A multiple-view visual study of the inner/outer region interactions of coherent motions in a turbulent boundary layer
Abstract
A time resolved multiple-view laser sheet and multiple location, multicolor dye injection flow visualization technique was used to observe the interactions between sublayer motions and the coherent motions above the wall in a turbulent flat-plate flow at R sub theta approximately = 805. Pairs of low-speed streaks were observed to form and evolve underneath passing typical eddies in the log region and lower portion of the outer region. The pairs of streaks were also found to be associated with the formation of a pocket between the streaks near their downstream end. Statistical characteristics of the observed coherent features and their interactions are presented. The streak pair spacing was found to be on average l sub + approximately = 75. Observations in laser sheets perpendicular to the flow direction consistently showed counter-rotating vortex pairs. Because of the highly resolved temporal record and the complementary spatial information, it was determined that the streamwise extent of these pairs was similar to the length characteristic of the typical eddies. Thus, they were not long axial vortices, or stretched hairpin legs. Instead they were identified as typical eddies. This was confirmed both by the dye with which they were colored, which indicated they were not connected to the wall, and by visual identification in the side view laser sheet of the typical eddy pattern. Correlations of the concurrent occurrences of the typical eddies and the sublayer features, pockets, and streak pairs, below them were examined. Results show that the average sizes of these coherent motions are similar, and that the dimensions of both sublayer features increase as the spanwise diameter of the typical eddy increases. The sampling of low-speed streaks using a classical counting method was also performed to confirm the universal mean streak spacing. An argument utilizing the statistical results of the typical eddy spanwise distribution spacing and the streak pair spacing enables one to recover the results that the overall streaky structure has a spacing of lambda sup + approximately = 100 at this Reynolds number. An overall picture of the inner/outer region interactions and the pinch-off phenomena of hairpin vortices at low Reynolds numbers is also presented.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1992
- Bibcode:
- 1992PhDT........25P
- Keywords:
-
- Counter Rotation;
- Flow Visualization;
- Turbulent Boundary Layer;
- Turbulent Flow;
- Vortices;
- Dyes;
- Injection;
- Laser Applications;
- Low Speed;
- Fluid Mechanics and Heat Transfer