Aeroacoustics in a Flow Pipe with a small, variable-length Cavity

Abstract

Pipes with corrugations or cavities are used in a wide variety of applications. In recent years the natural gas industry has experienced "singing" risers, where pipes transporting natural gas excite loud whistling sounds limiting the flow rate of which the gas can be transported. There has been a number of publications regarding this phenomenon, investigating corrugated pipes and pipes with one or more cavities. In this thesis the most basic situation will be studied; A smooth pipe with a single small cavity. This is studied by simulations and experiments. The effects of changing the length of the cavity, and the pipe section between the inlet and the cavity is investigated. The simulations were conducted in Palabos, a Lattice Boltzmann solver which proves to be a promising piece of software for acoustic simulations. The experiments were conduced using a metal pipe with variable inlet and cavity length. Initial vortices created at the inlet are amplified in the cavity by a cavity flow. The results strongly suggest that these inlet vortices are essential for the excitation of whistling sounds. The number of vortices traveling across a cavity at the time is called a hydrodynamic mode. When the frequency of vortices crossing a cavity coincides with an acoustic pipe mode, a whistling sound close to this frequency is excited. Cancellation of the whistling sound with an added cancellation frequency is possible for certain cavity and inlet lengths.