Structure of the turbulent boundary layer under static and dynamic impulsive roughness perturbation

Citation

Jacobi, Ian (2013) Structure of the turbulent boundary layer under static and dynamic impulsive roughness perturbation. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:07102012-152431583

Abstract

The zero-pressure gradient turbulent boundary layer at Reynolds numbers (based on momentum thickness) ranging from 2700--4100 was perturbed using an impulsively short patch of two-dimensional, spanwise roughness elements. A spatial perturbation was considered in which the roughness patch was held statically on the flat-plate, and the flow downstream of the perturbation was measured by hotwire and particle-image velocimetry. A dynamic perturbation, in which the roughness patch was actuated periodically in time, was also studied, and additional measurements were taken by phase-locking to the dynamic actuation itself.

The static perturbation distorted the boundary layer through the generation of a `stress bore' which modified the mean streamwise velocity gradient. The effect of this stress bore was observed in a modification of statistical and spectral measures of the turbulence, as well as a redistribution of coherent structures in the boundary layer. The characterization of the statically perturbed boundary layer provided a base flow from which to consider the dynamically perturbed flow. The dynamically perturbed flow manifested both effects analogous to the static perturbation, as well as a coherent, periodic, large-scale velocity fluctuation. The extent to which these two features could be treated as linearly independent was studied by a variety of statistical and spectral means. Moreover, the very large scale motion synthesized by the dynamic perturbation was isolated by phase-locked measurement, and its behavior was predicted with reasonable success by employing a resolvent operator approach to a forced version of the Orr-Sommerfeld equation.

The relationship between large-scale motions and an envelope of small-scale motions in the turbulent boundary layer was studied in both the unperturbed and perturbed flows. A variety of correlation techniques were used to interpret the interaction between the different scale motions in the context of a phase-relationship between large and small scales. This phase relationship was shown to provide a physically-grounded perspective on the relationship between the synthetic very large scale motion produced by the dynamic perturbation and the smaller scales in the flow, and was able to provide a foundation for thinking about new approaches to controlling turbulence through large-scale forcing.

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