The dual nature of the dead-water phenomenology: Nansen versus Ekman wave-making drags

Autor: Johan Fourdrinoy, Madalina Petcu, Julien Dambrine, Morgan Pierre, Germain Rousseaux
Přispěvatelé: Institut Pprime (PPRIME), Université de Poitiers-ENSMA-Centre National de la Recherche Scientifique (CNRS), Laboratoire de Mathématiques et Applications (LMA-Poitiers), Université de Poitiers-Centre National de la Recherche Scientifique (CNRS), Institute of Mathematics of the Romanian Academy [Bucharest] (IMAR), Mission Interdisciplinaire du Centre national de la recherche scientifique (CNRS), ANR-15-CE30-0017-04 associé au projet HARALAB (Hawking radiation at the Laboratory), Action Concertée Incitative (ACI) du Laboratoire Pprime, Projet OFHYS (Optimisation de formes en hydrodynamique à surface libre) du CNRS 80 Prime initiative, de la Mission pour les Initiatives Transverses CNRS
Rok vydání: 2020
Předmět:
Zdroj: Proceedings of the National Academy of Sciences of the United States of America
Proceedings of the National Academy of Sciences of the United States of America, National Academy of Sciences, 2020, pp.16770-16775. ⟨10.1073/pnas.1922584117⟩
Proc Natl Acad Sci U S A
ISSN: 1091-6490
0027-8424
DOI: 10.1073/pnas.1922584117
Popis: International audience; 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.
Databáze: OpenAIRE
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