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Interference Drag is when boundary layers mix/combine, usually it's caused by pieces of metal conjoining (wing struts/gear retracting).

Is it possible for interference drag to be created without having physical surfaces connecting? Rather, is it possible for boundary layers to form/mix without a physical surface?

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    $\begingroup$ Can you clarify? Do you mean boundary layers with no hard surface nearby, or boundary layers where the hard surfaces are close but not touching? $\endgroup$ Mar 14, 2020 at 14:09

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Yes, there can be interference without the surfaces in contact. It isn't limited to drag but can also alter lift and stalling/flow separation. Biplanes with closely spaced wings are the best example I can think of at the moment. Axial flow turbo fans and compressor blades are other cases. I remember some sort of smooth/bladeless turbine invented by Tesla that operated only on boundary layer interference between unconnected surfaces.

To be clear there is always some connection somewhere to maintain the separation distance. I really mean cases where the connection point is far removed from the area of interference.

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