Vortices on Sound Generation and Dissipation in Musical Flue Instruments

Musical flue instruments such as the pipe organ and flute mainly consist of the acoustic pipe resonance and the jet impinging against the pipe edge. The edge tone is used to be considered as the energy source coupling to the pipe resonance. However, jet-drive models describing the complex jet/pipe interaction were proposed in the late 1960s. Such models were more developed and then improved to the discrete-vortex model and vortex-layer model by introducing fluid-dynamical viewpoint, particularly vortex sound theory on acoustic energy generation and dissipation. Generally, the discrete-vortex model is well applied to thick jets, while the jet-drive model and the vortex-layer model are valid to thin jets used in most flue instruments. The acoustically induced vortex (acoustic vortex) is observed near the amplitude saturation with the aid of flow visualization and is regarded as the final sound dissipation agent. On the other hand, vortex layers consisting of very small vortices along both sides of the jet are visualized by the phase-locked PIV and considered to generate the acceleration unbalance between both vortex layers that induces the jet wavy motion coupled with the pipe resonance. Vortices from the jet visualized by direct numerical simulations are briefly discussed.

Aeroacoustic noise generated by flow around circular cyinder with permeable boundary

A new integral computational formulation is presented to evaluate the noncompact noise induced by low Mach number flows. Based on Howe’s Vortex Sound Theory and wave equation of Green’s function in free space, we obtain a new integral equation by choosing a permeable boundary as the integral boundary. The flow calculation is developed with second-order CFD solvers, and noise calculation is executed with a two-step method to solve scattered sources and far field pressure. Two- and three-dimensional circular cylinders are chosen as test examples. The pressure amplitude obtained with permeable boundary agrees well with that obtained with body-fitted high-order method. Numerical results indicate that the present method is valid and efficient to calculate noncompact noise, and the permeable boundary can replace the body surface as the integral boundary.

Study of Vortex Noise of Rotating Blades

The paper gives a brief overview of propeller noise generation mechanisms. The vortex component of the propeller noise is considered in detail. The results of the study of the vortex noise of rotating rods in open areas are presented. The spectral, integral and spatial characteristics of the acoustic field of the rotating rods are obtained. We established that the audibility of rotating rods considered is determined by radiation in the frequency range 250--1250 Hz. We summarized the results of the studies regarding the effect of the flow around the blade profile, characterized by the Reynolds number, on the vortex noise intensity. Findings of research show that the exponent of the dependence of the vortex sound intensity on the characteristic velocity around the blade profile in various ranges of the Reynolds number can vary significantly. When changing the value of lg Re in the range from 1.8 to 5, the final dependence of growth rate exponent first falls from 6 to 3, remains equal to 3 in the range of lg Re from 2.65 to 3.2, and then again increases to 6 for lg Re in the range from 3.4 to 3.7, and with an even larger increase, the exponent increases from 7 to 8 and above (to 11) to lg Re = 4.5. At higher Reynolds numbers, (over 106) corresponding to self-similar modes of flow around the blades of light propeller aircraft, the exponent is 5. Based on the study, we recommend using one of the well-known trailing edge noise models for calculating the vortex noise of propellers at the sketch design stage. The paper also introduces the main methods formulated for reducing the intensity of the vortex sound of rotating blades

Hybrid Aeroacoustic Computations: State of Art and New Achievements

This paper collects the state of the art and the tremendous progress that has been made in hybrid modeling of aeroacoustic sound. Hybrid modeling is defined such that flow and acoustics are modeled separate and connected by an aeroacoustic model. The contributions will be classified with respect to the aeroacoustic models being developed, covering Lighthill’s analogy, Ffowcs Williams and Hawkings, vortex sound, linearized Euler equations (LEE), and different perturbation equations modeling flow induced sound. Within each topic, specific applications, such as jet noise, aircraft noise, ground mobility, noise, fan noise and human phonation, are covered. We focus on the accomplishments and provide the authors’ contribution to aeroacoustic research. Eventually, a concise summary of the different methods and their capabilities is included.

Sound from aeroelastic vortex–fibre interactions

The motion of a line vortex moving past a one-dimensional flexible fibre is examined theoretically. A Schwarz–Christoffel conformal mapping enables the analytical solution of the potential flow field and its hydrodynamic moment on the flexible fibre, which is composed of a rigid segment constrained to angular motions on a wedge. The hydroelastic coupling of the vortex path and fibre motion affects the noise signature, which is evaluated for the special case of acoustically compact fibres embedded in a half plane. Results from this analysis attempt to address how the coupled interactions between vortical sources and flexible barbules on the upper surface of owl wings may contribute to their acoustic stealth. The analytical formulation is also amenable to application to vortex sound prediction from flexible trailing edges provided that an appropriate acoustic Green's function can be determined. This article is part of the theme issue ‘Frontiers of aeroacoustics research: theory, computation and experiment’.