Tecnoseal Answers - Resistance To Motion 3: Boundary Layer
We close today the speech started two weeks ago on the fluid resistance going to talk about the boundary layer.
With the term boundary layer (or also mechanical boundary layer) we refer to the thin zone of fluid located near the walls of the moving body which is affected by the change in speed of the same.
The boundary layer can be of the laminar type, where the fluid threads are laminae that follow the contour of the body, or turbulent where the fluid threads follow intricate lines. The boundary layer can also transit from laminar to turbulent through an area called the transition region, but will never spontaneously return to laminar. The transition from laminar to turbulent is favored by the increase in speed and surface roughness of the body and the less tapered shape of the body.
The figure illustrates the main differences in the boundary layer, in particular how the thickness of the turbulent boundary layer is significantly higher. The turbulent boundary layer has the disadvantage of generating a greater resistance than the laminar one, but the advantage of having a lower tendency to detachment from the smaller body.
The boundary layer is in fact particularly important to understand a type of resistance less relevant for the boats, but still interesting: the wave resistance. This is a form resistance that is generated when the body, due to its transonic or supersonic motion, encounters shock waves which cause the boundary layer to become detached, thus increasing the fluid resistance of the profile itself.
We define squat bodies, therefore, all those objects that, due to their shape, can not maintain the boundary layer attached along their entire surface. Examples of squat bodies are automobiles (anyhow they are made), trains, tubes, spheres, etc. Even a Formula One car is not a wing profile, since it produces a trail in its forward movement.
Furthermore, since a laminar flow, when compared to a turbulent one, is less able to maintain the boundary layer attached to the moving body in the fluid, in some cases for squat bodies it is advisable to have a boundary layer flow which is turbulent. For example, in the case of tennis balls, the hair they have on the surface allows to reduce their resistance to shape, keeping the boundary layer flow attached to the ball more than if it were smooth. Other examples are the golf balls, which have a series of footprints on the surface, and the cricket balls, divided in half between smooth and rough surface.