Model Tank Experiments Research Articles

Elastic response of a light-weight floating support structure of FOWT with guywire supported tower

To obtain fundamental knowledge on the elastic response characteristics of a light-weight floating support structure of a FOWT (floating offshore wind turbine) with guywire supported tower, basic load transmission mechanism was investigated. Static analysis with elastic frame model, numerical analysis and wave tank experiment with elastically and dynamically similar segmented backbone model were conducted to clarify the dynamic elastic response characteristics of the structure. In the numerical analysis, analysis code of a rotor-floater-mooring-control coupled response NK-UTWind developed in University of Tokyo is used. It is clarified that when the rigidity of the frame structural part is low compared with guywire, the load is mainly borne by guywire under pitch motion. It was found that the tension fluctuation of guywire becomes large at wave period of 6 s when the inertial force due to pitch motion is large, and at wave period of 18–20 s when inclination of tower is larger the tension fluctuation also becomes large due to the overturning moment.

Nonlinear Container Ship Model for the Study of Parametric Roll Resonance

Parametric roll is a critical phenomenon for ships, whose onset may cause roll oscillations up to +-40 degrees, leading to very dangerous situations and possibly capsizing. Container ships have been shown to be particularly prone to parametric roll resonance when they are sailing in moderate to heavy head seas. A Matlab/Simulink parametric roll benchmark model for a large container ship has been implemented and validated against a wide set of experimental data. The model is a part of a Matlab/Simulink Toolbox (MSS, 2007). The benchmark implements a 3rd-order nonlinear model where the dynamics of roll is strongly coupled with the heave and pitch dynamics. The implemented model has shown good accuracy in predicting the container ship motions, both in the vertical plane and in the transversal one. Parametric roll has been reproduced for all the data sets in which it happened, and the model provides realistic results which are in good agreement with the model tank experiments.

A comparison of Hankel transform algorithms’ performance in shallow-water waveguides and their effect on the determination of seabottom plane-wave reflection coefficients

Complex acoustic pressure data as a function of range may be decomposed into its constituent wave-number components by taking the Hankel transform. This paper investigates the accuracy of a number of discrete Hankel transform algorithms applied to computer simulated pressure data in a shallow water channel for a variety of seabed bottom types. The results are compared against the exact solution and the most accurate algorithm identified for shallow-water applications. The differing behavior of these algorithms is illustrated by their use in the Hankel transform method proposed by Frisk et al. [J. Acoust. Soc. Am. 68, 602–612 (1980)] for the determination of seabottom plane-wave reflection coefficients. The results from computer simulations and scale model tank experiments are compared.

A sample of controlled experiments in diff radion tomography1

AbstractVarious applications of a new geophysical reconstruction method, generalized acoustical diffraction tomography (GADT), which is based on transmission data as input are considered. Conventional diffraction tomography methods normally require linearization with respect to a uniform reference medium and regular sampling along a straight line. Thus, these methods will not work well when the background is strongly non‐uniform and/or the acquisition geometry is arbitrary. However, GADT can, in principle, handle both irregularly spaced data, curved acquisition lines, and non‐uniform background models.A number of controlled model tank and field experiments, where the model and the test object(s) are known a priori, have been carried out. After acquiring the tomographic data in each experiment, these are used to compute a reconstruction of the model, which can then be compared with the actual, known model. The method's ability to yield high‐quality images of the different targets is demonstrated.

A nondestructive bottom characterization with a model tank experiment

The theory of normal modes has been applied to the inverse scattering problem. The model accounts for multiple elastic and absorptive bottom sediments. The characterization is performed at a number of frequencies according to the depth of the water column. Transmission loss measurements with range are supplied to a modified Levenberg‐Marquardt minimization model at each frequency. The highest frequency is resolved first to characterize the uppermost sediment of the ocean floor. After convergence to a set of acoustic properties for that sediment, the next lower frequency is used to evaluate the acoustic properties of the next deeper sediment. Some experimental measurements by Hundley and Glegg [J. Acoust. Soc. Am. Suppl. 1 87, S132 (1990)] are used to test this inverse scattering method in a model tank with an epoxy sediment over a concrete subbottom. [Work supported by ONR.]

