AirShaper

To learn more about:
- Boundary layers: https://youtu.be/TwOxa9rAOfE
- Flow separation: https://youtu.be/HfICmKv9SzE
- Golf ball aerodynamics: https://youtu.be/KzXcglAhxps

In this video, we'll explain the aerodynamics behind curved free kicks, corners, and so on! Want to bend it like Beckham? Then stay tuned!

When you kick the ball off center, you'll give it a spin around the vertical axis. At one side, the surface is spinning against the airflow, so the relative velocity between the surface and the air is increased. This slows down the air and also moves the separation point upstream.

At the other side, the surface is spinning in the same direction as the airflow, reducing the relative velocity. This reduces friction and boundary layer buildup and pushing the separation point further downstream.

First way to explain the force - Bernoulli:
the faster moving air will feature lower pressure, creating a suction effect bigger than on the other side. This generates a net lateral force.

Second way to explain the force - momentum:
the wake behind the soccer ball is pulled towards the side of the forward spinning side. Because of Newton's law of action and reaction, this generates an equal but opposite effect on the ball.

The effect described in this video is called the magnus effect.


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The AirShaper videos cover the basics of aerodynamics (aerodynamic drag, drag & lift coefficients, boundary layer theory, flow separation, reynolds number...), simulation aspects (computational fluid dynamics, CFD meshing, ...) and aerodynamic testing (wind tunnel testing, flow visualization, ...).

We then use those basics to explain the aerodynamics of (race) cars (aerodynamic efficiency of electric vehicles, aerodynamic drag, downforce, aero maps, formula one aerodynamics, ...), drones and airplanes (propellers, airfoils, electric aviation, eVTOLS, ...), motorcycles (wind buffeting, motogp aerodynamics, ...) and more!

For more information, visit www.airshaper.com

Soccer aerodynamics - How to curve a ball

Part 1: https://youtu.be/K2jgsgihZ_w
Tuft testing on a Vokswagen Beetle: https://youtu.be/CGmqdXKgBiQ

In this second video on the Porsche Taycan Aerodynamics, we’ll take the car to the street to compare tuft movement to simulation data.
Special thanks to the Porsche Centre in Mechelen, Belgium, for letting us play with this fantastic car 😊
https://www.porsche.com/belgium-mechelen

In our last video, we had equipped the Porsche Taycan with a number of tuft

Rear window & spoiler

On the rear window and spoiler, none of the tufts move. This is because the flow in that area stays nicely attached & laminar. There is some slow-down of the flow due to the “negative” curvature of the window, but this doesn’t lead to separation.

Rear wheels
The rear wheels create a turbulent zone which extends downwind (causing a lot of movement of the tufts on the rear bumper) and also upward (creating movement above the wheel arch as well). The latter extends all the way to the rear spoiler.

A-pillar and side window
As the air makes its way from the front wind shield to the side window, it curves around the A-pillar. In doing so, a vortex is created which locally pushes the air upward, just behind the A-pillar. This upward motion extends further downwind at the top of the side window as well.

Front wheels
The front wheels also create a turbulent zone which splashes out upward. Further downstream, this zone is countered by the air coming off the front hood, which creates a zone of attached air curving downward on the flanks.

Headlamps
The headlamps are located in a “pocket” in the bodywork. As the air jumps out of this pocket, it locally separates from the bodywork and leaves behind a local bubble of separated, turbulent flow. This can be seen in both simulation & tuft movement.


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The AirShaper videos cover the basics of aerodynamics (aerodynamic drag, drag & lift coefficients, boundary layer theory, flow separation, reynolds number...), simulation aspects (computational fluid dynamics, CFD meshing, ...) and aerodynamic testing (wind tunnel testing, flow visualization, ...).

We then use those basics to explain the aerodynamics of (race) cars (aerodynamic efficiency of electric vehicles, aerodynamic drag, downforce, aero maps, formula one aerodynamics, ...), drones and airplanes (propellers, airfoils, electric aviation, eVTOLS, ...), motorcycles (wind buffeting, motogp aerodynamics, ...) and more!

