Abstract
Severe vibrations and sound production can occur in dry gas flow through corrugated pipes. The addition of very small amounts of liquid to the dry gas flow potentially mitigates these flow-induced vibrations (FIVs) and noise. The different mechanisms behind this whistling mitigation are studied in this work, where acoustic measurements are combined with flow visualization and droplet sizing. Different corrugation geometries are studied. It is shown that noise mitigation mainly occurs through a geometric alteration of the cavity mouth, resulting in a reduced acoustic source strength. Additional acoustic damping as a consequence of the presence of droplets has a very limited contribution to the mitigation of FIVs. A non-axisymmetric filling of the cavities of a corrugated pipe with liquid is more effective in reducing the acoustic output, compared to an axisymmetric filling. The liquid viscosity has a minor effect on the achieved noise mitigation. To predict the acoustic source strength for a particular cavity geometry a numerical method is developed, based on URANS simulations combined with Howe's energy corollary. An energy balance method is applied to obtain the acoustic source strength from experiments. The whistling frequencies are accurately predicted with the simulations, but the acoustic source strength is over-predicted by a factor 2. Trends in the source strength obtained from simulations, however, closely resemble the experimentally obtained results. The developed method provides an intuitive understanding of sound production by vortical flow structures and shows potential for the prediction of self-sustained oscillations in corrugated pipes.
Highlights
Flow-induced vibrations occur in various applications, ranging from wind-turbine noise to building acoustics
A similar reduction in source power is obtained from single-phase simulations, where only the cavity geometry is copied from the experiments
Experiments and simulations are carried out to assess the effect of liquid addition to vertical upward corrugated pipe flow
Summary
Flow-induced vibrations occur in various applications, ranging from wind-turbine noise to building acoustics. An application that is of particular interest in many fields, and closely related to cavity flow noise, is whistling in corrugated pipes. Corrugated pipes are widely used when a flexible connection is required to transport a gas or liquid from one place to another. The interaction of the internal gas flow with the corrugated pipe wall, which is essentially a sequence of individual axisymmetric cav ities, causes the formation of shear layers over the cavities. These shear layers separate the internal cavity flow from the bulk flow through the pipe. High amplitude acoustic noise is generated, which causes vibrations that can seriously damage
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