ABSTRACT
Two observations drawn from a thoroughly validated direct numerical simulation of the canonical spatially developing, zero-pressure gradient, smooth, flat-plate boundary layer are presented here. The first is that, for bypass transition in the narrow sense defined herein, we found that the transitional-turbulent spot inception mechanism is analogous to the secondary instability of boundary-layer natural transition, namely a spanwise vortex filament becomes a [Formula: see text] vortex and then, a hairpin packet. Long streak meandering does occur but usually when a streak is infected by a nearby existing transitional-turbulent spot. Streak waviness and breakdown are, therefore, not the mechanisms for the inception of transitional-turbulent spots found here. Rather, they only facilitate the growth and spreading of existing transitional-turbulent spots. The second observation is the discovery, in the inner layer of the developed turbulent boundary layer, of what we call turbulent-turbulent spots. These turbulent-turbulent spots are dense concentrations of small-scale vortices with high swirling strength originating from hairpin packets. Although structurally quite similar to the transitional-turbulent spots, these turbulent-turbulent spots are generated locally in the fully turbulent environment, and they are persistent with a systematic variation of detection threshold level. They exert indentation, segmentation, and termination on the viscous sublayer streaks, and they coincide with local concentrations of high levels of Reynolds shear stress, enstrophy, and temperature fluctuations. The sublayer streaks seem to be passive and are often simply the rims of the indentation pockets arising from the turbulent-turbulent spots.
ABSTRACT
OBJECTIVE: The undertreatment of ethnic minority children with ADHD prompted a study on the effects of methylphenidate (MPH) on the executive functions of African American children with ADHD. METHOD: Nineteen African American children with ADHD are tested on the Tower of Hanoi (TOH) and the Paired Associates Learning Task (PAL) in a double-blind crossover acute challenge of MPH and placebo. RESULTS: Under MPH, TOH rule breaks decrease, especially in the second testing session, and TOH planning time increases, particularly for incorrect solutions; PAL recall in the final learning trial improves with MPH. CONCLUSION: Similar to previous findings with predominantly majority samples of patients with ADHD, MPH enhances planning, precision, and persistence in African American children with ADHD.
