Noise Generated By A Turbulent Jet Interacting With A Rigid Plate

Abstract

Experimental observations are presented of the far field noise radiated when a diffusing, turbulent, circular jet passes over a rigid flat plate mounted parallel to the jet flow, spanning the jet through its axis. The results are compared with the expected characteristic variations of sound pressure level predicted from surface acoustic dipole, edge scattering and unsteady aerofoil lift analyses. It is found that when the plate leading edge lies in the self-preserving region, the noise radiated from large plates is due to acoustic dipoles near the leading edge. Plates of small finite chord positioned in this region tend to follow the behaviour expected in terms of the unsteady force dipole on a small aerofoil, although the results obtained here were limited by the ratio of plate chord to turbulence integral scale and also by Reynolds' number effects. When the plate leading edge was located in the initial region of the mixing jet, edge scattering effects influenced by higher local Mach numbers were evident. Large chord plates showed a steady increase in radiated sound with nozzle-plate separation in this region, while small chord plates tended to give almost constant radiated sound pressure level for varying small separations. Moving the plate from the initial to the self-preserving region characteristics gave rise to maximum sound pressure levels when the plate leading edge was located approximately eight diameters away from the nozzle. It appears that no single noise source mechanism analysis describes all the present observations, and that noise due to the interaction of a jet and a plate is generally influenced by several interacting noise source processes. The investigation relates to the noise of small jet/plate surfaces as occurring in production line plant, with nozzle Reynolds numbers up to 280 × 103 and Mach numbers up to unity. Choked nozzles and their associated shock structures were not considered.