The hairpin vortex illusion

Abstract

It has long been customary to assume that vortical structures in turbulent flows are synonymous with regions of rotational motion. Mathematical implementations of this idea using numerical and experimental velocity data from turbulent boundary layers reveal the presence of hairpin vortices, both singly and in groups called packets. However, vorticity may be present that does not directly cause rotation, and by failing to take this into account it is possible to be misled as to the true nature of the vortical structures. In this work a vortex filament scheme is applied to boundary layer flow that allows for a view of structures unrestrained by the requirement that they be regions of rotational motion. It is found that furrow-like streamwise aligned eruptions of the nominally spanwise near-wall vorticity overlying low-speed streaks are the primary structural feature of the transitioning boundary layer. These progress from an arch-like form at their upstream end to either one or two-lobed mushroom-shapes at their downstream end. The rotational motion associated with the furrows has the appearance of hairpins. Mushroom-shaped structures rapidly breakdown into complex forms in the post-transitional region that may have rotational signatures similar to that of packets.