## Meshing

The behaviour of airflow around an object is extremely complex. Phenomena such as turbulence, flow separation and eddy currents can be present and significantly affect aerodynamic performance. To accurately model the intricacies of this flow, CFD simulations need to run with finer meshes in these areas of interest.

<ImageWithCaption
 src="/assets/images/motorBike_before_refinement_slice_2.png"
 width="700"
 height="350"
 positionCenter={true}
/>

<ImageWithCaption
 src="/assets/images/motorBike_before_refinement_slice_zoomed_2.png"
 alt="A mesh around a motorbike which features a refined mesh close to the surface and a coarser mesh further away"
 width="700"
 height="362"
 positionCenter={true}
/>

In CFD, the volume around an object is divided into miniature 3D blocks called cells. Similar to pixels in a digital photograph, the smaller the cells, the more detail you can capture. However, a higher number of cells requires more processing power. Therefore, to balance this demand for detail with efficient run times, aerodynamicists refine the mesh in areas of complex flow behaviour, using a low resolution for the rest of the mesh.

- <Link href="/videos/computational-fluid-dynamics-explained/dyiREvdc4Gg">
 Learn how CFD simulations work
 </Link>

## Adaptive mesh refinement

However, it is impossible for even the most experienced aerodynamicists to accurately predict where the flow will be the most complicated, after all that’s why they’re using CFD. To help solve this problem, adaptive mesh refinement was invented.

This technique runs an initial simulation with a coarse mesh, determines where higher resolutions are required and then automatically refines the mesh in these areas. This process is repeated until the desired accuracy and run time are satisfied. It is also used to refine/unrefine a mesh dynamically during transient simulations, <ExternalLink href="https://www.youtube.com/watch?v=u-VV3euIsXo&ab_channel=HolzmannCFD">as in this Karman vortex street simulation.</ExternalLink>

At the core of AirShaper is an open source CFD software called OpenFOAM. It utilises adaptive mesh refinement but only refines the cells in the volume around an object and not on the object’s surface itself. AirShaper can work with any given 3D model, <Link href="/videos/save-weeks-run-aerodynamic-simulations-on-open-surface-models/12xA80foGXk">even if the geometry is not completely watertight</Link>, so having the capability to use adaptive mesh refinement for such a wide variety of 3D shapes is essential. Not being able to refine the surface detail of the model could lead to unrepresentative results.

To ensure that adaptive mesh refinement could be used for both the volume around an object and for the object’s surface in AirShaper, we developed our own code. Using the default mesher snappyHexMesh in OpenFOAM, we’re excited to announce that we have now made this part of the code open source.

## Open source repository

The <ExternalLink href="https://github.com/airshaper/adaptive-mesh-refinement">AirShaper adaptive mesh refinement repository</ExternalLink> works in a series of steps, as highlighted in the diagram below.

<ImageWithCaption
 src="/assets/images/adaptive_mesh_refinement_flow_chart.png"
 alt="Flow chart showing the stages of AirShaper's open source adaptive mesh refinement code"
 width="600"
 height="506"
 positionCenter={true}
/>

## Original mesh

The process for a regular snappyHexMesh is as follows:

- **Castellated phase:**

 The blockMesh is refined based on edge, surface and region refinement settings. These are manual refinement settings, which are different from adaptive mesh refinement. Also, any mesh inside an object is removed.

- **Snapping phase:**
 Here, the cut cells are snapped to the surface of the object.

For our adaptive mesh refinement solution, we wanted to keep the castellated mesh and store it separately before it is cut out and snapped. Therefore, the castellated phase and snapping phase are executed separately, with the following modifications:

- **Castellated phase:**

 To avoid the inner mesh being cut away, the patches are set to baffle type. This will leave the entire castellated mesh intact. A copy of this mesh is then stored in the “primal refinement” folder.

- **Snapping phase**
 This picks up the castellated mesh generated in the first step, cuts out the inside cells, and then snaps them to the object, just like it would normally.

This “original mesh” is now ready for a CFD simulation with the results stored in the “primal run” folder.

## Refined mesh

To adaptively refine the mesh based on the flow field, the results from the first CFD simulation are mapped onto the castellated mesh that was stored in the “primal refinement” folder. Then, a refinement parameter called “refVal” is calculated (the repository provides methods for both gradient of pressure and vorticity) using those fields and the castellated mesh. It’s important to do this on the castellated mesh, as refinement in snappyHexMesh cannot be done on a snapped mesh.

<ImageWithCaption
 src="/assets/images/motorBike_after_refinement_slice_curlU_2.png"
 width="700"
 height="311"
 positionCenter={true}
/>

<ImageWithCaption
 src="/assets/images/motorBike_before_refinement_slice_refVal_curlU_2.png"
 alt="Illustration of the refVal field based on flow vorticity. Notice how it nicely captures the wake"
 width="700"
 height="214"
 positionCenter={true}
/>

Once the refVal field has been created, the castellated mesh is refined by running pimpleFoam, which uses the refVal field. The user can play with the threshold values of refVal to select more or less cells for refinement in the dynamicMeshDict file.

Once this castellated mesh has been refined, it is sent back to the “primal run” folder. There, the snapping phase is executed again, but now starting from this refined mesh. In this way, all the cells on the surface will be refined and snapped to the object. Once this snapping phase is complete, a second CFD run is then executed with the newly refined mesh.