For more information, visit www.airshaper.com

Porsche Taycan Aerodynamics Part 2: Road testing

For part 2, check this link: https://youtu.be/kyHjWGnrByQ

In this video, we will go over the aerodynamic features of the Porsche Taycan and prepare it for road testing! Special thanks to the Porsche Centre in Mechelen, Belgium, for letting us play with this fantastic car 😊
https://www.porsche.com/belgium-mechelen

In our last video, we had optimized the rear of the Porsche Taycan using our Aerodynamic Shape Optimization technique (to have it generate more downforce), In this video, we get up close with all the other special features of this car (underfloor, diffuser, wheels, …)


Underfloor

At the front splitter, the air jumps underneath the car and accelerates as it gets into the channel formed by the road surface and the underfloor of the car. This increased velocity will reduce the pressure, creating a suction force pulling the car downward (which is good for handling).
On electric cars, the underfloor can be made very smooth & flat, as it sits below the battery pack and as they have very little exposed components. So the air maintain its velocity and connect with the diffuser at the rear, where it connects to the wake behind the car, reducing drag, increasing downforce and so on.


Wheels
Wheels are a major source of aerodynamic drag on cars. They are large, unshielded elements rotating through the air, acting like a mixer. Not only do they generate drag themselves, they also create turbulent air downstream, which can increase drag / reduce performance elsewhere.

Porsche has fitted the car with air curtains, which are channels in the front bumper. The opening at the front accepts the high pressure are at the nose of the car which is then sped up to exit at high velocity through a slot at the rear of the front bumper, just ahead of the front wheel. This creates a curtain, shielding the turbulent air coming off the rotating wheel.

Another feature are the aerodynamic rims. To limit their drag, they created a rim which is more “covered” than a regular rim. Secondly, the profile of the spokes has been designed in such a way that the air nicely slides across the spokes, instead of being pushed away by more blunt spoke shapes.

And finally, Porsche also air deflectors in front of the wheels: these are small “ramps” that divert the air so it doesn’t hit the wheels directly (where it would create a large wake). These are present both at the front & the rear wheels.


Rear spoiler
The active rear spoiler will automatically extend when you are going fast enough to improve downforce & aero balance of the car.


Coolers
Air is forced through the coolers and this represents an energy loss. So when you don’t need full cooling power, it’s good for efficiency to (partially or fully) close the entrance to the coolers. This will reduce the energy lost on air going through the coolers and will make the air go around the car instead.


Ride height
The Porsche Taycan has an extremely low drag coefficient of 0.22 and one of the tricks to achieve this is to lower the ride height of the car. Lowering the ride height at low speeds would be impractical towards ground clearance, speed bumps etc, so it is lowered dynamically as the car picks up speed. A lower ride height also has a positive impact on the handling of the car, as downforce typically increases with reduced ride height.


Tufts
We equipped the car with tufts, which are small strands of wool that will align themselves with the local orientation of the airflow. This allows us to analyse flow patterns and see where the air has detached and/or has become turbulent.


Next video
Stay tuned for our next video, where we’ll take the car for a ride and compare the tuft patterns with what we saw in simulations!


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The AirShaper videos cover the basics of aerodynamics (aerodynamic drag, drag & lift coefficients, boundary layer theory, flow separation, reynolds number...), simulation aspects (computational fluid dynamics, CFD meshing, ...) and aerodynamic testing (wind tunnel testing, flow visualization, ...).

We then use those basics to explain the aerodynamics of (race) cars (aerodynamic efficiency of electric vehicles, aerodynamic drag, downforce, aero maps, formula one aerodynamics, ...), drones and airplanes (propellers, airfoils, electric aviation, eVTOLS, ...), motorcycles (wind buffeting, motogp aerodynamics, ...) and more!

For more information, visit www.airshaper.com

Porsche Taycan Aerodynamics Part 1: aero features explained

For more about measurement techniques, check this series on how wind tunnels work:
- Part 1: https://youtu.be/L1AYo9Mk1EI
- Part 2: https://youtu.be/qqYiz1eo-F4
- Part 3: https://youtu.be/yA9FdqJdiKk


Pitot tubes are small devices used on aircraft, turbo machinery and even Formula One cars to measure flow velocity, or rather, measure pressure to calculate flow velocity.

