Enthalpy said:
On many planes, drag does result from pression rather than skin friction. That's why such planes (so many planes) have smooth forms behind their main section, instead of reducing their skin area as soon as possible.
This is wrong. On
all planes, drag is a result of a combination of pressure drag and skin friction drag (and at increased Mach numbers, wave drag). Planes have smooth forms behind them to reduce pressure drag but this is completely independent of the attempts to lower skin friction. It is not an either/or thing.
On a typical commercial airliner such as a Boeing 737 or Airbus A320, skin friction still accounts for roughly 50% of the total drag on the aircraft
1. The rest is covered by things such as form drag, induced drag and wave drag. This is, in part, due to the fact that planes
are streamlined and so the form drag is greatly reduced. Like I said, though, that is independent of efforts to reduce skin friction drag.
Enthalpy said:
Dimples on a golf ball serve to promote flow separation.
Quite the opposite, as has already been discussed. Perhaps this is a typo?
Enthalpy said:
But a well-designed plane fuselage is already better than a sphere improved by dimples.
It doesn't even need to be specially designed. The most common shape for a fuselage since the beginning of flight, the tapered tube, is just naturally a better shape for this than a sphere. You can improve it further, but it doesn't take a whole lot of special design considerations to get better than a sphere.
Enthalpy said:
The third, less obvious reason is that it reduces drag by providing a well-defined wake whose pressure isn't as negative.
Assuming you are talking about differential pressure, I will let the negative pressure comment slide. However, this example does not reduce drag. In fact, it increases drag compared to letting the boundary layer separate later. The advantage, though, is maintaining a fixed separation point and therefore maintaining predictable controls, particularly in regards to accelerating and braking. If the separation point fluctuated, the car would be much more difficult to control. This was a problem in the Audi TT, which is why they had to add the rear spoiler.
Enthalpy said:
As for energizing the layer: this works only if you create an exchange with the air outside the layer by provoking vortices with their axis parallel to the flow. Dimples don't do that. Small tilted plates near the leading edge of a wing do.
You can energize the boundary layer a number of ways, including streamwise vortices (which come with their own set of problems). You can also do it with dimples or bumps
2. Large dimples and bumps actually do create a hairpin vortex which can lead directly to boundary layer transition, feed energy into unstable modes that eventually transition or can do limited modification of the laminar boundary layer (which can also lead to transition). Small bumps or dimples can also help force unstable modes and lead to transition. Once the boundary layer transitions, the mixing that is the hallmark of a turbulent boundary layer will bring energy into the boundary layer from the free stream. This is essentially the mechanism of dimples on a golf ball. The promote boundary-layer transition and a turbulent boundary layer is more resistant to separation than a laminar boundary layer.
Enthalpy said:
Corrugated skins have long been studied because people imagined sharks and swordfish swim quickly thanks to their skin. It didn't work well and fish' explanation is nearly abandoned, but there is still research in this topic.
Corrugated skins have some really interesting properties. They are very good at preventing the spread of turbulent spots once they form, so they can successfully help delay transition at least globally over the surface and reduce the drag increase that comes with a turbulent boundary layer. The trade-off is that they have a lot more surface area and therefore have a lot higher base level of viscous drag. Like you mentioned, I have yet to see anyone get a net gain out of this.
Enthalpy said:
An other direction is to have elastic skins (possibly piezoelectric with electric damping, or even active) that hinder the formation of turbulence. Seen no magic result up to now.
This is a very bad idea. There has been a pretty large body of work on active cancellation of unstable waves in the boundary layer, particularly Tollmien-Schlichting waves, and it is really a fool's errand. It is a neat academic study and can be done, but it will almost certainly never be practical. It is much easier and more cost effective to design the surface to be subcritical to T-S waves through the use of natural laminar flow techniques. That just isn't as sexy.
References:
[1] Robert JP. 1992. Drag reduction, an industrial challenge, AGARD Report 786.
[2] Arcarlar MS, Smith CR. 1987. A study of hairpin vortices in a laminar boundary layer. Part 1. Hairpin vortices generated by a hemisphere protuberence.
J. Fluid Mech..
175:1-41.