The aeroacoustics of a steam kettle

The whistle in a steam kettle provides a near-perfect example of a hole tone system, in which two orifice plates are held a short distance apart in a cylindrical duct. This setup leads to distinct audible tones for a large range of flow rates. The main objective of the current paper is to understand the physical mechanism behind the generation of hole tones (whistling of steam kettles). A variety of experiments were undertaken, primarily focusing on how the acoustics of the hole tone system varied depending on the flow rate, whistle geometry, and upstream duct length. These were supplemented by flow visualisation experiments using water. The results show that the whistle's behaviour is divided into two regions of operation. The first, occurring at Reynolds numbers (based on orifice diameter and jet velocity) below Re_δ ≈ 2000, exhibits a near-constant frequency behaviour. A mathematical model based on a Helmholtz resonator has been developed for this part of the mechanism. The second, for Reynolds numbers greater than Re_δ ≈ 2000, the whistle exhibits a constant Strouhal number behaviour. A physical model has been developed to describe this part of the mechanism where the resonant modes of the upstream duct are coupled with the vortex shedding at the jet exit.