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An apparatus for attenuating acoustic resonance generated by flow over a cavity in a surface comprises a plurality of flat flaps proximate to an upstream edge of the cavity. The flaps are disposed in an array spaced in a width direction of the cavity edge, and are oscillated by the flow in two degre

An apparatus for attenuating acoustic resonance generated by flow over a cavity in a surface comprises a plurality of flat flaps proximate to an upstream edge of the cavity. The flaps are disposed in an array spaced in a width direction of the cavity edge, and are oscillated by the flow in two degrees of freedom solely by the flow, independent of an actuation mechanism. Each flap includes a first hinge generally coextensive with the surface for enabling oscillation in a first degree of freedom and a second hinge orthogonal to the first hinge and forming a tab for enabling oscillation in a second degree of freedom. The hinges are constructed with torsional spring constants that provide predetermined oscillation frequencies and magnitudes. The apparatus can include a deployment mechanism for moving each flap between a stowed position wherein it is generally flush with the surface and a deployed position wherein the flap can be oscillated by the flow.

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What is claimed is: 1. Apparatus for attenuating acoustic resonance generated by flow over a surface with a cavity therein, the apparatus comprising: a plurality of flaps proximate to the cavity at an edge thereof upstream in the direction of the flow, said flaps being disposed in an array spaced i

What is claimed is: 1. Apparatus for attenuating acoustic resonance generated by flow over a surface with a cavity therein, the apparatus comprising: a plurality of flaps proximate to the cavity at an edge thereof upstream in the direction of the flow, said flaps being disposed in an array spaced in a width direction of said upstream edge; and a mounting arrangement for mounting each said flap for oscillation relative to the flow, wherein each said flap is constructed to oscillate with two degrees of freedom and the oscillation of each said flap is driven solely by the flow independent of an actuation mechanism. 2. Apparatus as in claim 1, wherein each said flap is a flat sheet mounted at an angle of attack α=−20° to +20° relative to the direction of the flow over the surface and at an angle φ=10° to 90° relative to said surface. 3. Apparatus as in claim 2, wherein φ is preferably between 20° to 45°, and α is preferably between −5° and +5°. 4. Apparatus as in claim 3, wherein α=0°. 5. Apparatus as in claim 1, wherein each said flap includes a first hinge generally coextensive with said surface for enabling oscillation in a first degree of freedom and a second hinge orthogonal to said first hinge and forming a tab for enabling oscillation in a second degree of freedom. 6. Apparatus as in claim 5, wherein each of said first and second hinges has at least one predetermined torsional spring constant for a given range of angular deformation to provide a desired magnitude and frequency of oscillation when driven by the flow. 7. Apparatus as in claim 6, wherein: each said flap is a flat sheet having a leading edge and a trailing edge and a chord length c between said leading and trailing edges; and said torsional spring constants provide a frequency of oscillation fo in both degrees of freedom at a flow velocity V∞ over said surface, such that V∞/(c·fo)=O(10-100). 8. Apparatus as in claim 1, wherein: each said flap is a flat sheet having a leading edge and a trailing edge and a chord length c between said leading and trailing edges; and each said flap is configured to oscillate at a frequency fo at a flow velocity V∞ over said surface, such that V∞/(c·fo)=O(10-100). 9. Apparatus for attenuating acoustic resonance generated by flow over a surface with a cavity therein, the apparatus comprising: a plurality of flat flaps proximate to the cavity at an edge thereof upstream in the direction of the flow, said flaps being disposed in an array spaced in a width direction of said upstream edge; a mounting arrangement for mounting each said flap for oscillation relative to the flow, wherein each said flap is constructed to oscillate with two degrees of freedom and the oscillation of each said flap is driven solely by the flow independent of an actuation mechanism; and a deployment mechanism for moving each said flap between a stowed position wherein said flap is generally flush with said surface and a deployed position wherein each said flap is disposed at a position for oscillation by the flow. 10. Apparatus as in claim 9, wherein: each said flap when in said deployed position is oriented at an angle of attack α=−5° to +5° relative to the direction of the flow over the surface and at an angle φ=10° to 90° relative to said surface; each said flap includes a first hinge generally coextensive with said surface when said flap is in said deployed position, said first hinge enabling oscillation in a first degree of freedom, and a second hinge orthogonal to said first hinge and forming a tab for enabling oscillation in a second degree of freedom; and each of said first and second hinges has at least one predetermined torsional spring constant for a given range of angular deformation to provide a desired magnitude and frequency of oscillation when driven by the flow. 11. Apparatus for attenuating acoustic resonance generated by flow over a surface, the apparatus comprising: at least one flap disposed for presenting a leading edge to the flow; and a mounting arrangement for mounting said flap for oscillation transverse to the direction of the flow, wherein said flap is constructed to oscillate with two degrees of freedom and the oscillation of said flap is driven solely by the flow independent of an actuation mechanism. 12. Apparatus as in claim 11, further comprising a plurality of said flaps spaced from each other in an array disposed transverse to the direction of the flow.