What are the little scallops for?
Look carefully at the exhaust of the turbofan engine on the Boeing 787 pictured and you will see it has a scalloped edge, what Boeing calls chevrons.
Turbofans vs turbojets
A turbojet engine has a compressor section which draws air in. Fuel is added and burned which expands toward the rear of the engine. It first passes through a turbine section which produces the power for the compressor. Then, the exhaust leaves at the rear and creates the thrust pushing the engine forward.
Turbofans add a large fan at the front of the engine which also produces thrust using power produced by the turbine. The turbofan exhaust flows around the body of the core combustion and turbine sections. At the tail end of the engine, both the hot jet exhaust and the cold fan exhaust mix.
Regulations requiring lower noise levels have been introduced at many airports, challenging aircraft designers, particularly since it is at takeoff and landing that the most power is typically required.
This mixing of fan and jet exhaust produces a great deal of noise. The chevrons on the nacelle break up the mixing into smaller segments, reducing noise. Principle – Segmentation
“The relatively small projection of the chevron apexes … into the jet produces aerodynamically smooth flow guidance to force an accelerated mixing process of jet-to-jet or jet-to-ambient air, and generates significant streamwise vorticity which further accelerate the mixing process. The resultant shorter mixed-jet plume length and reduced plume perimeter results in lower levels of audible jet noise and significantly reduced infrared signature over the prior art.”
1997 US Patent US6360528B1
Segmentation and vortexes – other examples
Now that I’ve seen an example of the usefulness of segmenting airflow, it helps explain other aerodynamic treatments, like the increasingly complex (i.e. segmented) Formula1 car wings which have the benefit of creating more downforce with less drag.
The same applies to the little chevron details called vortex generators you see on the new Civic Type R.
Golf balls have little dimples that produce small vortexes that reduce drag. The principle is thought to have been discovered when the first iteration of golf balls which started life smooth became worn over their lifetime. Avid golfers at the time discovered that the balls traveled further and more predictably. Soon the dimple pattern became standard.
Finally vortexes don’t just happen in air, they happen in any fluid. A shark’s skin is covered in small details called denticles – they look like little teeth – which serve the same vortex producing purpose.