Asymmetric Doppler amplitudes in the surface scatter channel for crosswind transmitter–receiver geometry

Doppler asymmetries have been observed in model tank experiments in the signal received after being scattered from a wind-driven surface. A distinct spectral shift was observed when the plane containing the transmitter and receiver was perpendicular to the wind direction. This paper examines the conjecture that this shift arises from the fundamental asymmetry observed in a wind-driven surface, i.e., from the fact that the upwind slopes tend to be shallower than the downwind slopes. Three different situations are considered: (1) a deterministic surface, (2) a random surface with large Rayleigh parameter, (3) a random surface with small Rayleigh parameter. All three models show that the slope asymmetry results in Doppler asymmetry; however, the amount of this asymmetry corresponding to realistic values of surface parameters is very small. Hence the slope asymmetry does not completely explain the occasionally observed larger spectral shifts, and other mechanisms must be considered.

MODEL STUDIES ON SOME ASPECTS OF RESISTIVITY AND MEMBRANE POLARIZATION BEHAVIOUR OVER A LAYERED EARTH*

AbstractElectrolytic model tank experiments to study resistivity and time domain induced polarization (IP) response over layered earth models were initiated primarily to facilitate the understanding of field results. Alternate layers of clay and sand (or clay‐coated sand) with, in some cases, a surficial layer of water were assembled in the tank and resistivity and IP measurements made for a range of electrode spacings using the Wenner configuration. Graphite and silver‐silver chloride electrodes were used as current and potential electrodes respectively.Clay‐coated (3% by weight) sand was found to generate stronger polarization than either clay or sand alone. Apparent chargeability ma was observed to be positive for a nonpolarizable surface layer. For a polarizable surface layer, the sign of IP was controlled by the polarizability, the thickness of the second layer, and the spacing of the electrode spreads. The apparent chargeability ma can theoretically change sign from positive to negative and vice versa with a gradual increase in electrode spacing, and such negative IP effects were obtained in a few observations. A simultaneous decrease in IP and an increase in resistivity, which is a qualitative diagnostic feature for the occurrence of clean freshwater sand aquifers, could also be generated in the model tank experiment.Combined resistivity and IP soundings were carried out near Fredericton Junction and Tracy, New Brunswick, Canada. Field curves are presented along with the model curves for qualitative comparison and understanding of IP behaviour over a layered earth. Twenty‐five out of twenty‐seven soundings show only positive apparent chargeabilities, whereas two show chargeability sign changes (positive/negative/positive). The model study gives reason to believe that surface soils and Quaternary gravel boulder deposits near Fredericton Junction are relatively non‐polarizable.As an auxiliary experiment, sand and clay were taken in different proportions by weight and mixed thoroughly with water in a cement mixer. The mixtures were then compressed with a suitable die and plunger under 3.6 Pa pressure to prepare cylindrical samples of height 18 cm and diameter 15.5 cm. IP measurements were done on the flat faces using the Wenner configuration with a= 2 cm. Chargeability was found to be negative for 100 and 90% clay mixtures. It reached a positive maximum for an 80% clay‐20% sand mixture and then decreased gradually with increasing sand and decreasing clay content.

Sound propagation in a wedge-shaped ocean with a penetrable bottom

Modal cutoff during up-slope propagation in a wedge-shaped ocean is studied using the parabolic equation model; theoretical results are compared with some model tank experiments.

Range‐dependent propagation in coastal waters

The parabolic equation (PE) is used to study sound interaction with a sloping ocean bottom and some of the results are compared to a model tank experiment. The bottom, as inputted in the PE model, is characterized by sound speed, density, and attenuation. Practical running times have been achieved for these coastal water studies by utilizing a real time computer system with an array processor. Up and down slope propagation is studied demonstrating mode conversion and mode cutoff. For the upslope propagation we find that each mode cuts off by radiating a discrete beam into the ocean bottom and that there is very little conversion into the next lower mode. Mode cutoff occurs at approximately the depths predicted by normal mode theory but the cutoff takes place over a finite distance which essentially provides an aperture of many wavelengths for radiation of the beam into the bottom. An important conclusion that one can draw from this study is that mode‐coupling theories should also include the coupling into the local continuous modes which would then provide the description of the radiation into the bottom.

