The Very Near Pressure Field of Single- and Multi-Stream Jets

We present experimental data towards the development of a low-order model for the jet noise source for predictions of isolated and installed noise. In the proposed scheme, the source is prescribed on a radiator surface defining the boundary between the inner rotational jet flow and the outer linear pressure field. The source consists of wavepacket-type partial fields whose noise emission can be computed using well-established linear propagation methods. Experiments on singleand dualstream jets are used to define the radiator surface and determine relevant quantities on it, including the convective velocity, axial correlation scale, and azimuthal coherence. It is found that the rotational/irrotational boundary is characterized by negative skewness of the pressure field, leading to a convenient criterion for defining the radiator surface. Space-time correlations of the pressure fluctuation on the radiator surface yield the distribution of convective velocity Uc of the partial fields. In coaxial jets, the outer flow effectively silences the eddies of the primary shear layer in the vicinity of the nozzle exit. This is manifested by a large drop of Uc in the initial region of the jet. Offsetting the nozzles prolongs the low-Uc region on the side of the thickened secondary flow. The resulting reduction in radiation efficiency is deemed the primary reason for the noise benefit of eccentric jets. Circumferential coherence measurements indicate that the partial fields are very localized azimuthally.