Normal waves of the anisotropic Earth—ionosphere waveguide in the VLF-ULF range

A series of wave basin experiments were undertaken to better understand the selection of groin spacings and lengths. Rather than obtaining edge waves with the same period as the normal incident waves, subharmonic edge waves were produced with a period twice that of the incoming waves and a wave length equal to the groin spacing. Rip currents were therefore not formed by the interactions of the synchronous edge waves and normal waves as proposed by Bowen and Inman (1969). Rips were present in the wave basin but their origin is uncertain and they were never strong enough to cause beach erosion. The generation of strong subharmonic edge waves conforms with the work of Guza and Davis (1974) and Guza and Inman (1975). The subharmonic edge waves interacted with the incoming waves to give an alternating sequence of surging and collapsing breakers along the beach. Their effects on the swash were sufficient to erode the beach in some places and cause deposition in other places. Thus major rearrangements of the sand were produced between the groins, but significant erosion did not occur as had been anticipated when the study began. By progressively decreasing the length of the submerged portions of the groins, it was found that the strength (amplitude) of the edge waves decreases. A critical submerged groin length was determined whereby the normally incident wave field could not generate resonant subharmonic edge waves of mode zero with a wavelength equal to the groin spacing. The ratio of this critical length to the spacing of the groins was found in the experiments to be approximately 0.15 to 0.20, and did not vary with the steepness of the normal incident waves.

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EROSION AROUND A PILE DUE TO CURRENT AND BREAKING WAVES

Coastal Engineering Proceedings ◽

10.9753/icce.v21.102 ◽

1988 ◽

Vol 1 (21) ◽

pp. 102 ◽

Cited By ~ 2

Author(s):

E.W. Bijker ◽

C.A. De Bruyn

Keyword(s):

Shear Stress ◽

Breaking Waves ◽

Velocity Profiles ◽

Long Waves ◽

Orbital Velocity ◽

Bottom Shear Stress ◽

Normal Waves ◽

Short Waves ◽

A Current

Tests have been performed on a vertical pile subject to current only and to a combination of current with normal waves and current with breaking waves. The scour around the pile produced by current only is decreased by normal short waves superimposed upon that current and increased when breaking waves are superimposed upon the current. After analysis of the velocity profiles in the undisturbed area upstream of the pile and next to the pile, the following explanation is found for this phenomenon. When normal short waves are superimposed upon a current, the bottom shear stress of the combination of current with waves is increased more in the undisturbed area than next to the pile in the scour area. This results in a decrease of the scour around the pile. Due to the large values of the orbital velocity under breaking waves this effect is reversed for the combination of a current with breaking and relatively long waves. This results in an increase of the scour around the pile.

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COMPARISON OF THE EFFECTS OF MEDIUM MOTION AND ITS INHOMOGENEITY ON SOUND PROPAGATION IN THE OCEAN

Journal of Computational Acoustics ◽

10.1142/s0218396x96000143 ◽

1996 ◽

Vol 04 (04) ◽

pp. 385-397

Author(s):

NATALIE S. GRIGORIEVA

Keyword(s):

Sound Wave ◽

Gulf Stream ◽

Sound Pressure ◽

Adiabatic Approximation ◽

Sound Propagation ◽

Medium Density ◽

Normal Waves ◽

Linear Partial Differential Equations ◽

Oscillation Velocity ◽

Moving Fluid

The purpose of this paper is to compare the effects of medium motion and its inhomogeneity on sound propagation in the ocean at frequencies from a few tens to a hundred Hz for the sound paths up to several hundreds kilometers in length. It is considered the acoustic propagation passing through a cyclonic eddy and the Gulf Stream current. In moving fluid, the sound propagation is described by a system of seven linear partial differential equations for seven unknown elements of a sound wave. These are the sound pressure, the particle oscillation velocity in a sound wave as well as the changes in medium density, its entropy and concentration of the salt caused by the passage of a sound wave. All these elements of a sound wave are seek in the form of a sum of the quasi-normal waves using the modification of the method of horizontal rays/vertical modes. The numerical simulations are carried out on the base of formulas taking into account the first correction to the adiabatic approximation.

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