Basic theory
The Bernoulli stated that the pressure goes gown as the velocity goes up, when following a particle along its flow path. For incompressible fluids, this can be stated using the following equation:

P_static + 0.5*rho*v² = P_total

P_static: this is the static pressure, which is the pressure you would feel when traveling along with the flow without disturbing it.
0.5*rho*v²: this is the dynamic pressure, which basically relates to the kinetic energy of the fluid.
P_total: this is the total pressure, which is the pressure you would measure when you bring the flow to a complete standstill – therefore, it’s also called the stagnation pressure.

The interesting thing about this equation is that we can reshuffle it to calculate the velocity based on the static pressure and total pressure:
V = sqrt( (2*(P_total-P_static) ) / (rho) )


Measurement probes
So how do we measure the static and total pressure? Well, you guessed it – we can use a pitot tube!
At the front of the pitot tube, which points directly into the wind, the air comes to a complete standstill. This means that the pressure measured there equals the total pressure.
At the side of the pitot tube, the openings are parallel to the flow and so it is not slowed down. A “static ring” as it is called is often used in these kinds of probes.  The pressure measured here equals the static pressure.

Logging the pressure difference
The next step is to obtain the difference between the static pressure and the total pressure to calculate the dynamic pressure. You can do this in an analogue way, by connecting both tubes coming from the measurement openings to a manometer or a membrane. The difference in water height or membrane deflection can then be used to calculate the pressure delta and then the velocity. Or you can hook the tubes to a pressure transducer and use the electric output signal to log the data.

Limitations
Keep in mind that a pitot tube can only measure the relative wind speed, so the difference in velocity between the object and the air (called the airspeed in aviation terms). So if you want to know the absolute speed of your object versus the real world (called the ground speed in aviation), you’ll also need to measure the velocity of the wind itself. Or cheat, and use your GPS 😊
Also important is to keep the pitot tube pointing straight into the wind, although more advanced versions are able to measure not just wind speed but also the angle of attack of the wind. –7
Pitot tubes come in various shapes and forms, like this elbow version I got from the lovely Katharina Kreitz of Vectoflow – they actually make pitot tubes using 3D laser sintering to create complex internal channels for custom applications.

Use in applications
Pitot tubes are used in countless applications. Airplanes are the most obvious, where they log the airspeed. But they are also used on the front of Formula One cars, to have a good reference velocity to compare to wind tunnel data. They’re also key to measure & calibrate the wind speed of wind tunnels, just ahead of the test section. And I’ve been told pitot tubes have even been used to measure the airflow in kitchen cooker hoods – not kidding you!

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The AirShaper videos cover the basics of aerodynamics (aerodynamic drag, drag & lift coefficients, boundary layer theory, flow separation, reynolds number...), simulation aspects (computational fluid dynamics, CFD meshing, ...) and aerodynamic testing (wind tunnel testing, flow visualization, ...).

We then use those basics to explain the aerodynamics of (race) cars (aerodynamic efficiency of electric vehicles, aerodynamic drag, downforce, aero maps, formula one aerodynamics, ...), drones and airplanes (propellers, airfoils, electric aviation, eVTOLS, ...), motorcycles (wind buffeting, motogp aerodynamics, ...) and more!

For more information, visit www.airshaper.com

Pitot Tubes - Measuring Flow Velocity

To see actual show cases of adjoint shape optimization:
- Porsche Taycan render: https://youtu.be/-fBXwx_n10I
- Aptera optimization: https://youtu.be/WpFNwfHwFn4

In this video, we'll discuss Aerodynamic Shape Optimization using the adjoint technique.


Aerodynamic Optimization
In aerodynamic simulations, the airflow around an object is chopped up in small pieces called cells within a mesh. The surface of the object is also chopped up into small pieces called surface cells with nodes on the corners.

To obtain the total drag or lift of the object, we need to integrate the pressure & friction forces on all these local surface cells to get the total value. This indicates that the position of each individual node has an influence on the final drag & lift on the object.

