Simulation of hydrodynamic oscillation and visualization of sound generation in the cylindrical cavity of police whistles

摘要

It has been experimentally known that under the certain conditions, the police whistle and its components generate the sound whose frequency is linearly proportional to the amount of blowing. Among the whistle itself and its components, the cylindrical cavity, i.e, one of the whistle components, generates the sinusoidal sound whose frequency is proportional to the flow speed in the wide flow range. I.e., the cylindrical cavity of a police whistle has the function of an excellent flowmeter of frequency detection type. But, it is not clarified how fluid behaves inside the cylinder of the whistle when whistling, nor why the sound generates. This paper is aimed at describing to clarify these phenomena occurring in the cylinder by the fluid flow visualization experiment and the simulation. At first, experimental visualization results that show only the part of fluid flow phenomena were compared with the corresponding simulation results. Both results show the same characteristics. Next, the details that cannot be examined by experiments but can be obtained by the simulations were analyzed. Finally it was clarified that the cause of the sound generation is due to the cyclic returning of fluid to the inside from the cylinder outside at the outlet part of the cylinder.