Experimental Study of Noise Propagation in a High Velocity Flow Field

A great deal of effort has been devoted to the study of noise suppression in the jet engine air intake. Noise suppression by sonic choking is well known, but noise suppression by subsonic choking is less understood. Sound waves generated by the compressor fan are attenuated while travelling upstream in high Mach number flow. The limiting case is when a sonic plane is generated in the inlet. In this case, theoretically speaking, no sound waves can travel through the sonic plane. There are many factors which influence subsonic choking. The nonlinear behavior of the spinning waves generated by the compressor fan and the distortion of the flow due to these spinning waves cause noise attenuation, as described by Tam. Noise is substantially attenuated due to the increasing throat Mach number and velocity gradient in ducted rotors. Experimentally, Lumsdaine found that sizeable attentiat1.on of a plane wave is possible by changing the velocity gradients in the diffusion section of a variable area duct. Analytically, Tam studied the transmission of spinning acoustic waves in a slightly non-uniform duct and found that for a given throat geometry, a smaller fraction of the energy of the incident spinning acoustic wave will be transmitted if the axial Mach number is increased or if the frequency of the wave mode is increased. The main motivation of this experimental investigation is to show the effect of throat Mach number as well as velocity gradient on noise attenuation of spinning waves and verify the limited theoretical work. The primary concern is to study the attenuation in the overall sound pressure level and blade passage tone resulting from high throat Mach numbers accompanied by sharp pressure gradients. A detailed discussion of the aerodynamics of the near sonic inlets is given in a series of other related experimental studies by Lumsdaine. The test was conducted at the NASA-Langley anechoic chamber facility. The sound and air source was a single-stage transonic compressor