Climate neutral aviation? Now boarding!
At TU Delft we are convinced that it is possible to make aviation climate neutral. That’s why we’re working so hard on it. Our drive? We know that climate change is real. And although aviation contributes a great deal to the prosperity and well-being of people all over the world, it also contributes to climate change. If we do not make this sector more sustainable more quickly, this contribution will increase exponentially. Of course: making more conscious choices about when and how we travel is necessary. But: as Delft engineers, we also believe that with our knowledge, technology, innovation and a new generation of aviation engineers, we can make a crucial contribution to climate-neutral aviation in 2050. So that aviation can continue to connect the world for future generations.
Dean Faculty of Aerospace Engineering TU Delft
How do we work on making aviation sustainable?
We make aircraft and aircraft engines as efficient as possible. Our revolutionary Flying-V aircraft design for example aims at saving 20% on fuel consumption compared to state-of-the-art long-haul aircraft, like the Airbus A350, just as a result of improved aerodynamics and reduced weight. But lightweight aircraft materials and new systems for Air Traffic Management, such as a Single European Sky, so that aircraft can always fly the shortest distance possible, will also save energy.
We want aviation to run completely on a mix of sustainable energy and propulsion technologies. We for example work on electric and electric/hybrid aircraft. But also on the sustainable production of green aviation fuels, such as synthetic kerosene, LNG and hydrogen. By re-using CO2 for the production of green fuels, we make aviation CO2 neutral. All options need to be on the table. Sustainable fuels are better suited for long-haul flights and electric aircraft for shorter routes.
This covers topics ranging from sustainable airports in healthy environments – including clean ground operations, such as electric taxiing - to sustainability-focused Air Traffic Management systems and fleet management (we can fly more on electric aircraft if they make more stops on the way to recharge). In addition, we also study the effect of new aircraft and fuels on the earth’s atmosphere. By doing so we can optimise route and cruise altitude to minimise climate effects, and to greatly reduce air and noise pollution.
We work on minimising the impact of aircraft and spacecraft materials and structures on our planet. To this end, we follow different approaches including design for life cycle, development of bio-based and bio-degradable aerospace-grade materials, development of energy- and resource-efficient manufacturing technologies, and of reuse and recycling methods, thereby making the circular aerospace industry of the future a reality. This topic also includes sustainable aircraft maintenance. Condition-based maintenance, for example, where the aircraft’s health is constantly monitored through sensors and Artificial Intelligence, allows airlines to replace components when they’re truly at the end of their lifetime, rather than during preventive inspections, which reduces waste.
Vordering ‘slim én duurzaam’In 2018 publiceerden twintig transportorganisaties en kennisinstellingen samen het actieplan ‘Slim en duurzaam’ om de CO2-uitstoot van de luchtvaart te reduceren. Daarna volgden twee aanvullingen om inzicht te geven in activiteiten en behaalde resultaten. Vandaag is er een derde actualisatie beschikbaar die de huidige stand van zaken weergeeft.
BBC Future: the epic attempts to power planes with hydrogen“During the Cold War, both the US and USSR researched liquid hydrogen as a way to fuel aircraft. Could this cleaner fuel finally be around the corner?” BBC Future journalist Mark Piesing investigates this question in an informative longread published on 21 March 2022 on BBC Future.
Brazil and TU Delft strengthen collaboration for independence from fossil sources and clean aviationToday Sergio Freitas de Almeida, Vice Minister of Science Technology and Innovation of Brazil and Rob Mudde, Vice President of Delft University of Technology, agreed to intensify their collaborations on research and innovation to make renewable energy and clean aviation more rapidly available. This builds on present programmes to mitigate climate change and supports efforts to replace fossil gas needs.
4TU presents second opinion sustainability aviation to Dutch House of RepresentativesOn 17 February, Minister Harbers presented a second opinion on the subject of sustainable aviation, drawn up by 4TU, to the Dutch House of Representatives. In the second opinion, three Full Professors of the universities of technology discuss the fact sheet 'Future of sustainable aviation: an update'.
In-depth knowledge and getting things done: TU Delft and aerospace start-ups successfully collaborate on sustainable aviationMaking aviation more sustainable by improving the aerodynamics of aircraft. That is the objective of two new collaborative projects between researchers from TU Delft (Aerospace Engineering), aerospace start-ups from the Aerospace InnovationHub@TUD and industry.
Irene Dedoussi new member of the Young Academy 2022The Young Academy is welcoming ten new members. They are researchers who work in a variety of disciplines, have been selected for their scientific achievements, and received their doctorates less than ten years ago. One of them is Dr. Irene Dedoussi, Associate Professor in the field of climate impact of aviation.
