Propulsion mechanism employing flapping foils
AssigneeMassachusetts Institute of Technology
InventorsMichael S. Triantafyllou, David S. Barrett
Life of this patent
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Abstract
A propulsion system for use in a fluid, the system utilizing at least one foil which is both oscillated at a frequency f with an amplitude a in a direction substantially transverse to the propulsion direction and flapped or pitched about a pivot point to change the foil pitch angle to the selected direction of motion with a smooth periodic motion. Parameters of the system including Strouhal number, angle of attack, ratio of the distance to the foil pivot point from the leading edge of the foil to the chord length, the ratio of the amplitude of oscillation to the foil chord width and the phase angle between heave and pitch are all selected so as to optimize the drive efficiency of the foil system.
Claims24
Apparatus for providing propulsion in a fluid, said propulsion being in a selected direction at a speed U, comprising:
- at least one foil having an average chord c, an average span S, a leading edge facing on average in said selected direction, a trailing edge facing in a direction opposite said leading edge and a pivot point spaced by a distance b from said leading edge;
- a heave mechanism for oscillating said at least one foil at a frequency f and with an amplitude a in a direction substantially transverse to said selected direction;
- and
- a pitch mechanism for flapping said at least one foil about its pivot point to change its pitch angle to said selected direction with a smooth periodic motion at substantially said frequency f through an angle from +.theta..sub.o to -.theta.0.sub.o, there being a phase angle .psi. between the pitch angle of the foil and its transverse oscillation, and the total excursion A of the trailing edge of the foil being such that ##EQU6## the apparatus substantially conforming to at least one of the following relationships:
- ##EQU7## where .alpha. is the nominal angle of attack b.apprxeq.10% to 40% of c(2)
- a/c>1 (3).
A method for providing propulsion in a fluid, said propulsion being in a selected direction at a speed U, the method utilizing at least one foil having an average chord c, an average span S, a leading edge facing on average in said selected direction, a trailing edge facing in a direction opposite said leading edge and a pivot point spaced by a distance b from said leading edge, the method comprising the steps of:
- oscillating said at least one foil at a frequency f and with an amplitude a in a direction substantially transverse to said selected direction;
- and
- flapping said at least one foil about its pivot point to change its pitch angle to said selected direction with a smooth periodic motion at substantially said frequency f through an angle from +.theta..sub.o to -.theta..sub.o, there being a phase angle .psi. between the pitch angle of the foil and its transverse oscillation, and the total excursion A of the trailing edge of the foil being such that ##EQU8## the method substantially conforming to at least one of the following relationships:
- ##EQU9## where .alpha. is the nominal angle of attack b.apprxeq.10% to 40% of c(2)
- a/c>1 (3).
A method as claimed in claim 23 wherein the method also substantially conforms to at least one of the relationships:
- St=fA/U.apprxeq.0.20 to 0.45 where St is the Strouhal number
- .psi..apprxeq.70.degree. to 110.degree. for forward propulsion
- and -70.degree. to -110.degree. for reverse propulsion.
A method as claimed in claim 22 wherein said method conforms to all of the relationships (1)-(3) in claim 23 and to both relationships in claim 24.
Dependencies are inferred from the claim text.