Coherent structures in supersonic twin-rectangular jet flow

Supersonic jet noise is a major cause of hearing impairment among aviation personnel working on aircraft carriers. While the mechanisms of noise generation are increasingly well-understood in axisymmetric jets, these mechanisms may have only limited applicability to jets from military-style nozzles. For example, in a twin-rectangular jet, which lacks azimuthal symmetry, coherent structures are not azimuthal Fourier modes. Rather, the D₂ symmetry of the nozzle imposes reflection symmetries on these structures about the major and minor axes. Recent investigations into twin-rectangular jets have focused on the highly energetic screech phenomenon, and the coherent structures responsible for it. As a step towards a global understanding of twin-rectangular jet physics, D₂-symmetric spectral proper orthogonal decomposition (SPOD) is performed on data from a large-eddy simulation of a Mach 1.5, nominally ideally-expanded, unforced twin-rectangular jet. The energy spectrum consists of a mixture of discrete tones and broadband turbulence. To control spectral leakage while ensuring adequate statistical convergence, a recently developed adaptive-multitaper version of SPOD is used. Dominant three-dimensional coherent structures at a broad range of frequencies of interest are cataloged and analyzed.