Wind-wave growth over a viscous liquid

Experimental and theoretical studies on wind-wave generation have focused primarily on the air-water interface, where viscous effects are small. Here we characterize the influence of the liquid viscosity on the the growth of mechanically generated waves. In our experiment, wind is blowing over a layer of silicon oil, of viscosity 20 and 50 times that of water, and waves of small amplitude are excited by an immersed wave-maker. We measure the spatial evolution of the wave slope envelope using Free-Surface Synthetic Schlieren, a refraction-based optical method. Through spatiotemporal band-pass filtering of the surface slope, we selectively determine the spatial growth rate for each forcing frequency, even when the forced wave is damped and coexists with naturally amplified waves at other frequencies. Systematic measurements of the growth rate for various wind velocities and wave frequencies are obtained, enabling precise determination of the onset of wave growth and the marginal stability curve. From these measurements, we show that Miles' model, which is commonly applied to water waves, offers a reasonable description of the growth rate for more viscous liquids. We finally discuss the scaling of the growth rate of the most amplified wave and the critical friction velocity with the liquid viscosity.Comment: subm. to Phys Rev Fluid

Étude de la rétrodiffusion des surfaces d'eau en bande Ka à faible incidence

The evolution of the altimetric techniques from Ku-band to Ka-band and the wide swath al-timetry in the context of the SWOT mission (« Surface Water Ocean Topography », CNES/NASA) raises new scientific questions about the validity of the backscattering models from water sur-faces in such a frequency band and errors in estimating water heights from time-evolving water surfaces. A backscattering model (GO4) adapted to the SWOT configuration is introduced. It preserves the accuracy of the referencial Physical Optics model while maintaining the simplicity of the clas-sical Optical Geometrics model. In addition to the classical slope parameter, it introduces another parameter called as « effective curvature » (msc). This model allows the inverson of the surface slope and curvature parameters under certain conditions which are developped in this manus-cript. The joint validity of the backscattering models in Ka-band and from water surfaces is che-cked from controlled wind-wave tank radar measurements . In a last part, the temporal properties of the backscattered signal is studied, in particular the correlation time and the Doppler shift induced by waves motion. Influence of the latters on the non focused SAR synthesis is studied in the context of the SWOT system.L’évolution des techniques altimétriques de la bande Ku Nadir vers la bande Ka et l’interféro-métrie large fauchée proche Nadir dans le contexte de la mission SWOT (« Surface Water Ocean Topography », CNES/NASA) soulève de nouvelles questions scientifiques quant à la validité des modèles de rétrodiffusion des surfaces d’eau dans cette bande de fréquence et les erreurs sur les estimations de hauteurs d’eau dues aux mouvements de ces surfaces au cours du temps. Un modèle de rétrodiffusion (GO4) adapté à la configuration SWOT est présenté. Il conserve la précision du modèle de référence de l’Optique Physique tout en gardant la simplicité du modèle plus couramment employé de l’Optique Géométrique. En plus du paramètre classique de pente, il introduit un paramètre supplémentaire, dit de « courbure effective » (msc). Le modèle permet l’inversion des paramètres de pente et de courbure de la surface sous certaines conditions déve-loppées dans ce manuscrit. La validité des modèles conjoints de rétrodiffusion en bande Ka et de surface d’eau a été vérifiée sur des mesures radar effectuées en soufflerie dans un environnement contrôlé. Dans une dernière partie, les propriétés temporelles du signal rétrodiffusé ont été étudiées, en particulier le temps de corrélation et le décalage Doppler induit par le mouvement des vagues. Nous étudions l’influence de ces quantités sur les performances de la synthèse SAR non focalisée du système SWOT

EFFECT OF WINDSPEED TO WAVE HEIGHT ON CONTINENTAL SHELF MALAYSIA

Correlation between wind speed and wave height need to take into consideration when designing offshore platform in order to optimize the usage of the platform material and optimize the operational period. In this study, the wave characteristic effected by wind generation is determined by using descriptive statistics analysis. The result of the correlation were then being analysis to know the sea development state in Continental Shelf Malaysia by comparing ratio of significant wave height with respect to the corresponding wind speed with each calculated significant wave height from P-M Spectrum Model, JONSWAP Spectrum and NALL Spectrum. The MetOcean Data recorded at three (3) operating regions located in Malaysian waters which are Peninsular Malaysian Operation, Sarawak operation and Sabah Operation were used in order to make a realistic descriptive correlation between wind speed and wave height. The finding were also compared between seasonal monsoons and non-seasonal monsoon to make a comparable analysis of the different between the monsoons on the Continental Shelf Malaysia. From the analyzes results, calculated significant wave height form NALL Spectrum Model and JONSWAP Spectrum Model shows lowest ratio with respect to the ratio of SEAFINE MetOcean data