Conditioned sandy soils as an envelope to prevent silting-up in drain tubes

A sand model tank experiment under ponded water conditions was carried out in order to compare the effectiveness of three filters either flax straw of 1 cm thickness, or conditioned sand with bitumen emulsion or conditioned sand with PAM solution as envelopes of 4 cm thickness put around the drain tubes. All of these treatments have prevented the silting-up completely. After 24 hours, when conditioned soil is used as a filter around the drain tube the water discharge is 1·3 times bigger than from the drain without filter but 1·16 times less than using flax straw.

Geophysical subsurface probing with radio-frequency interferometry

The radio-frequency interferometry method can be used to probe interiors of celestial bodies and terrestrial areas with low conductivity. Several glaciers have been studied with this technique. An experiment based on this method was designed for Apollo 17 to examine the lunar subsurface. In order to interpret the interference patterns, We have studied theoretically the electromagnetic fields due to a dipole antenna on the surface of a horizontally stratified <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</tex> -layered medium. Three approaches are used to calculate the interference patterns: 1 ) direct numerical integration; 2) asymptotic evaluation by the saddle point method; and 3) residue series approach. The asymptotic approach leads to the geometrical optics interpretation. The residue approach leads to modal analysis. Validity of the formulation is checked by comparisons with analogue model tank experiments and actual field data obtained from glaciers.

Model tank experiments for resistivity measurements on non-conducting and conducting sheets

Model tank experiments for conducting and non-conducting sheets have been described and the results compared with those of other workers.

Frequency Spectra of Forward-Scattered Sound from the Ocean Surface

This study consists of a theoretical analysis, a model tank experiment, and an ocean experiment to determine the frequency spread of surface-reflected sound. The theoretical study is based on the concept that the phase and amplitude of the surface waves modulate the acoustic signal. The amount of spreading is a function of the incident acoustic frequency, the surface-wave spectrum, and the geometry of the experiment. The ocean experiment was conducted in winter off the coast of Bermuda. Two frequencies (750 and 1500 Hz) were emitted simultaneously from a fixed projector. The tones were surface reflected and received on a narrow-beam fixed bottom-mounted vertical array. Oceanographic and meteorological data were obtained in the vicinity of the insonified area. The time variation of the surface elevation was obtained from a wave staff, and this information has been processed to obtain the one-dimensional ocean spectrum. The experimental results are compared with theoretical predictions. It is shown that the comparison is excellent for both the model tank and the ocean experiments. Under all conditions for the ocean experiment, the frequency spread is less than 1.0 Hz, and under low sea-state conditions, the spread is about 0.2 Hz.

Acoustic Spectra of Specular and Near-Specular Scattering from a Three-Dimensional Traveling Sinusoidal Surface

A model tank experiment has been conducted that investigated the spectral characteristics of surface reflected and scattered underwater sound. The major experimental objectives were to determine the frequency spectrum of specular and near specular scattering from a traveling sinusoidal surface. The spectral characteristics were investigated for various directions of surface wave propagation and also for a two frequency surface. A narrow-beamed 500-kHz CW signal was projected at the surface and the reflected and scattered sound was received, bandshifted, and recorded on magnetic tape for spectral analysis. A vibration generator produced gravity waves: wavelength and wave-height measurements were made over the insonified area. A scattering integral developed in the literature was evaluated for a traveling sinusoidal surface to compare the experimental results. Experimental and theoretical results show that for large surface-to-acoustic wavelength ratios, the specularly reflected signal is amplitude- and phase-modulated by the near-specular scattering regardless of the surface directionality. The frequency spectrum is symmetrical about the carrier in both amplitude and frequency. This symmetry holds for a multifrequency surface if the surface frequencies are not commensurable. The spectrum of off-specular scattering is Doppler shifted and is a function of surface wave directionality.

