(57) An improved acoustic signal generator has rotor and stator elements, each having
a plurality of radially-extending lobes and intervening ports relatively positioned
and configured to establish fluid dynamic forces that bias the generator into an open
position, thereby imparting a «stable open» characteristic to the generator. The rotor
is located downstream of the stator, and rotor lobes are outwardly tapered in the
downstream direction and have underlap relative to the upstream stator lobes. The
invention is especially suited for use in oil industry MWD operations to communicate
downhole measurement data to a well surface during drilling. In one embodiment, undercuts
on the rotor lobes impart a flutter action which clears debris.
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