How did Ludwig Prandtl suggest the condition known as the Prandtl Condition?
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.
Prandtl condition
The Prandtl condition was suggested by the German physicist Ludwig Prandtl to identify possible boundary layer separation points of incompressible flows.
Ludwig Prandtl
Ludwig Prandtl (4 February 1875 – 15 August 1953) was a German engineer.
 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.
Theodor Meyer
Theodor Meyer (July 1, 1882 – March 8, 1972 in Bad Bevensen, Germany) was a mathematician, a student of Ludwig Prandtl, and a founder of the scientific discipline now known as compressible flow or gas dynamics. Mixing length model
In fluid dynamics, the mixing length model is a method attempting to describe momentum transfer by turbulence Reynolds stresses within a newtonian fluid boundary layer by means of an eddy viscosity.
 The model was developed by Ludwig Prandtl in the early 20th century.
 Prandtl himself had reservations about the model, describing it as, "only a rough approximation," Stephen Timoshenko
Timoshenko was born in the village of Shpotivka in the Chernigov Governorate of Ukraine.
 He studied at a "realnaya" school (Russian: ) in Romny, Poltava Governorate (now in Sumy Oblast) from 1889 to 1896.
 In Romny his schoolmate and friend was future famous semiconductor physicist Abram Ioffe.
 Timoshenko continued his education towards a university degree at the St Petersburg Institute of engineers Ways of Communication.
 After graduating in 1901, he stayed on teaching in this same institution from 1901 to 1903 and then worked at the Saint Petersburg Polytechnical Institute under Viktor Kyrpychov 1903–1906.
 In 1905 he was sent for one year to the University of Göttingen where he worked under Ludwig Prandtl. Lifting-line theory
The Prandtl lifting-line theory is a mathematical model that predicts lift distribution over a three-dimensional wing based on its geometry.
 It is also known as the Lanchester–Prandtl wing theory. Tesla turbine
The Tesla turbine is a bladeless centripetal flow turbine patented by Nikola Tesla in 1913.
 It is referred to as a "bladeless turbine".
 The Tesla turbine is also known as the "boundary layer turbine", "cohesion-type turbine", and "Prandtl layer turbine" (after Ludwig Prandtl) because it uses the "boundary layer effect" and not a fluid impinging upon the blades as in a conventional turbine.
 Bioengineering researchers have referred to it as a multiple disk centrifugal pump.
 One of Tesla’s desires for implementation of this turbine was for geothermal power, which was described in "Our Future Motive Power". Ludwig-Prandtl-Ring
The Ludwig-Prandtl-Ring is the highest award of the Deutsche Gesellschaft für Luft- und Raumfahrt (German Society for Aeronautics and Astronautics), awarded "for outstanding contribution in the field of aerospace engineering."
 The award is named in honour of Ludwig Prandtl. Prandtl number
The Prandtl number (Pr) or Prandtl group is a dimensionless number, named after the German physicist Ludwig Prandtl, defined as the ratio of momentum diffusivity to thermal diffusivity.
 That is, the Prandtl number is given as: Membrane analogy
The elastic membrane analogy, also known as the soap-film analogy, was first published by pioneering aerodynamicist Ludwig Prandtl in 1903.