Spanwise growth of vortex structure in wall turbulence

Recent studies of the structure of wall turbulence have lead to the develop ment of a conceptual model that validates and integrates many elements of p revious models into a relatively simple picture based on self-assembling pa ckets of hairpin vortex eddies. By continually spawning new hairpins the pa ckets grow longer in the streamwise direction, and by mutual induction betw een adjacent hairpins the hairpins are strained so that they grow taller an d wider as they age. The result is a characteristic growth angle in the str eamwise-wall normal plane. The spanwise growth of individual packets implie s that packets must either merge or pass through each other when they come into contact. Direct numerical simulations of the growth and interaction of spanwise adjacent hairpins shows that they merge by the vortex connection mechanism originally proposed by Wark and Nagib (1990). In this mechanism t he quasi-streamwise legs of two hairpins annihilate each other, by virtue o f having opposite vorticity, leaving a new hairpin of approximately double the width of the individuals. PIV measurements in planes parallel to the wa ll support this picture. DNS of multiple hairpins shows how the spanwise sc ale doubles when the hairpins form an array.