The Effect of Core Flow Turbulence on Planar Lobed-Mixer Nozzle Effectiveness

Abstract

4 , based on the average velocity and lobe wavelength). The freestream turbulence level on the high velocity stream was augmented from 0.6% to 9.5% using a passive grid while the low velocity stream turbulence level remained constant at 1.5%. In addition to the experimental measurements a computational study was performed with the same inlet conditions for the two contrasting turbulence cases. The primary effect of adding a turbulence-generating grid to the high velocity stream of the mixer was that the upstream boundary layer remained attached in the diffusing portion of the mixer lobe geometry to a greater extent than for the low turbulence case. This was due to two factors: decreased boundary layer thickness with the addition of the grid and increased boundary layer mixing with the heightened freestream turbulence. The result was a larger difference in velocities at the interface of the two flows, a feature conducive to more rapid mixing of the two streams. The shear layer in the low velocity lobe of the mixer did not show a significant effect of the elevated freestream turbulence in the region where measurements were taken. The companion computational study, using a steady Reynolds Averaged Navier Stokes solver with an RNG k-e turbulence model, achieved a high degree of similarity with the experimental results. Other turbulence models did not fare as well in this study. The implication is that the proper use of current computational methods can predict the effect of elevated freestream turbulence on lobed mixer performance reasonably well.