Ludwig Prandtl (German pronunciation: [ˈluːtvɪç ˈpʁantl̩]; 4 February 1875 – 15 August 1953) was a German fluid dynamicist, physicist and aerospace scientist. He was a pioneer in the development of rigorous systematic mathematical analyses which he used for underlying the science of aerodynamics, which have come to form the basis of the applied science of aeronautical engineering. In the 1920s, he developed the mathematical basis for the fundamental principles of subsonic aerodynamics in particular; and in general up to and including transonic velocities. His studies identified the boundary layer, thin-airfoils, and lifting-line theories. The Prandtl number was named after him.

Prandtl was born in Freising, near Munich, on 4 February 1875. His mother suffered from a lengthy illness and, as a result, Ludwig spent more time with his father, a professor of engineering. His father also encouraged him to observe nature and think about his observations.

Prandtl entered the Technische Hochschule Munich in 1894 and graduated with a Ph.D. under guidance of Professor August Foeppl in six years. His thesis was "On Tilting Phenomena, an Example of Unstable Elastic Equilibrium" (1900),

After university, Prandtl went to work in the Maschinenfabrik Augsburg-Nürnberg to improve a suction device for shavings removal in the manufacturing process. While working there, he discovered that the suction tube did not work because the lines of flow separated from the walls of the tube, so the expected pressure rise in the sharply-divergent tube never occurred. This phenomenon had been previously noted by Daniel Bernoulli in a similar hydraulic case. Prandtl recalled that this discovery led to the reasoning behind his boundary-layer approach to resistance in slightly-viscous fluids.

In 1901 Prandtl became a professor of fluid mechanics at the technical school in Hannover, later the Technical University Hannover and then the University of Hannover. It was here that he developed many of his most important theories. On August 8, 1904, he delivered a groundbreaking paper, Über Flüssigkeitsbewegung bei sehr kleiner Reibung (On the Motion of Fluids in Very Little Friction), at the Third International Mathematics Congress in Heidelberg. In this paper, he described the boundary layer and its importance for drag and streamlining. The paper also described flow separation as a result of the boundary layer, clearly explaining the concept of stall for the first time. Several of his students made attempts at closed-form solutions, but failed, and in the end the approximation contained in his original paper remains in widespread use.

The effect of the paper was so great that Prandtl would succeed Hans Lorenz as director of the Institute for Technical Physics at the University of Göttingen later in the year. In 1907, during his time at Göttingen, Prandtl was tasked with establishing a new facility for model studies of motorized airships called Motorluftschiffmodell-Versuchsanstalt (MVA), later the Aerodynamische Versuchsanstalt (AVA) in 1919. The facility was focused on wind tunnel measurements of airship models with the goal of shapes with minimal air resistance. During WWI, it was used as a large research establishment with many tasks including lift and drag on airfoils, aerodynamics of bombs, and cavitation on submarine propeller blades. In 1925, the university spun off his research arm to create the Kaiser Wilhelm Institute for Flow Research (now the Max Planck Institute for Dynamics and Self-Organization).

Due to the complexity of Prandtl's boundary layer ideas in his 1904 paper, the spread of the concept was initially slow. Many people failed to adopt the idea due to lack of understanding. There was a halt on new boundary layer discoveries until 1908 when two of his students at Gottingen, Blasius and Boltze, released their dissertations on the boundary layer. Blasius' dissertation explained what happened with the boundary layer when a flat plate comes in parallel contact with a uniform stream. Boltze's research was similar to Blasius' but applied Prandtl's theory to spherical shapes instead of flat objects. Prandtl expanded upon the ideas in his student's dissertations to include a thermal boundary layer associated with heat transfer.

There would be three more papers from Gottingen researchers regarding the boundary layer released by 1914. Due to similar reasons to Prandtl's 1904 paper, these first 7 papers on the boundary layer would be slow to spread outside of Gottingen. Partially due to World War I, there would be a lack of papers published regarding the boundary layer until another of Prandtl's students, Theodore Von Karman, published a paper in 1921 on the momentum integral equation across the boundary layer.