In memoriam Prof. Hans Grönig (1931–2021)

Professor Hans Grönig passed away on July 14, 2021, surrounded by his family in his home at Walheim, Aachen, at the age of 90. He died 5 months after the passing of his beloved wife Inge. Many colleagues and scientists remember Hans and Inge Grönig as diligent and enthusiastic participants of the Shock Wave Symposia and other conferences, as well as extremely lovable guests and hosts. It was their unbiased openness to all people of different nationalities and backgrounds that led to numerous long-lasting close national and international friendships. With the death of Hans Grönig, we lose an outstanding colleague in the field of high-temperature gas dynamics and shock wave physics, who made ground-breaking and lasting contributions, as well as an exceptional human being, mentor, and friend.

Anyone who wants to describe Hans Grönig will probably, without much hesitation, list the following attributes: friendly, calm, generous, patient, very well-read (not only in his field of expertize), competent, helpful, mindful of harmony, and often showing subtle yet profound humor. Those who have known Hans Grönig professionally or privately for some time would have experienced the mentioned character traits in many forms. One Australian colleague, Professor Brian Milton from the University of New South Wales in Sydney, summed it all up very briefly and aptly in a conversation a few years ago when he said that Hans Grönig was a real gentleman.

Hans Grönig was born on February 10, 1931, at Mönchengladbach-Rheydt, a historic city in West Germany. As a child, he showed great interest in physics while attending school in the difficult times of World War II and the post-war period. In 1951, he moved to Aachen to study physics, and in 1956, after finishing his diploma degree, he started his research career as a research assistant under the guidance of Professor Fritz Schultz-Grunow at the Institute of General Mechanics, RWTH Aachen University. There he earned his doctoral degree (Dr. rer. nat.) in 1961, followed by his habilitation in 1967. In between he worked as a postdoctoral fellow at GALCIT (Graduate Aeronautical Laboratories of the California Institute of Technology) with Professor Hans W. Liepmann as his mentor (1961–1962). In 1968 he became Professor of High-Temperature Gas Dynamics and in 1971 Head of Shock Wave Laboratory at RWTH Aachen, continuing until his retirement in 1996.

In his doctoral thesis, Hans Grönig used theoretical methods to determine the electron distribution in strong shock waves, which he verified experimentally using Aachen’s first shock tube where the shock velocity was measured with the help of commercial spark plugs. With hydrogen as high-pressure gas and argon as low-pressure gas, a shock Mach number of around 10 could be achieved. The gas heated by this shock wave reached a temperature of about 10,000 K at the end wall of the tube. Its bluish-white glow could be observed clearly as a glass plate served as the tube end wall. This work concluded that diffusion of electrons into the cold gas ahead of a strong shock was a dominant precursor in conventional shock tubes due to the limited Mach numbers achieved. Photoionization from radiation was assumed to be the main precursor for very strong shock waves, yet no evidence from experiments had been observed. At that time, Professor Liepmann and colleagues at GALCIT reported their success in magnetically driven cylindrical shock waves, which inspired Hans Grönig to conduct postdoctoral studies at GALCIT. In his experiments, a Mach number of around 60 was produced in a similar cylindrical magnetic shock chamber with a radial tube extension for measurements using the space-charge-limited current of two diodes. The results demonstrated that precursors consisted of both radiation and electron diffusion at this very high Mach number corresponding to an energy of 4.5 eV, i.e., a 52,000 K equilibrium temperature behind the shock. Returning to Aachen, he continued the research on strong shock waves via magnetic inverse pinch and spectrometric methods measuring the energy of both precursor photons and electrons. This led to a theory for determining different physical quantities in a high-density photoionization chamber for his habilitation thesis.

