Linear absorbing boundary conditions for surface water wave propagation

Summary The meaning of linear absorbing boundary conditions for periodic water waves in elliptical approaches is discussed. It is found that the direction of propagation of the waves must be known for reliable boundary conditions and that the energy dissipation of waves running parallel to a boundary in general cannot be modelled in the framework of a linear theory. Work presented at the Euromech Colloquium 240 on ?Dispersive waves in dissipative fluids?, Bologna, August 30–September 2, 1988.     

Thermal expansion of a rig

Summary We have estimated and measured the daily thermal expansion of an oceanographic platform. This has been found negligible as far as the calibration of a satellite altimeter is concerned.     

水下噪声及其控制技术进展和展望

本文主要介绍近年来船舶行业水下噪声研究进展情况，包括水下噪声形成机理和预报；水下噪声控制技术；水下噪声近代测量分析技术，并提出对水下噪声研究工作的展望。     

人类对超声的听觉感知及应用前景

本文对人类通过骨传导所产生的超声听觉感知，就以下四方面进行介绍：１．感知的方法、频率和强度范围；２．感知机理；３．可能的应用；４．值得研究的问题。     

On the behaviour of artificial head transfer system applied to acoustic evaluation researches for halls

The author has made tests for an artificial head transfer system whichis modelled on a Chinese male on its main physical behaviour such as frequencyresponses,frequency response differences between the front and the rear inci-dences,as will as on its important hearing behaviour such as localization,theperception of distance and room dimensions.The results verify that this kind ofartificial head transfer system can be used as one of effective tools of acousticevaluation researches for halls.     

On the incoherent scattering of an acoustic or electromagnetic wave

A random configuration of objects in space, or a stochastically rough boundary, is considered to scatter an incident acoustic or electromagnetic wave having harmonic time dependencee ^iwt . In the case of a stochastic surface, Beckmann has compared the Kirchhoff solution with his approach, which employs random walk. The latter approach is used to demonstrate the Rayleigh-distributed amplitude of a field scattered by a very rough surface. This demonstration requires the conjecture that large standard deviations in the random phases of the scattered elementary waves result in an incoherent scattered field. Beckmann's conjecture has not been rigorously proven. However, in this paper, incoherence of the scattered field and broad distributions, over many cycles, in the phases of the elementary waves are both shown to be implied by a third condition, which is defined. Furthermore, the random phase of an incoherent field is shown to be statistically independent of its amplitude and uniformly distributed on a 2-rad interval.     

The effect of free surface on the stability of geophysical flows: a note for a quasi-geostrophic ocean

Summary The role of the free surface on the stability of a wind driven, quasi-geostrophic ocean having an Ekman bottom layer is studied analytically using a perturbative procedure. It is shown that the growth rate range of the developing disturbances is reduced by an amount depending on the energetics of the free surface. In other words, the main free surface effect is to increase the stability of the whole system if this is compared with the rigid-lid ocean model.

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Riassunto Viene analizzato il ruolo della superficie libera nella stabilità di un oceano nell'approssimazione quasi-geostrofica, forzato dal vento ed interessato dallo strato di Ekman sul fondo. Nell'ambito di un approccio lineare, si dimostra che la presenza di detta superficie aumenta la stabilità del sistema quando la si confronta con quella di un oceano nell'approssimazione di ?rigid lid?.

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Резюме Используя процедуру теории возмущений, аналитически исследуется роль свободной поверхности на устойчивость океана в квази-геострофическом приближении. Показывается, что главный эффект свободной поверхности сводится к увеличению устойчивости всей системы по сравнению с моделью океана в приближении ?жесткой крышки?.