National Geographic Magazine: 'How Green, How Soon?National Geographic Magazine's October cover story 'How green, how soon?' is about sustainable aviation. In the spring, top photographer Davide Monteleone visited the faculty to take spectacular photos of the Flying-V and the SmartX-alpha flexible aircraft wing for the magazine.
Study of scenarios: aviation is - currently - unlikely to support Paris goalsWe all know that emissions from aviation are increasing. The Paris climate agreement does not actually specify climate goals for aviation, but targets for reducing the climate impact from aviation do exist in other policies, regulations and research. Is the pace of innovation and policies high enough, however, for aviation to support the Paris agreement goals?
AeroDelft back at the faculty of Aerospace EngineeringThis summer, AeroDelft is moving their Phoenix prototype to the Materials & Structures Laboratory at the faculty of Aerospace Engineering to continue working on their mission: to prove that emission-free aviation is possible by developing the world's first manned aircraft powered by liquid hydrogen.
Faster and more radical innovation essential for climate-neutral aviation in 2050The NRC of 23 June2021 contained an article claiming that the aviation sector cannot innovate its way out of the climate crisis (‘De luchtvaart kan zich niet uit de klimaatcrisis innoveren’). The article was written in response to a factsheet and presentation that two scientists gave to the Dutch House of Representatives. The scientists paint an honest picture, but we do not share the conclusion that the journalist came to. On the contrary: with faster and more radical innovation the aviation sector can be made climate neutral by 2050.
TU Delft opens the first open-access intelligent knowledge databank to predict the future state of composite aircraft structuresAn intense two year test campaign on aerospace composite structures at the Aerospace Structures & Materials Laboratory of Aerospace Engineering Faculty of the TU Delft (NL) and at the Department of Mechanical Engineering & Aeronautics of University of Patras (GR) has resulted in a unique Structural Health Monitoring (SHM) Database.
New Online Course: Sustainable AviationThe experts of The Faculty of Aerospace Engineering at TU Delft have authored this new online course to share the current and future developments and strategies to achieve climate-neutral aviation with learners of various backgrounds (and not only technical ones).
Sustainable alternatives to kerosene: the factsWhy are CO2 emissions from flying increasing while aircraft are becoming more fuel efficient? What would be a logical distance to fly on a battery-powered electric plane? In what ways can you fly on hydrogen? The Netherlands Aerospace Centre (Royal NLR) and TU Delft have answered these and other pressing questions about sustainable aviation by setting out the facts in the fact sheet 'Sustainable Alternatives to Kerosene'.
SUSTAINair focuses on circular aviation for green transitionHorizon 2020 project SUSTAINair was launched recently to research and develop solutions to increase resource efficiency and aircraft performance while cutting down on waste and material costs throughout the aircraft life cycle, what is known as circular aviation.
Royal NLR and TU Delft present vision for route to sustainable air transportIn the white paper published today NLR - Royal Netherlands Aerospace Centre and Delft University of Technology present their joint vision on the most promising technologies for achieving a climate neutral air transport system by 2050.
Smart certification for sustainable aviationRapid innovation is needed to reach the 2050 sustainability goals for the aviation sector. But rather than a lack of ideas, or the effort needed to bring these to fruition, the main bottleneck in reaching these goals are the existing, very lengthy certification procedures. Three TU Delft professors therefore propose a radically different approach towards certification in aviation.
Looking over the shoulder of the pilot of a Flying-VThe first scale model test proved last year that the Flying-V is an airworthy, stable and controllable aircraft. In early November, it was time for the next step: the first simulator flight. Test pilot Thibaut Cappuyns had the honour of taking control of the stick of the SIMONA simulator. The pilot's experience of this revolutionary type of aircraft is crucial.
Is hydrogen-powered air travel the future?Hydrogen is a sustainable alternative to kerosene-powered air travel because there are no CO₂ emissions. But hydrogen is also associated with challenges and risks, such as the danger of explosion. TU Delft scientist Dr. Ivan Langella aims to use mathematical models, high-fidelity computational simulations and experiments to develop a zero-emission hydrogen-powered engine that will enable aircraft to fly safely and efficiently.
Airplanes cross borders, so do their environmental effectsWhen it comes to the health effects of aviation, reducing total fuel burn may not necessarily be the optimal strategy. And, perhaps even more surprisingly, your flight from Amsterdam to Rome contributes to premature deaths in Asia. Dr. Irene Dedoussi models the global human health impact of air pollution from aviation emissions, helping both airplane designers and policy makers in weighing the various mitigation strategies to make aviation sustainable.
