Ludwig Prandtl contributed significantly to transport phenomena and boundary layer concepts. He introduced the boundary layer concept to study turbulent flow, modified the Navier-Stokes equations to fit boundary layers, defined the Prandtl number as the ratio of thermal to momentum diffusivity which determines relative boundary layer thicknesses, proposed the mixing length theory which uses a mixing length to model Reynolds stress similar to the mean free path of molecules, and collaborated with von Karman on the universal logarithmic velocity distribution for flow in the inertial sublayer.