<ImageWithCaption
 src="/assets/images/motorBike_after_refinement_slice_zoomed_curlU_2.png"
 alt="Example of a mesh refined using the flow vorticity – both volume and surface mesh are refined"
 width="700"
 height="337"
 positionCenter={true}
/>

## What experts say

We’ve had the honour of receiving feedback from a number of leading OpenFOAM experts. Here’s what they had to say about the repository:

- Jozsef Nagy (Founder of <ExternalLink href="http://www.eulerian-solutions.com/solutions.html">eulerion-solutions</ExternalLink> / <ExternalLink href="https://www.youtube.com/channel/UCjdgpuxuAxH9BqheyE82Vvw">OpenFOAM YouTube Channel</ExternalLink> / <ExternalLink href="http://wiki.openfoam.com/">OpenFOAM Wiki</ExternalLink>):
 _"This is a great addition to the community contributions of OpenFOAM. Mesh refinement is an important topic in order to intelligently find the balance between grid resolution and simulation runtime. Try it out!"_

 Jozsef even created a <ExternalLink href="https://wiki.openfoam.com/Adaptive_Mesh_Refinement_by_Air_Shaper">dedicated AirShaper wiki page for this repository </ExternalLink>

- Robin Knowles (Founder of <ExternalLink href="https://www.cfdengine.com/about/">CFD Engine</ExternalLink>):
 _"This is exactly the kind of thing that makes me smile about OpenFOAM - taking the standard "building blocks" & doing something really clever with them.
 The repo's not just useful for those looking to do this kind of refinement, but it's also a practical demonstration of "OpenFOAM thinking" & problem solving.
 Well worth digging into 🙏"_

- Joel Guerrero (CTO of <ExternalLink href="http://www.wolfdynamics.com/">Wolf dynamics</ExternalLink> / Assistant Professor at the <ExternalLink href="https://unige.it/">University of Genova</ExternalLink>):
 _"This workflow is a clever (but not so easy to follow) way to do a-posteriori adaptive mesh refinement exploiting OpenFOAM tools. But the most important feature is that you can refine the backGround mesh using the reference refinement value and then use the new backGround mesh to generate a better body-fitted mesh with snappyHexMesh. This means smaller meshes (because we are not doing unnecessary volume mesh refinement), better boundary layers, and no more missing feature edges, No doubt that I will use this workflow to get better meshes with snappyHexMesh."_

## Conclusion

We’ve created an open-source repository to provide the OpenFOAM community with access to a part of our adaptive mesh refinement code. We are excited to be part of this open-source community and look forward to future contributions from the OpenFoam community.

Interesting links:

<ExternalLink href="https://github.com/airshaper/adaptive-mesh-refinement">
 Github repository
</ExternalLink>

 

<ExternalLink href="https://wiki.openfoam.com/Adaptive_Mesh_Refinement_by_Air_Shaper">
 Wiki page
</ExternalLink>

Open source adaptive mesh refinement

Wind Engineering

## Introduction

Matěj Karásek is co-founder and CEO of <ExternalLink href="https://flapper-drones.com/wp/">Flapper Drones</ExternalLink>. He's a mechanical engineer with a PhD in Engineering Sciences. He has 10 years of experience in development and research of flapping wing aerial vehicles at the <ExternalLink href="https://www.tudelft.nl/">Delft University of Technology</ExternalLink> and at the <ExternalLink href="https://www.ulb.be/">Université Libre de Bruxelles</ExternalLink>.

## What is a flapper drone?

Flapper is a robotic bird that flies by flapping its wings. It can fly just like a drone: it can hover in place, fly forward or sideways and climb and descend vertically. But, unlike conventional drones, it does not have any propellers. Instead, it flaps its wings back and forth, and adjusts this motion further in order to maneuver, like a hummingbird.

<YouTubeEmbed videoId="ybk1CWpduKI" title="Flapper Drones" />

## Bio-inspired flight

The inspiration comes, obviously, from nature and more specifically from hummingbirds and flying insects such as moths and flies. While all the flying animals flap their wings in order to propel themselves through air, it is only these smaller ones that are capable of prolonged hovering. Hummingbirds are the only birds capable of sustained stationary flight and this is due to scaling – for larger birds this simply requires too much power.

## Comparison to regular drones

The main benefits of Flappers are their inherent safety, agility and their better societal acceptance, stemming from their natural appearance and sound.

The propellers of conventional drones are fast spinning, blade-thin objects that need to be protected. Interaction with a propeller in flight is not only dangerous for humans (and their hands or faces) but any impact will also slow the propeller down, which can lead to an abrupt loss of stability and crash. Moreover, propellers are the source of the characteristic, high-pitched buzzing noise typical for drones.

Flappers fly with wings that are soft and move back and forth. In case of a mid-air collision, the wings bounce gently off of objects, causing no harm while the Flapper maintains stable flight. The flapping wings also produce a lower-frequency humming sound that is less disturbing. And finally, since flapping flight works better at smaller scales, the Flappers are lightweight with a considerable wing area that slows the fall in case of a power failure.

<ImageWithCaption
 src="/assets/images/flapper-drones-closeup.jpg"
 alt="Flapper Drone - close-up (image credit: Flapper Drones)"
 width="800"
 height="533"
 positionCenter={true}
/>

## Efficiency / flight time / pay load

Efficiency remains an open research question, as flapping wing designs are still very young, and we are far from reaching an optimal match between motor, transmission, wing motion and wing shape.
Nevertheless, our 100-g Flapper can hover for about 6 minutes with its standard battery (300 mAh, 18 grams). Maximum power efficiency is achieved in forward flight, where we can gain additional 10-20% of flight time.

If we use all the payload capacity for just the battery, the longest hovering flight we achieved with a 950 mAh battery was nearly 18 minutes long, which is pretty good for such a lightweight drone. We are certain this can be further improved.

