What is the boundary layer

By boundary layer we mean that portion of fluid in contact and in the immediate vicinity of a solid surface. In this area, in fact, the transition takes place between the outermost undisturbed flow and the very slow flow in contact with the solid wall. The air sheet in contact with the solid wall does not flow, on the contrary, it tends to slow down all subsequent air sheets. By convention, the thickness of the boundary layer is defined as the portion of fluid whose velocity differs by 1% from the velocity of the undisturbed fluid (asymptotic velocity). Or, in other words, it reaches 99% of the asymptotic velocity value of the undisturbed external flow. The boundary layer originates where the solid surface begins.

The boundary layer can be of type:

    • laminar : the fluid sheets follow and follow the contour of the solid surface
  • turbulent: the fluid sheets follow lines and trajectories that are no longer linear and stationary, generating the formation of turbulences, detachments and eddies

Of course, the quality and size of the boundary layer vary, first of all, with the increase in air velocity with respect to the body, or with the body with respect to the air (identical velocities in relative motion) and on the basis of the conformation of the solid body itself (more or less straight and tapered shape, roughness, etc.). However, the quality of the boundary layer also depends on the size of the solid surfaces: the larger a surface, the thicker and slower the boundary layer will be. Furthermore, the boundary layer can pass from laminate to turbulent, never from turbulent to laminate spontaneously. This, in general features, the boundary layer; as is obvious, the investigation around this fluid dynamic phenomenon includes and contemplates complex equations, which, however, we will not deal with.

The boundary layer, therefore, is present every time there are high air velocities and solid surfaces in contact with this fluid. The effects therefore change primarily according to the speed. It goes without saying that in the aeronautical field, where speeds are significantly higher than traditional competition cars (but the gap is narrowing compared to some record “monsters”), the effects of the boundary layer make you feel your weight, both at subsonic and above all supersonic speeds. 

Not to mention the aerospace field, in which the boundary layer produces stresses (starting with the thermal ones) which can be fatal and lethal in the event of failure. Just to mention, think of a capsule or the Space Shuttle returning to Earth. Therefore, the effects of the formation of the boundary layer on a competition car (both with open and covered wheels) will be very mild, not at all heavily harmful, often unnoticeable and negligible. However, as we will see, there is no shortage of cases in which we tend to exclude, evacuate and energize the very thin boundary layer for the benefit of aerodynamics and engine cooling.

 

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