Effects of the upstream-propagating guided jet waves on the mixing layers of Mach number 0.9 free jets

The effects of the upstream-propagating guided jet waves on the mixing layers of free isothermal jets at a Mach number of 0.9 are investigated for four jets computed using large-eddy simulations, three ones with laminar nozzle-exit boundary layers and one with disturbed boundary layers. For the initially laminar jets, the velocity spectra obtained in the mixing layers upstream and downstream of the first stage of vortex pairings are dominated by multiple peaks associated with the azimuthal modes n(theta) = 0, 1 and 2. For the dominant peaks and most other ones, the frequencies are located inside or near the frequency bands of the free-stream upstream-propagating guided jet waves. In this case, standing-wave patterns are found in the root-mean-square fluctuation fields on both sides of the shear layers. Otherwise, the peak frequencies are harmonics of the frequency of the dominant peak. Therefore, the guided jet waves not only excite the Kelvin-Helmholtz instability waves growing close to the nozzle [1], but also govern the mixing-layer development farther downstream during the laminar-turbulent transition. For the initially disturbed jet, the velocity spectra in the mixing layers are broadband. For n(theta) = 0, however, one small peak seems to appear several diameters downstream of the nozzle. This peak is at a Strouhal number approximately of 0.41, similar to that of the tone emerging in the near-nozzle pressure spectra in the frequency band of the first radial mode of the free-stream upstream-propagating guided jet waves. This suggests that the turbulent structures of the jet mixing layers are affected by the guided jet waves at this frequency.