Turbulent mixing of two non-parallel streams

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.