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Numerical Study of Noise Characteristics in Overexpanded Jet Flows

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Memorandum rept. Jul 2014-Mar 2015

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Noise characteristics in shock-containing jets at an overexpanded jet condition have been investigated. Total temperature ratios of 1.0 cold, 2.0, and 3.0 are considered. The cold jet is a highly screeching jet. Frequency-wavenumber Fourier analysis is employed to examine the wave characteristics of pressure waves along the lip line and also along a near-field conical surface. It is found that the radiating portion of the pressure wave intensity increases with the jet temperature, but the hydrodynamic portion is much less sensitive to the change of the jet temperature. The near-field noise intensity associated with the Mach wave radiation is observed over a large axial distance, and the Mach wave radiation extends to much higher frequencies in heated jets. The peak radiation direction in the cold jet is dominated by the axisymmetric mode, but the directions around the sideline show a much weaker azimuthal dependence. Furthermore, the axial locations of lip-line pressure peak intensities at the screech frequency are near the axial locations of shock-cell tips. A reinforcing loop between upstreamdownstream propagating waves and the induced shock-cell coherent oscillatory motion is observed in the highly screeching jet. The formation of this reinforcing loop requires a match of the peak phase velocities between upstream and downstream propagating waves. This phase velocity match exists in the highly screeching cold jet, but not in the weakly screeching heated jets. It appears that the phase velocity match that sustains the reinforcing loop is important to the screech generation, and the phase velocity mismatch in heated jets is believed to be an important cause of the screech intensity reduction.

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  • Fluid Mechanics

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