Directional Irregular Waves Generated in a Long Tank

Takezawa, Seiji; Kobayashi, Kentaro; Kasahara, A

doi:10.2534/jjasnaoe1968.1988.222

Cited by 12 publications

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“…The wider the tank, the broader the region where the target wave spectrum can be achieved. On the other hand, in a narrow tank, a previous study [22], positively utilizing the side wall reflections, has found that (i) it is possible to generate a symmetric random directional wave field suitable for sea-keeping tests; (ii) the anticipated cross-tank standing wave modes do not appear for short-crested random waves; (iii) the wave statistics of the short-crested random wave, such as the significant wave height and average wave period, agree well with that of the longcrested wave. Concerning the effects of the paddles in the generator, it was pointed out in [23] that the segmented wave maker generates unidirectional waves, in addition to the intended directional wave.…”

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confidence: 99%

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Statistical Properties of Directional Ocean Waves: The Role of the Modulational Instability in the Formation of Extreme Events

et al. 2009

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We discuss two independent, large scale experiments performed in two wave basins of different dimensions in which the statistics of the surface wave elevation are addressed. Both facilities are equipped with a wave maker capable of generating waves with prescribed frequency and directional properties. The experimental results show that the probability of the formation of large amplitude waves strongly depends on the directional properties of the waves. Sea states characterized by long-crested and steep waves are more likely to be populated by freak waves with respect to those characterized by a large directional spreading. DOI: 10.1103/PhysRevLett.102.114502 PACS numbers: 47.35.Bb, 47.55.NÀ An important task in the study of surface gravity waves is the determination of the probability density function of the surface wave elevation. The knowledge of the probability of the occurrence of large amplitude waves is essential for different engineering purposes such as the prediction of wave forces and structural responses or the design of offshore structures. A deep comprehension of the physical mechanisms of the generation of such waves is also a first step towards the development of an operational methodology for the probabilistic forecast of freak waves. It is well known that surface gravity waves obey nonlinear equations and, to date, a universal tool suitable for deriving the probability distribution function of a nonlinear system has not yet been developed. Fortunately, water waves are on average weakly nonlinear [1,2] and solutions can be generally written as power series, where the small parameter, in the case of deep water waves, is the wave steepness ". Strong departure from Gaussian statistics of the surface elevation can be observed if third order nonlinearities are considered. At such order it has been shown numerically [3] and theoretically [4] that, for long-crested waves, a generalization of the Benjamin-Feir instability [5] (or modulational instability [2]) for random spectra can take place [6]. This instability, that corresponds to a quasiresonant four-wave interaction in Fourier space, results in the formation of large amplitude waves (or rogue waves) [7] which affect the statistical properties of the surface elevation (see, for example, [8]). This is particularly true if the ratio between the wave steepness and the spectral bandwidth, known as the Benjamin-Feir Index (BFI), is large [4]. We mention that rogue waves have also been recently observed in optical systems [9] and in acoustic turbulence in He II [10] where giant waves are observed during an inverse cascade process.We emphasize that in many different fields of physics (plasmas [11,12], nonlinear optics [13,14], chargedparticle beam dynamics [15,16]) the modulational instability plays an important role; under suitable physical conditions a nonlinear Schrödinger equation can be derived and the modulational instability can be analyzed directly with this equation [2]. A major question which has to be addressed (and is the subject of the pre...

