放射状に広がる不足膨張噴流からの放射音について

抄録

Supersonic jets are jets with flow velocities exceeding the speed of sound. They are used as assist gases for the laser cutting and cooling jets for heat-strengthened glass production. After impinging on an object, the jet spreads radially over the surface of the object at supersonic velocities. In this study, we experimentally reproduce a radially spreading supersonic free jet. The ends of circular tubes are placed facing each other, and a radial jet issues from the gap between them at a critical pressure ratio or higher. The resulting underexpanded radial jets are visualized and the noise emitted from the jet are measured. Visualization results show that the first cell length of the jet increases with increasing pressure ratio, and becomes shorter downstream. The acoustic measurement results show multiple dominant frequency components. The frequency components with a broadband are considered to be harmonics components of other frequencies. Using the frequency prediction equation, the proportionality constant was calculated from the experimental results. The frequencies and cell lengths that agree well with the proportionality constants for the rectangular jet are α = 0.44 for L₁ and f₂, and α = 0.66 for L₂ and f₄. The sound emitted from the jet is visualized as a density wave, so the frequency was calculated from the density wave, and the radiated sound from L₂ was found to be in good agreement with the frequency component of f₄.