Introduction

Abstract

The development of modern fluid mechanics is closely connected to the name of its founder, Ludwig Prandtl. In 1904 it was his famous article on fluid motion with very small friction that introduced boundary-layer theory. His article on airfoil theory, published the following decade, formed the basis for the calculation of friction drag, heat transfer, and flow separation. He introduced fundamental ideas on the modeling of turbulent flows with the Prandtl mixing length for turbulent momentum exchange. His work on gas dynamics, such as the Prandtl–Glauert correction for compressible flows, the theory of shock waves and expansion waves, as well as the first photographs of supersonic flows in nozzles, reshaped this research area. He applied the methods of fluid mechanics to meteorology, and was also pioneering in his contributions to problems of elasticity, plasticity, and rheology. Prandtl was particularly successful in bringing together theory and experiment, with the experiments serving to verify his theoretical ideas. It was this that gave Prandtl’s experiments their importance and precision. His famous experiment with the tripwire, through which he discovered the turbulent boundary layer and the effect of turbulence on flow separation, is one example. The tripwire was not merely inspiration, but rather was the result of consideration of discrepancies in Eiffel’s drag measurements on spheres. Two experiments with different tripwire positions were enough to establish the generation of turbulence and its effect on the flow separation. For his experiments Prandtl developed wind tunnels and measuring apparatus, such as the Göttingen wind tunnel and the Prandtl stagnation tube. His scientific results often seem to be intuitive, with the mathematical derivation present only to serve the physical understanding, although it then does indeed deliver the decisive result and the simplified physical model. According to Werner Heisenberg, Prandtl was able to “see” the solutions of differential equations without calculating them.