NUMERICAL STUDY ON THE GENERATION POSITION OF SCREECH TONE IN AXISYMMETRIC SUPERSONIC JET BASED ON MODAL DECOMPOSITION

For the imperfectly expanded supersonic jet, the quasi-periodic shock-cell structures in jet core interacts with the coherent structures in shear layer to generate shock-associated noise. Screech tone is the shock-associated noise component with discrete frequency and high intensity, and it propagates primarily toward upstream direction. Its generation is driven by a nonlinear acoustic feedback loop. The exact nature of screech-generation mechanism, including source positions, has remained an open question. Accurately locating the sound source position of screech tone is a key point to quantitatively understand the screech feedback loop mechanism and to develop exact screech prediction model. In this paper, numerical simulations of underexpanded supersonic cold jet issuing from a circular sonic nozzle are carried out through solving axisymmetric compressible Navier-Stokes equations directly, using fifth order finite difference weighted essentially non-oscillatory scheme and third order total variation diminishing Runge-Kutta scheme. The fully expanded jet Mach numbers are 1.10 and 1.15. The present numerical result is compared and in good agreement with the experimental result in the literature. The axisymmetric A₁ mode and A₂ mode screech tones are captured. The time sequential pressure field and velocity field of the jet are analysed through the Fourier mode decomposition, the proper orthogonal decomposition and the dynamic mode decomposition. The spatial evolution of screech-associated coherent flow structures are studied and the sound source positions of axisymmetric A₁ mode and A₂ mode screech tones are accurately located. The results show that each screech-associated coherent flow structure has its own saturation state region, where the screech waves generate and radiate outward. It is also found that the effective source positions of axisymmetric A₁ mode and A₂ mode screech tones are the trailing edges of the fourth and the third shock-cells respectively for the jet Mach numbers considered.