Wall jets are often used to enhance the heat and mass transfer from a surface, to provide super-circulation and to delay boundary-layer separation from lifting surfaces. A wall jet, flowing over the outer surface of a circular cylinder has the unique ability to wrap itself around the surface detaching itself at a direction that is opposite to the nozzle from which the jet emanated. In the example shown below, a jet of momentum J, emanating to the right from a slot located on top of the cylinder turns around it before separating to the left at the bottom of this cylinder. The turning of the flow generates a low pressure region on the right hand surface creating a side force that is almost equal to twice the value of J. This force is currently used to prevent the autorotation on a NOTAR type of a single rotor helicopter eliminating the need for a "tail-rotor".
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