Directivity Measurements of Low Frequency Sound Field Radiated from an Open Cylindrical Pipe with a Hot Mean Flow 2016-01-1822

Radiation of sound from an open pipe with a hot mean flow presents one of the classic problems of acoustics in inhomogeneous media. The problem has been especially brought into focus in the last several decades, in the context of noise control of vehicle exhaust systems and jet engines. However, the reports on the measurements of the radiated sound field are still rare and scattered over different values of subsonic and supersonic flow speeds, cold and hot jets, as well as different sound frequency ranges. This paper focuses on low Mach number values of the mean flow speed and low frequencies of the incident (plane) sound waves inside an unflanged cylindrical pipe with a straight cut. It presents the results of the far-field radiation pattern measurements and compares them with an existing analytical model from the literature. The mean flow inside the pipe reached Mach number values up to 0.25 and temperature up to 300°C. Acoustic measurements were done in an anechoic chamber using a semi-circular microphone array. The plane wave inside the pipe was generated using a low-frequency loudspeaker placed in a side-branch pipe. In general, the results coincide very well with the analytical model, with the exception of refraction effects, such as the occurrence of radiation minimum at the pipe axis (i.e. “the cone of silence”).