Aeroacoustics of corrugated pipes

In thin walled pipes corrugations provide local stiffness while allowing global flexibility. This unique characteristic makes corrugated pipes convenient for applications ranging from domestic appliances to natural gas transportation. At critical conditions, however, the flow through these pipes drives self-sustained flow oscillations that lead to high-amplitude sound generation, called whistling. While the literature provides crucial information on the whistling of corrugated pipes, there has been no attempt until now to develop a quantitative prediction method for the whistling of corrugated tubes. The main objective of the thesis is to develop a physical understanding of aeroacoustic sound generation due to self sustained flow oscillations in ducted cavities and to provide a quantitative prediction method for the whistling in corrugated pipes. The presented work combines experimental, numerical and analytical approaches to achieve this goal. Experiments have been performed not only for corrugated pipes but also for multiple side branch systems and axisymmetric cavities in a pipe. These different setups are designed to address different aspects of the whistling in corrugated pipes. During experiments the emphasis has been on an accurate determination of the acoustic and hydrodynamic boundary conditions, which is essential for the numerical method. The extensive set of experimental data provides information on the effect of a number of geometrical parameters on the whistling namely, the length of the pipe, the cavity depth, the cavity width, the cavity edge radius and the separation distance between the cavities. Experiments also provide an understanding of the nature of the acoustic sources and the effect of velocity profile on the whistling. In corrugated pipes the cavities are small compared to the wave length of the acoustic waves, which allows the use of a simplified approach. A numerical method that combines 2D-axisymmetric incompressible flow simulations with Vortex-Sound Theory is proposed to determine the time averaged acoustic source power produced by single or multiple axisymmetric cavities. The proposed numerical method is a computationally efficient approach. Thus, it was possible to use it extensively to address most of the aspects that have been investigated experimentally. Once equipped with realistic acoustic and hydrodynamic boundary conditions, the numerical method appears to be very successful in predicting many aspects of the whistling including: Strouhal number ranges of acoustic energy production and absorption, the Strouhal number of maximum acoustic energy production (peakwhistling Strouhal number), the nonlinear saturation mechanism responsible for the stabilization of the limit cycle oscillation, the effect of the velocity profile on the whistling, the hydrodynamic interference observed between successive cavities. Using an energy balance whistling amplitude can be predicted within a factor two for moderate-high pulsation amplitude range. The sound radiation from a short corrugated pipe segment (Hummer), used as a musical instrument, has been investigated. An analytical radiation model is proposed for the prediction of the observed frequency and the sound pressure level at the listener position. The radiation model also qualitatively explains the amplitude modulation, which provides the chorus like sound quality of this instrument.