Turbulent Spots as a Basic Building Block for the Turbulent Boundary Layer

This thesis presents evidence that turbulent spot structures – dense concentrations of vortices that were first observed in the transitional region – populate the developed turbulent region of the canonical turbulent boundary layer. The qualitative similarity of these spots in the turbulent region to the transitional spots initially motivated the idea that they could be a basic building block element for the turbulent boundary layer. The goal of this work was to investigate turbulent spots as a basic building block for a kinematic description of the turbulent boundary layer.

The turbulent spots are systematically extracted from the rest of the flow using an algorithm that identifies spatially connected regions of high swirling strength. Temporal tracking of the identified spot structures shows that they are locally generated in the turbulent region and are not simply transitional spots advected far downstream. Our initial evidence for the physical importance of these turbulent spots is the high degree of spatial intermittency of the swirling strength close to the wall. Probability distributions of length scales show that these regions of high swirling strength are coherent in space and tend to be wall-attached. Further evidence of the spot physical importance is the strong fluctuations in enstrophy and Reynolds shear stress observed in the spot regions. Based on these results, we conclude that the turbulent spots are markers of intense physical events and could be meaningful building blocks for the turbulent boundary layer.

To support the development of a spot-based boundary layer theory, we study the geometry of the spots and the flow in their cores. We find that the spot bounding-boxes are self-similar and larger spots tend to have more complex shapes. Proper Orthogonal Decomposition identifies sweep-like characteristics as the most energetically important part of the flow in the spots.