Near-field and far-field spectral analyzis of supersonic jet with and without fluidic injection

Paper in proceeding, 2014

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 exitMach number of M = 1.56 and a Reynolds number of Re = 2.46×106 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.