Aerospace students revive Leonardo da Vinci’s aerial screw (and turn it into an electric personal aerial vehicle for today)In the midst of the corona pandemic, five aerospace engineering students from Delft University of Technology designed a vertical take-off and landing vehicle based on Leonardo da Vinci’s Aerial Screw and demonstrated its feasibility and physics.
Sustainable aviation starts on the groundFlying needs to become more sustainable, quieter and more efficient. For this you need to think far beyond the aircraft itself: airports for example, can contribute as well. In the newly launched Airport Technology Lab, TU Delft researchers are testing their ideas, from better weather forecasting models to faster baggage handling. All of these ideas contribute to improved efficiency in aviation, and a more sustainable industry. Already before the current coronavirus crisis, the aviation industry was facing huge challenges in areas such as sustainability, capacity and noise nuisance. The goal of the Airport Technology Lab (ATL) is to contribute to solving these problems. Since recently, it offers a special environment at Rotterdam The Hague Airport, where new services and products can be developed and tested under realistic and “live” conditions. Knowledge institutions as TU Delft, government bodies such as the City of Rotterdam, and the business community such as the airport and its innovation foundation RHIA, are collaborating closely. Fieldlab for aviation innovation “In other words, ATL is a fieldlab for innovations in aviation, where smart technologies are conceived, developed, tested and put into production", says project manager Elise Bavelaar from TU Delft. “We actually embarked on this course back in 2016 with the Innovation Airport initiative launched by Deltas, Infrastructures & Mobility Initiative (DIMI) and the faculty of Aerospace Engineering. This originated from the need to align all airport-related expertise at TU Delft and to link it together smartly. Of course the ultimate goal is to share this knowledge with parties beyond the university. An important part of Innovation Airport is our ambition to create a Fieldlab and the collaboration with the innovation foundation Rotterdam The Hague Innovation Airport.” The sector remains strongly convinced of the need for innovation, to be honest, I think even more than before the corona crisis. Read more Huge puzzle Airport Technology Lab is meeting this ambition and is thus an important follow-up from the Innovation Airport initiative. “All in all it has been a long journey to get the ATL to take off. It has taken us more than 18 months”, says Bavelaar, who has been involved with Innovation Airport from the start. “An important part of the process was our successful application for ERDF (European Regional Development Fund) funding. It was a huge and complex puzzle to coordinate everything and everyone, with on the one hand the many parties and areas of expertise (within TU Delft alone three faculties are involved, AE, EEMCS and IDE, plus the Innovation & Impact Centre), and on the other hand the different aspects that need to be addressed, ranging from financial affairs to legal issues. A key question was for example whether there was any unlawful state aid for the project.” Personal passion This made the ATL a very special environment for Bavelaar, who has a background in technology. She graduated five years ago from the Faculty of Aerospace Engineering at TU Delft. “Yes, it's a completely different job I have now, but I see that it is a considerable advantage to be well up-to-date on advancements in technology and engineering.” “It is precisely the combination of technology with other aspects that appeals to me. I experienced this in Germany during an internship for my Master's degree. I was working for Air Berlin and focused on improving airport processes. During that internship I discovered I like being involved with more than just the technology.” “My personal passion is to translate academic knowledge into practice. It is important that scientific insights can have a quicker impact on the real world.” Improved forecasting Back to ATL, which was officially opened at the end of May 2020. What makes this specific project unique? “For the most part this is because of the access to relevant airport data that we can use to test and develop new innovations. Of course appropriate measures related to privacy issues have been taken.” Meanwhile, the first tangible research projects have kicked off. “We have started working on three topics”, explains Bavelaar. “They all involve technology to make ground and air activities at airports more efficient and more sustainable in the near future. The first project is on expanding and refining the radar system at the airport. An extremely accurate model for current weather forecasting is being developed which will give Air Traffic Control increased insight into the current weather situation. This model can be used to predict possible turbulence between aircraft under changing weather conditions and this will ultimately lead to more efficient take-off and landing procedures. This part of the ATL project primarily involves the faculty of EEMCS.” Pleasant working environment In the second project, researchers are developing a new tool that can predict airside disruptions using machine learning techniques. This information can be used by planners at the airport to help them make tactical and operational decisions which will also lead to more efficient procedures. As part of the first project, the ‘flight-to-gate planning’ module is being tested. And finally, a tool is being developed which can simulate the efficiency, safety and resilience of processes in the airport terminal. Among other things, this tool enables development of applications for a call-to-gate strategy and passenger flow optimisation. In addition, this tool could be used to assess how the baggage drop-off points impact the flow of passengers in the terminal. According to Bavelaar: “The researchers’ initial experiences are positive. The airport has proven to be a pleasant working environment, with good accessibility and opportunities to