Linear and Nonlinear Optical Effects in High Carrier Concentration Oxides and Nitrides at Epsilon-Near-Zero

Nonlinear optics has been an important method for achieving ultrafast light manipulation. Recently, ENZ material have gained interest due to inherent advantages such as slow light, improved confinement, and ideal relaxation times, the nonlinear response of these materials, such as the intensity-dependent-refractive-index, are ultra-large yet remain ultra-fast. This experimental discovery of epsilon-near-zero enhancement has thus opened new avenues in nonlinear optics research in recent years, and while experiments have continued to progress a theoretical understanding of the processes and origins of nonlinear optical enhancement at epsilon-near-zero has lagged. To fill this gap, the work herein focuses on uncovering the mechanisms that drive the nonlinear interactions of Drude-based epsilon-near-zero materials. This framework utilizes knowledge of a given material’s electronic band structure in energy-momentum space to understand the kinetic motion of free electrons under intense optical irradiation, realizing a fully feed-predictive simulator without fitting parameters. From this, two types of nonlinearities are elucidated, intra- and inter-band, whose overall effect on the optical properties are rooted in the non-parabolic dispersion of energy bands. Moreover, these effects are shown to induce opposing changes on the optical permittivity leading to distinctively different outcomes that can be used individually or together to sculpt the material’s optical properties in time and space. Experimentally, both intraband and interband nonlinearities are interrogated through these methods with the first known multi- beam deflection studies in epsilon-near-zero materials. Through this holistic study, improved prediction power is available for finding the ideal nonlinear films, and effects can be explored to optimize them

Left-right ambiguity resolution of a towed array sonar

In this work, a method is proposed for resolving the Left-Right Ambiguity in Passive Sonar Systems with towed arrays. This problem arises in source localization when the array is straight. In practice, the array is not straight and a statistical analysis within the Neyman-Pearson framework is developed for a monochromatic signal in the presence of random noise, assuming that the exact array shape is known. For any given array shape, an expression for the Probability of Correct Resolution (PCR) is derived as a function of two parameters; the signal to noise ratio (SNR) and an array-lateral-displacement parameter. SNR measures the strength of the signal relative to the noise and the second parameter the curvature of the array relative to the acoustic wavelength. The PCR is calculated numerically for a variety of array shapes of practical importance. The model results are found to agree with intuition; the PCR is 0.5 when the array is straight and is increasing as the signal is becoming louder and the array more curved. It is explained why the method is useful in practice and the effects of correlation between beams are discussed. Furthermore, we look at the acoustical Sharpness Methods; these methods have originated from optical imaging, where they were used successfully to correct atmospherically degraded optical images of telescopes. The method entails that an appropriate function, called `Sharpness Function' is supposed to be maximised when the shape of the towed array, used to construct the beamformer image, is the true one. Reviewing carefully previous literature, which indicated that the method has good chances of producing a very good estimate of the array shape, we prove that the proposed Sharpness Function is in fact not maximised and we deduce that the Sharpness Method does not seem appropriate in the sonar context. Therefore, we conclude that, with the proposed Sharpness function, the Sharpness Method does not work, unlike what has been suggested in the literature. We prove however that, most crucially, Optical Imaging is the same as Beamforming. This opens avenues for further exploration of the sharpness analogy between optics and acoustics imaging

Signal Processing for Synthetic Aperture Sonar Image Enhancement

This thesis contains a description of SAS processing algorithms, offering improvements in Fourier-based reconstruction, motion-compensation, and autofocus. Fourier-based image reconstruction is reviewed and improvements shown as the result of improved system modelling. A number of new algorithms based on the wavenumber algorithm for correcting second order effects are proposed. In addition, a new framework for describing multiple-receiver reconstruction in terms of the bistatic geometry is presented and is a useful aid to understanding. Motion-compensation techniques for allowing Fourier-based reconstruction in widebeam geometries suffering large-motion errors are discussed. A motion-compensation algorithm exploiting multiple receiver geometries is suggested and shown to provide substantial improvement in image quality. New motion compensation techniques for yaw correction using the wavenumber algorithm are discussed. A common framework for describing phase estimation is presented and techniques from a number of fields are reviewed within this framework. In addition a new proof is provided outlining the relationship between eigenvector-based autofocus phase estimation kernels and the phase-closure techniques used astronomical imaging. Micronavigation techniques are reviewed and extensions to the shear average single-receiver micronavigation technique result in a 3 - 4 fold performance improvement when operating on high-contrast images. The stripmap phase gradient autofocus (SPGA) algorithm is developed and extends spotlight SAR PGA to the wide-beam, wide-band stripmap geometries common in SAS imaging. SPGA supersedes traditional PGA-based stripmap autofocus algorithms such as mPGA and PCA - the relationships between SPGA and these algorithms is discussed. SPGA's operation is verified on simulated and field-collected data where it provides significant image improvement. SPGA with phase-curvature based estimation is shown and found to perform poorly compared with phase-gradient techniques. The operation of SPGA on data collected from Sydney Harbour is shown with SPGA able to improve resolution to near the diffraction-limit. Additional analysis of practical stripmap autofocus operation in presence of undersampling and space-invariant blurring is presented with significant comment regarding the difficulties inherent in autofocusing field-collected data. Field-collected data from trials in Sydney Harbour is presented along with associated autofocus results from a number of algorithms

