Ludwig Prandtl

Born: 4 February 1875 in Freising, near Munich, Germany
Died: 15 August 1953 in Göttingen, Germany

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Ludwig Prandtl's parents were Alexander Prandtl (1840-1896) and Magdalena Ostermann. Alexander Prandtl was a Professor at the Agricultural College Weihenstephan in Freising. Alexander and Magdalena were married in 1874, when Alexander was 34 years old and Magdalena was 18. Ludwig was their first child, born in his maternal grandmother's home at Hauptstrasse 64; his grandmother Maria Ostermann was, by this time, a widow. Ludwig was baptised when he was exactly one week old. His father Alexander carried out research on milk production and wrote scientific papers such as On the theoretically expected effect of creaming caused by centrifugal forces (1877) and The effect of currents caused by heating or cooling milk (1879). Alexander Prandtl had got to know Magdalena since he had lodged with her mother Maria Ostermann. After Ludwig was born his father set up home at Hauptstrasse 41, in the same road as his mother-in-law lived. His parents had two further children; a boy born in April 1877 only lived for one week and a girl born in January 1879 only lived for two weeks. Ludwig was, therefore, essentially an only child with his mother suffering a number of stillbirths and miscarriages over the following years.

A bright child, Ludwig could count to 10 by the time he was four years old and he could read by the age of five. He started attending elementary school in the autumn of 1881 when he was six years old. He was an outstanding pupil, coming top of the class of 82 pupils in his first year and continuing to be ranked first throughout his elementary schooling. He began his secondary education at the Gymnasium at the Freising Domberg in September 1885. His mother, who suffered mentally following the deaths of her children, the stillbirths and the miscarriages, went into a psychiatric clinic in Munich in 1888. After six months in the clinic she returned home but was completely bedridden. Soon she was taken into a nursing home in Neufriedheim. These unfortunate events had, of course, quite a serious impact on Ludwig during his time in secondary education. His mother, however, had a sister Marie who was unmarried and she effectively stepped into the role of mother to the young schoolboy. His father spent a lot of time with his young son and taught him much physics. Ludwig loved to sketch, particularly when on holiday in the South Tyrol, and also show considerable musical talent, having piano lessons from the age of nine.

In 1888 Prandtl was sent to the Ludwig Gymnasium in Munich. This was the second oldest Gymnasium in Munich, founded in 1824. He lived in a dormitory at the school but this was not a good experience for the 13-year old Prandtl who was bullied by the older boys. After one year at the school, his father decided that Prandtl should return to the Gymnasium at Freising. After two years, he returned to the Ludwig Gymnasium in Munich where now he was more able to fit in and make good friends. He excelled at Latin, Greek and particularly natural sciences. He gained much respect from his classmates. He graduated from the Gymnasium in July 1894 and he went to Nuremberg where he spent three months gaining practical experience working in a foundry. Returning to Munich, he entered the Technical University, the Königlich Technische Hochschule, in the autumn of 1894. He studied there for four years, attending lectures by August Föppl (1854-1924), Professor of Theoretical Mechanics and Graphical Statics, Sebastian Finsterwalder (1862-1951), Professor of Mathematics, Leonhard Sohnke (1842-1897), Professor of Physics, and Moritz Schröter (1851-1925), Professor of Theoretical Mechanical Engineering. He graduated with a degree in Mechanical Engineering in 1898. During his undergraduate years, both his parents died. His father, who had moved to live with his sister Anna in Dingolfing, died in March 1896 and his mother died two years later.

After graduating, Prandtl remained at the Königlich Technische Hochschule for a year having been appointed as an assistant to August Föppl. In order to allow him to take up this position, Föppl had requested that he be allowed to defer his military training. The request was allowed and Prandtl worked as Föppl's assistant from the beginning of October 1898 to the end of November 1899 [4]:-