The objective function, therefore, has thousands if not millions of input variables: the X, Y and Z coordinate of each individual node. To obtain the local gradient, you would need to move one node individually and then re-run the entire calculation to calculate the local gradient. With that information, you know whether to push the node inward or outward to increase or decrease the objective function. Once you have obtained all these local gradients, you can put them together into what is called a surface sensitivity map.

As you can imagine, re-running the entire simulation for each node displacement can become very expensive. Luckily, there is something called the adjoint technique.

Adjoint
The adjoint technique allows you to calculate all of these local gradients, or sensitivities, in one single simulation. Here's how this works:

- Normal aerodynamic simulation (primal or forward simulation):
After the computational mesh has been generated, the flow field is typically initialized using a fixed value (zero or something else). Then, the properties defined at the boundary conditions (like an inlet flow velocity for a virtual wind tunnel) propagate through the domain. After enough small time steps, the solution will have converged to a stable flow field.

- Adjoint simulation (dual or backward simulation)
Starting from the aerodynamic simulation data, the adjoint simulation will take small "time" steps. It calculates backward from the objective function to see how each node position affects this objective function. Based on these results, we can then calculate all the local gradients and construct the sensitivity map.

So instead of running a full aerodynamic simulation for each local gradient calculation, which would be prohibitively expensive, we can now calculate all the gradients with just two simulations: the normal (primal) and the adjoint (dual) simulation.

Morphing
The next step is to use the input of the sensitivity map and start implementing it: move the surface inward or outward based on what the map tells you. Once you have morphed the 3D object, you can re-run the aerodynamic simulation to see how much the objective value has changed. You can then also re-run the adjoint simulation to create a new sensitivity map and to the morphing again.

With each cycle, you will get closer to the most optimal shape for your object. You can do this for the entire object or you can define a design space in which the algorithm can operate.

Porsche Taycan
Interestingly enough, the adjoint optimization technique - when asked to increase downforce - starts to create the iconic ducktail spoiler that Porsche has been applying for years.

For more information or to run your own optimization project, just contact AirShaper at info@airshaper.com


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The AirShaper videos cover the basics of aerodynamics (aerodynamic drag, drag & lift coefficients, boundary layer theory, flow separation, reynolds number...), simulation aspects (computational fluid dynamics, CFD meshing, ...) and aerodynamic testing (wind tunnel testing, flow visualization, ...).

We then use those basics to explain the aerodynamics of (race) cars (aerodynamic efficiency of electric vehicles, aerodynamic drag, downforce, aero maps, formula one aerodynamics, ...), drones and airplanes (propellers, airfoils, electric aviation, eVTOLS, ...), motorcycles (wind buffeting, motogp aerodynamics, ...) and more!

For more information, visit www.airshaper.com

Aerodynamic Shape Optimization - The Adjoint CFD Method

Learn more about:
- Reducing pickup truck drag: https://youtu.be/E0ass_AxloM
Road testing the Porsche Taycan: https://youtu.be/kyHjWGnrByQ

How to use coastdown testing for drag measurement

What is coastdown testing?
A coastdown test is one of the most common tests in the automotive industry. It is a way of measuring the mechanical and aerodynamic resistance that acts on a vehicle. 
The vehicle is accelerated to a particular speed and once constant, the clutch is engaged. The vehicle is then allowed to slow or ‘coast’ down in neutral. By recording the distance travelled and the time taken, the aerodynamic drag can be calculated (see also: https://airshaper.com/videos/what-is-aerodynamic-drag/video/DbIIw-WryHY)

How to calculate the drag coefficient
See also: https://airshaper.com/videos/what-is-a-drag-coefficient/video/bEgoZ_dAg7o
For simplicity, let’s assume the vehicle is slowed down by only two forces: 1) mechanical resistance and 2) aerodynamic resistance. Mechanical resistance is primarily due to the rolling resistance of the tyres and therefore remains constant, regardless of the vehicle’s velocity. Whereas aerodynamic resistance changes with the square of speed. In other words, drag force has less of an affect at low speeds, but a large affect at high speeds.

