Numerical Simulation of Supersonic Twin-Jet Noise with High-Order Finite Difference Scheme

In this study, the noise generated from a cold supersonic circular twin jet is simulated with a high-order finite difference solver. The three-dimensional compressible Favre-filtered Navier-Stokes equations in Cartesian form are solved by computational aeroacoustics methods. To handle the complex geometry of two closely spaced circular nozzles, the grid block interface flux reconstruction method for high-order finite difference scheme is used. The supersonic twin jet with fully expanded Mach number 1.358 is simulated in this study. A single jet operating at the same condition is also computed for comparison. It is found that the two coupling jets are both oscillating in flapping mode but out of phase with each other. Therefore the near-field pressure is symmetric about two planes, the midplane separating the two nozzles and the plane involving the two jets' axes. The computed noise spectra are compared with the experimental data of Walker ["Twin Jet Screech Suppression Concepts Tested for 4.7% Axisymmetric and Two-Dimensional Nozzle Configurations," AIAA Paper1990-2150, 1990]. The dominant and first harmonic modes in the twin jet are amplified dramatically comparing with the single jet. An increment of 12dB for the dominant component is observed, whereas 17dB for the first harmonic. The predicted amplitudes of the screech tone and its harmonic agree well with the experimental data. Comparing with the single jet, a frequency shift of the screech tone in the twin jet is observed. The pressure field of the jet plumes is analyzed with the dynamic mode decomposition (DMD) method. The eigenvalues and most energetic eigen-modes are presented. The coupling mechanism of the twin jet is analyzed based on the DMD results. The upstream shift of the DMD modes in the twin jet indicates stronger interactions between the instability waves and the shock cells, which generate stronger tone noise. The stronger interactions in turn alter the shock-cell length and the convective velocity of the instability waves and result in the frequency shift of the screech tone in the twin jet.