The Aachen Shock Wave Laboratory at that time, founded by Professor Schultz-Grunow in 1956, was located in a former anti-air-raid underground bunker near the main railway station and soon no longer met the space and operating pressure safety requirements of increasingly powerful shock tubes. For these reasons, plans started in 1963/64 for the construction of a new shock wave laboratory. Hans Grönig significantly influenced the design and became the lead for the construction as he took the professor position in 1968. The construction of the current laboratory, completed in 1970 in Aachen’s suburb Laurensberg-Seffent in a rural setting, ensured not only significantly enhanced safety for both environment and staff but also enabled the installation of larger systems. As Hans Grönig took the lead for this newly built, unique Shock Wave Laboratory, numerous new shock tubes were built and put into operation. This included a high-pressure shock tube system which was also used as the first shock tunnel with an operation pressure up to 1000 bar (TH1), several conventional stainless steel shock tubes, an ultra-high-vacuum shock tube that could be heated to 450 $^{\circ }$C, a low-temperature shock tube for temperatures down to − 193 $^{\circ }$C, and a water shock tube. Immediately after completion of the new laboratory building, Hans Grönig led the design and construction of a new shock tunnel (TH2), designed for an operating pressure up to 2000 bar. This facility became operational in 1975 and was, at the time, the largest and most powerful shock tunnel in Europe. It is still in use today, with an operational pressure of 1500 bar. Several transient measurement methods were introduced, including a new variant of the Mach–Zehnder interferometry technique developed by him in 1967. Within the framework of national research programmes, numerous experimental investigations on heat flux, force, and pressure distributions on lifting-body re-entry vehicles were carried out in both shock tunnels until 1976. Thereafter, hypersonic re-entry research activities were discontinued in Germany, but Hans Grönig succeeded in using the facilities in the laboratory for other research projects.

In the following years, the Shock Wave Laboratory led by Hans Grönig made globally significant contributions in the fields of shock wave physics, hypersonic aerothermodynamics, and detonation. The research covered topics such as strong shock wave structure, ionization phenomena in shock-heated gases, dissociated boundary layer behind shock waves, rotational relaxation in molecular gases, hypersonic non-equilibrium expansion flow, reflected shock wave structure, blast and shock focusing, stability of converging waves in air and water, shock dynamics and detonation in gas–particle flow, cavitation bubbles in water, shock wave and transient waves in solids, and transient profile fluid dynamics in a water-Ludwieg tunnel. These works were well documented in over 150 publications.

With the start of the European spacecraft program HERMES in 1987, the shock tunnel TH2 was re-activated and modernized with up-to-date measurement and flow visualization techniques and was used intensively in the HERMES program in the following years. The high-quality results were reported in numerous publications in international journals. Some of the results have been included in international databases and are still used today for validation purposes, among other applications. The developed modern diagnostics, including instruments for ultra-fast heat flux, force and pressure measurement, and visualization techniques such as color schlieren, were also adapted for worldwide supersonic flow and hypersonic spacecraft studies. In addition to the extensive experimental work, numerous theoretical and numerical studies were initiated, either as independent projects or in conjunction with experiments. The knowledge and skills acquired under the leadership of Hans Grönig in the field of hypersonic research led to further scientifically demanding studies in the national and European HERMES successor programmes.

In 1981, Hans Grönig initiated transient shock wave studies in gas–particle flows. In 1987, the shock tunnel TH1 was converted into a solid particle–gas detonation tube. Of interest were the unresolved questions of the transition from slow deflagration to detonation and whether a stable, self-sustaining detonation can exist in a reactive dust–gas mixture. Conclusive experimental proof could be provided due to the large dimension of the tube, which had an inner diameter of 141 mm and a length of 21.7 m. The results, which showed, for the first time, a detailed structure of stable spinning detonation, ended the decades-long suspicion regarding the existence of heterogeneous detonation of solid particle–gas flow characterized by a transverse wave structure. Based on the results obtained, the design and construction of a twofold scaled-up solid particle–gas detonation tube began in 1992. This was sponsored by German industry as these studies served, among other applications, to clarify safety issues during the transport of organic, metal, and reactive powders of all kinds in long piping systems. The new stainless steel detonation tube, with an inner diameter of 300 mm and a length of 40 m, could be operated at pressures up to 250 bar. In this facility, numerous significant contributions were made on deflagration-to-detonation transition as well as spinning and cellular detonation structures (cell width of the order of 0.3–1 m) in aluminum, anthraquinone, and cornstarch particle–air flows, which attracted great interest in the international academic communities and industries.

Flow visualization was another area that Hans Grönig was very interested in. While waiting for the delivery of a commercial Mach–Zehnder interferometer in the 1960s (the delivery time of these individually produced instruments exceeded 2 years), he and his team decided to design and build their own smaller version, which they used until the commercial unit became available. In later years, the development and extension of other visualization techniques such as color schlieren, shearing interferometry, and holographic interferometry followed.

