810"
 height="405"
 positionCenter={true}
/>

"The length of the tunnel also means that there are no wind effects so the flow distribution is much more uniform than in a conventional wind tunnel,' explains Jon Paton, Group Leader at TotalSim. 'This means there is no need to implement flow conditioning techniques such as <Link href="/videos/how-wind-tunnels-work-contraction-test-section-diffuser-fan-turning-vanes-and-settling-chamber/L1AYo9Mk1EI">meshes and grids</Link> to reduce perturbations in flow velocity.'

By improving the repeatability of tests, engineers are then able to isolate and compare smaller differences in measured forces, knowing that any changes are real and not just measurement uncertainties. This allows for more detailed design iterations where every marginal gain can add up to a substantial performance improvement.

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## How does it compare to wind tunnel testing?

Wind tunnels are also designed to overcome some of the problems of real world testing. They aim to eliminate the same repeatability problems and add some additional benefits. Forces can be monitored via a <Link href="/videos/how-wind-tunnels-work-measurement-techniques-piv-pressure-taps-force-balance-pressure-probe/yA9FdqJdiKk">force and moment balance</Link> and many different set ups of the vehicle can be analysed within a short time frame, such as aero mapping.

- <Link href="/videos/how-aero-maps-can-improve-race-car-performance/u-GyTRblNmc">
 How aero maps can improve car performance
 </Link>

However, nothing comes for free, so there are also a few downsides to using wind tunnels. 'This is where the unusually large width of the Catesby Tunnel can add real value to a test programme,' highlights Paton. 'The cross-sectional area of 40m² is considerably larger than a typical car. Take the Tesla Model 3 for example, which has a frontal surface area of around 2.3m², this means the blockage ratio is less than 6%.'

'The large width of the Catesby Tunnel also helps to minimise wall interactions which would normally affect the quality of the airflow. The Tesla Model 3 is around 1.85m wide compared to the 6m wide roadway in the tunnel.'

<YouTubeEmbed
 videoId="_Z6_QYlbS8c"
 title="The transformation of Catesby Tunnel"
/>

Another benefit to testing at Catesby is that the vehicles are actually being driven. As a vehicle moves through air, it drags along a certain amount of air – an effect called “added mass”. This will be captured more accurately in the Catesby Tunnel then it would be in a conventional wind tunnel.

Although, wind tunnels are also used in the early stages of a vehicle's design life; testing models with no chassis components. Whereas at the Catesby Tunnel, cars have to be fully drivable and so have to have a functional drivetrain. To get round this issue, customers can use an electric driving platform and mount items that they want to test, such as 3D printed bodywork designs, to this platform.

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## What types of tests can be done?

Alongside full scale aerodynamic performance testing, the Catesby Tunnel can also be used for vehicle performance evaluation. Low and high speed, acceleration and braking can all be tested along with the thermal performance of the powertrain, radiators and brakes. Aero-acoustics such as cabin noise, engine sound and exhaust systems can also be tested, removing the need for manufacturers to visit specific acoustic testing facilities.

<ImageWithCaption
 src="/assets/images/catesby-tunnel-cyclist.jpg"
 alt="The Catesby Tunnel can also be a useful facility for optimise the aerodynamics of athletes. CREDIT: www.catesbytunnel.com"
 width="810"
 height="387"
 positionCenter={true}
/>

- <Link href="/videos/designing-an-aero-bike/DqlDpUFcScc">
 How to design an aero bike to win the Ironman
 </Link>

Catesby can also be used for emissions testing, with Paton hinting that the tunnel may also support manufacturers towards WLTP certification. WLTP (World Harmonized Light Vehicles Test Procedure) is a global standard which determines fuel consumption, range and emission figures in a more representative way. The controlled environmental conditions offered by the Catesby Tunnel could, over time, also make it suitable to conduct the necessary WLTP tests.

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## How will accurate measurements be achieved?

Similar to real world testing, measurements are currently limited to on-vehicle or in-vehicle test equipment. For force measurements, Paton and his team are currently developing force plates which can be integrated into the ground, measuring the force as the vehicle drives over them.

'We may also include a system that will measure pressure differences on the wall of the tunnel to capture the pressure reflections of a vehicle passing by,' says Paton. 'By modelling the same vehicle passing through CFD [Computational Fluid Dynamics] simulations, we can then determine a correlation factor between pressure measurements and vehicle drag. This will allow us to estimate the drag on the car.'

There is also a plan to install an ultra-wide band indoor GPS system to measure the position and speed of the vehicle inside the tunnel with a precision of 0.1 km/h. This will allow for more accurate coast-down testing to measure vehicle drag and rolling resistance.

- <Link href="/videos/drag-measurement-coastdown-testing/Gip3iG2A2zE">
 How to use coastdown testing to measure drag
 </Link>

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## How will testing be kept confidential?

Confidentiality is always a huge consideration for OEMs and other users of facilities like this. Even at some of the most secure outdoor testing facilities, spyshots from unathorised photographers can be leaked, but this is impossible at Catesby.

"The tunnel is effectively a closed-off room that customers can hire and seal, so the Catesby team ensures full confidentiality," says Paton. "Vehicles are unloaded behind closed doors and then enter the tunnel directly. With 99% of measurements expected to be on-vehicle, we anticipate very few confidential setup and component changes to happen outside of the tunnel itself."

<ImageWithCaption
 src="/assets/images/catesby-tunnel-facility.jpeg"
 alt="A science and technology park will be built adjacent to the tunnel, offering CFD, 3D scanning and other services. CREDIT: www.catesbytunnel.com"
 width="810"
 height="458"
 positionCenter={true}
/>

Overall, the Catesby Tunnel is an all-in-one cost effective solution which allows customers to test many aspects at the same time and on the same vehicle. "We have developed the Catesby Tunnel to offer a number of unique advantages in terms of repeatability, real-world conditions and cost-effectiveness," concludes Paton. "This makes it ideal for range and downforce optimization as well as achieving those all important marginal gains. By blending CFD simulations with real world testing, customers can conduct advanced vehicle development programmes and ultimately improve the performance of their vehicles."

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##### Interesting links:

 

<ExternalLink href="https://catesbytunnel.com/">Catesby Tunnel</ExternalLink>

 

<ExternalLink href="https://www.wltpfacts.eu/">WLTP</ExternalLink>

 

<Link href="/videos/how-to-run-an-aerodynamic-simulation/O_DNDxCtI_I">
 How to use coastdown testing to measure drag
</Link>

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