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.

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.