test innovations. Moreover, the airport staff and the other stakeholders are more than happy to work with us.” The coronavirus situation demanded a great deal from the students’ capacity for improvisation. Nonetheless, in virtually no time at all they made the necessary practical adjustments, as did the other researchers in the project. This is really something to be proud of. Student involvement “So we're making good progress”, concludes Bavelaar. “An important factor is that we continue to reinforce the vision of DIMI within the project and in particular the emphasis on a multidisciplinary and holistic research approach. Of course there is the link with teaching at TU Delft. For example several student groups of the Interactive Technology Design course, at the faculty of Industrial Design Engineering, have already worked on airport assignments.” “The coronavirus demanded a great deal from the students’ capacity for improvisation. However, in no time at all they made the necessary practical adjustments, as did the other researchers in the project. This is really something to be proud of.” Bavelaar is aware that the current times have huge consequences for the aviation sector as a whole. “Yet the impact on the ATL project seems less bad than we feared, and if anything the coronavirus crisis has reinforced the need for innovation.” Read more stories of Aerospace Engineering Project Manager ir. M.E. (Elise) Bavelaar M.E.Bavelaar@tudelft.nl More stories More stories
The first Flying-V test flightSince 2017, a team of researchers from TU Delft has been working on a new, more sustainable type of passenger aircraft, the Flying-V. After the research phase, July saw an exciting and significant next step in the programme: the scale model’s first test flight. Would the aircraft remain stable when airborne, even at low speeds?
Lockdown presents unique opportunity to study sustainable taxiing at SchipholStarting a PhD on the first day of the Covid-19 lockdown was a bit challenging for Bieke von den Hoff. But the lockdown also gave her an unexpected and unique opportunity. As part of a trial programme at Schiphol Airport, she was asked to measure noise levels of a sustainable aircraft towing vehicle called the TaxiBot.
Flying in a VThe lorry pulls up very slowly, en route from the Faculty of Aerospace Engineering to Schiphol. The cargo on board is precious: the 3-meter-wide flying scale model of the revolutionary Flying-V aircraft. Chief engineer Malcom Brown: “The Flying V will use far less energy thanks to its aerodynamic V-shape.”
Bringing silent aviation closerHundreds of millions of people living near airports are still at increased risk of developing hearing problems cardiovascular diseases. Roberto Merino-Martinez, who himself lives near Rotterdam Airport, is researching a relatively new topic in the field of aerospace engineering: Aeroacoustics. An annoying whistle Although aircraft engine noise has decreased substantially over the past decades, air flow around the airplane structure itself is still a considerable source of noise. In wind turbines, this mechanism causes the typical swooshing sound. During the first part of his PhD, in which Merino-Martinez focused on flyover measurements at Schiphol airport, he was surprised to find that many types of planes produced a huge tonal peak at a frequency of 1720 Hz. That is a sound about as annoying as a heart monitor flatlining. “It was especially loud for the Airbus A320, one of the most popular passenger aircraft of all time,” Merino-Martinez says. “It turned out to be caused by a cavity in the nose landing gear, acting like a whistle. It is my most cited research paper to date, and I believe Airbus may fix this in their newer models.” An array of van microphones “The noise models used when designing new planes are simply not good enough. They lack sufficient detail and often predict noise levels to be lower than they actually are”, explains Merino-Martinez. “Improvements in acoustic imaging and its algorithms are essential. A single microphone suffices to measure a general noise level. Add a second and some directionality can be obtained. But to really understand aircraft noise, to make a high-resolution image of all its sound sources, you need to use a microphone array.” Noise created by aircraft wings As it turned out, Merino-Martinez worked most of his PhD on integrating a versatile microphone array into the existing TU Delft laminar flow anechoic wind tunnel, in order to be able to perform the desired measurements. “The solution we came up with allows for the microphones to be rearranged depending on the details of the experiment,” he says. “Sometimes you want to move them further apart for a higher resolution image, or closer together to improve the array’s frequency response.” He also spent a lot of time improving the algorithm for reconstructing the sound sources. “I wanted to accurately measure the sound levels created by airfoils, such as the wings of an airplane or the blades of a wind turbine. A lot of noise can be created by instabilities in the airflow at the trailing edge.” Most acoustic imaging algorithms are based on point sources because they are mathematically simpler. Merino-Martinez rewrote the algorithm so that it was also possible to accurately reproduce line sources of sound. "Our approach gives better results than existing solutions, even those developed by NASA." The silent flight of owls Using the new array and algorithm, Merino-Martinez and his colleagues performed measurements on variations of two noise-reduction techniques. “They are both inspired by the silent flight of owls,” he says. “Their feathers create a saw-tooth profile, called serrations, which interferes with the scattering of the airflow at the trailing edge.” The researchers also investigated the insertion of porous materials or metal foams at the trailing edge, causing a similar interference. “It’s a great solution from a scientific point of view,” he says, “but real-world application is difficult, as these materials can get dirty or attract lightning.” Whereas serrations are already being commercially used by some wind turbine manufacturers, it may take another