Drag reduction, turbulent systems characteristics

The addition of polymers has been investigated also for twenty years. Polymers reduce the drag of the turbulent boundary layer, without avoiding transition. Since the layer remains turbulent, there is no stringent requirement for surface smoothness, as there is for many of the other techniques. Although much is not known about the detailed mechanism, practical success can be achieved more or less routinely. As there would be in the application of any drag reduction technique, there are practical problems that must be solved to make polymers operational these include such problems as how and with what and when to mix the polymer and how to eject it, and how to mate the propulsion system to the altered body characteristics. 

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