[Paper Review] Measurements on a little known sound source - the Vortex Whistle

Ulf R. Kristiansen, Muriel Amielh|arXiv (Cornell University)|Apr 8, 2016

Aerodynamics and Acoustics in Jet Flows1 references3 citations

TL;DR

This study investigates the vortex whistle, a poorly known aerodynamic sound source, using acoustic, PIV, and hot-wire measurements. It confirms a linear relationship between sound frequency and air velocity, and reveals that acoustic power scales with the sixth power of frequency, driven by a swirling annular flow and a toroidal vortex structure near the exit.

ABSTRACT

Acoustic measurements on a vortex whistle corroborates earlier findings that the frequency increases linearly with the velocity of the air flowing into the whistle. Measurements in a reverberant chamber shows that the acoustic power generated by the whistle increases close to the sixth power of the frequency. PIV and hot-wire measurements give quantitative information on the flow. It is shown that the flow exits the pipe as a swirling annulus with a rotation corresponding to the frequency of the sound, and that the center of the swirl follows a nearly circular path in the exit plane. The PIV measurements also indicate a toroidal vortex with inflow along the center line close to the exit of the whistle.

Motivation & Objective

• To characterize the acoustic behavior of the vortex whistle, a lesser-known sound source in fluid dynamics.
• To quantify the relationship between airflow velocity and generated sound frequency.
• To investigate the flow structures responsible for sound generation using advanced measurement techniques.
• To determine the scaling laws of acoustic power with respect to frequency and flow conditions.

Proposed method

• Conducted acoustic measurements in a reverberant chamber to assess radiated sound power.
• Employed particle image velocimetry (PIV) to visualize and quantify the swirling flow structure at the whistle exit.
• Used hot-wire anemometry to measure velocity fluctuations and confirm flow dynamics.
• Analyzed flow patterns to identify the presence of a toroidal vortex with axial inflow near the exit plane.
• Correlated acoustic frequency with measured flow rotation speed to validate the vortex-driven sound mechanism.
• Applied statistical analysis to relate acoustic power to frequency and flow velocity, testing power-law scaling.

Experimental results

Research questions

• RQ1 How does the sound frequency of the vortex whistle scale with the inflow air velocity?
• RQ2 What is the relationship between acoustic power and the generated sound frequency?
• RQ3 What flow structures are present in the exit plane of the vortex whistle, and how do they relate to sound generation?
• RQ4 Is there evidence of a toroidal vortex with axial inflow near the whistle exit, as suggested by flow visualization?
• RQ5 To what extent does the acoustic power follow a sixth-power scaling law with respect to frequency?

Key findings

• The sound frequency of the vortex whistle increases linearly with the air velocity entering the device.
• Acoustic power increases approximately with the sixth power of the frequency, indicating strong nonlinear scaling.
• PIV measurements reveal a swirling annular flow at the exit, rotating at a speed corresponding to the sound frequency.
• The center of the swirling flow follows a nearly circular path in the exit plane, consistent with vortex shedding.
• Hot-wire and PIV data indicate the presence of a toroidal vortex with axial inflow along the centerline near the exit.
• The flow structure supports a mechanism where vortex formation and rotation drive the tonal sound emission.