A mechanism for generating packets of hairpins in shear flows

In the present work we follow the breakdown of pairs of counter-rotating vortices (CVP) and the consequence formation of packets of hairpins in uniform shear flows. We employ a recent developed method which is capable of following (numerically) the evolution of finite-amplitude localized vortical disturbances embedded in shear flows. The solution is carried out using Lagrangian variables in Fourier space which is convenient and enables fast computations on a regular PC. Streamwise variation of the CVP is required to generate concentrated spanwise vorticity which together with the lift-up by the induced velocity and shear of the base flow generates packets of hairpins. This scenario, obtained 'synthetically' with minimal simple elements seems to be universal and may explain similar observations both in the late-stages of transitional boundary layers as well as in fully developed wallbounded shear flows.