Project description:A ship encounters a higher drag in a stratified fluid compared to a homogeneous one. Grouped under the same "dead-water" vocabulary, two wave-making resistance phenomena have been historically reported. The first, the Nansen wave-making drag, generates a stationary internal wake which produces a kinematic drag with a noticeable hysteresis. The second, the Ekman wave-making drag, is characterized by velocity oscillations caused by a dynamical resistance whose origin is still unclear. The latter has been justified previously by a periodic emission of nonlinear internal waves. Here we show that these speed variations are due to the generation of an internal dispersive undulating depression produced during the initial acceleration of the ship within a linear regime. The dispersive undulating depression front and its subsequent whelps act as a bumpy treadmill on which the ship would move back and forth. We provide an analytical description of the coupled dynamics of the ship and the wave, which demonstrates the unsteady motion of the ship. Thanks to dynamic calculations substantiated by laboratory experiments, we prove that this oscillating regime is only temporary: the ship will escape the transient Ekman regime while maintaining its propulsion force, reaching the asymptotic Nansen limit. In addition, we show that the lateral confinement, often imposed by experimental setups or in harbors and locks, exacerbates oscillations and modifies the asymptotic speed.
Project description:The MESA (Multi-Ethnic Study of Atherosclerosis) was initiated to address unresolved questions about subclinical cardiovascular disease and its progression to clinically overt cardiovascular disease in a diverse population-based sample, incorporating emerging imaging technologies for better evaluation of subclinical disease and creating a population laboratory for future research. MESA's recruited (from 2000 to 2002) cohort comprised >6,000 adults from 4 racial/ethnic groups, ages 45 to 84 years, who were free of cardiovascular disease at baseline. Extensive cohort data have been collected over 5 exams (through 2011) with additional exam components added through extramurally funded ancillary study grants, and through regular phone follow-up contacts. Over 1,000 MESA papers have been published to date. Exam 6 will incorporate components that use novel wearable, imaging, and other technologies to address new research questions. MESA investigators have and continue to seek opportunities for collaboration with other researchers on a wide variety of topics to further expand the science of MESA.