ten years for the conservative airplane industry to follow suit. “Safety first.” Roberto Merino-Martinez +31 (0)15 27 81736 email@example.com This is a story from Aerospace Engineering An annoying whistle Although aircraft engine noise has decreased substantially over the past decades, air flow around the airplane structure itself is still a considerable source of noise. In wind turbines, this mechanism causes the typical swooshing sound. During the first part of his PhD, in which Merino-Martinez focused on flyover measurements at Schiphol airport, he was surprised to find that many types of planes produced a huge tonal peak at a frequency of 1720 Hz. That is a sound about as annoying as a heart monitor flatlining. “It was especially loud for the Airbus A320, one of the most popular passenger aircraft of all time,” Merino-Martinez says. “It turned out to be caused by a cavity in the nose landing gear, acting like a whistle. It is my most cited research paper to date, and I believe Airbus may fix this in their newer models.” An array of van microphones “The noise models used when designing new planes are simply not good enough. They lack sufficient detail and often predict noise levels to be lower than they actually are”, explains Merino-Martinez. “Improvements in acoustic imaging and its algorithms are essential. A single microphone suffices to measure a general noise level. Add a second and some directionality can be obtained. But to really understand aircraft noise, to make a high-resolution image of all its sound sources, you need to use a microphone array.” Noise created by aircraft wings As it turned out, Merino-Martinez worked most of his PhD on integrating a versatile microphone array into the existing TU Delft laminar flow anechoic wind tunnel, in order to be able to perform the desired measurements. “The solution we came up with allows for the microphones to be rearranged depending on the details of the experiment,” he says. “Sometimes you want to move them further apart for a higher resolution image, or closer together to improve the array’s frequency response.” He also spent a lot of time improving the algorithm for reconstructing the sound sources. “I wanted to accurately measure the sound levels created by airfoils, such as the wings of an airplane or the blades of a wind turbine. A lot of noise can be created by instabilities in the airflow at the trailing edge.” Most acoustic imaging algorithms are based on point sources because they are mathematically simpler. Merino-Martinez rewrote the algorithm so that it was also possible to accurately reproduce line sources of sound. "Our approach gives better results than existing solutions, even those developed by NASA." The silent flight of owls Using the new array and algorithm, Merino-Martinez and his colleagues performed measurements on variations of two noise-reduction techniques. “They are both inspired by the silent flight of owls,” he says. “Their feathers create a saw-tooth profile, called serrations, which interferes with the scattering of the airflow at the trailing edge.” The researchers also investigated the insertion of porous materials or metal foams at the trailing edge, causing a similar interference. “It’s a great solution from a scientific point of view,” he says, “but real-world application is difficult, as these materials can get dirty or attract lightning.” Whereas serrations are already being commercially used by some wind turbine manufacturers, it may take another ten years for the conservative airplane industry to follow suit. “Safety first.” Roberto Merino-Martinez +31 (0)15 27 81736 firstname.lastname@example.org This is a story from Aerospace Engineering Read more stories of Aerospace Engineering Related stories The responsibility gap of self-driving cars Roboats in Amsterdam The flying V
Can ancient algae help replace chromium-6 in coatings?Timelapse: corrosion protection of the letters ‘TUDelft’ What seemed like a wild idea in 2014, using the external skeletons of algae to prevent corrosion, has now been shown to provide long term protection of aluminium used in airplanes. In a few years’ time, it may provide a safe and environmentally-friendly replacement for the use of chromium-6. “Because of its toxicity, the European Commission has forbidden the use of chromium-6,” says Paul Denissen, PhD researcher in the Novel Aerospace Materials group at the faculty of Aerospace Engineering. “Use of chromium-6 is only still tolerated in situations where good alternatives are lacking, for example to protect airplanes against corrosion.” He explains that the aluminium alloy most used in aviation is especially susceptible to corrosion because of the copper that has been added to increase material strength. Typically, multiple boundary layers are applied to protect this aluminium against weathering. One of these layers is a primer coating loaded with chromium-6. “Our research focusses on using the external skeletons of a sort of algae to develop an environmentally friendly alternative for the use of chromium-6 in this layer.” Challenging chromium-6 Chromium-6 is a so-called active corrosion-inhibitor. When a treated surface is damaged, for example by scratching, the chromium-6 atoms will be released from the primer layer. They will create a thin layer of chromium oxide on the exposed metal surface, preventing further corrosion. After their release, chromium-6 atoms can continually redistribute themselves, providing continuous protection of the damaged area. “There are a number of alternative corrosion-inhibitors that are also very good at creating a protective barrier,” Denissen explains. “Unlike chromium-6, however, they can oxidize only once, and the protective layer they create is not permanent. Long-term protection therefore requires the continuous release of these inhibitors. More importantly, these alternative inhibitors may already chemically react with the primer coating at the time of its fabrication or application, thereby weakening their anti-corrosive power.” Quite some challenges to overcome, with a possible solution coming from the world of algae. Various shapes of the external skeleton of diatom algae Source : https://paleonerdish.files.wordpress.com/2013/06/diatoms.jpg Pill-box protection Diatoms are a group of microalgae that have been roaming the earth for more than 100 million years. These single cell organisms come in various sizes, ranging from one to a few tens of micrometres. They have a hard, inorganic shell to protect them from the environment. This cell wall is made out of silica, the same material as glass, and contains many nanometre-sized pores. Inspired by the pill-box shape of these shells ( see figure ), it was Santiago Garcia, associate professor in the same group and the supervisor of Denissen, who came up with the idea to use them for active corrosion protection in coatings. Garcia explains that “my idea was to fill these shells with alternative corrosion-inhibitors, and then add these loaded shells to the primer coating. I envisioned the pill-box structure to prevent the unwanted chemical reaction between inhibitors and coating.” He also imagined the pores to allow the immediate and sustained release of these inhibitors when the protective layers are damaged, and the metal surface is exposed. “And these algae shells are easily available at low-cost,” Denissen adds. Rapid development Denissen explains that his 2015 master’s thesis was merely a feasibility study, to see if this approach could be successful. “Now, we are three years into my PhD and despite limited resources we have just shown corrosion protection potentially equalling that of chromium-6. We still use our first pick of algae shells, but we have substantially increased their filling with inhibitors as well as their release efficiency, leading to a much-improved protection.” 30-day protection by algae coating with corrosion-inhibitors Testing in Paris After intensive work in Delft to proof the concept, the researchers travelled to Paris for a challenging experiment. “We were curious as to the long-term protective power of our coating for large damages, as required by several companies,” Denissen says. Together with their collaborators from the group of Polina Volovich at Chimie ParisTech, they applied a 1 mm wide scratch to samples of aluminium used for airplanes, covered in a variety of their test ‘algae-coatings’. These samples were subsequently immersed in large volumes of a highly corrosive environment. The researchers got what they bargained for ( see figure ). “We were astounded,” continues Denissen, “what we saw was full protection against corrosion, even after thirty days of immersion. Only a couple of alternative solutions come this close to the results obtained with chromium-6. It’s an amazing result after only such a short period of development.” Visualising corrosion protection Denissen and Garcia have also developed a novel method to study the onset and development of corrosion. It allowed them to gain a detailed understanding of the results they obtain with their algae shells, guiding further optimisation. “It is relatively simple technology, using a basic optical camera,” Garcia explains. “Optical techniques have traditionally been used to obtain qualitative information or to make beautiful pictures. What we have shown is that optics can be used to monitor and quantify local corrosion processes at a very high resolution, in real time. It is mature technology, allowing us to analyse any coating, commercially available or still in development.” Optimal protection “We use our experimental findings to build a computer model for further optimization of our coatings,” Denissen says. This can prove very beneficial as these algae shells come in more than 100.000 sizes and shapes. And there are more variables to tune, such as the type of corrosion-inhibitor used, whether or not to add an outside layer to the algae shell to even better regulate inhibitor release, or the optimal concentration of shells in the coating. “We may for example want to use disc-shaped shells to reduce our protective layer to the thickness currently used by the industry,” Denissen explains. “We are also looking into using combinations of inhibitors and shells in our coatings, further improving corrosion protection.” A small revolution It is not an easy task to replace chromium-6. “There are many barriers, resulting in a lack of good alternatives,” Denissen says. “For example, the Dutch Ministry of Defence wants proof that alternatives will provide twenty-year protection of their military equipment. But there are no good methods to accelerate this evaluation, to validate it in only a limited time-span.” More importantly, he explains, many of the tests used to validate the efficiency of new coating materials are designed specifically for chromium-6. “It is not a level playing field. It means that you have to prove your alternative coating to behave similar to chromium-6, rather than prove that it provides adequate protection.” Nevertheless, a small revolution has recently taken place. Rather than waiting for coating manufacturers to replace chromium-6, airplane manufacturers are now actively developing their own solutions as well. “At the moment, we are already talking to both.” Future perspective Despite very promising results, Denissen stresses that “we need a few more years to develop and demonstrate our algae-based coating before it can be used on planes, bridges or any metal surface that needs protection against corrosion. Does our coating protect sufficiently against scraping and scratching? Can it withstand frequent variations in outside temperature? Will it bond well with the other protective layers?” Garcia adds that “our main commitment is to find solutions to societal problems. We are currently talking to several industry partners about collaboration. Together we can speed up the development and launch of our technology and we expect to be ready for operational experiments by 2022, on an airplane.” Until completion of those experiments and passing the required certifications, the airplane industry may require the European Commission to again extend its leniency, tolerating the use of chromium-6 for the time being. You can find scientific publications, related to this research, here in Corrosion Science and in Electrochimica Acta .
