Abstract
Physical model of self-sustained infrasonic air oscillations related to interaction of fresh gale with choppy sea surface is proposed. It is shown that air infrasonic oscillations are expected inside moving 3D cavities in sea surface generated by gale and detected far from its region. Interaction of wind with moving sea wave crests is shown to be of weaker impact on oscillations in far field. For wind velocity in the range from 10 to 40 m/s deepest cavities acquire resonance frequencies in the range of 3.0 - 0.7 Hz, i.e. frequencies much lower than their quarter wavelength resonance frequencies. In the course of oscillations effective wind velocity applied to cavities can achieve value from 0.4 to 0.6 of wind velocity, while air self-sustained oscillations velocity amplitude can run up in the range from 0.2 to 0.3 of wind velocity. Wind intensification leads to oscillations frequency decrease and oscillation energy losses increase with wind velocity cubed. Cavities natural frequencies are transformed due to air attached mass and volume elasticity additional transformation under wind influence in the range from 1.05 to 1.9 with respect to resonance frequencies at rest. Amplitude of self-sustained oscillation in atmosphere is expected to increase with wind velocity cubed, while cavity air oscillation velocity-linear with wind velocity. Wind velocity threshold of an order of 25 - 30 m/s overcome is necessary to observe effect. Spectral peaks on resonance frequencies in the range 0.7 - 2.5 Hz are expected in effect observation. Infrasonic signals observable far from whole gale in atmosphere, sea water thickness and earth crust on self-sustained oscillation frequency and its harmonics frequencies beginning from third harmonic 2.1 - 7.5 Hz are regarded as phenomenon signs.
Highlights
Infrasonic signals observable far from whole gale in atmosphere, sea water thickness and earth crust on self-sustained oscillation frequency and its harmonics frequencies beginning from third harmonic 2.1 - 7.5 Hz are regarded as phenomenon signs
Troubled surface interaction with wind is bound up with high-speed atmosphere turbulent stresses spectral components propagating with velocity exceeding atmosphere sound velocity [14]. Such properties could be expected from not bearing wind turbulence basic power turbulent fluctuation spectrum “tails” only. Even those statistical in nature continuously depending on wind velocity processes can not explain phenomenon of sudden appearance of powerful infrasonic signals observed far from heavy gale region in conditions where wind velocity overcome a kind of threshold-so called “voice of sea”
Powerful infrasonic field generated in atmosphere over sea surface penetrate in sea layer due to cavity walls oscillations and through sea bottom penetrate to earth crust
Summary
Problem of wind flow interaction with troubled ocean surface attracts scientist attention for a long time [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33]. Observations evidence that such flow regime leads to rapid vortex formation and edge velocity decreases To explain this phenomenon in hydrodynamics the principle [7] is used in accordance to which realistic flow tends to avoid infinite flow velocity transforming flow near these points to discontinuity surfaces. As it is already specified, radiation requires sea surface disturbance (say, wind disturbance) Fourier integral decomposition component wavelength exceeding air sound wavelength This condition possible exception is related to marginal effects only, observed mainly in wave interaction with shore line [4]. In the same time besides stochastic turbulent tangential stresses on ocean surface situated close to matched layer mentioned above there are localized low frequency sound sources related to wave crests and cavities on troubled sea surface generating self-sustained oscillations under wind influence
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