The lifeguard from Lake Constance who became a Professor in Odense
Portrait of Achim Schroll – retiring professor at the Department of Mathematics and Computer Science, SDU.
Everything seems carefully prepared, yet at the same time warm and genuine – much like the man himself.
Achim Schroll, dr.sc.math.habil., has been professor of Computational Science at SDU since 2009. He is a trained mathematician with a doctorate from the prestigious ETH Zurich and a habilitation from RWTH Aachen – both top European universities. He has held positions and visiting appointments at universities in five countries and has taught in five languages: German, English, Norwegian, Swedish, and Danish. And he has sailed through it all with the helm firmly in his hand.
He turned 63 this August and is stepping down from his professorship to continue as Professor Emeritus. He will not let go of his academic curiosity – or his eye for detail and depth – he will simply continue within freer framework.
- I will miss the academic conversations with colleagues. But not the administrative meetings. I have rarely had a whole day for research without either teaching or some meeting. That is not compatible with serious research, he says dryly with a laugh.
A mathematician who started out as a lifeguard
-My very first summer job was as a lifeguard at an outdoor swimming pool by Lake Constance, he tells me, then adds with a little smile: - So perhaps you could call me ‘the lifeguard from Bodensee who became a professor in Odense.
The humorous tone often accompanies his stringent way of thinking, but it is precisely this mix of precision and having both feet firmly on the ground that characterizes him. In his research, he has specialized in numerical analysis; a field he describes as the art of making mathematical models work in reality. It is about computing solutions to equations that describe flows, constructions, and physics – and keeping track of the errors that always arise in numerical simulations.
- The more powerful a computer is, the worse it gets if your calculations are wrong. Because then the errors just accumulate faster. That’s why you must know exactly how your errors behave. That is what numerical analysis is about, he explains.
Numerical Analysis
Numerical analysis is the study of how to translate mathematical models into computations that can be carried out by a computer. It is used, for example, to simulate real phenomena such as airflow over a wing, blood flow in the human body, wave impact on offshore wind turbines, and traffic flows in urban planning. When equations become so complicated that they cannot be solved with pen and paper, numerical analysis steps in.
Achim Schroll explains it like this: -Mathematics is always about equations – and many of them do not have an exact solution in the form of a formula. That’s why we replace them with a discrete version, something we can actually compute. But then errors arise. And numerical analysis is about keeping those errors under control.
A special focus area in Schroll’s research is so-called hyperbolic conservation laws, which occur in systems where physical quantities such as mass, energy, or momentum must be preserved – for example, in fluid flows, shock waves, or traffic models. Here the goal is to develop algorithms that not only produce an answer but a trustworthy one – even when reality is complex.
-The faster a computer is, the more important it becomes that we don’t make stubborn mistakes. Otherwise, the wrong result just gets worse the more the machine works on it. That’s why developing good numerical methods requires both theoretical understanding and practical precision.
Numerical analysis is not just a calculation tool – it is a fundamental discipline in modern science and technology. And according to Schroll, it is only becoming more important as the world grows more complex: -Nature has no problem figuring out how, for example, weather and climate develop – but it is not as easy for us. That is why research in this field is needed.
An academic journey across five countries
His academic journey began in Konstanz, where he started in teacher training for mathematics and physics, but he was quickly drawn to the logical beauty of mathematics. He changed course and devoted himself to pure mathematics – but the wish to use it in practice became decisive for his career. Numerical analysis became his calling. Not as an engineer or “calculator operator” for others, as he puts it – but as someone who wanted to develop the methods himself.After completing his diploma at the University of Konstanz, he followed his supervisor, Professor Rolf Jeltsch, to ETH Zurich in Switzerland, where he defended his dissertation. Later, he went to Norway to work with Aslak Tveito, who later founded the Simula Research Laboratory (www.simula.no), and then on to Sweden and Denmark. He has been a postdoc in Oslo, professor in Aachen, Trondheim, and Lund, and finally at SDU in Odense.
-Claus Michelsen, who was head of department at IMADA at the time, had a vision of strengthening applied mathematics and numerical analysis in Odense, and that matched my own ambition to build something from the ground up, Achim says.
A passion for sailing and structure
Sailing is Achim’s great passion. - Something very special happens when wind, water, and boat interact, he says with joy in his voice. -It is actually exactly what I work with in my field.
He describes fluid-structure interaction – how boat, air, and water constantly influence each other. He has worked with computational fluid dynamics and structural dynamics for years. -What’s fascinating is that it all happens at once. In nature it just works. But when we simulate it, it can take days on a supercomputer, he says. -So there are still things we haven’t fully understood. It shouldn’t be that complicated!