In a breakthrough experiment Marios Kotsonis used plasma to actively interfere with the airflow on the wings of jet airliners.
Using plasma forces to improve airplane fuel efficiencyIn a breakthrough experiment Marios Kotsonis used plasma to actively interfere with the airflow on the wings of jet airliners.
A wealth of information on noiseIn aircraft, noise is an unwanted side-effect of propulsion. In sonar, it is a useful tool for conducting accurate depth measurements. But irrespective of whether it involves a disruptive noise or the useful application of sound, it always tells you something about the source and the surrounding environment. As Associate Professor in the Aircraft Noise and Climate Effects (ANCE) group, Mirjam Snellen specialises in techniques for imaging sound and using sound for imaging. She is only happy when all of the information has been extracted from the measured noise, often using detectors developed by the group. With its noise research, the small and close-knit ANCE research group, led by Prof. Dick Simons, is deliberately positioned at the interface between metrics and modelling. “If all you do is measure and take readings, you will not have the knowledge to design quieter aircraft,” says Mirjam Snellen. “Other groups are specialised in calculation. They build models that use the shape and material of an aircraft fuselage to predict the noise that it will produce at different speeds. Of course, this then needs to be validated using measurements.” As far as the ANCE group is concerned, even an accurate noise reading on a scale model in a wind tunnel is not sufficient. They prefer to measure it in practice, in the most realistic conditions possible. A key factor in this process is to ensure that the source of the noise, in this case the fuselage, can be properly separated from other noise sources, such as the engines. Mirjam Snellen: “We can achieve that in ANCE, using our arrays”. Separating out sound A single microphone only enables you to measure the volume and pitch of the sound. This changes if you use an array of several microphones, spread across a surface. For example, the noise that comes from the aircraft's nose cone reaches the closest microphone first, followed shortly by the microphones slightly further away. Software can be used to reconstruct the different sources of noise. “This is called beamforming,” explains Mirjam Snellen. “We can use it to separate the measured noise over time according to source location, pitch (frequency) and volume. We then make corrections for background noises, noise reflections on the ground and the Doppler effect, if necessary.” Certain preconditions are required for an array of this kind. To achieve a good resolution of the low pitches, which have a long wavelength, the microphones need to be slightly further apart. For the higher pitches, they need to be slightly closer together, to prevent distortion. For this reason, an array generally has a mixture of greater and smaller distances between microphones. An array is also limited in size in order to enable transportation. A large white surface The researchers in this group have mastered the beamforming techniques down to the finest details. In the summer, they took their first readings using the newly-designed array of 64 microphones, over a surface of 4x4 square metres. With it, they can also distinguish between the different sources of noise on small aircraft. Before setting off to work, the researchers always call the airport. “After all, you're going to be covering a large white surface there. Even now, someone from the airport always comes along, but that's just out of interest.” Array readings of this kind produce impressive pictures that clearly show which part of an aircraft produces which noise. The airflow around the wings typically results in slightly lower sound frequencies whereas the engines and the blast behind them produce higher frequencies. “Our measurements test the predictions made by models. If we are not expecting much noise from the undercarriage and our measurements show something different, the model may need to be revised.” In other words, different types of aircraft have different ‘fingerprints’ and this calls for a different way of reducing noise nuisance. Quieter aircraft Schiphol and Lelystad airports are reaching their permitted noise limits and are committed to quieter aircraft for the future. In 2015, Prof. Symons joined forces with the German Aerospace Center (DLR) in organising a workshop with experts from across the world. The key question was to what extent can aircraft become quieter, and how can this be achieved. “It's not easy,” says Mirjam Snellen. “For example, one option is ultra-high bypass engines. They are much quieter, but also extremely large and therefore heavy. The additional propulsion required to take off and fly could potentially end up increasing noise nuisance. Using alternative materials can alleviate the problem slightly.” The group is working on this together with the wind energy group in the same faculty, which is exploring the use of porous materials for wind turbines. “They could be used to make the nose and wing flaps or the landing gear of an aircraft slightly less noisy.” New developments like this are first tested in a wind tunnel. For this research, the group placed a microphone array