Theory of Flow Access With Apparent Obstacles: Cascades, Jumps, Roll Waves, and Turbulence

This article addresses the fundamental question of whether a flow system that has freedom to morph also exhibits the natural tendency to configure itself to provide greater access to its currents. The reason for questioning this evolutionary tendency in nature is that there are several flow architectures that seem to contradict the tendency. First, the flow of rain water on an inclined street covered with gravel does not carve a smooth channel with uniform depth and width. Instead, it carves a straight path with cataracts, which are periodic “dams” interspaced with pools and waterfalls. The pools are most evident after the rain. Second, sheet flow that is smooth and rectilinear terminates itself into a “wall” posed by a thicker layer of fluid that flows more slowly. This is known as a “hydraulic jump.” Are these abrupt morphological changes hindering or liberating the flow? By questioning the directionality of these changes we have an excellent opportunity to check and validate the constructal law of evolutionary design toward greater access.