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Resumen:
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The near-wall region is characterized by the presence of organized structures [1], but the way in which they interact to maintain the local dynamics and to extract energy from the mean flow is still controversial. The two dominant structures are the streamwise velocity streaks and the quasi-streamwise vortices. The former consist of long (x + ~ 1000) wavy arrays of alternating streamwise jets with an average spanwise wavelength of z + ~ 100. The latter are vortical structures almost aligned with the mean flow but slightly tilted away from the wall. While it is generally accepted that the vortices induce streaks by transferring mean streamwise momentum towards and away from the wall [2], there is less consensus on the generation mechanism of the streamwise vortices. Two conceptual models, widely discussed in the literature, are illustrated in Fig. 1: A) the streamwise vortices are a consequence of the breakdown of the streaks, probably due to inflectional instabilities of the local velocity profiles; B) the streamwise vortices are formed directly, or at least triggered, by the amplification of outer flow perturbations. Hypothesis A assumes that an autonomous wall cycle exists, while B predicts that wall turbulence depends on the existence of an outer flow.
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