Ludwig Prandtl: Real Fluids Explained

Prior to Ludwig Prandtl (1875 - 1953), analytic analysis of fluids with an early form of Computational Fluid Dynamics (CFD), confined to paper and pencil, assumed ideal inviscid potential flow. However, this approach was less than ideal at predicting drag (d'Alembert's paradox) and airfoil stall, which was a pressing problem for engineers at the dawn of powered heavier-than-air flight. Prandtl developed practical theories for real fluids that found favor with aircraft designers. Also, like Osborne Reynolds before him, Prandtl had a non-dimensional number, the Prandtl Number, named after him.

The Life and Times of Ludwig PrandtlTold by Eberhard Bodenschatz

Prandtl proposed that real fluids have viscosity, which cannot be ignored (or assumed insignificant) as it leads to the formation of boundary layers wherever a fluid flows past a surface, which in turn creates a force - drag. With a blend of careful experiments (using early water and wind tunnels) and theory, Prandtl popularized the concept of the boundary layer and showed how it gave rise to flow separation. This approach elegantly explained the easily observed flow separation behind a sphere. From this foundation Prandtl went on to provide practical theories for airfoil design, thus restoring faith in analytical approaches to aerodynamics.

Ludwig Prandtl 1904Ludwig Prandtl 1904At the University of Gottingen. Licensed under Public domain via Wikimedia Commons

Prandtl tutored a number of notable fluid dynamists who went on to become major contributors in their own right, such as:

In his later years Prandtl focused on experiments and theory to better understanding the fundamental structures found in turbulent flow. This was a continuation of his work on boundary layers, only at a more microscopic level.

Thanks to Prandtl and other pioneers we now have analytic tools available in the form of CFD that can provide realistic insights into complex fluid flow prior to constructing physical objects.