He not only assisted in classes, but also participated in a number of experiments and theoretical projects concerned with stability and elasticity. One of these was later to become the subject matter of his doctoral thesis.
Ludwig Föppl, the professor's son, recalled Prandtl at this time:-
My first encounter with Ludwig Prandtl, which I still remember, was in 1898. The twenty-three-year-old assistant to my father, August Föppl, was invited by my parents from time to time to Sunday dinner. He also gladly stayed after dinner to drink a cup of coffee, particularly since he received motherly affection from my mother. This practical woman with a warm-hearted disposition took the young Prandtl, who was so alone and abandoned to the world without female support, under her wing from time to time. She gave him practical suggestions, which certainly provided him with valuable advice for coping with the problems of everyday life.
Prandtl wrote his doctoral thesis Lateral displacement phenomena, a case of unstable elastic equilibrium while working as Föppl's assistant. It was a piece of work which he carried out without any formal thesis advisor. The topic was of his own choosing. Although he had completed his thesis by 1899, the Königlich Technische Hochschule did not have the right to award doctorates so he submitted his thesis to the University of Munich. Ferdinand von Lindemann was appointed as an examiner and the oral conducted on 29 January 1900. He was given the grade "very good" and graduated with his doctorate. The thesis was published in early 1901 at Nuremberg and Prandtl immediately sent a copy to August Föppl. He replied to Prandtl on 21 February 1901:-
I was very pleased with the fine piece of work which you kindly sent to me and which I immediately read from cover to cover. I was especially surprised by the detailed and circumspect discussion of all the various cases related to tilting phenomena. Before reading your dissertation, I was not aware that your study would encompass such a large range of phenomena. It is the first time that one of my students has carried out such a proficient piece of work and this gives me all the more pleasure.
After working as Föppl's assistant he had to complete his military service which had been delayed for a year. Before the award of his doctorate, Prandtl applied for a position at the Maschinenbaugesellschaft in Nuremberg where he took up a position on 1 January 1900. While in this position he was assigned a task which led him to investigate fluid flow, the first time he had worked in this topic. However, he did not work in Nuremberg for long since he was soon appointed to a professorship in mechanics at the Technische Hochschule at Hanover, the Technical University of Hanover. This came about since, when the professorship became vacant, August Föppl had been asked to advise on who might be worthy of being appointed. He made a number of suggestions including Prandtl. The person who was most influential in making the appointment was Carl Runge who had been a professor at the Technische Hochschule at Hanover since 1886. He was extremely impressed with Prandtl, despite his youth, and strongly advised that he be appointed. He received the offer of the position on 21 August 1901. He gave up his position in Nuremberg at the end of September and moved to Hanover to take up the professorship. He became the youngest professor in Prussia.

Prandtl continued to work on fluid flow and, within four years, he had answered the outstanding open problem regarding fluid flow with vanishing viscosity by introducing the boundary layer, or, as Prandtl described it, the friction layer on the walls of the container. He presented his remarkable result at the International Congress of Mathematicians held in Heidelberg in August 1904. Before this meeting, however, he had been approached by Felix Klein who had offered him the position of Head of the Institute for Technical Physics at the University of Göttingen. However, Runge was determined to keep Prandtl at Hanover and Karl Schwarzschild, having had a letter from Runge, wrote to Klein saying that he felt there was little chance that Prandtl would accept. Prandtl was in two minds about whether to accept and Klein wrote to the Ministry in Berlin with a strong recommendation concerning Prandtl's ability (see, for example [4]):-

Prandtl's work stands out due to the combination of his specialist knowledge and his grasp of mathematics, a marked intuitive ability and the originality of his thought. At the same time, he also has a great interest in education.
Prandtl went to Berlin to negotiate over the offer and, after making the difficult decision, Prandtl accepted Klein's offer on 1 July 1904; by the end of the month, all the paperwork had been completed. It is clear that Prandtl held Klein in high regard and this was one factor in his decision to move to Göttingen. Indeed, after Prandtl moved to Göttingen he became a close friend of Klein. Another factor must have been the fact that Klein also approached Carl Runge and persuaded him to also accept a position at Göttingen.

Although Göttingen provided Prandtl with outstanding research opportunities, he was less happy with the teaching duties he was asked to perform. Therefore when he was approached by the Technical University Stuttgart and offered a full professorship, he used this to persuade the ministry to give him a full professorship at Göttingen. At Göttingen, in 1907, he organised the setting up of a wind tunnel where he could carry out practical results and it was constructed in the following year. By 1909 he had been given:-

... responsibility for the whole area of scientific aeronautics in both lectures and practical courses.
Prandtl had kept up his friendship with August Föppl and Föppl's son Ludwig came to Göttingen in 1908 to take courses by David Hilbert, Felix Klein, Prandtl and Runge. In December of that year, Ludwig told Prandtl that his sister Gertrud was soon to be engaged. Ludwig later said how sad Prandtl looked to receive this news. However, Prandtl received a letter from August Föppl shortly after the New Year telling him that Gertrud's engagement had been broken off soon after it was announced. In January 1909 another of August Föppl's sons, Otto Föppl, came to Göttingen taking up a position as Prandtl's assistant. In April 1909, Prandtl proposed marriage to Gertrud in a letter. On 11 September 1909, Prandtl married Gertrud Föppl in Munich. They had two daughters, Hildegard born 1914, and Johanna born in 1917. Johanna is the author of [4]. Before the birth of their first child they had moved into a larger apartment which was, at that time, on the edge of the town.

World War I saw Prandtl involved in a large aerodynamics laboratory which was created to support the German army and navy. Several of Prandtl's assistants who had been drafted for military duties were brought back to Göttingen to assist in this which was considered of major importance for the war effort. In 1915, another wind tunnel project was started and completed in 1917. This more powerful, 300 horse-power, tunnel gave Prandtl more scope for research. Although financed from war funds and intended for military research, nevertheless these facilities also gave Prandtl and his team scope to undertake research with no obvious military aims which they happily undertook.