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confidence: 99%

“…This problem is alleviated as the tank width increases and the number of plungers per width increases. Both studies [22,23] considered short duration experiments. As far as we know, there is not a lot of literature available discussing the role of nonlinearity and the effect of sidewall reflections for long-term experiments.…”

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Statistical Properties of Directional Ocean Waves: The Role of the Modulational Instability in the Formation of Extreme Events

et al. 2009

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“…It is known for irregular waves that the statistical properties of the random waves are more homogeneous in a narrow channel than in a rectangular basin with two wavemakers at normal angle (Takezawa, Kobayashi & Kasahara 1988). Indeed, the sidewall reflections fill in the diffraction region as described by Dalrymple (1989).…”

Section: Experimental Conditionsmentioning

confidence: 97%

Third-order resonant wave interactions under the influence of background current fields

et al. 2015

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A series of experiments were conducted in a wave basin (50 m long, 10 m wide and 5 m deep) generating two waves propagating at an angle by a directional wavemaker. When the two waves were selected from a resonant triplet, an initially non-existing wave grew as the waves propagated down the tank. The linear growth rate of the resonating wave agreed well with third-order resonance theory based on Zakharov's reduced gravity equation. Additional experiments with opposing and coflowing mean current with large temporal and spatial variations were conducted. As the flow rate increased, the linear growth was suppressed. As reproduced numerically with Zakharov's equation, the resonant interaction saturated at time scales inversely proportional to the magnitude of the forced random resonance detuning. It is conjectured that the resonance is detuned by the variation and not by the mean of the current field due to wavelength-dependent Doppler shift and to the refraction of wave rays. Further analysis of the spectral evolution revealed that while discrete peaks appear at high frequencies as a result of dynamical cascading, a continuously saturated spectrum develops in the background as the current speed increases. Additional experiments were conducted studying the evolution of the random directional wave on a dynamical time scale under the influence of current. Due to random resonance detuning by the current, the spectral tail tended to be suppressed.

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“…The wave data measured in running condition consist of wave energy in encounter frequency base. Wave frequency and encounter frequency are related by the following equation : ( 3 ) In following sea condition encounter wave angle chi (x) varies from -90 deg to +90 deg and hence encounter frequency becomes zero at wo=g/Vcosx and needs special attention. The distribution of encounter frequency along with that of wave energy over encounter…”

Section: Analysis Of Experimental Datamentioning

confidence: 99%

“…The input time series data for a particular wave spectrum and a particular directional distribution function are calculated by a main frame computer and are transferred to a micro computer which converts the digital data into analog signals for the wave maker units every 100 miliseconds (for details see ref. 2,3). In the present study wave signal data were prepared using single summation method for P-M type ITTC spectrum with mean period (T02) of 11.3 sec (1.21 sec in model scale) and significant wave height of 6.7 m (7.6 cm in model scale) .…”

Section: Introductionmentioning

confidence: 99%

Towed Ship Motion Test in Directional Spectrum Waves in a Long Tank (Part 2)

Takezawa

Hirayama

Acharyya

1989

J. SNAJ, Nihon zousen gakkai ronbunshu

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As a continuation to our previuos report on tank testing results of ship motions in directional spectrum waves this paper presents further test results under 'following sea' condition. Uniform irregular waves with arbitrary directional spectrum have been generated in the whole area of towing tank of Yokohama National University by the newly installed snake type wave maker and running tests have been carried out at Froude numbers 0.2 and 0.275. The experimental data have been analyzed at encounter frequency base using both Fast Fourier Transformation (FFT) and Maximum Entropy Method (MEM) to obtain the power spectrum of motions and the results are presented along with the theoretical estimates. Directional transfer functions of motions have been calculated using well known New Strip Method (NSM) and analyzed directional wave spectra obtained from experimental data by Maximum Likelihood Method (MLM) have been used to estimate theorectical motion spectra. The compared results show fairly good agreement. This paper also presents short term prediction of motions calculated for both 'head sea' and 'following sea' conditions over venous range of wave periods.

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Directional Irregular Waves Generated in a Long Tank

Cited by 12 publications

References 3 publications

Statistical Properties of Directional Ocean Waves: The Role of the Modulational Instability in the Formation of Extreme Events

Statistical Properties of Directional Ocean Waves: The Role of the Modulational Instability in the Formation of Extreme Events

Third-order resonant wave interactions under the influence of background current fields

Towed Ship Motion Test in Directional Spectrum Waves in a Long Tank (Part 2)

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