Mach Wave Radiation from High-Speed Jets

Extensive experimental evidence is now available to support the observation that there are two components of jet mixing noise. They are the fine-scale turbulence noise and the noise from the large turbulence structures of the jet flow. The large turbulence structure's noise radiates primarily in directions with a large inlet angle around the downstream axis of the jet. The fine-scale turbulence noise dominates in the sideline and upstream directions. This study investigates the mechanism by which large turbulence structures radiate noise. It is believed that the mechanism is Mach wave radiation. Theoretical model results and physical reasoning are presented to support the Mach wave mechanism. They are further supported by experimental measurements both in the far field and in the near acoustic field. These measurements include peak noise direction, noise-source distribution along the jet column, and near-field pressure-contour pattern. A signature pattern of the near-field pressure contours associated with Mach wave radiation is identified.