光学CT法を用いた超音速噴流騒音の断層可視化(流体工学,流体機械)

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  • Tomogram Visualization of Supersonic Jet Noise Using an Optical CT Method(Fluids Engineering)
  • Tomogram Visualization of Supersonic Jet Noise Using an Optical CT Method(Fluids Engineering)

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Supersonic jet noise was visualized using a newly developed optical CT (Computed Tomography) method. From a converging round nozzle whose exit diameter is 8.0 mm, air is injected into the atmosphere. The pressure ratio is set at 2.25 (M_j=1.14), and an under-expanded jet is formed. A sound wave is pressure fluctuation, and the density fluctuates at the same time, Using a kind of schlieren method, the fluctuation of the density gradient is detected. A He-Ne laser beam passes through the supersonic jej, and the beam direction is slightly refracted due to the density gradient inside and around the supersonic jet. The beam refraction is converted to a brightness signal using a knife edge and a photo sensor. The signal obtained with the photo sensor is the integrated value along the laser beam path. Using a modified Abel inversion, the local refraction of the laser beam is calculated and the tomogram is re-constructed. It is shown that, using the optical CT method, nonintrusive acoustic measurement is possible even inside the supersonic jet. It is shown that the sound source of the screech tone exists around the 4th shock cell of the under-expanded jet, and that intensive fluctuation is observed not only in the shear layer but also along the center axis of the jet, implying the existence of the traveling shock wave.

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