## Complex Aerodynamics

The aerodynamics of a flapping wing are unsteady and so the challenge lies in developing an appropriate model that would capture all the phenomena, including the fluid-structure interactions, i.e. interaction of the aerodynamics and structural dynamics of the flexible wings. In fact, the wings of Flappers also interact with each other as they “clap” and “peel” to further improve the power efficiency, which adds further complexity to the modeling problem.

But, for the flapping-flight, these inherently complex aerodynamics are actually beneficial. The most prominent flow feature is a leading-edge vortex, a tornado-like structure that remains attached to the flapping wing throughout the entire stroke, increasing its lift coefficient. The interaction of the wing with the vortices shed in the previous stroke can help recover some of their energy, improving further the overall power efficiency.

## Blending high-tech academics and hands-on creativity

Because of these complexities, we rely mostly on experiments and (systematic) trial and error approach. Although the aerodynamic forces keep changing substantially throughout each wing stroke, we can evaluate the performance of a particular wing using only the flap-averaged forces. This is in particular true for smaller fliers, where the flapping frequency is high and well above the bandwidth of the fliers’ body dynamics. And these averaged forces we can measure using force balances, as long as care is taken as to how the data is filtered and averaged.

However, further insights can be gained through flow visualizations. During my research at the TU Delft we were able to develop methods that enabled recording the 3D flow structures in flight. To achieve that, we used tiny helium-filled soap bubbles illuminated with lasers and recorded with multiple high-speed cameras simultaneously from different angles. The recorded data can also be used to develop better aerodynamic data in the future, but for now, we need to also rely on intuition and gut-feeling.

<ImageWithCaption
 src="/assets/images/flapper-drones-flow-visualization.jpg"
 alt="Delfly, the predecessor of Flapper Drone - 3D flow visualization using laser-illuminated soap bubbles (image credit: TU Delft)"
 width="800"
 height="535"
 positionCenter={true}
/>

## Scaling physics

Looking into nature we see a clear trend. The smaller (and lighter) the animal, the faster its wings need to flap and the easier it is to hover. For large animals, hovering flight is too power-demanding, and they can sustain it only briefly e.g. when perching. Currently, we can build Flappers as small as 25 centimeters /30 grams, to go even smaller we are mostly limited by the available actuator technology, but the physics of flying would be even more favorable there.
For carrying large payloads on long distances, we should stick to gliding flight, which is what large birds master incredibly well. However, if hovering flight and also agility are important, this gets better at small scales. Our 100-gram Flappers can carry an extra payload of 25 grams, so carrying additional sensors or a vision system for higher autonomy is certainly possible.
When it comes to noise, higher frequencies can be more irritating, think of a mosquito flapping its wings up to 1000 times per second. This is also the frequency range where propellers of conventional drones operate, just the sound pressure is higher as it scales with weight. The lower frequency humming sound of hummingbirds, and Flappers, is much more pleasant for a human ear.
Nevertheless, for stealth operation, stopping the wings from flapping and switching to gliding flight is the best option, and here we can also learn from nature: night predators such as owls evolved their wings to be virtually silent.

## Target markets

We started Flapper Drones with the idea of creating a new type of drone show. A swarm of robotic birds flying in choreography with music at festivals, or robotic fairies flying around people in theme parks.

<ImageWithCaption
 src="/assets/images/flaper-drones-artistic.jpg"
 alt="Flapper Drone - artistic 3D printed shell"
 width="800"
 height="558"
 positionCenter={true}
/>

However, with the COVID-19 pandemic we—hopefully just temporarily—changed our focus to consumers. We are working on a DIY kit that people can assemble with simple tools, before taking the assembled Flapper for a flight. This way, the users can learn about the inner-working of the Flapper as well as about bio-inspired flight in general in a very hands-on way.
Ultimately, we believe flapping-wing robots will become a new class of drones, especially suitable for indoor applications where you need a safe and un-intrusive robot that can fly near people and that is resilient to minor collisions. Next to the entertainment applications, we see opportunities in stock monitoring in warehouses, in greenhouses to monitor—and later possibly even pollinate—plants, but also applications like a personal flying guide in a museum or a store.

## New materials & production techniques

While we employ lightweight materials such as carbon fibre composites and apply techniques such as 3D printing, the reason why we see the first hover-capable flapping wing fliers only now are the technological advances made in the past 20 years, driven by mobile technology. We now have high-energy-density batteries, and powerful yet lightweight and energy-efficient micro-computers and sensors.
Especially the miniature and lightweight MEMS IMU sensors, which inform your smartphone when to flip your screen, are essential.
Hovering flapping flight is unstable and requires active stabilization that estimates the attitude — the orientation in space — using these sensors. The on-board computer then sends signals to the actuators, at rates well over 100 times per second, which keep adjusting the motion and geometry of the wings.
These actuators need to be fast enough to keep the Flapper balanced, and it is what currently limits us from making these Flappers even smaller. Despite the recent developments in drones which brought lighter and more powerful DC motors, for now, their power efficiency drops at small scales making the achievable flight times short and impractical below wingspans of about 15 centimeters.

## The ultimate goal

We are moving towards a roboticized society and drones will eventually become part of our environments. We would like to show that drones do not need to be scary and annoying. By employing bio-inspiration, we want to introduce a new class of flying robots that are useful and, at the same time, beautiful and fun to watch, just like natural fliers!

Interesting links:

<ExternalLink href="https://flapper-drones.com/wp/">
 Flapper Drones
</ExternalLink>
 

<ExternalLink href="http://www.delfly.nl/">Delfly</ExternalLink>
 

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

Flapper Drones - Bio-inspired Flight

## AirShaper’s new Aerodynamic Shape Optimization software

Did you know that professional cyclists spend over 80% of their energy pushing through air? While over 50% of the energy from fuel or batteries in vehicles is used to overcome air resistance at highway speeds?

