EMISSION OF SOUND FROM TURBULENCE CONVECTED BY A PARALLEL MEAN FLOW IN THE PRESENCE OF A CONFINING DUCT

An approximate method for calculating the noise generated by a turbulent flow within a semi-infinite duct of arbitrary cross-section is developed. It is based on a previously derived high-frequency solution to Lilleyʹs equation, which describes the sound propagation in a transversely sheared mean flow. The source term is simplified by assuming the turbulence to be axisymmetric about the mean flow direction. Numerical results are presented for the special case of a ring source in a circular duct with an axisymmetric mean flow. They show that the internally generated noise is suppressed at sufficiently large upstream angles in a hard-walled duct, and that acoustic liners can significantly reduce the sound radiated in both the upstream and downstream regions, depending upon the source location and Mach number of the flow.