The disintegration of wave trains on deep water Part 1. Theory

Journal of Fluid Mechanics - Tập 27 Số 3 - Trang 417-430 - 1967
T. Brooke Benjamin1, J. E. Feir1
1 Department of Applied Mathematics and Theoretical Physics, University of Cambridge

Tóm tắt

The phenomenon in question arises when a periodic progressive wave train with fundamental frequency ω is formed on deep water—say by radiation from an oscillating paddle—and there are also present residual wave motions at adjacent side-band frequencies ω(1 ± δ), such as would be generated if the movement of the paddle suffered a slight modulation at low frequency. In consequence of coupling through the non-linear boundary conditions at the free surface, energy is then transferred from the primary motion to the side bands at a rate that, as will be shown herein, can increase exponentially as the interaction proceeds. The result is that the wave train becomes highly irregular far from its origin, even when the departures from periodicity are scarcely detectable at the start.In this paper a theoretical investigation is made into the stability of periodic wave trains to small disturbances in the form of a pair of side-band modes, and Part 2 which will follow is an account of some experimental observations in accord with the present predictions. The main conclusion of the theory is that infinitesimal disturbances of the type considered will undergo unbounded magnification if \[ 0 < \delta \leqslant (\sqrt{2})ka, \] where k and a are the fundamental wave-number and amplitude of the perturbed wave train. The asymptotic rate of growth is a maximum for δ = ka.

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Tài liệu tham khảo

Benjamin, T. B. & Feir, J. E. 1967 The disintegration of wave-trains on deep water. Part 2. Experiments.J. Fluid Mech. (to appear).

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Journal of Fluid Mechanics

Tập 27 Số 3

417-430

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