Model tank experiments for resistivity measurements on sloping and vertical non conducting dykes

Model resistivity experiments over sloping and vertical, non-conducting dykes have been carried out and reported in this paper. The results of vertical dykes as compared to the sloping dykes are very interesting and not so far reported in the available literature. Of particular interest is distinct resistivity high in the centre of the two vertical dykes, followed by lows over the dykes and again by highs on either side. As expected the curves are symmetrical. As against this, in the cases of sloping dykes, there is a distinct low, in between the dykes, followed by highs over the dykes, the curves being asymmetrical.

Frequency Spectra of Forward-Scattered Sound from a Time-Varying Surface

This study consists of a theoretical analysis, a model tank experiment, and an ocean experiment to determine the frequency spread of surface-reflected sound. The theoretical study is based on the concept that the surface waves' phase- and amplitude-modulate the acoustic signal. The amount of spreading is a function of the incident acoustic frequency, the surface wave spectrum, and the geometry of the experiment. The ocean experiment was conducted in winter off the coast of Bermuda. Two frequencies (750 and 1500 Hz) were simultaneously emitted from a fixed projector. The tones were surface reflected and received on a narrow-beam, fixed, bottom-mounted vertical array. Oceanographic and meteorological data were obtained in the vicinity of the insonified area. The time variation of the surface elevation was obtained from a wave staff, and this information has been processed to obtain the one-dimensional ocean spectrum. The experimental results are compared with theoretical predictions. It is shown that the comparison is excellent for both the model tank and the ocean experiments. Under all conditions for the ocean experiment, the frequency spread is less than 1.0 Hz; and under normal conditions, the spread is about 0.2 Hz.

Analysis of Underwater Acoustic Backscattering from a Random Surface

This paper is essentially a continuation of work presented earlier [J. Acoust. Soc. Am. 36, 1987(A) (1964)]. A mathematical model is hypothesized that yields the backscattered power as a function of incident angle, transducer-beam pattern, frequency, and the waveheight spatial-correlation function. This model considers the surface to be composed of differential facets with dipole beam patterns. The average backscattered power is obtained by squaring the spatial integration of the time-varying signal and averaging in the statistical domain. Curves are presented that compare results from model tank experiments with the analytical results. [This investigation is supported by the Acoustics Program, U. S. Office of Naval Research, and the U. S. Naval Underwater Ordnance Station, Newport, R. I.]

Rippled-Boundary Acoustic Experiment

A ripple structure on a sandy ocean bottom produces important changes in the reflection characteristics of the boundary. This effect is analogous to the focusing produced by an optical-reflection grating. The acoustics problem has been studied since the time of Lord Rayleigh, with successes being limited to the case of extremely small corrugation amplitudes. This paper presents some results obtained from a model-tank experiment wherein a scale factor of about 20:1 is applied to the acoustic frequency and linear dimensions. The model boundaries were constructed of a plaster-sand mixture and of very fine sand in both flat and corrugated forms. In the experiments, the incidence angle was set while a complete semicircular reflection pattern was recorded for the whole region above the sonified boundary section. The incidence angle was varied from grazing to normal, so that families of reflection patterns were produced for the flat and corrugated-plaster and for the flat and corrugated-sand boundaries. These patterns are shown in the paper, as well as some curves showing the specular and backscatter [toward the sound source] amplitude dependence upon incidence angle and boundary contour. The experimental work is still in progress.

MODIFIED TRIPOTENTIAL PROSPECTING METHOD*

AbstractA modification to the tripotential method of Carpenter (1955) and Carpenter and Haberjam (1956) is suggested and its theoretical implications and practical advantages are discussed. A specially designed switchbox is described which enables model tank experiments to be made with the modified tripotential method and with conventional methods. The results broadly show that a horizontal distance of the configuration at which the γ—resistance becomes negative directly corresponds to the depth to the interface. Discrepancies between the observed and actual depths are due to the bottom and wall effects of the wooden tank.