August Föppl resigned from his position at the Technical University in Munich in 1920 and, although he wanted to continue to be able to undertake research in the laboratory, he gave up all teaching duties. The professorship was offered to Prandtl but, after much thought, he rejected the offer. After making the position more attractive to Prandtl, it was again offered in 1922 and this time he accepted. However, Göttingen were not going to give up easily and they countered with a promise of establishing a new Institute of Hydrodynamics at Göttingen if he stayed. This was really hard decision for the Prandtl family and negotiations dragged on for about a year. Eventually in June 1923 he made a decision to stay in Göttingen. However, over the following months the promise if the Institute became uncertain causing him again to favour Munich and only in December was his decision definite when the decision to built the new Institute at Göttingen was finalised. We note that the Gesellschaft für Angewandte Mathematik und Mechanik was founded in 1922 and Prandtl became the Society's first president. He continued in this role until 1945.

Let us now look at Prandtl's research. Busemann writes [8]:-

... he was not even a passenger in an airplane until about 1930, and his aerodynamical laboratory flourished without any connexion to an airfield. Prandtl preferred a small staff of rational thinkers and was almost afraid of the emotional interference which comes with the touch of the stick and with real flying. In no way does that mean that all play and fun were taboo at work. On the contrary, Prandtl's own way to tackle the Navier-Stokes equations was to toy with flows far beyond the range of practical application, showing the pleasure and also the marvelling observation of a child, till a phenomenon crystallized that asked for explanation. Spheres and cylinders were used as models - there would be no 'Karman vortex street' without that-sharp-edged corners were preferred to complicated shapes. He liked to reduce the number of arbitrary shape parameters, etc., so the brain would have no excuse for misinterpretation. The growth in time of starting vortices or of the curling ends of discontinuity layers, or the steady supersonic flow around the corner, are famous examples without any geometrical length, as indeed is the development of the boundary layer from the edge of a flat plate.
We have made a complete list of Prandtl's publications at THIS LINK and this gives a good guide to the range of his work. In 1930, Prandtl received the Daniel Guggenheim Medal [11]:-
Years ago Dr Ludwig Prandtl, professor at the University of Göttingen, Germany, studied how air rushes around moving planes. His work was so far ahead of his time and so accurate that it forms the basis of much present aerodynamic theory. Now Dr Prandtl is awarded the second Guggenheim gold medal for notable achievement in aeronautics. Orville Wright, who with his brother made the first successful flight, was presented the first medal last month.
When the Nazis came to power in 1933 then, on the one hand they were keen on aeronautical research, but of course there were attacks on mathematicians and theoretical physicists. It was not only those who were Jewish who were attacked. For example, although he was in no way Jewish, Heisenberg was subjected to frequent attacks in the press describing him to be of "Jewish style" and his appointments were blocked. Prandtl stood up for Heisenberg and others including Einstein.

In September 1938 Prandtl attended the Fifth International Congress for Applied Mechanics in Cambridge, Massachusetts, USA. There he met with G I Taylor and von Kármán among others and had useful discussions. Although these discussions were very pleasant, in general there was a difficult atmosphere at the meeting between the German scientists and others. After the meeting, G I Taylor wrote to Prandtl (see for example [4]):-

Now I must ask you to believe that, whatever happens between our countries, the friendship and admiration which I, in common with aerodynamical people in all other countries, feel for you, will remain unchanged. I realized that you know nothing of what the criminal lunatic, who rules your country, has been doing, and so you will not be able to understand the hatred of Germany which has been growing for some years in every nation, which has a free press.
The Nazis investigated Prandtl since he did not believe in the Nazi "German physics" and "German mathematics" and had spoken out against such political attempts to control science for their own ideology. Their report stated (see [1]):-
Professor Prandtl is a typical scientist in an ivory tower. He is only interested in his scientific research which has made him world famous. Politically, he poses no threat whatsoever ... Prandtl may be considered one of those honourable, conscientious scholars of a bygone era, conscious of his integrity and respectability, whom we certainly cannot afford to do without, nor should we wish to, in light of his immensely valuable contributions to the development of the air force.
When the war ended, teaching at Göttingen started again. Although he was now 70 years old, Prandtl again began lecturing to students. Of course Germany was now occupied by the allies and another difficult period followed. Although he had taken little interest in politics all his life, now he did become interested and joined the Liberal Party. In November 1950 he suffered a stroke and lost the power of one arm and a leg. However, he recovered so that he could again walk, but with a slight limp. In May 1952 he spent three weeks at the health resort at Bad Gastein but felt it did him more harm than good. In August he suffered another stroke and after a period with some improvement, his health began to deteriorate quite steadily.

Prandtl received many honours for his contributions. He was elected to the Royal Society of London in 1928, having been awarded their Gold Medal the previous year, and received a honorary degree from the University of Cambridge in 1936. Many other universities gave him an honorary degree including the Technische Hochschule Danzig, the Technische Hochschule Zürich, the Technische Hochschule Prague, the Technische Hochschule Trondheim, the University of Bucharest, and the University of Istanbul. He was given honorary membership of the Royal Aeronautical Society and awarded its gold medal. In total over twenty academies elected him to membership. He received a large numbers of medals and other awards.

Article by: J J O'Connor and E F Robertson

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