Optimizing aerodynamics can be the key to unlocking performance, improving efficiency and reducing waste & greenhouse gas emissions. As we move towards a more sustainable future, industries are recognizing the important role aerodynamics can play in achieving energy efficiency. But how can this be done quickly, easily and in a cost-effective way?

AirShaper have developed a brand new Aerodynamic Shape Optimization tool which revolutionizes the conventional approach to aerodynamic design. Innovative algorithms automatically optimize a design's shape; helping you achieve aerodynamic performance targets and reach the optimum solution faster.

<YouTubeEmbed
 videoId="-fBXwx_n10I"
 title="Aerodynamic Shape Optimization - Launch trailer*"
/>

**‘Our mission at AirShaper is to make aerodynamic simulations accessible to everyone regardless of the industry you work in,’** says Wouter Remmerie, CEO at AirShaper. ‘This type of optimization software is usually only available within expensive software packages and normally requires skilled engineers to set up. But our new solution offers a quick, easy alternative that still achieves accurate and reliable results.’

## Revolutionizing the conventional approach

Improving aerodynamic performance requires a detailed and iterative approach which usually involves CFD simulations and wind tunnel tests. Typically, CFD is used to analyse the initial performance of an object which engineers then use to manually tweak its design. These changes need to be re-evaluated with further CFD simulations or wind tunnel tests to quantify the improvements and the process repeats.

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

Over the last decade, CFD has become the tool of choice amongst engineers. Substantial improvements in computing power have meant faster run-times and more complex modelling techniques. Allowing engineers to replicate more realistic flow conditions along with finer meshes.

However, this aerodynamic development cycle still requires substantial budgets and long development times. Problems which shape optimization algorithms can help to solve.

## How does Aerodynamic Shape Optimization work?

There has been a huge surge of development in shape optimization algorithms over recent years, making them now more reliable than ever.

<VideoEmbed
 poster="/assets/images/rear-end-optimization"
 videos={[
 {
 src: 'https://airshaper.storage.googleapis.com/website/video/aerodynamic_shape_optimization.webm',
 type: 'video/webm;codecs="vp8, opus"',
 },
 {
 src: 'https://airshaper.storage.googleapis.com/website/video/aerodynamic_shape_optimization.mp4',
 type: 'video/mp4',
 },
 ]}
/>

Utilizing the adjoint optimization technique, these algorithms create accurate sensitivity maps of an object’s surface in just two simulations - instead of the thousands usually required. These sensitivity maps are then used to automatically morph the shape of an object which is then re-evaluated and the process starts again. This optimization cycle automatically repeats until the shape of the object has been transformed to meet its performance targets.

<YouTubeEmbed
 videoId="cZAhPQFINZ8"
 title="Aerodynamic Shape Optimization - How it works*"
/>

This automated optimization sequence means that users no longer have to manually adjust the shape of an object and re-run simulations. All you need to do is upload a 3D model, specify the orientation, wind speed, morphing space and optimization goal and the algorithms will deliver a fully optimized 3D model!

## The benefits

The beauty of this technique is that it can be applied to any application where drag or lift are relevant.

#### Electric vehicles

Several electric vehicle projects have utilized this shape optimization approach to reduce drag by 7%. The less drag an electric vehicle experiences, the less energy it requires and the further it can travel on one charge.

#### Motorbikes

Companies such as <ExternalLink href="https://www.mvagusta.com/" >MV Agusta</ExternalLink> have used our tool to increase top speed and downforce of their motorbikes. Improving handling both on the road and on track.

#### Sports

Our Aerodynamic Shape Optimization tool has also been used for <ExternalLink href="https://www.decathlon.com/" >Decathlon</ExternalLink>, a major sports equipment manufacturer, to optimize the performance of their upcoming Van Rysel bicycle helmet. Taking into account comfort, safety and ventilation constraints, the helmet was optimized to reduce aerodynamic drag as much as possible. Saving even just a tenth of a Newton in drag force can lead to seconds of difference on a time trial.

<VideoEmbed
 poster="/assets/images/decathlon-helmet-optimization-small"
 videos={[
 {
 src: 'https://airshaper.storage.googleapis.com/website/video/decathlon-helmet-optimization-small.webm',
 type: 'video/webm;codecs="vp8, opus"',
 },
 {
 src: 'https://storage.googleapis.com/airshaper/website/video/decathlon-helmet-optimization-small.mp4',
 type: 'video/mp4',
 },
 ]}
 ratio="16x9"
 caption="Decathlon - Van Rysel helmet optimization to reduce aerodynamic drag"
 width={1080}
 height={272}
/>

#### Aviation

As the aviation sector pushes to reduce carbon emissions, maximizing the efficiency of fuel or batteries is crucial. Shape optimization can be used to generate more lift for a higher payload capacity or to design for higher efficiency to extend range.

#### Renewable energy

Shape optimization has also been used to reduce wind loads on renewable energy equipment, such as solar panels and offshore structures. Improving the lift & drag forces and moments of these devices is key to guarantee safety, operability and efficiency.

#### Start-up companies

AirShaper’s optimization platform allows small companies to make fast gains in aerodynamic performance without having to invest in complex software packages or trained engineers.

**To streamline your aerodynamic optimization process, whatever industry you are in, <Link href="/contact">get in touch</Link> to sign up for AirShaper’s new Aerodynamic Optimization software**


 *The optimization shown in these animations is an independent study based on a
 3D scan provided by{' '}
 <ExternalLink href="https://www.a2mac1.com/">A2MAC1</ExternalLink> and is not
 endorsed by any OEM.