Experimental and numerical study of planing hulls in waves

Il lavoro presentato nella presente tesi e\u300 stato svolto nell\u2019ambito del progetto SOPHYA (Seakee- ping Of Planing Hull YAchts). Il progetto di ricerca e sviluppo e\u300 finalizzato all\u2019indagine delle prestazioni imbarcazioni plananti da diporto in termini di tenuta al mare, sicurezza e propul- sione in condizioni di mare medie. Questa indagine e\u300 stata condotta attraverso tre approcci complementari: esperimenti in scala modello, prove in mare e simulazioni numeriche. La tesi si concentra principalmente sulla tenuta al mare degli scafi plananti, in particolare, buona parte del lavoro e\u300 stata dedicata allo studio sperimentale e all\u2019analisi dei dati delle prove su modello e scala reale. I risultati sono stati utilizzati come dati di riferimento per le simulazioni CFD al fine di sviluppare un procedura affidabile per le simulazioni di scafi plananti su onda. E\u300 stato condotta un\u2019indagine sperimentale estensiva sullo studio del comportamento non linea- re di scafi plananti in onde regolari e l\u2019effetti dei parametri geometrici dello scafo, della velocita\u300 e della pendenza dell\u2019onda sulla risposta ultra armonica dei moti e delle accelerazioni. Ilcontributo principale dello studio e\u300 rappresentato dall\u2019ampia analisi della seconda e terza risposta armonica del moto e delle accelerazioni. I risultati rappresentano inoltre un notevole contributo ai dati della serie sistematica di Napoli e forniscono un riferimento dettagliato per futuri studi computazionali. Le prove di tenuta al mare al vero sono state pianificate ed eseguite, lo yacht e\u300 stato com- pletamente strumentato e una boa ondametrica e\u300 stata utilizzata per la misurazione direzionale dello stato del mare. E\u300 stato sviluppato un metodo innovativo di analisi integrata delle misure di moti a bordo e d\u2019onda al fine di migliorare la stima delle funzioni di trasferimento in scala reale ed e\u300 stato investigato l\u2019effetto dei modelli di distribuzione direzionale dell\u2019onda sul risultato. Le funzioni di trasferimento al vero ottenute con l\u2019analisi integrata proposta sono confrontabili con quelle in scala modello. Sono state eseguite simulazioni in acqua calma e di seakeeping di uno scafo planante utilizzando un codice open source basato su foam-extend. In seguito e\u300 stata sviluppata una procedura di pre- processing automatizzata che riduce drasticamente i tempi di pre-simulazione. Questa procedura genera una griglia raffinata ad-hoc per simulazioni di idrodinamica navale e prepara una cartella del caso da simulare pronta per l\u2019uso. La procedura utilizza strumenti di generazione griglie compresi in openFOAM ed altri svilupati appositamente. Le simulazioni di acqua calma hanno mostrato una una sottostima dell\u2019assetto in corsa e della resistenza alle alte velocita\u300, tuttavia le simulazioni di seakeeping mostrano risultati molto piu\u300 fedeli ai dati sperimentali in termini di funzioni di trasferimento. Sono inoltre stati implementati nuovi strumenti per le simulazioni che debbano riprodurre fedelmente le condizioni della vasca navale, come accelerometri virtuali e nuove condizioni cinematiche piu\u300 fedeli al reale fissaggio del braccio di rimorchio. I solutori e gli strumenti sviluppati sono stati in seguito utilizzati per studiare l\u2019effetto delle variazioni della forma dello scafo sulle prestazioni di tenuta del mare di otto diverse varianti. I risultati sperimentali dello scafo con le migliori prestazioni sono comparabili con quelli ottenuti dalle simulazioni.The work presented in this thesis has been carried out as part of the SOPHYA (Seakeeping Of Planing Hull YAchts) project. The research and development project is aimed at the investigation of the performances of planing pleasure-boats in terms of sea-kindliness, safety and powering in mild weather conditions. This investigation is pursued via three complementary approaches: model scale experiments,sea trials and Numerical simulations. This thesis is mainly focused on the study of planing hulls seakeeping, in particular, a large part of this work has been dedicated to experimental investigations and data analysis both in model and full scale. The results of towing tank tests have been then used as reference data for CFD simulations with the aim of to develop a reliable computational framework for seakeeping simulations of planing hulls. A novel experimental work has been carried out in order to investigate nonlinear behavior of planing hulls in regular waves and the effects of geometrical parameters, speed and wave steepness on higher harmonic response of hull motions and accelerations. An extensive analysis of the second and third harmonic response of motion and accelerations is presented, adding valuable results to the Naples Systematic Series data set and providing detailed benchmark data for future computational studies. Full scale calm water and seakeeping trials have been carefully planned and executed, the yacht has been fully instrumented and a wave buoy has been used for directional sea state mea- surement. An innovative integrated analysis method of on-board and wave measurements has been developed in order to improve full scale transfer functions estimation. The effect of wave directional distribution models on the estimated transfer functions has been investigated. The results show a remarkable agreement with model scale experimental results. Planing hull calm water and seakeeping simulations have been performed using open source CFD code. An open source fully automated pre-processing procedure has been developed to drastically reduce time-to simulations. The procedure generate a mesh refined ad-hoc for planing hullsimulations and a ready-to-run case folder using OpenFOAM built-in and custom tools. Calm water simulations showed an underestimation of the running trim angle and resistance at high speeds. Seakeeping simulations however showed much better agreement with experimental results in terms of motion transfer functions. New simulation tools have been implemented for the specific case of reproducing towing tank conditions, such as virtual accelerometers and new restraint capabilities that are more faithful of the real towing arm restraint conditions. The tested simulation framework has been used to investigate the effect of hull form variations on seakeeping performances of eight alternative hulls. Experimental results on the best performing hull alternative show comparable results with simulations