Newtons 2nd law states that the acceleration of an object is directionally proportional to the sum of the forces acting on it. Applying this to our simplified coastdown test, we get an equation which includes a rolling resistance coefficient, Crr and a drag coefficient, Cd.
So how do you calculate the value of two coefficients from a single equation with only one measurement? First, let’s look at two extremes:

1)   No aerodynamic resistance
If the vehicle completed a coastdown test in a vacuum, where there was no air resistance, then only the mechanical resistance would slow it down. Therefore, the car would decelerate at a constant rate, resulting in a linear deceleration curve.

2)   No mechanical resistance
If the vehicle had no rolling resistance and so only the aerodynamic resistance was slowing it down, then the deceleration curve would be non-linear. The car would decelerate faster at high velocity and slower at low velocity.

In reality, the measured curve from your coastdown test would be a combination of both. You can then adjust the coefficients to alter the shape of the curve until the measured curve matches the predicted curve. This process is called least squares regression. Overall, you will end up with a value for aerodynamic drag and a value for mechanical resistance.

As ever in aerodynamics, it is unfortunately not that easy. In reality, the equations mentioned above are much more complex. They need to account for factors such as mechanical resistance in the drivetrain, ambient conditions, wind direction, gravitational force and rotational inertia.

Yet, despite its complexity, coastdown tests are an industry standard for determining the performance of a vehicle. So, if you want to find out the drag coefficient of your car, why not use your cell phone’s GPS and some Excel magic to conduct your own coastdown test?

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The AirShaper videos cover the basics of aerodynamics (aerodynamic drag, drag & lift coefficients, boundary layer theory, flow separation, reynolds number...), simulation aspects (computational fluid dynamics, CFD meshing, ...) and aerodynamic testing (wind tunnel testing, flow visualization, ...).

We then use those basics to explain the aerodynamics of (race) cars (aerodynamic efficiency of electric vehicles, aerodynamic drag, downforce, aero maps, formula one aerodynamics, ...), drones and airplanes (propellers, airfoils, electric aviation, eVTOLS, ...), motorcycles (wind buffeting, motogp aerodynamics, ...) and more!

For more information, visit www.airshaper.com

Drag Measurement  - Coastdown Testing

How to use coastdown testing for drag measurement

For more videos on how wind tunnels work:
- Part 1: https://youtu.be/L1AYo9Mk1EI
- Part 2: https://youtu.be/qqYiz1eo-F4

In this third video in our series "How Wind Tunnels Work" we look at wind tunnel measurement techniques - both some of the most widely used as well as some innovative ones.

Forces & moments
One of the most important output parameters of a wind tunnel test are the forces & moments on the object. To obtain these, the object is typically connected to a 3D force balance, which is a device that can measure the forces and moments around three perpendicular axes.

It's important to make sure that the influence of the connection part does not disturb the airflow too much, which is why they're often given an aerodynamic profile.

Surface pressure measurement
To measure the static pressure on the surface, you can use pressure taps. These are small holes drilled perpendicularly to the surface with a tube fitted to each hole in the model. These tubes are then sent to a pressure transducer which is typically located outside the wind tunnel, where the pressures are measured & logged.

If you want to measure the pressure on many locations on the surface of your object, you'll quickly end up with a huge set of cables that needs to be led outside. This is something you need to take into account during the design of your prototype.

Alternatively, you can work with miniaturized pressure transducers that fit within your model and then convert the measurements into digital data which is then sent to the outside world via a single cable to save space.

3D flow measurement
- Smoke: one of the oldest & most common techniques is to release smoke upstream of the object to trace the path of the air. This can lead to very insightful information in terms of where the separation point is located, what the size is of certain vortex structures and so on. But it's quite difficult to determine local flow velocities, so over the years, a number of alternatives have been developed.
- PIV - particle image velocimetry: tracer particles are released upstream of the area of interest. The size, density, ... of these particles are chosen in such a way that they follow the real flow nicely, without dropping to the ground or disturbing the flow pattern. They are then illuminated, typically in a 2D plane, using a laser. Then, they're filmed perpendicularly to that plane by high-speed cameras. By tracking & tracing the position of the individual particles between consecutive shots, one can reconstruct their path and calculate the velocity vector and flow field.
- 3D measurement probes: by measuring the differential pressures at the head of these multi-hole probes, you can obtain the static pressure and total pressure, the angle of attack and the velocity. If you then trace the position of the probe itself in 3D, you can reconstruct a 3D flow field.