Hans Grönig maintained an active exchange with scientists from numerous other countries, which led to a variety of research cooperations. Examples included scholars from Israel, South Africa, Australia, France, Poland, Canada, and the USA. The contacts to research institutions from all over the world led to close collaborations and also to personal friendships that continue to this day. The Aachen Shock Wave Laboratory became a Mecca for high-temperature gas dynamics and shock wave physics during Hans Grönig’s time, with an environment that encouraged collaborative work and the exchange of research efforts. In his mind, the invitation of various worldwide known scholars and experts to Aachen’s Laboratory provided valuable direct personal interactions that significantly benefited research and students in a broad vision of knowledge and research experience.

He established his first contacts with Chinese scientists in 1979, when he hosted the first visit of Professor Hongru Yu of the Institute of Mechanics, Chinese Academy of Sciences, and in 1985 of Professor Zhaoyuan Han from the University of Science and Technology of China, Hefei. A few years later, an intensive scientific collaboration was also established with the University of Science and Technology in Nanjing. During his visit in 1988, Professor Yu worked on the concept of a gas detonation driver for the TH1 shock tube, leading to a close cooperation to develop and construct a detonation driver for the shock tunnel TH2 as an alternative to a free-piston driver. This facility was set into operation in 1994 and allowed a significant increase in the performance of the TH2 shock tunnel.

Hans Grönig was also a source of inspiration to shock wave and detonation researchers around the world, and he was invited as a visiting professor by many universities and academic institutes. Among the lectures he held abroad in his earlier times were those at the Chinese Academy of Sciences in Beijing (1981), the Japan Society for Promotion of Science (1981—Kyoto University, Kyushu University, Nagoya University, University of Tokyo, Tsukuba University, Institute of Space and Astronautical Science Tokyo, and Yokohama National University), the USSR Academy of Sciences (1985—Institutes at Moscow, Novosibirsk, and Saint Petersburg), several courses in the USA, and a number of plenary lectures at International Symposia since 1975.

For many of us, the 16th International Symposium on Shock Tubes and Waves is remembered as an outstanding event, which took place in Aachen in 1987 under the direction of Hans Grönig, who did most of the organizational work himself, supported mainly by his wife and his secretary, Ms. Christiane von Hoegen. Very early on he paid great attention to establish contact with foreign scientists from all over the world. During the symposium planning, he worked intensively with the relevant organizations to ensure that scientists from the USSR could also participate in this symposium. Thanks to his persistence, this became eventually possible. After all the particularly strict and extensive visa requirements existing at that time had been met, seven scientists from the USSR took part in this meeting after having been absent from earlier installments for 10 years. This laid the foundation to a cooperation that resulted in numerous visits of Soviet and then Russian scientists at the Shock Wave Laboratory, which led to high-quality scientific contributions in the fields of shock wave propagation in dusty gases, ignition behavior, and detonation transition in gas mixtures as well as shock wave kinematics such as shock diffraction and reflection.

Hans and Inge Grönig discovered their passion for Japan at a very early time. Shortly before the inauguration of the new Shock Wave Laboratory building in 1971, Professor Kunio Terao of the Yokohama National University visited the laboratory. Four years later, Hans Grönig traveled to Japan for the first time and took part in the 10th International Shock Tube Symposium in Kyoto. Here he established numerous contacts with Japanese scientists, resulting in close cooperation in various fields of shock and detonation wave physics. Hans and Inge Grönig visited Japan several times, with their first several-month-long stay in 1981 at Yokohama National University. At that time, the university did not have a guest hotel, and so the Grönigs lived in the student dormitory. They thoroughly enjoyed the warm hospitality they received, the only minor problem being that Hans Grönig was, for Japanese standards, extraordinarily tall, which led to some issues regarding the length of beds and the height of doors. Many mutual long-period scientific visits and cooperations with numerous universities and institutes of Japan followed, and an intensive exchange of Japanese and German scientists developed. In addition to scientific discussions, Hans Grönig regularly tried to convince his Japanese guests of the advantages of wine from the Moselle river, located about 100 km south of Aachen. One Japanese merchant in Yokohama eventually started importing this wine from Hans Grönig’s favorite vineyard to Japan.

