Stop Whistling! A Note on Fluid Driven Whistles in Flow Ducts

The generation mechanism and possible counter measures for fluid driven whistles in low Mach number flow duct networks are discussed. The vortex sound model, where unstable shear layers interact with the acoustic field and act as amplifiers under certain boundary conditions, is shown to capture the physics well. Further, for the system to actually whistle an acoustic feedback to the amplifying shear layer is also needed. The demonstration example in this study is a generalized resonator configuration with annular volumes attached to a straight flow duct via a number of small holes, perforations, around the duct’s circumference. At each hole a shear layer is formed and the acoustic reflections from the resonator volumes and the up and downstream sides provides a possible feedback to them. Not only the Helmholtz mode but also ring modes in the annular volumes provide a feedback to sustain whistles. The attenuation properties as well as the whistling frequencies at varying inlet mean flow velocities for this system are studied both numerically and experimentally showing that good quality predictive simulations are possible using the vortex sound theory. Finally a few countermeasures against whistling are tested. Both the feedback and the shear layers are manipulated. Best effect was found disturbing the shear layers by covering the holes with a coarse mesh.