Direct simulations have become indispensable tools in turbulence research, and, in the last two decades, especially so for the study of wall-bounded turbulence. Early simulations dealt mostly with the viscous and buffer layers near the wall, because their Reynolds numbers had to be necessarily limited (Kim et al.,
). They led very soon to a fairly complete description of the kinematics of this part of the flow (Robinson,
), and, later, to the qualitative understanding of their dynamics (Jiménez et al.,
). While most of those studies were carried out in turbulent channels, the results are generally believed to apply to all attached wall-bounded turbulent flows, because the time scales of the near-wall region are too fast to interact strongly with the slower processes of the non-universal outer layers.