On the use of microjets to suppress turbulence in a Mach 0.9 axisymmetric jet

Abstract

We have experimentally studied the effect of microjets on the flow field of a Mach 0.9 round jet. Planar and three-dimensional velocity field measurements using particle image velocimetry show a significant reduction in the near-field turbulent intensities with the activation of microjets. The axial and normal turbulence intensities are reduced by about 15% and 20%, respectively, and an even larger effect is found on the peak values of the turbulent shear stress with a reduction of up to 40%. The required mass flow rate of the microjets was about 1% of the primary jet mass flux. It appears that the microjets influence the mean velocity profiles such that the peak normalized vorticity in the shear layer is significantly reduced, thus inducing an overall stabilizing effect. Therefore, we seem to have exploited the fact that an alteration in the instability characteristics of the initial shear-layer can influence the whole jet exhaust including its noise field. We have found a reduction of about 2 dB in the near-field overall sound pressure level in the lateral direction with the use of microjets. This observation is qualitatively consistent with the measured reduced turbulence intensities.

Influence of the Nozzle-Exit Boundary-Layer Thickness on the Flow and Acoustic Fields of Initially Laminar Jets

“…Axial position of the end of the potential core zc , and peak rms values of axial and radial fluctuating velocities u ′ z and u ′ r along the centerline. As previously for the mean flow features, comparisons with measurements by Lau et al, 32 Arakeri et al 33 and Fleury et al 34 are made. Albeit significantly scattered, in particular for the radial velocity component, the experimental data are somewhat closer to the results obtained for the jets with thinner nozzle-exit boundary layer.…”

“…For comparison with experimental data, the centerline velocity decays and jet spreadings obtained by Lau et al, 32 Arakeri et al 33 and Fleury et al 34 for Mach 0.9 jets at Reynolds numbers Re D ≥ 5 × 10 5 are depicted in figure 13. Despite uncertainties and possible variations in the experimental initial conditions, the jet of Arakeri et al 33 being for instance characterized by δ θ = 0.05r 0 and rms values of u ′ z around 10% u j at the nozzle exit (refer to table 4), the measurements are rather close.…”

“…Despite uncertainties and possible variations in the experimental initial conditions, the jet of Arakeri et al 33 being for instance characterized by δ θ = 0.05r 0 and rms values of u ′ z around 10% u j at the nozzle exit (refer to table 4), the measurements are rather close. This probably results from the fact that at such Reynolds numbers the jets are certainly all initially turbulent.…”

“…For the aerodynamic fields, the data obtained by Hussain and Zedan, 4 and Husain and Hussain 6 in axisymmetric mixing layers, and by Lau et al, 32 Arakeri et al 33 and Fleury et al 34 in round jets at Mach number 0.9 and Reynolds numbers higher than 5 × 10 5 will be used. In the same way, for the acoustic fields, the sound pressure levels and spectra provided by Zaman 7,8 for initially laminar jets, and by Mollo-Christensen et al, 35 Lush, 36 Tanna 37 and Bogey et al 38 for jets at M ≃ 0.9 and Re D ≥ 5 × 10 5 will be considered.…”

“…The inflow turbulent intensities are however higher with respect to the simulations, except for JetD005p2000 in which the peak rms axial fluctuating velocity is around 2% of the jet velocity as in Hussain and Zedan. 4 Regarding the seven jets [32][33][34][35][36][37][38] at Mach numbers around 0.9, they can be assumed to be initially turbulent because their high Reynolds numbers.…”