SPL Sound Pressure Level

This paper describes a study of sound generated by supersonic jets such as those emanating from the exhausts of high-performance military aircraft. Four different nozzles are employed: one convergent nozzle and three convergent-divergent nozzles with design Mach numbers of 1.3, 1.5 and 1.65. All four nozzles have been built as laboratory-scale test articles and large-eddy simulations have been applied to jet flows from the C-D nozzle with a design Mach number of 1.5. They are all studied at their design Mach number and at an underexpanded condition. The convergent-divergent nozzles are also studied at an overexpanded condition. CFD and shadowgraph both show that these nozzles produce a shock at the nozzle throat which is present whether the nozzle is operated at design or off-design. Acoustic measurements also show that shock-associated noise is generated even at the design condition. Examination of the curve of OASPL as a function of fully-expanded Mach number has found no minimum at or near the design Mach number