Accession Number : ADA193386

Title :   Theory of Sound Production by Vortex-Airfoil Interaction,

Corporate Author : BBN LABS INC CAMBRIDGE MA

Personal Author(s) : Howe, M S

PDF Url : ADA193386

Report Date : Jan 1988

Pagination or Media Count : 53

Abstract : An analysis is made of the sound produced when a field of vorticity is cut by an airfoil in low Mach number flow. A general formula is given for the acoustic pressure when the airfoil is rigid and the chord is acoustically compact. This expresses the radiation in terms of an integral over the region occupied by the vorticity; the integrand contains factors describing the influence of the thickness, twist and camber of the airfoil. Explicit analytical results are derived for the case of a rectilinear vortex, having small core diameter and finite axial velocity defect, which is 'chopped' by an airfoil of large aspect ratio. The acoustic signature generally comprises two components associated respectively with the axial and azimuthal vorticity, the latter being determined by the velocity defect distribution within the core. Sound is generated predominantly when the core is in the neighborhoods of the leading and trailing edges. The contribution from the trailing edge is usually small, however, because of destructive interference between sound produced by edge-diffraction of near field energy of the vortex and that produced by vorticity shed into the wake of the airfoil in order to satisfy the unsteady Kutta condition that the pressure and velocity should be bounded at the edge.

Descriptors :   *AIRFOILS, *VORTICES, ACOUSTIC SIGNATURES, ASPECT RATIO, AZIMUTH, CAMBER, CORES, DESTRUCTION, DIAMETERS, DISTRIBUTION, EDGES, ENERGY, SUBSONIC FLOW, FORMULAS(MATHEMATICS), INTERFERENCE, LEADING EDGES, NEAR FIELD, PRODUCTION, SOUND, SOUND PRESSURE, THICKNESS, TRAILING EDGES, VELOCITY, VORTEX SHEDDING, WAKE, AERODYNAMIC NOISE, INTERACTIONS, PULSES, TWIST(AERODYNAMICS), ROTOR BLADES(ROTARY WINGS), THEORY, RIGIDITY

Subject Categories : Acoustics

Distribution Statement : APPROVED FOR PUBLIC RELEASE