Aerodynamic Shape Optimization

## The return of wind powered vessels

 

### Ship aerodynamics

Large vessels have been sailing our oceans for many centuries, transporting goods to all parts of the world. They were, in their beginnings, solely powered by wind, which made the success of their journeys highly dependent on meteorological conditions. With the industrial revolution, engines were developed and introduced into these vehicles to provide a more reliable constant source of power. Two centuries after the first steam-engine sailed the seas, an excess of the use of fossil fuels is forcing this generation to come up with innovative ideas that will allow world-wide transportation to be continued but with a reduced carbon footprint. It is in the amidst of this necessity that the <ExternalLink href="https://www.oceanbirdwallenius.com" >Oceanbird</ExternalLink> is born, a joint project between <ExternalLink href="https://www.walleniusmarine.com" >Wallenius Marine</ExternalLink>, the <ExternalLink href="https://www.kth.se/en" >KTH Royal Institue of Technology</ExternalLink> and <ExternalLink href="https://www.sspa.se" >SSPA</ExternalLink>.

### Dr. Laura Marimon Giovannetti

For this blog, we have interviewed Dr Laura Marimon Giovannetti, who is working at SSPA as a senior researcher. Prior to moving to Sweden, she spent many years at the University of Southampton in the UK, where she studied and completed her PhD and a postdoc. During her PhD, she was be able to work in the design team of the British Challenger for the 35th America’s Cup (Land Rover Ben Ainslie Racing). <Link href="/blog/sailing-yacht-aerodynamics-and-hydrodynamics">Learn more about sailing yacht aerodynamics here</Link>.

<ImageWithCaption
 src="/assets/images/profile-picture-laura-marimon-giovannetti.jpg"
 alt="Dr. Laura Marimon Giovannetti"
 width="400"
 height="400"
/>

Alongside her whole professional career, she has also been an elite athlete in sailing, representing Italy and then the British Sailing Team at World and European championships. She shared with us that her knowledge of sailing as an athlete as well as an engineer has definitely helped her shape her path in high-performance sports and now also in wind propelled vessels. "The latter especially reflects not only my passion for sailing but also my personal involvement in sustainability, as I would really like to admire the sea that once again is used to transport goods in a clean and sustainable way".

<YouTubeEmbed
 videoId="pZVDs4n_a0M"
 imgSize="default"
 title="The Wallenius Marine Oceanbird"
/>

## Wing sail design

### Airfoil sections

The wing sails are essentially aerofoil sections. Contrarily to aeroplanes that only need to create lift in one direction, in a ship the wings would need to create lift, or side-force in this case, in both left and right direction, commonly defined in a marine environment as port and starboard, respectively. It is therefore evident that ships are not actually able to sail straight against the wind. Therefore, the wings need to rotate independently to achieve the best combination of overall aerodynamic drive to allow the ship to move forward. The wing sections also need to be symmetrical to sail both directions – or tacks. Moreover, contrarily to an aeroplane where you would have a main wing of long span and a short rear tail wing, in the Oceanbird the wings are all of the same size and it is extremely important to understand the interaction effects that would change the overall drive created.

<ImageWithCaption
 src="/assets/images/oceanbird-top-view.jpg"
 alt="Top view of the Oceanbird sails featuring a symmetric airfoil section - Image credit: Oceanbird"
 width="1000"
 height="562"
/>

### Aerodynamic interaction

Aerodynamic knowledge is key when we start talking about interaction effects between the wings. Indeed, we need to understand that if one wing produces a certain amount of lift, the interaction between that wing and the one behind or in-front will change the overall lift created. Another effect of interaction is seen on the effective angle of attack or stall angles as the wind bends around the front wing to then reach the ones behind.
Other aerodynamic knowledge comes to our advantage when we want to design the top part of the deck. As the ship travels forward, the wings, which are sitting on the top deck, are affected by the superstructure. So, investigating new deck designs is key to achieve a consistent flow direction avoiding aerodynamic losses in the lower part of the wings.

### The tallest sails in the world

The largest wings built at the moment are for the F50, a high-performance sailing vessel racing in the Sail GP circuit. The main difference is the necessary drive force to move forward a heavy car-carrier compared to a relatively light sailing boat. Current wings for sailing boats are optimised on a vessel mostly made of carbon fibre, that has a very high stiffness to weight ratio. Traditional commercial ships are instead made almost entirely in metal and are undergoing strict safety regulations, so the overall weight is brought up to be able to withstand all the necessary loads. The wings in Oceanbird are therefore designed as tall to be able to drive forward a commercial vessel around a specific route always maintaining a minimum speed to be able to deliver the goods in time.

<ImageWithCaption
 src="/assets/images/oceanbird-tall-sails.jpg"
 alt="The Oceanbird sails will reach a height of 105 meters above the waterline - Image credit: Oceanbird"
 width="879"
 height="1080"
/>

### Telescopic sails

“Reefing the sails” is a normal manoeuvre in sailing. It is necessary to reef the sails, effectively reducing the surface area of the wings, when the side force generated is too large due to strong winds. In those conditions, as the side force is acting at the centre of effort of the wings, so approximately at half-span, the heel moment becomes too large to sustain safely and without causing damage to the content of the ship and the crew. Therefore, we need to lower the wings down to deck level to lower the centre of effort, still being able to produce drive force but in a much safer way. The possibility of reefing the wings and lowering them completely to the deck is also important in in-port operations and when we encounter bridges en-route.