Sendai and Tohoku University was another place that deeply impressed the Grönigs. Hans Grönig visited Professor Kazuyoshi Takayama of the Institute of Fluid Science in 1982. After a short time, a close scientific connection developed between them with frequent mutual visits. At the end of the 1980s, both came to realize that the rapidly growing international activities in the fields of shock wave physics including detonation and explosion could no longer be adequately presented in the existing journals. It was therefore necessary to have a new international journal to provide a forum especially for these topics. They had numerous discussions on this initiative and regular meetings with members of the Springer Verlag at Heidelberg. Eventually the first issue of the Shock Waves Journal was published in March 1991. Hans Grönig served as the Managing Editor from the first issue of the journal until the end of 1994 and from then as the Editor-in-Chief until 2003.

In the middle of the 2000s, Hans Grönig, jointly with Professor Takayama and Professor Yasuyuki Horie of the Air Force Research Laboratory Eglin as the Series Editors, founded a Springer book series, the Shock Wave Science and Technology Reference Library. This book series was conceived in the style of the famous German “Handbuch der Physik” and had the principal motivation to assemble state-of-the-art archival reference articles by leading scientists in shock wave research and its applications. Over ten volumes have been published so far, and many young scientists and engineers have been educated and inspired through this reference book series. Due to his remarkable scientific achievements in the aforementioned areas, Hans Grönig was awarded two honorary professorships and one honorary doctorate during his active career.

One of the remarkable character traits of Hans Grönig was that he was always passionate in encouraging and supporting young scientists by allowing them to attend various academic conferences and platforms as well as providing exchange opportunities with other institutions. A total of 35 PhD candidates studied under him, and many went on to be leaders in education and research, in universities, industry, and government agencies throughout the world, thus continuing Hans Grönig’s legacy. In the 26 years of leadership of Aachen’s Shock Wave Laboratory, he created a collegial, friendly, and productive atmosphere in which everyone had a large amount of freedom to solve encountered scientific or technical problems in their own way, even if this way was not exactly what Hans himself would have chosen to do. It is no coincidence that most of the doctoral theses completed under his guidance contained personal acknowledgments emphasizing this particular feature of the work in the Shock Wave Laboratory. He was constantly willing to share his own extensive knowledge when asked for advice—he had an open-door policy, and anyone who knocked on the door of his office to discuss a technical question was always welcome and left the room with good indications of a solution path.

The pleasant atmosphere that Hans Grönig had created in the laboratory was also evident in other events: after the end of the aforementioned 16th International Symposium on Shock Tubes and Waves held in Aachen, Hans Grönig privately organized and financed a trip to the Moselle river, including an extensive wine tasting, for all Shock Wave Laboratory staff and some international guests. This trip is still, more than 30 years later, a topic of conversation at almost all alumni gatherings.

Hans Grönig met the love of his life, Inge, at school in Mönchengladbach-Rheydt, and since then, they stayed together for 70 years. As Inge said “Zusammen können wir Alles schaffen (together we can do everything),” she supported her husband’s efforts every step of the way. Their friendly home at Walheim attracted friends and visitors of the shock wave communities around the world; while Inge offered all kinds of generous hospitalities, he (as a wine connoisseur) brought from the cellar his beloved dry Pfalz Spätlese Rieslings.

In 2002, already well after his retirement, Hans and Inge Grönig organized the 16th Mach Reflection Symposium (nowadays this conference series continues as the Shock Interaction Symposium) in Aachen. Apart from its robust scientific program, all participants still have fond memories of the symposium excursion and accompanying persons’ program, organized and led by Inge, as well as the memorable banquet in the Brunnenhof restaurant in Walheim with its charm and atmosphere of a 300-year-old country house. Needless to say that Hans’ Rieslings were in unlimited supply.

Hans Grönig was very well-read and had many other interests outside his field of expertize, including geography, history, and art from all around the world. Visitors have always been impressed by the extensive library at their home, which stretched over several rooms. He often surprised his guests with detailed knowledge on topics that one would not have expected him to be interested in. The Grönigs loved traveling to various parts of the world as mentioned before. In spite of his battle with medical issues in recent years, they showed their courage and kind souls by facing life’s challenges with a smile. Inge passed away peacefully on February 10, 2021, and Hans followed her 5 months later.

We are all deeply affected by the loss we suffered with the death of Hans Grönig and his wife Inge. They had a predominantly beautiful and long life, and we consider ourselves lucky that we were able to know them and accompany them for a part of their life journey. As their influence to our life continues, Hans and Inge Grönig will stay in our hearts and minds forever. May their souls rest in peace.