This fascination began long before his research. -I’ve always been drawn to the sea. The dynamics, the unpredictability, but at the same time so physically tangible. The fact that nature makes it work without calculation – that inspires me.
He and Petra took up sailing again after some years in Denmark. -We bought a boat and joined the sailing club in Faaborg. We’ve been members for more than ten years now. We’ve taken part in regattas, communal dinners, parties, all that, he says. But adds with a crooked smile: -In the beginning, we were just ‘those Germans with the boat.’ It took time before we became part of the community.
Sailing also gives room for the careful craftsmanship he values. -I’ve actually considered volunteering at a heritage shipyard in Middelfart, where they still build traditional dinghies. There’s something deeply satisfying about working with your hands and refining a surface through eight layers of varnish.
He compares it to his academic work: -Both in woodworking and numerical methods, it’s about preparation and precision. It’s about attention to detail. That suits me.
As a child, he dreamed of becoming a carpenter. -I liked wood. Shaping something physical. But my parents said I had to finish high school first. And then life took another direction, he adds.
But the passion for craftsmanship and precision has followed him – both as captain of his boat and in his research.
Computational Fluid Dynamics (CFD) & Structural Dynamics
Computational Fluid Dynamics (CFD) and structural dynamics are two closely connected fields within applied mathematics and engineering where numerical analysis plays a decisive role. They deal with understanding how fluids and solid structures move and affect each other – and how these complex dynamics can be simulated on a computer.
CFD is used to calculate how air, water, and other fluids flow around objects. It is applied in the design of aircraft, cars, wind turbines, and medical devices, where it is crucial to predict flow patterns, pressure distributions, and turbulence. Structural dynamics concerns how solid bodies – such as bridges or buildings – react to loads and motion.
When these two systems are coupled, it is called fluid-structure interaction (FSI). It is precisely in this field that Achim Schroll has found his niche. He explains: -Everything is dynamically coupled. The flow is influenced by the geometry – and the flow in turn affects the structure. We see it, for example, in a sailboat, where the wind, the boat’s shape, and the waves continuously interact.
Schroll has worked on developing methods that can simulate these interactions with great precision. Today, CFD and structural dynamics are used in everything from climate and ocean models to the design of artificial heart valves and high-speed trains. And in all cases, it requires physical understanding, mathematical modeling, and refined numerical methods – a combination that lies at the core of Schroll’s work.
Observation, abstraction, and concretization
Teaching has played a major role in Achim Schroll’s academic life. In Lund, he participated in an intensive, long-term university didactics program, which he now describes as decisive for his pedagogical approach. -It was hard to find time for it back when I was a young and busy researcher. But it really gave me something. It made me understand what teaching is really about, he says.
It is not, as he says, about transferring knowledge: -As a teacher, it’s not about transmitting information, but about supporting learning.
He works with three pedagogical building blocks: observation, abstraction, and concretization. -First you observe a phenomenon. Then you try to find the laws behind it – you abstract. And finally, you use those same laws in a completely different context. That is where real learning happens.
He likes to use linear algebra as an example: -You start by learning to compute with vectors and matrices. Then you understand the concept of vector spaces. And suddenly you recognize the structure in a completely different place – maybe in particle physics. Then you know you’ve learned something.
He shares this insight with his students every year: -I say it again and again: I can’t learn for you – but I can help you learn. He emphasizes that students should understand both the practical tools and the underlying theory. -The most important thing is not that they can run a simulation – but that they understand why it works. And where it can break down, he says.
During his time at SDU, he has developed teaching in applied mathematics so that today, already in their third bachelor year, students work with 2D and 3D simulations of the Navier-Stokes equations in complex geometric domains – a Clay Millennium Problem (navier-stokes-equation).
He describes it as a joy to see students succeed. -When it all suddenly comes together for them – when they get that ‘aha’ moment. That is what makes it all worthwhile, he adds.
A professional and personal legacy
At IMADA, Achim has been part of building up the research section in Computational Science. Today he has colleagues like Kristian Debrabant, Ralf Zimmermann, and Benjamin Jäger, all working in numerical analysis at SDU. -I didn’t have formal responsibility for them, but I advocated that they should be part of the group. I could see their potential – and it was important that we weren’t just one researcher in the field.
He has also supervised a number of PhD students. One, who came from Ethiopia, stands out in his memory, as Achim traveled there several times during the project. -It was a very meaningful experience. I could feel that what I contributed really made a difference for her – and maybe for her whole family. In Ethiopia, large parts of the population don’t have electricity in their homes. Even at the hotel in the city, there wasn’t running water all day. It puts things into perspective.