### CFD simulations on sails & ships

<ImageWithCaption
 src="/assets/images/oceanbird_cfd_1.jpg"
 alt="CFD analysis of the sails - starboard view (note the wing tip vortices) - Image credit: SSPA Sweden AB"
 width="1000"
 height="590"
/>

In our partnership between Wallenius Marine, SSPA and KTH we use a large number of different design approaches, varying from low-confidence to high-confidence levels. The higher the confidence level grows (i.e. moving toward wind tunnel tests or accurate CFD modelling) the higher the costs involved and the time to get the results. So using a combination of different fidelity models is key to a successful design spiral.

<ImageWithCaption
 src="/assets/images/oceanbird_cfd_2.jpg"
 alt="CFD analysis of the sails - port view - Image credit: SSPA Sweden AB"
 width="1000"
 height="590"
/>

### Wind conditions.

The sail rotate independently to be able to provide aerodynamic drive in all conditions, aside when heading directly toward the wind. There is an area of plus-minus 30 degrees of true wind angle that in sailing is a “no-go” zone as the ship is not able to sail directly toward the wind but is instead using a zig-zag approach to sail toward the wind direction.

### Environmental break-even

We’re currently working with material selection and production techniques for the wing sails, for example investigating the possibility to use recycled composite materials. It is therefore at this phase too early to state the break-even period (the time needed to save as much CO2 / energy as was needed to build the ship in the first place).

## The future of wind powered shipping

<ImageWithCaption
 src="/assets/images/oceanbird-front-view.jpg"
 alt="The Oceanbird - the future of shipping? - Image credit: Oceanbird"
 width="1000"
 height="562"
/>

### Sails versus kites

Kites are very large soft-sails that are deployed in certain wind conditions. They are mostly efficient at “pushing” the ship forward when the wind comes from behind. Our wings instead work producing lift, like aeroplane wings, so their preferred angles of attack are lower, below stall.

### Future challenges

One major challenge that we face is to prove the concept and the possibility of effectively reducing to a minimum the fuel consumption. Being able to do it will create a positive movement where we can prove that even with wind powered ships we can deliver goods across the world in time. Another important point is crew training for the new systems onboard.

### Scaling down

There are some projects currently looking at small passenger ferries that could be fitted with sails. I believe that once we have proven the concept and we will be sailing with the Oceanbird then more projects will follow!

### The first cross-Atlantic journey

The target launch for the first Oceanbird concept (the Orcelle), which is currently under development together with Wallenius Wilhelmsen, is 2024/2025.

Interesting links:

<ExternalLink href="https://www.oceanbirdwallenius.com">Oceanbird</ExternalLink>
 

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

The Oceanbird Wind Powered Vessel

## What is vehicle water management? 📢

When was the last time you saw the message ‘clean me’ on the back of a dirty car, truck or lorry? If it’s winter where you are, then it probably wasn’t long ago. But why do some vehicles seem to get dirtier than others? Ok, some owners are not the best at keeping their vehicles clean, but aerodynamics also has a major role to play.

To find out the physics behind vehicle water and dirt management and how simulation can help, we spoke to our two partners <ExternalLink href="https://www.nextflow-software.com/">Nextflow Software</ExternalLink> and <ExternalLink href="https://www.a2mac1.com/">A2MAC1</ExternalLink>.

### What is vehicle soiling?

Water management is a huge topic. It encompasses everything from designing efficient windscreen wipers to adding aerodynamic devices to control the exterior airflow around a vehicle.

‘It’s essentially all about the interaction of liquid with the exterior surface of the car,’ explains Vincent Keromnes, Dynamic Benchmarking Domain Leader at A2Mac1. ‘This can include rain, water wading and water injection. It’s crucial to monitor where water can get through on a vehicle. If it accumulates in the engine bay, battery pack or on a camera, it can cause major issues.’

It’s not just water that’s a concern though. Dirt, debris, stones and other particles also need to be monitored. Due to the miniature nature of these particles, they are strongly affected by the behaviour of the exterior airflow. Therefore, the aerodynamics need to be designed to minimise these particles either hitting the surface or being ingested in intakes and openings.

‘There are two main sources of soiling,’ highlights Keromnes. ‘Firstly, there is self-soiling which is from the vehicle itself. Wherever there is flow separation, such as on the sideview mirror or at the rear of the car, a wake is generated which causes contamination. Then there is third party soiling which is caused by the interaction of wind, rain and dirt with other obstacles on the road.’

**Did you know?**
The bigger the rear wake of a vehicle, the dirtier it will get. This is why SUV’s get dirtier quicker than other types of cars - because it has a much larger wake. <Link href="/blog/mercedes-eqc-drag-coefficient">Find out why SUV’s aren’t aerodynamic here.</Link>

<ImageWithCaption
 src="/assets/images/SUV-rear-soiling.jpg"
 alt="Accumulation of contaminant on the rear of an SUV after 75 s, obtained using a UV fluorescence method in the FKFS thermal wind tunnel - Image credit: FKFS / Adrian Gaylard"
 width="640"
 height="480"
 positionCenter={true}
/>

### Why is vehicle water management important?

We all like to see where we’re going when driving, no matter what the conditions are outside. If you’ve ever experienced a faulty windscreen wiper on a motorway, then you’ll have a particular appreciation for this.

‘The most important reason is obviously safety,’ explains Keromnes. ‘Drivers have to have clear visibility through the windscreens, windows and side mirrors, so avoiding dirt deposition on those areas is critical.'

'Also, with more ADAS systems onboard modern vehicles, cameras and LIDAR sensors are very sensitive to dirt. Just a droplet of water on such a tiny camera can seriously affect its performance. This is becoming increasingly important as we move towards more autonomous driving. These types of devices need to stay clean or be easy to clean.’

