  Key flow regions linked to jet noise sources are investigated through
comparison of convection velocity and turbulence measurements in high speed
three-stream nozzles. A time-resolved Doppler global velocimetry (DGV)
instrument was applied in the Nozzle Acoustic Test Rig (NATR) at NASA's
Aero-Acoustic Propulsion Lab to measure 250 kHz repetition laser scattering
signals arising from seeding particles at 32 points in high speed flow.
Particle image velocimetry (PIV) measurements previously reported by NASA
provided mean velocity and turbulence intensity. Results for convection
velocity of the particle concentration field were obtained from the DGV data
for three-stream nozzle configurations using a cross-correlation approach. The
three-stream cases included an axisymmetric and an asymmetric, offset nozzle
configuration. For the axisymmetric case, areas of high convection velocity and
turbulence intensity were found to occur 4 to 6 area-equivalent nozzle
diameters downstream from the nozzle exit. Comparison of convection velocity
between the axisymmetric and offset cases show this same region as having the
greatest reduction in convection velocity due to the offset, up to 20%
reduction for the offset case compared to the axisymmetric case. These findings
suggest this region along the centerline near the end of the potential core is
an important area for noise generation with jets and contributes to the noise
reductions seen from three-stream offset nozzles.
