Vortices are of interest in many situations in fluid mechanics. One type of vortex with which the gas turbine designer is concerned is the line vortex springing like a miniature tornado from a point on the ground (or from a nearby aircraft surface) and entering the engine intake. It is possible for these vortices to suck stones up into the engine (Refs. 1, 2) and also to reduce the compressor surge margin (Refs. 3, 4). They are unsteady, in that their point of origin wanders about and so does their point of entry into the intake. This unsteadiness makes it very difficult to measure them, and so far no one known to the author has reported measurements of ground vortex strength, in the course of several investigations of the phenomenon (Refs. 1-3, 5-8).
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