<ImageWithCaption
 src="/assets/images/tesla-sensor-locations-small.jpg"
 alt="Tesla Model 3 sensor locations - Image credit: System Plus Consulting"
 width="640"
 height="406"
 positionCenter={true}
/>

‘Aesthetics are also a part of it. This is why some manufacturers develop their designs to drive soiling into areas that are not so visible. There’s also the comfort side of things. You can imagine if you’re a truck driver, you don't want to get covered in dirt every time you grab the door handle.'

### How do manufacturers manage dirt and water?

Dirt and water management are a key consideration when designing a vehicle’s exterior. Early in the design process, designers analyse the vehicles aerodynamics to control the behaviour of water and dirt to keep key areas of the vehicle clean. However, in areas where this can’t be done, engineers have to develop systems that remove dirt and water.

‘It’s difficult to find an area on a car that stays completely clean. Sometimes your only option is to put a cleaning system in place,’ says Keromnes. ‘That’s why in the 80s and 90s Mercedes cars had a wiper system on the headlamps.'

'As well as wiper systems, carefully controlled spraying techniques can also be used which are efficient and inexpensive. This has become so important that some manufacturers now have specific departments dedicated to developing these spray systems. It's interesting to know how fast a car can get dirty, but it's also interesting to know how fast you can clean it. Both of these can be done through simulation.’

### How do you simulate the flow of water and dirt particles?

Accurately modelling the behaviour of a consistent airflow over a vehicle is challenging enough. Add in the effect of wind, the dynamics of water and the trajectories of dirt particles, and you can begin to appreciate the difficulties of simulating water and dirt.

This is why such analysis requires the combination of aerodynamic and hydrodynamic simulations. As the water droplets on the vehicles surface are so small, these local flow fields don't have a major effect on the vehicle’s aerodynamics. Therefore, computational fluid dynamics (CFD) simulations evaluating the global aerodynamic performance of the vehicle are completed first. Then, this flow field is used for separate simulations where liquid particles are injected into the simulation to predict the forces and trajectories of water and dirt particles.

<YouTubeEmbed
 videoId="VYQkWA2MWj0"
 title="Sensor Soiling Analysis on a Model Y 3D scan"
/>

‘If we need to conduct a more detailed analysis we can also add in some particle refinement algorithms. This allows us to evaluate the wettability effect such as surface tension, dynamic contact angle etc of smaller particles in more specific areas,’ says Julien Candelier, the technical team leader at Nextflow Software.

Of course, such complex simulations require a substantial amount of computing power. ‘The main challenge is memory management,’ highlights Candelier. ‘That’s why everything is modelled as a steady flow using a mean value of diversity of car wake. Even with that we are dealing with gigabytes of data.'

'To cope with this, we do some parallel computing for the liquid phase to achieve efficient simulations. For example, a full wheel soiling simulation can run in 20 hours on a regular desktop.’

The ability to accurately model liquid flows quickly and efficiently means that these simulation techniques can also be used for other flows within a vehicle. For example, e-motors can rotate up to 15,000rpm, generating airflows that can significantly effect oil flows within the transmission.

<ImageWithCaption
 src="/assets/images/gearbox-lubrication-simulation.jpg"
 alt="Gearbox lubrication simulation - Image credit: Nextflow Software"
 width="477"
 height="424"
 positionCenter={true}
/>

‘We use the same technology to study the cooling and lubrication performance of rotating gearboxes,’ reveal Candelier. ‘Using a similar approach, we assume the oil flow doesn't influence the overall airflow, but the airflow does influence the oil flow.’

‘A Formula E team used our software to investigate a gearbox issue where the oil temperature was rising by 1degC or 2degC per lap. The engineers were able to reproduce this phenomenon with our software, test, validate and develop a new design which solved the issue.’

So, next time your car gets dirty – just think how many more times you would have to wash your car if it wasn’t for simulation.

Interesting links:

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

 

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

 

<ExternalLink href="https://www.nextflow-software.com/">
 Nextflow Software
</ExternalLink>

Vehicle Water Management

## Ever wondered how the drag coefficient changes when you drive an SUV backwards? 📢

Imagine you are the project manager for the exterior design of the new Mercedes EQC electric SUV. Your job is to balance what designers want (an aesthetic car) with what engineers want (an aerodynamic car).

For months you have been running a variety of wind tunnel test programmes. You've experimented with aerodynamic tricks such as active grille shutters, specialised wheel hubs and smoother underfloors. All with the aim of optimising that vital Cd value.

By the end of the programme, the Mercedes EQC <ExternalLink href="https://airshaper.com/videos/what-is-a-drag-coefficient/bEgoZ_dAg7o">drag coefficient</ExternalLink> is 0.27 - bring out the champagne! However, the marketing department want to complete one last run, with two dogs in the vehicle. Well, they need marketing material and if this engages audiences then why not.

<ImageWithCaption
 src="/assets/images/mercedes-eqc-dogs.png"
 alt="Mercedes Wind Tunnel Test with Dogs - Image credit: MERCEDES BENZ"
 width="898"
 height="504"
/>

## The real joke: reverse aerodynamics

But then the real prankster of the team decides to take it a step further. He wants to run the car backwards too. Not entirely sure what that will achieve, but again – why not? It’s been a long day.

As the dogs begin enjoying themselves, the Cd value appears on the screen. It is only 7% higher than the original Cd value of 0.27. To add insult to injury, the prankster removes the rear spoiler and rear wind deflectors. The Cd value this time is exactly the same as when the car drives forwards. Time to put the champagne away.

Of course, (apart from the dogs) this is a hypothetical story. However, Vincent Keromnes of <ExternalLink href="https://portal.a2mac1.com/">A2MAC1</ExternalLink> did complete these tests virtually using the AirShaper platform. Using a high quality 3D scan of the Mercedes EQC, Vincent ran a series of AirShaper simulations with the car in reverse.

