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

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

Large Eddy Simulation of High Reynolds Number Jets with Microjet Injection

Large eddy simulations of two isothermal Mach 0.75 jets have been performed, one of a clean jet and one of the same jet fitted with eight equally spaced microjets. The microjets have a pressure ratio of 2.38, with a fully expanded Mach number of 1.19. The Reynolds number of the main jet in both simulations, based on the jet core velocity and diameter, is 1.3 million. The simulations were performed on a cylindrical, structured, multiblock mesh created for the clean round jet. The microjets are introduced as pressure inlet areas within the computational domain, so avoiding the complication of modelling the microjet feed pipes. Results of the clean jet simulation agree well with experimental data. The simulation shows the microjets penetrating into the jet core and disrupting the otherwise circular nature of the shear layer in the early flow development regions, though no change in mean flow variables is noticed by the end of the potential core. Two-point two-time correlation are performed on both cases and compared. The results show the microjets reduce the second and fourth order correlation amplitudes and turbulent lengthscales even at large axial locations downstream of the nozzle exit, where the effect of the microjets on the mean flow field is not present. This gives evidence as to how the microjets are able to reduce jet noise levels