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The AirShaper videos cover the basics of aerodynamics (aerodynamic drag, drag & lift coefficients, boundary layer theory, flow separation, reynolds number...), simulation aspects (computational fluid dynamics, CFD meshing, ...) and aerodynamic testing (wind tunnel testing, flow visualization, ...).

We then use those basics to explain the aerodynamics of (race) cars (aerodynamic efficiency of electric vehicles, aerodynamic drag, downforce, aero maps, formula one aerodynamics, ...), drones and airplanes (propellers, airfoils, electric aviation, eVTOLS, ...), motorcycles (wind buffeting, motogp aerodynamics, ...) and more!

For more information, visit www.airshaper.com

How Wind tunnels Work - Measurement techniques, PIV, pressure taps, force balance, pressure probe.

To accurately understand the behaviour of the airflow around an object requires a variety of wind tunne measurement techniques ranging from pitot tubes and kiel probes to flow visualisation strategies such as smoke jets and PArticle Image Velocimetry (PIV).

Unveiling the secrets behind wind tunnel measurement

For more videos on how wind tunnels work:
- Part 1: https://youtu.be/L1AYo9Mk1EI
- Part 3: https://youtu.be/yA9FdqJdiKk

In this video, we explain the huge variations in size, speed and accuracy of low-speed aerodynamic wind tunnels. This is mainly due to variations in:
- object size
- object velocity
- required accuracy


Object size
When you place an object in the wind tunnel, the air needs to curve around it and fit between the object and the walls of the wind tunnel. This creates 2 problems:
- Blockage ratio: around the object, the cross-section through which the air can flow is reduced. This means the air will locally speed up compared to the reference velocity, which is typically set at the undisturbed beginning of the tunnel using a pitot tube.
To compensate for this, you can calculate the blockage factor. Typically, it's recommended to keep this value below 5-10%.
- Wall effects: a boundary layer of slower moving air is present close to the wall. In reality, this slower moving air is usually not present and can bias the results. Also, the walls can interfere with the flow & pressure patterns around the object if they're too close. In that regard, it can be very interesting to first run a CFD simulation to analyse the flow and see exactly how large this "zone of disturbed air" is to know how far it should form the walls. One example are the vortex structures around wingtips.

In some cases, you'll even see wind tunnels employing an open test section, partially to overcome the challenges of blockage ratio & wall effects.


Object speed
The velocity of the application can vary greatly, with a cyclist going 10 m/s and an airplane 250 m/s. Ideally, you select a wind tunnel with the wind speed that you need. In reality, there will be 2 main constraints:
- Budget: an automotive tunnel can easily cost 5.000€ per hour. Building one also costs over 100 million euros in some cases.
- Availability: tunnels are often overbooked and sometimes they just don't exist in the size/speed combination that you need.

Scaled model testing can help in that case. But keep in mind that you'll need to respect certain similarity numbers - the most important one being the Reynolds number. If you want to keep the Reynolds the number the same when scaling down your model by a factor X (so length_scaled = length_full / X), then you need to scale up the velocity by that same factor X (so velocity_scaled = velocity_full * X). This can make you hit the speed limit or the wind tunnel or even the limit of compressible flow (around Mach 0.3).


Accuracy
The higher the flow quality or the more realistic the setup needs to be, the more tricks you need to pull, like cooling the air, working with a moving ground or even employing gases other than air at different pressures / temperatures.

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The AirShaper videos cover the basics of aerodynamics (aerodynamic drag, drag & lift coefficients, boundary layer theory, flow separation, reynolds number...), simulation aspects (computational fluid dynamics, CFD meshing, ...) and aerodynamic testing (wind tunnel testing, flow visualization, ...).