     

Theoretical investigation of an acoustic wave in molecular magnets via electromagnetically induced transparency

Using the Schrödinger-Maxwell equations, we theoretically investigate the propagation properties of a transverse acoustic wave in a crystal of molecular magnets in the presence of two strong ac resonant magnetic fields and a weak acoustic wave. The acoustic wave can freely propagate in the magnetic molecule medium (under appropriate conditions) due to quantum interference. Furthermore, using the slowly varying envelope approximation, we discuss the propagation equation of the acoustic wave, which includes the high order nonlinear term. The results show that a crystal of molecular magnets can support the propagation of acoustic wave solitons via electromagnetically induced transparency. We also obtain the analytical expressions for the phase shift and absorption coefficient of the acoustic wave within certain parameters.     

Sea-level response to impulsive north winds in the Gulf of Trieste. Event on 25th September 1987

Summary The detailed behaviour of the sea level in the Gulf of Trieste, in the presence of a sharp wind impulse at the first hours on 25th September 1987, is described. For the first time the data have been obtained by means of an ultrasound tide gauge, with a digital record and one minute scanning.

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Riassunto Il lavoro descrive il comportamento del livello del mare nel golfo di Trieste in relazione ad un impulso meteorologico che ha avugo luogo nelle prime ore del 25 Settembre 1987. Per la prima volta i dati sono stati ottenuti con un mareografo ad ultrasunoni, a registrazione digitale, con una frequenza di acquisizione di uno al minuto.

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Резюме Описывается подробнхое поведение уровня моря в заливе Триеста при наличии резкого импульса ветра в первые часы 25 сентября 1987 г. Впервые получены данные с помощью ультразвуковых приливных датчиков с цифровой регистрацией и с одноминутым сканированием.

     

A Kalman-filter estimate of the tidal harmonic constants

Summary A Kalman-filter estimate of the tidal harmonic constants is proposed in order to take into account their stochastic behaviour. The filter algorithm has been applied to a state-space model of a stochastic system in which the state is defined by the harmonic constants themselves. The results, analysing Trieste sea-level data, have demonstrated that this approach is very suitable for such a purpose, since good estimates and excellent resolution capabilities have been obtained. Furthermore, this method can be very useful also from a practical point of view because real-time computation of the harmonic constants can be developed where an opportune sea-level data acquisition system is available. In conclusion, this paper has emphasized that tidal harmonic constants have to be treated like random variables and, in consequence, new methods of analysis can be used.

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Riassunto Si propone un metodo basato sul filtro di Kalman per la stima delle costanti armoniche di marea. Tale metodo tiene conto del comportamento stocastico riscontrato nelle stesse costanti armoniche. L'algoritmo di filtraggio è stato applicato ad un modello stocastico a variabili di stato nel quale lo stato è definito dalle stesse costanti armoniche. I risultati, analizzando dati di livello marino della stazione mareografica di Trieste, dimostrano che tale metodo risulta molto adatto allo scopo in quanto sono state ottenute stime molto consistenti delle costanti armoniche ed una ottima capacità di risoluzione. Inoltre, il metodo può essere molto utile anche dal punto di vista applicativo in quanto è possibile ottenere una stima in linea delle costanti armoniche disponendo di un opportuno sistema di acquisizione di dati di livello marino. In conclusione questo lavoro dimostra che, considerando le costanti armoniche come variabili aleatorie, possono essere usati nuovi metodi di analisi in alternativa al semplice impiego dei minimi quadrati.

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Резюме Предлагается оценка с помощью фильтра Кальмана приливных гармонических постоянных с целью объяснения их стохастического поведения. Алгоритм фильтра применяется к модели пространства состояний стохастической системы, в которой состояния определяются с помощью гармонических постоянных. Анализируя данные на уровне моря в Триесте, показывается, что предложенный подход является удобным для оценки. С помощью этого метода получается хорошее разрешение. Кроме того, этот метод очень удобен с практической точки зрения, так как вычисление гармонических постоянных может быть проведено в реальном времени, когда имеются соответствующие данные на уровне моря. В заключение отмечается, что приливные гармонические постоянные можно рассматривать как случайные переменные и, следовательно, использовать новые методы анализа.