Control of laminar flow in fluids by means of acoustic energy
First Claim
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1. A method for retarding the onset of turbulent boundary layer flow around an airfoil moving with respect to an air mass through the infusion of acoustic energy into the boundary layer comprising the steps of:
- (a) electronically generating acoustic energy at or near the surface of an airfoil moving through an air mass and having a laminar flow boundary layer over at least a portion of said surface of said airfoil,(b) providing a plurality of predetermined acoustic energy emitting locations upon said airflow surface over which said boundary layer flow is laminar, and(c) directing said electronically generated acoustic energy into said boundary layer from said plurality of predetermined acoustic energy emitting locations,wherein the step of electronically generating includes generating said acoustic energy as a band of random noise having wavelengths not greater than one half of a predetermined critical wavelength of Tollmien-Schlichting oscillatory disturbance waves present within said boundary layer in the absence of said generated acoustic energy, such that said band of random noise forms wave patterns that destructively interfere with the formation of said Tollmien-Schlichting oscillatory disturbance waves.
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Abstract
Retarding of the point of transition from laminar flow to turbulent flow in aerodynamic boundary layers on the surfaces of aircraft is accomplished by radiating acoustic energy at frequencies greater than twice the critical Tollmein-Schlichting frequency into the boundary layer. The acoustic energy interferes with the formation of Tollmein-Schlichting waves, thereby increasing the incidence of laminar flow and reducing aerodynamic drag.
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11 Claims
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1. A method for retarding the onset of turbulent boundary layer flow around an airfoil moving with respect to an air mass through the infusion of acoustic energy into the boundary layer comprising the steps of:
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(a) electronically generating acoustic energy at or near the surface of an airfoil moving through an air mass and having a laminar flow boundary layer over at least a portion of said surface of said airfoil, (b) providing a plurality of predetermined acoustic energy emitting locations upon said airflow surface over which said boundary layer flow is laminar, and (c) directing said electronically generated acoustic energy into said boundary layer from said plurality of predetermined acoustic energy emitting locations, wherein the step of electronically generating includes generating said acoustic energy as a band of random noise having wavelengths not greater than one half of a predetermined critical wavelength of Tollmien-Schlichting oscillatory disturbance waves present within said boundary layer in the absence of said generated acoustic energy, such that said band of random noise forms wave patterns that destructively interfere with the formation of said Tollmien-Schlichting oscillatory disturbance waves. - View Dependent Claims (2, 3, 4, 5)
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6. Apparatus for retarding the onset of turbulent boundary layer flow around an airfoil moving with respect to an air mass through the infusion of acoustic energy into the boundary layer comprising:
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an airfoil moving with respect to an air mass and having laminar boundary layer flow over at least a portion of said airfoil, a plurality of electro-acoustic transducers, each said transducer having a sound radiating element for generating acoustic energy in a frequency band having wavelengths not greater than one half of the predetermined critical wavelengths of Tollmien-Schlichting disturbance waves present within said boundary layer in the absence of said generated acoustic energy, means for mounting said electro-acoustic transducer upon the inner side of the surface of said airfoil, such that said sound radiating elements are in acoustic communication with at least a portion of said laminar flow boundary layer, sound generator means for generating an electrical signal for said electro-acoustic transducers, and circuitry means for connecting said plurality of electro-acoustic transducers to said sound generator means. - View Dependent Claims (7, 8, 9, 10, 11)
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Specification