Surface additives such as polymers, microbubbles and solid particles have been particularly studied in recent years, due to their virtue of reducing the aero/ hydrodinamic drag by inhibiting the fundamental processes that cause turbulent transition (Berman, 1978).
Effects on Viscous Drag
The effect becomes evident at just a few parts-per- million (ppm), and gradually increases to values that depend on the molecular nature of the dilute suspension. The molecular weight seems to be playing an important part in the drag reduction characteristics. Most of the additives used are polymers of high molecular weight (for ex. polyetylene-oxide).
There are reports of as much as 80 % skin friction drag reduction in internal flows, and 60 % in external flows ! – Such values open up fantastic opportunities for both pipelines and marine applications (high speed vehicles, submarines).
For example long polymers derived from alfa-olefins are used for drag reduction in commercial pipelines for crude oil and refined oil products (gasoline, diesel, etc.). Pipeline performance is greatly enhanced with the injection of the polymer at each pumping station.
The dilute solution may vary from 1 ppm (part-per-million) to several ppm. Drag savings of 25-30 % (sometimes more) are reported. This means that at constant pumping power, there is a corresponding increase in throughput, or at constant throughput the pumping power can be reduced (by reducing for example the number of pumping stations).
In aircraft applications the drawback, though, would be the amount of additives that must be released from the surface and the power required to run the system.