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.

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Flow-induced acoustics are a well-known phenomenon, occurring in a broad variety of applications, as well as in nature. In many applications, the produced acoustics are purposeful, e.g. for communication and in musical instruments. In other circumstances, however, the sound and vibrations are a nuisance, and even harmful to human beings and the environment. Pipes with internally grooved or corrugated walls can be such a source of sound production. These pipes find broad application due to the local stiffness that is combined with larger scale flexibility of the pipes. The main industrial use of corrugated pipes is as flexible connections, transporting a gas or liquid between e.g. ships and onshore storage facilities, or between subsea bore-wells and floating production facilities.@en

Two-phase flow in ribbed or corrugated pipes is of interest in many industrial applications. Experiments are performed to assess the flow regime characteristics in upward annular flow through vertical smooth and corrugated pipes. From high speed recordings, the flow regime and temporal film characteristics are obtained. A novel implementation of a planar laser-induced fluorescence (PLIF) method is used to measure the film thickness, preventing strong reflections from deteriorating the measurements. Liquid accumulation between the ribs of the corrugated pipe is also measured using a PLIF technique. Furthermore, droplet sizing is performed combining shadowgraphic and interferometric techniques to capture a large droplet size range. The measurements show that the presence of pronounced corrugations at the pipe wall causes a strong increase in entrainment of liquid into the gas flow. The entrainment is correlated to the filling of the corrugations with liquid; it is significantly reduced (from 90% entrainment to 50%) when the corrugations are entirely filled with liquid. The amount of liquid filling of the corrugations is related to the superficial liquid film flow velocity. The liquid filling fraction (α) scales with the Weber and liquid Reynolds number, and the obtained scaling also holds when the experiments are repeated with a different liquid (mono-ethylene glycol) and with a larger corrugation geometry. Droplets occurring in corrugated pipe flow are 30–50% larger compared to the smooth pipe, as a consequence of the locally (at the locations of the cavities) increased film thickness.

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Sound production can occur in corrugated pipes subject to an internal dry gas flow (whistling). The sound originates from an interaction between the shear layers spanning the axisymmetric cavities along the pipe and an acoustic resonance of the pipe system. Mitigation of whistling in these pipes can be obtained by the addition of small fractions of liquid to the incoming flow. Experiments are carried out to investigate the effect of droplets in the pipe and liquid accumulation within the cavities along the pipe. Shadowgraphy was combined with an interferometric technique to obtain droplet size distributions in the corrugated pipe. The additional acoustic damping associated with the obtained droplet distributions could not explain the mitigation of whistling. Measurements of the amount of liquid accumulating within the cavities using laser-induced fluorescence show a strong correlation between the filling of the cavities and the reduction of acoustic amplitude for different gas flow rates. A dry gas flow through newly fabricated corrugated pipes with a geometry based on the measured liquid filling profiles did not exhibit any whistling within the studied parameter range. A reduction of only the depth or only the length of the cavities did not lead to the same mitigation of whistling. It therefore seems that the geometric alteration of the corrugation geometry by the presence of liquid is the most important mechanism leading to mitigation of whistling in corrugated pipes.@en

When a corrugated pipe is subject to a dry gas flow, high amplitude sound can be produced (so-called ‘whistling’). It was shown previously that liquid addition to corrugated pipe flow has the ability to reduce sound production. Small amounts of liquid are sufficient to mitigate whistling entirely. One of the mitigation mechanisms, cavity filling, is studied experimentally. Acoustic measurements are combined with a planar laser-induced fluorescence technique to measure the liquid accumulation in the cavities of a corrugated pipe. Using this technique, it is shown that the amount of filling of the cavities with liquid increases with increasing liquid injection rate and with reducing gas flow rate. The reduction in whistling amplitude caused by the liquid injection is closely related to the cavity filling. This indicates that the geometric alteration of the pipe wall, caused by the accumulation of liquid inside the cavities, is an important factor in the reduction in whistling amplitude.@en