<ImageWithCaption
 src="/assets/images/mercedes-eqc-aerodynamics-backward.png"
 alt="Mercedes EQC in reverse (configuration 1 - original car) - Aerodynamic Simulation by AirShaper (pressure clouds shown)"
 width="1174"
 height="387"
/>

## Matching the original Mercedes EQC Drag Coefficient

Interestingly, the result was a minor 7% increase in drag coefficient. Vincent then wanted to see how close he could get the drag coefficient in reverse to the original Cd value. He removed the rear spoiler and the rear wind deflectors as these were significantly affecting aerodynamic performance. He then repeated the simulations and astonishingly, achieved the same drag coefficient.

<ImageWithCaption
 src="/assets/images/mercedes-eqc-aerodynamics-backward-modified.png"
 alt="Mercedes EQC in reverse (configuration 2 - rear spoiler and wind deflector removed) - Aerodynamic Simulation by AirShaper (pressure clouds shown)"
 width="908"
 height="285"
/>

## The real message: traditional SUV shapes simply aren't aerodynamic

This may have started as an amusing story, but the underlying message is extremely important. The long nose and bulky rear end of modern SUV’s is the exact opposite of the typical aerodynamically efficient teardrop shape. A good example of this is the <ExternalLink href="https://airshaper.com/videos/lowie-vermeersch-2-the-lightyear-one-solar-car/TKHjyC_L0OM">Lightyear Solar One</ExternalLink>. Despite the Mercedes EQC being electric and therefore benefitting from the packaging opportunities of batteries and electric motors. Its shape remains bulky and far from aerodynamic.

The question is why? Of course, the long nose houses the crash structure and the appeal of SUV’s is that they are spacious and comfortable. But the simple fact is that SUV&#39;s are not aerodynamic.

It seems that to ease the transition from internal combustion vehicles to electric vehicles, manufacturers are developing cars that look exactly the same as their predecessors. So they are missing out on potential opportunities to improve aerodynamic efficiency. Yet, if the aerodynamic performance of a vehicle is improved, it requires less energy to travel the same distance.

If manufacturers want us to take their claims of developing greener vehicles seriously, they need to step away from these traditional SUV shapes. Instead, they need to deliver vehicles which maximise efficiency, to minimise the amount of energy these vehicles consume. That&#39;s the definition of a true environmentally friendly vehicle.

Interesting links:

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

 

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

The Case of the Mercedes EQC Drag Coefficient

## About the Author 📢

James Aarestad, the owner of [Eagle Eye Photography](https://blimpguy.com/), is busy building a variety of camera pods for aerial imagery on manned aircraft. His business specializes in helping his customers obtain information from the air using a variety of unique cameras attached to aircraft. And with aerodynamics being crucial to the performance & safety of aircraft, the effects of objects attached to the outside need to be checked properly.

## Finding Gas Leaks

Every day, pipelines around the world carry natural gas through millions of miles of pipe. Many people take for granted the complex infrastructure in place to route natural gas to homes and businesses for heating. As these pipelines age, small gas leaks can develop and go undetected for years. As this impacts the environment, safety and profitability, detection of these leaks is crucial.

## Specialized cameras

So the gas and oil industry needed a better way to find these leaks - one that would be flexible and efficient enough to inspect the many miles of pipeline that often cross rough & remote terrain. That's why in 2019, I began work on a camera pod that would detect these dangerous natural gas leaks from the air. This required the use of an extremely specialized camera. This camera is heavy and large, making it a challenge to attach to the exterior of an airplane. As you might imagine, attaching camera systems to Cessna aircraft is no easy task.

<ImageWithCaption
 src="/assets/images/camera-pod-aerodynamics-6.jpg"
 alt="The Eagle Eye Camera Pod attached to the fuselage of a Cessna"
 width="1323"
 height="361"
/>

## Camera Pod Aerodynamics

Our biggest design hurdle was keeping the aerodynamic footprint as small as possible. This was critical not only for aircraft control but also for certification requirements. The Federal Aviation Administration in the USA has strict regulations that must be following when modifying an aircraft and this includes aerodynamics.

We used the AirShaper platform to determine how much wind resistance the camera pod would generate at various speeds. Using this data, we were able to adjust the size of the camera pod for optimal performance. The data helped us determine the maximum allowable airspeed the pilot could safely fly with the camera pod attached. The reports helped confirm our engineers' aerodynamic estimations and helped us with certification requirements.

<ImageWithCaption
 src="/assets/images/camera-pod-aerodynamics-5.jpg"
 alt="AirShaper CFD Analysis of the Camera Pod - 3D Streamlines"
 width="953"
 height="767"
/>

## The Future

We are currently working on two more camera systems which we'll of course subject to aerodynamic analysis again. With each iteration, we'll be able to reduce the aerodynamic loads for a given camera size (reducing the drag coefficient Cd).

Interesting links:

<ExternalLink href="https://blimpguy.com/">Eagle Eye Photography</ExternalLink>
 

[Running your own simulation](/videos/how-to-run-an-aerodynamic-simulation/O_DNDxCtI_I)

AirShaper BlogPassion for aerodynamics

Open source adaptive mesh refinement

Wouter Remmerie

Wind Engineering

Xerxes Chong Xian

Flapper Drones - Bio-inspired Flight

Matěj Karásek

Aerodynamic Shape Optimization

Wouter Remmerie

The Oceanbird Wind Powered Vessel

Claudia Jiménez Cuesta

Vehicle Water Management

Wouter Remmerie

The Case of the Mercedes EQC Drag Coefficient

Wouter Remmerie

Camera Pod Aerodynamics

James Aarestad

Trusted By