Abstract:
Turbulent mixing layers have been investigated experimentally to study the effect of
velocity ratio and merging angle on them. Two types of mixing layers were produced
from merging of two streams at 0° and 18°. Each type of mixing layers was produced
for three different velocity ratios 0.7, 0.8 and 0.9 keeping the high speed velocity
constant at 10 mls. At the farthest downstream station for a velocity ratio of 0.7, the
Reynolds numbers based upon downstream distance and convection velocity were
1.3x 106 for parallel stream (0°) and Ux 106 for non-parallel stream (18°) mixing layers.
The hot-wire traces of the fluctuating component of the streamwise velocity in all the
mixing layers at 10 mm upstream and 8.3 mm from the splitter walls indicated that the
initial boundary layers were in turbulent state. The boundary layers were also untripped
in all the cases. The initial and operating conditions were documented, and the
streamwise pressure variations were measured. The time history of the hot-wire signals
were recorded for each of the mixing layers at different streamwise locations: 5, 107,
507,907, 1217, 1617 and 2017mm from its initiation. These time history records of the
hot-wire signals were analyzed and then mean streamwise and cross-stream velocities,
Reynolds streamwise, cross-stream and primary shear stresses were calculated.
The calculated data of mean flow have been analyzed further to study the isovel,
mixing layer thickness, momentum thickness, free-stream velocity and mean velocity
defect in the flow. Self-preservation and scaling of the flow properties of both mean and
turbulence quantities have been studied. The results for parallel stream mixing layers
were compared with the available results of others and found to have agreement as well
as discrepancy.
In both types of mixing layers, the results showed that the splitter wake played a
dominant role on their development. Both types of mixing layers attained self-similarity
for velocity ratios 0.7 and 0.8 but failed for 0.9 within the measurement domain. With
the increasing velocity ratio, the development distance increased and the mixing layer
thickness decreased for both types. The comparison of the results of two types of
mixing layers showed that the development distance and Reynolds stress level were
less, but the growth was higher for 18° than for 0°. The isovels for normalized velocity
of 0.9 in both types of mixing layers were found to spread more into the high speed
region with increasing velocity ratio. Scaling of the mixing layer thickness was found to
be effective for all three velocity ratios for both types of mixing layers and the newly
scaled parameter was also found to be effective in removing the dependence of merging
angle on the scaled mixing layer growth. But scaling of the peak Reynolds stress levels
was found to be effective only for velocity ratios 0.7 and 0.8 for both types of mixing
layers.