in the relatively quiet vertical wind tunnel. The walls were covered with absorbent material to minimise sound reflection. Mirjam Snellen: “The challenge is to only measure the object. The array itself cannot be allowed to cause sound reflections and the microphones need to be positioned to enable precise measurements and beamforming.” Virtual noise It is easy to forget that the listener also plays an important role in noise nuisance and this also points the way towards new developments. If a group of test subjects are not disturbed by the hum from the fuselage, nothing needs to be adapted. Dick Simons was closely involved in the development of the Virtual Community Noise Simulator by the Netherlands Aerospace Centre (NLR). In it, you can experience the noise of a landing aircraft in a completely simulated natural environment. This requires carefully measured or modelled sound, something that the ANCE group can provide. Mirjam Snellen: “In a simulator, you can switch off the noise from the wings and engines only or change the pitch, for example.” Although the latter may seem futuristic, groups are already trying to achieve it using smart modifications in shape and material. Safety in shipping Beamforming also plays an important role in using echo sounding to determine the depth of water, which can improve safety in shipping. For example, precise information about the current water depths provides useful input for the frequency of dredging in and around the port of Rotterdam, ensuring it remains accessible even to the largest ships. Identifying water depths in this way can be done efficiently using multi-beam echo sounders (MBES). An MBES is an echo sounder that emits several narrow beams of sound simultaneously (a ‘ping’). Each of the beams propagates through the water, reflecting on the bottom before being recorded by the MBES again. Combined with the noise profile as a function of the depth, half of the time between sending and receiving a ping gives you the distance from the MBES to the bottom. Each ping identifies the bottom across a wide line, perpendicular to the direction of travel of the ship being studied. The entire bottom of the channel of water can be charted by using several parallel movements. Mirjam Snellen: “in the Netherlands, we are one of very few groups with a good knowledge of all the technical details of MBES; the system, its impact on the environment and the algorithms.” The MBES systems are complicated and expensive but achieve the level of precision required by government of just a few centimetres at depths of up to several tens of metres. Any new systems and cheaper alternatives will also need to prove they can meet these criteria before they will be permitted for use by a dredging company, for example. Mirjam Snellen: “We have developed a model that determines whether measurements meet these standards, given a set of measurement systems.” In addition, it is possible to obtain more information from the current systems, for example by also looking at the intensity of the echo signal alongside its two-way travel time. Mirjam Snellen: “In recent years, we have developed several methods to not only determine the water depths, but also the composition of the seabed using MBES measurements. For example, would it be a suitable site for a wind farm?” Using all of the data It was her determination to obtain all of the information available from sound that resulted in a method for improving the accuracy of MBES depth measurements. The raw data from an MBES system has to be corrected for many variables, including the underwater sound speed profile. This depends on the pressure (depth), temperature and salt content. In estuaries or tidal areas in particular, local variations in the sound speed can result in inaccurate readings. Mirjam Snellen: “This lack of precision increases towards the edges of the strip being measured. In the past, this was simply accepted or the measurements were taken again. But that is not necessary if you realise that the strips measured partly overlap.” She developed an algorithm that uses the extra data from this overlap to calculate the sound speed profiles. “It's not an estimate, it's physics,” says Mirjam Snellen. “With the occasional re-measurement, of course,” she adds with a laugh. Measurement is the key to knowledge Her algorithm for the underwater sound speed profile was recently implemented in commercial hydrographic software, but actually originates from 2009. “As in other fields, it can take time for solutions to be applied in practice,” says Mirjam Snellen. Another current topic to which this applies is the calculation of aircraft noise impact at airports. For this, so-called noise power distance tables are used. “No one knows how accurate the use of these tables is. This is 2018, why not position some arrays at the airport and actually measure this noise experience?” Find more information about the ANCE group here.
Mirjam SnellenIf you can precisely identify the noise that an aircraft produces, you can make smart use of this information in designing sustainable aircraft. This is the aim of Associate Professor in Acoustics Mirjam Snellen and her colleagues in the Aircraft Noise and Climate Effects (ANCE) research group, who find it fun to explore acoustic puzzles.
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