Near-Field and Far-Field Spectral Analysis of Supersonic Jet with and without Fluidic Injection

Abstract

In the presented study the time-dependent flow features of a supersonic jet with and without steady microjet injection are investigated. The flow field is sampled at various axial and radial locations in the supersonic region and its near surroundings. The jet is emitted from a sharp-throat converging diverging nozzle operated at a nozzle pressure ratio (NPR) of 4.0, which gives a jet exit Mach number of M = 1.56 and a Reynolds number of Re = 2.46 × 10 6 based on the jet exit diameter. Large Eddy Simulation (LES) is used to obtain the fully three dimensional instantenous turbulent flow field and the Kirchhoff surface integral method is applied to obtain the far-field radiated noise. Both the near-field flow dynamics and the far-field noise obtained from the LES are in good agreement with experimental data. The noise components in the far-field noise are identified and compared with the spectra obtained from the probe-locations within the jet. The effect of micro-jet injection on the spectral characteristics within the jet and the far-field noise is analyzed. The screech tone appearing in the far-field noise is clearly established also in the jet-plume. Two point cross-correlations within and outside the supersonic region of the jet-plume revealed two types of moving phenomenon. These where found to be turbulent structures and acoustic waves. The odd thing at first sight was that the acoustic waves appeared to be traveling upstream within the supersonic region, which sounds contradictory. However, it was showed that the acoustic wave was traveling in the form of a helical mode which allows the phase velocity of the pressure wave to be higher than the flow velocity, even at supersonic flow speeds. The fluidic injection was shown to disrupt and weaken the helical pattern which resulted in a lower far-field screech tone noise. Upon sufficient dissipation of the injection, a few nozzle diameters downstream of the nozzle exit, the helical pattern picks up strength again. However, the feed-back loop mechanism associated with the screech tone is still disabled.

Near-field spatial variation in similarity spectra decomposition of a Mach 1.8 laboratory-scale jet

“…As the line arrays are parallel to the jet centerline and the shear layer of the jet is expanding, the microphones at larger x, in Figure 2a, are closer to the flow. The presence of the hydrodynamic field 11,12,13 causes a boost in the low-frequency content of the spectrum as can be seen at z = 20 and 30 on the 5 line array (Figure 4a) and at z = 30 on the 10 line array (Figure 4b). As the hydrodynamic field is not present in the far-field measurements used to develop the similarity spectra, its effect is disregarded when fitting the similarity spectra to near-field data.…”