|Ludwig Prandtl fluid dynamics|
Calculations of the dynamics of fluids require values for the pressure on the fluid, and its inertia, velocity, density, and viscosity. Ludwig Prandtl simplified calculations of aerodynamic flow by dividing a flow over a bounding surface into two areas, the first inside a boundary layer near the surface, where the flow is affected by viscosity, and the second outside the boundary layer, where the effects of viscosity can be ignored.
Heat transferring from a surface also has a boundary layer. Design of airplanes, ships, submarines, and offshore drilling platforms. The blades of the Tesla turbines and pumps. Oscillating objects and rotating fluids. produce laminar or turbulent flows and all have boundary layers.
The line where the speed of a flow is slowed by the drag on a surface exactly one percent. The 49th parallel north. Where it was in my grandfather’s day. In this business, one can sometimes choose any number as long as it’s always the same number. Especially, of course, if it comfortably corresponds to agreed-upon reality.