We then use those basics to explain the aerodynamics of (race) cars (aerodynamic efficiency of electric vehicles, aerodynamic drag, downforce, aero maps, formula one aerodynamics, ...), drones and airplanes (propellers, airfoils, electric aviation, eVTOLS, ...), motorcycles (wind buffeting, motogp aerodynamics, ...) and more!

For more information, visit www.airshaper.com

How Wind tunnels Work – Blockage factor, wall effects, scaled model, similarity number, moving floor

To ensure the accuracy of scale model wind tunnel testing, the models are now highly detailed and consist of moving parts. Modern stings and support designs allow the models to be moved continuously through different attitudes to maximise wind on time.

Scale model wind tunnel testing explained

For more videos on how wind tunnels work:
- Part 2: https://youtu.be/qqYiz1eo-F4
- Part 3: https://youtu.be/yA9FdqJdiKk

In this first video of our series on wind tunnels, we’ll discuss the basic layout.

History
Wind tunnels were invented in the late 19th century and even the Wright Brothers used a simple wind tunnel to study different shapes for their Wright Flyer, the first plane. The principle of a wind tunnel is quite simple: Use a single or multiple fans to force air across an object to study its aerodynamic properties.

Layout
In an open return tunnel, the air is drawn from the environment in which the tunnel is located. This type of wind tunnel is typically cheaper to construct and doesn’t suffer too much from smoke or exhaust gases accumulating inside. It is however quite expensive to operate, as it constantly needs to accelerate “new” air from standstill. It’s also more challenging to achieve a good flow quality as the air needs to curve into the inlet. And the open exit makes it quite noisy as well.

Then there’s the closed return tunnel, in which the exit air is fed back to the inlet. This design is much more energy efficient as the fan only needs to overcome the losses and it’s quieter too and it has the potential for a higher flow quality. Downsides are a higher construction cost and because the air circulates, it can heat up quite a lot and smoke & exhaust gases can accumulate.

Goals
Whichever return type you go for, it’s very important to have good flow characteristics at the test section. Typically, the goal is to have a low turbulence intensity, a uniform flow velocity and a thin boundary layer.

Components
Let’s have a look at how the typical components of a tunnel are set up to achieve this: first, the fan pushes the air forward. Then, in the corners, turning vanes are added to help straighten the flow and reduce pressure losses.

After the last corner, the air hits the settling chamber, where it first passes through a honeycomb structure. This is basically a set of tubes to force the air to move in a parallel way to reduce the swirl. After exiting the honeycomb, the air goes through a number of screens, which are composed of thin wires interwoven to form square or rectangular meshes. These screens impose a static pressure drop which is proportional to the velocity squared, penalizing speeds above average and boosting speeds below average. This helps to reduce variations in longitudinal flow velocity. They also break up large eddeys into smaller ones, which decay faster.


To reduce the pressure drop in the entire circuit and settling chamber, the cross-section is quite large and thus the flow velocity relatively low. To speed up the air and further improve flow uniformity, after the settling chamber the air is then sent through a contraction just before it enters the test section. After the test section, it is slowed down again in the expanding diffuser before it enters the fan again.


So that was it for our first video on how a wind tunnel works. Stay tuned for more as we discuss sizing challenges and measurement techniques!

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The AirShaper videos cover the basics of aerodynamics (aerodynamic drag, drag & lift coefficients, boundary layer theory, flow separation, reynolds number...), simulation aspects (computational fluid dynamics, CFD meshing, ...) and aerodynamic testing (wind tunnel testing, flow visualization, ...).

We then use those basics to explain the aerodynamics of (race) cars (aerodynamic efficiency of electric vehicles, aerodynamic drag, downforce, aero maps, formula one aerodynamics, ...), drones and airplanes (propellers, airfoils, electric aviation, eVTOLS, ...), motorcycles (wind buffeting, motogp aerodynamics, ...) and more!

For more information, visit www.airshaper.com

How Wind tunnels Work – Contraction, Test Section, Diffuser, Fan, Turning Vanes and Settling Chamber

The accuracy and reliability of the wind tunnel test section can be affected by many factors such as blockage and boundary layer control. However, new technologies and approaches help to solve these issues and achieve repeatable testing.

Our Latest Videos Gain aerodynamic insight by watching our videos