Aerodynamical sounding mechanism in flue instruments: Acceleration unbalance between the jet vortex layers

Abstract

According to PIV (Particle Image Velocimetry) measurement applied to the sound production in organ flue pipes and flutes, the vortex sheddings at the pipe edge proposed by Howe (1975) are not observed but the formation of the vortex layer is clearly observed along both sides of the jet flow. This has been confirmed in various sounding conditions with different blowing pressures and resulting pitches. The acceleration unbalance is generated from an incomplete cancellation of the aeroacoustical source term (ω×U) between both sides of the jet, where U is the jet velocity and ω (= rotU) the vorticity. In addition, the vortex layer is essentially unstable because it is formed along the inflection point of the lateral jet-velocity profile. Therefore, the acceleration unbalance and inflection instability of the vortex layer activates the jet wavy motion to reinforce the inward or outward acoustic velocity u at the pipe mouth. Phase relations between acoustic quantities approve conventional acoustical models based on the volume-flow drive and momentum drive. Since ω×U can also activate the jet movement in edge-tone generation, the vortex-layer formation may be regarded as the fluid-dynamical mechanism common to the edge-tone generation and the pipe-tone generation. © 2013 Acoustical Society of America.