High-Speed Research: 1994 Sonic Boom Workshop: Atmospheric Propagation and Acceptability Studies

The workshop proceedings include papers on atmospheric propagation and acceptability studies. Papers discussing atmospheric effects on the sonic boom waveform addressed several issues. It has long been assumed that the effects of molecular relaxation are adequately accounted for by assuming that a steady state balance between absorption and nonlinear wave steepening exists. It was shown that the unsteadiness induced by the nonuniform atmosphere precludes attaining this steady state. Further, it was shown that the random atmosphere acts as a filter, effectively filtering out high frequency components of the distorted waveform. Several different propagation models were compared, and an analysis of the sonic boom at the edge of the primary carpet established that the levels there are bounded. Finally, a discussion of the levels of the sonic boom below the sea surface was presented

Atmospheric Circulation of Terrestrial Exoplanets

The investigation of planets around other stars began with the study of gas giants, but is now extending to the discovery and characterization of super-Earths and terrestrial planets. Motivated by this observational tide, we survey the basic dynamical principles governing the atmospheric circulation of terrestrial exoplanets, and discuss the interaction of their circulation with the hydrological cycle and global-scale climate feedbacks. Terrestrial exoplanets occupy a wide range of physical and dynamical conditions, only a small fraction of which have yet been explored in detail. Our approach is to lay out the fundamental dynamical principles governing the atmospheric circulation on terrestrial planets--broadly defined--and show how they can provide a foundation for understanding the atmospheric behavior of these worlds. We first survey basic atmospheric dynamics, including the role of geostrophy, baroclinic instabilities, and jets in the strongly rotating regime (the "extratropics") and the role of the Hadley circulation, wave adjustment of the thermal structure, and the tendency toward equatorial superrotation in the slowly rotating regime (the "tropics"). We then survey key elements of the hydrological cycle, including the factors that control precipitation, humidity, and cloudiness. Next, we summarize key mechanisms by which the circulation affects the global-mean climate, and hence planetary habitability. In particular, we discuss the runaway greenhouse, transitions to snowball states, atmospheric collapse, and the links between atmospheric circulation and CO2 weathering rates. We finish by summarizing the key questions and challenges for this emerging field in the future.Comment: Invited review, in press for the Arizona Space Science Series book "Comparative Climatology of Terrestrial Planets" (S. Mackwell, M. Bullock, and J. Harder, editors). 56 pages, 26 figure