Swirling Flow Effects on the Aeroacoustic Signature of an Aerospike Nozzle

Abstract

Supersonic nozzles are not always operated at design conditions. The total pressure, temperature, and velocity distributions at the nozzle inlet plane are often characterized by inhomogeneities, conditions dictated by the operating regime of the turbine or combustion chamber. In particular, a swirling flow motion can be induced by these components. While homogeneous inflow conditions are well documented for a large range of supersonic nozzles, data on the aeroacoustics of supersonic swirling jets is scarce. Large eddy simulations are deployed to simulate the swirling flow of a nonideally expanded three-dimensional, cold, axisymmetric aerospike nozzle at a nozzle pressure ratio (NPR) of 3. Three swirl numbers are considered and compared with the baseline case. Near-field acoustic analyses are completed by far-field acoustic computations based on the Ffowcs Williams–Hawkings (FWH) equation. Swirling flow shortens the potential core of the jet and leads to an annular shock cell length increase. Two-point space-time cross correlations of pressure data acquired in the annular shear layer indicate an enhancement of the azimuthal modes. Similar cross correlations in the circular jet shear layer further downstream show that screech tones are suppressed. Power spectral density of the radial velocity at monitoring points in the vicinity of the nozzle trailing edge allows to identify the oscillation modes of the annular shock cell structure. The far-field spectra exhibit lower mixing noise with the increasing swirl number. The global sound pressure level (SPL) decreases, while the nozzle thrust remains at 99% of the baseline thrust at low swirl numbers.