Volumetric visualization of coherent structure in a low Reynolds number turbulent boundary layer

4. Discussion

The scalar structures observed in the volumetric data set are similar to those seen in numerous experimental investigations: two-dimensional cross sections of the observed three-dimensional scalar structures, when compared to previous two-dimensional flow visualization studies, demonstrate this conclusively. In spite of the relatively small number of scalar structures observed, and the high degree of variability in their size and shape, certain aspects of the structure and dynamics of the scalar field occurred with regularity, and were felt to be representative of turbulent boundary layers in general.

The apparent large-scale spanwise organization of individual scalar structures (streamwise and spanwise scale ~ ) along diagonal lines in the x-z plane was very clear (this organization occurs throughout the complete 1600 volume data set). The lines were inclined to the freestream within the range ±50 . The spacing of the structures along the lines show significant variability, but is approximately 1 in the outer portion of the boundary layer. This spanwise organization of boundary layer structure has not been documented extensively, although evidence supporting this observation may be found in previously published flow visualization studies.

It is important to note that the x-z data slices and the resliced x-y images, which are typical of visual investigations of turbulent boundary layers, tend to present a more complex picture of the boundary layer than actually exists. Considering the stereoscopic time series, it is apparent that, while complicated, the data set is populated by a relatively small number of large scale structures. The two-dimensional images, because they do not incorporate the third dimension, sometimes cut a single contorted structure into several "pieces" which may not appear to be connected. The slicing may also miss (in whole or in part) a large scale structure directly adjacent to a bright streak due to a spanwise offset, hence underestimating or neglecting the possible influence of the structure on the motion of the near-wall fluid. The resulting picture of the boundary layer may overestimate the variety of structures present, or miss important aspects of the dynamics, particularly with respect to inner/outer layer interactions that are distributed in the spanwise direction.

For example, the ejections near the wall appeared to be spatially organized, and related to the passage of the large-scale motions. During the ejection process, it was seen that a single streak can be responsible for the ejection of more than one distinct fluid packet. Whether the packets should be considered as separate ejections, or components of a single, spatially distributed ejection is not clear at this point. In addition, the sinuous spanwise deformation of the streaks during oscillation and lift-up indicate the three-dimensionality of the entire process, and underscores the value of three-dimensional time-resolved flow visualizations of the kind presented here.

Abstract

1. Introduction

2. Acquisition and Visualization of the Volumetric Data Set

3. Results: Two- and Three-Dimensional Visualizations

4. Discussion

Next Section : Acknowledgments

References