Vortex Packets and the Structure of Wall Turbulence Extended Abstract

Abstract

In the classical view of statistically homogeneous turbulence large-scale eddies break down into smaller eddies which break down into still smaller eddies, and so on, thereby creating a cascade of energy from large scale to small scales. The cascade concept is incontestable for homogeneous flow, but spatially inhomogeneous turbulence it must be modified to account for the flow of energy between different regions in space. Turbulent flow created by fluid moving over a smooth wall is intrinsically inhomogeneous with small scales being favored close to the wall and large scales favored farther away from the wall. Moreover it is known that most of the energy production occurs in the near-wall region, so that energy must flow from smaller eddies to larger eddies. Recent work on the structure wall turbulence leads to a mechanistic picture of the eddies in this flow and the means by which the energy cascade process is modified. First it was shown that the long-suspected hairpin-shaped eddy is, indeed, the structural building block of wall-turbulence. Second, a mechanism was found by which small hairpins form close to the wall an assemble into much larger packets of hairpins. The hairpin packets are capable of assembling into still larger structures, creating energy flow from small to large. One formed, the large scales may undergo instability resulting in energy cascade from large to small. The simple scenario provided by the hairpin vortex packet paradigm conceptually explains many puzzling and heretofore unexplained aspect of wall turbulence, and it offers a substantially different view of fundamental characteristics.