Examples for the benefits and importance of having a Boundary Layer

Question

I understand what a boundary layer is and how it is formed. Mostly these boundary layers are related to increasing skin friction drag of the body. Please could you give some examples that show the positive effects of having a boundary layer i.e why boundary layers might be essential and actually beneficial for certain applications? For instance, I read somewhere that airplanes wouldn't be able to fly without a boundary layer.

Answer 1

Since boundary layers are impossible to avoid, only theoretical considerations without practical proof are the base of the claim that a boundary layer is needed for flight.

The general idea goes like this: In purely potential flow lift is only possible when the rear stagnation point is artificially fixed at the trailing edge. Without the loss of energy in the boundary layer, the flow around an airfoil approaching the trailing edge would just negotiate it and combine with the flow around the opposite side at a point where no circulation results. Only the slowing at the surface by friction will make this flow around the trailing edge impossible and force the rear stagnation point to the trailing edge.

I once witnessed a presentation in which the researcher tried to show that even in potential flow the sharp trailing edge will cause impossible flow conditions. In the end it looked to me like the scholasticism of the medieval monks: By changing the grid resolution, the flow parameters at the trailing edge changed, too. My conclusion is that the impossibility of lift in the absence of friction is impossible to prove conclusively.

Since friction saps energy, it is bad for efficiency. A more helpful example for a beneficial boundary layer might be temperature boundary layers which help to lower the wall temperature in a gas turbine below that of the core gas flow.