Achim’s research contributions revolve especially around hyperbolic conservation laws and numerical methods. Together with Professor Jens Lorenz (University of New Mexico), he laid the theoretical foundation for a class of methods that preserve the properties of these laws in computations. -We worked on ensuring that the errors that inevitably arise in a discrete model don’t grow uncontrollably – but instead are dampened and stabilized. That requires deep mathematical insight.
Under Achim’s supervision, students later developed and implemented the methods. This led to a number of master’s theses and PhD dissertations.
He calls his collected manuscript his “life’s thesis”: a 120-page work gathering his thoughts and results from an entire career. -I began working on it back in Konstanz during my diploma. Back then I didn’t understand much – but now, 89 semesters later, I can see how it all fits together: It’s not about being groundbreaking all the time, he says. -But about understanding something so deeply that you can pass it on with clarity.
A deep and meaningful contribution
When I finally ask Achim what he hopes to have left behind, he says: -I’ve helped build something. I’ve passed something on. Not only methods and knowledge, but also a way of thinking. A depth, perhaps. A striving to understand, he says, and concludes: -I’m probably not a leadership type, because I’ve never wanted to control people – only calculation errors.
Achim has found his home – and now also his retirement – here on Funen. No other university has managed to keep him as long as SDU has. He has been a driving force behind the Applied Mathematics program and brought numerical mathematics to Odense with both sharpness and enthusiasm. His teaching has united mathematical numerical theory and modern programming – with a poetic and humorous touch that makes the heavy lighter. And no matter where I have traveled with Achim – conference, guest stay, research collaboration – we have always met someone who knows and cherishes Achim. When old friends and colleagues meet him, it is with a glow in their eyes and warmth in their voice. One clearly feels that he has left his mark, and he has done so with heart, humor, and academic depth. Achim, thank you for your contributions, your puns, your crooked smiles, and your sharp mind. We hope that you and Queen Petra, after your abdication, will enjoy a wonderful retirement among waves, books, and brews.
Christian Valdemar Lorenzen, PhD in Applied Mathematics, Software Engineer at Unicontrol, and former student at IMADA.
Achim is not only an excellent researcher who always strives for deep knowledge, but he is also an unusually collegial and helpful person who has never hesitated to set aside personal interests for the benefit of the department, as well as for younger colleagues. He has also shown an extraordinary dedication to supervising students. I admire Achim’s visionary foresight and creativity as well as his perseverance in standing up for what he believes is right.
Kristian Debrabant, Associate Professor, Computational Science, IMADA.
When I became head of department in 2007, applied mathematics at IMADA was a declining program that could not attract students. With the appointment of Achim, IMADA gained an experienced and dedicated professor in applied mathematics.
I particularly recall how, during a visit to Simula Research Laboratory in Oslo, Achim introduced me to scientific computing and made me aware of applied mathematics in health research. This led to us jointly offering a course at the Danish Adult Education Association, where Achim gave the lecture “Differential Equations on the Doctor’s Desk,” and to Achim’s involvement in the BioMat project on the interaction between mathematics and biology in high schools, where he introduced teachers to “the mathematics of the heart.” It is very much thanks to Achim that IMADA today, quite uniquely for a mathematics department in Denmark, has attractive programs and a strong research group in applied mathematics. I greatly value my many years of collaboration with Achim and our good conversations with both professional and personal benefits.
Claus Michelsen, Professor Emeritus and former Head of Department at IMADA.
I have mostly worked with Achim in the teaching committee, where he has been a strong voice on behalf of Applied Mathematics, and where he has often drawn on his broad experience with mathematics education in several countries. In general, he has shown strong commitment both to the quality of IMADA’s study programs and to the well-being of the students.
A special task I have shared with Achim has been the work on the biannual educational reports, which many consider difficult. It has been a pleasure that Achim always tackled them head-on and used the reports constructively to improve and refine the study programs. Based on this, he also helped improve the study environment, even beyond the purely academic. Having worked in all three Scandinavian countries, he has, without reservation, developed his own charming version of Norwegian-Swedish-Danish, which he has fearlessly used, and which the students have embraced – yet another example of Achim’s “fearlessness” and willingness to learn and engage in order to be able to teach.
It has been a pleasure – always cozy and educational, even when we did not agree – to have Achim as a colleague. I will miss him at IMADA, but at the same time I wish him and his family – and his sailboat! – fair winds ahead.
Hans Chr. Petersen, Associate Professor, IMADA, SDU.
