Showing papers in "Ocean Engineering in 1990"
TL;DR: In this article, a closed-form solution for the velocity potential resulting from the interaction of second-order Stokes waves with a large vertical circular cylinder was developed, which was then used to determine the theoretical free surface elevations around the cylinder consistent to second order.
Abstract: A closed-form solution is developed for the velocity potential resulting from the interaction of second-order Stokes waves with a large vertical circular cylinder. At first-order, the solution is the usual linear diffraction theory. At second-order, the solution consists of forced wave motions, due to nonlinear wave-wave interactions in the free surface boundary condition, plus scattered free wave motions, due to the interaction of the forced waves with the fixed cylinder. The velocity potentials are then used to determine the theoretical free surface elevations around the cylinder consistent to second-order. Second-order terms are found to significantly alter wave envelopes around the cylinder as a result of nonlinear diffraction. For example, the maximum wave crest run-up on the cylinder from the nonlinear theory is found to exceed that predicted by the linear diffraction theory by up to 50%. A brief comparison of the nonlinear theory with the measured run-up data is found to largely confirm the theoretical solution.
76 citations
TL;DR: In this paper, phase control is used to increase the energy absorption from irregular waves by oscillating water column (OWC) devices, from regular and irregular waves, in a modified version of the self-rectifying axial-flow Wells air turbine.
Abstract: The paper deals with phase control as a method of increasing the energy absorption by oscillating water column (OWC) devices, from regular as well as from irregular waves. The power take-off machine considered is a modified version of the self-rectifying axial-flow Wells air turbine, whose rotor blades are of variable setting angle; this allows the air pressure and flow rate to be controlled independently from each other. Results of numerical simulations are presented for three different control strategies applied to energy absorption from irregular waves by an OWC device of simple, two-dimensional geometry. Experimental data from a turbine model are used in the simulation.
74 citations
TL;DR: In this article, the authors measured the impact pressure and resulting wall deflection from breaking oscillatory waves on a vertical wall with 1/10 foreshore slope and found that the longerlasting low impact forces are more effective in producing the larger wall deflections.
Abstract: Laboratory experiments are conducted to measure the impact pressures and resulting deflections from breaking oscillatory waves on a vertical wall with 1/10 foreshore slope. The maximum impact pressure data on the wall are statistically analysed and the relationships between the magnitudes of impact pressures and forces, and their durations, are investigated. The maximum impact pressures, among the 90 wave impacts, are found to vary between 1.37 × 10 4 and 28.3 × 10 4 Pa . The maximum impact pressures are shown to reasonably satisfy the log-normal probability distribution and they occur most frequently slightly below the still-water level. The greatest wall deflection at the point of measurement is caused by an impact which has a maximum pressure of 3.6 × 10 4 Pa , corresponding to 50% probability in the log-normal distribution. It is found that the longer-lasting low impact forces are more effective in producing the larger wall deflections. In this respect, the maximum impact pressures in the range between 2.5 × 10 4 and 5 × 10 4 Pa obtained in this study are found to be the most effective. The upper limit of this range (when non-dimensionalised by the specific weight of water and deep-water wave steepness) is suggested as a design value for vertical walls.
69 citations
TL;DR: In this article, the interaction of a current-free plane surface wave train of fixed frequency and a uniform wave-free current normal to the wave crests in the same or opposite direction to wave propagation is described.
Abstract: The interaction of a current-free plane surface wave train of fixed frequency and a uniform wave-free current normal to the wave crests in the same or opposite direction to wave propagation is described. Neglecting dissipation, the combined field after the encounter is assumed stable, uniform, steady and irrotational. The parameters describing the wave-current field generated by the interaction, namely the wave height, wave length, stream current and water depth are calculated numerically by solving a set of nonlinear equations. These equations are obtained on satisfying, to second order, conservation of mean mass, momentum and energy flux, as well as a dispersion relation on the free surface of the after-encounter field. Possible solutions are identified a posteriori. Contrary to other approaches the additional contribution to the change in the mean value of the current and mean water depth is not neglected in this work. Numerical results are presented which illustrate the changes in the wave length, wave height, current speed and water depth. A comparison with the numerical and experimental results reported by G.P. Thomas [J. Fluid Mech.110, 457–474)] is also provided.
58 citations
TL;DR: In this paper, an experimental and numerical study is made of the forces and flow about a circular cylinder steadily advancing beneath the free-surface, showing that the drag coefficient abruptly decreases from 1.3 to 1.0 and the Strouhal number simultaneously increases from 0.19 to 0.30 at the depth-radius ratio of 1.7 when the depth of submergence of the circular cylinder is gradually reduced.
Abstract: Experimental and numerical study is made of the forces and flow about a circular cylinder steadily advancing beneath the free-surface. Force measurements at a Reynolds number of 4.96 × 104 show that the drag coefficient abruptly decreases from 1.3 to 1.0 and the Strouhal number simultaneously increases from 0.19 to 0.30 at the depth-radius ratioof 1.7 when the depth of submergence of the circular cylinder is gradually reduced. Pressure measurements, flow visualization and numerical simulations indicate that at the shallowly-submerged condition the difference of the flow in the vacinities of the top and bottom of the cylinder causes asymmetric vortex generation and that this results in a smaller pressure reduction on the backward face of the cylinder, a lower drag coefficient of the cylinder, and a higher frequency of vortex shedding.
54 citations
TL;DR: In this article, the effects of the dynamic coupling of a liquid-tank system are studied analytically for a two-dimensional, rectangular rigid tank with no baffles, where the governing equations of the liquid motion are derived with reference to a moving coordinate system which is fixed to the moving container.
Abstract: It has been observed that the unrestrained free surface of a container can create relatively large liquid movements for even very small motions of the container. This excessive movement may endanger the stability as well as the maneuvering quality of the transporting vehicle. Therefore, the effects of the dynamic coupling of a liquid-tank system are of great concern. This dynamic coupling problem is studied analytically for a two-dimensional, rectangular rigid tank with no baffles. The governing equations of the liquid motion are derived with reference to a moving coordinate system which is fixed to the moving container. With the liquid forces generated by the fluid motion as the external exciting force for the tank, the motions of the liquid-tank system can be described according to Newton's Second Law of Motion. By using the Laplace transformation technique, the dynamic responses of the coupled system can be examined in detail. Numerical results for various types of external excitations and the resultant motions of the fluid-tank system are presented and compared with the equivalent non-shifting cargo system. The results of the comparison indicate that the discrepancy of responses in the two systems can obviously be observed when the ratio of the natural frequency of the fluid and the natural frequency of the tank is close to unity. Also, the amount of fluid inside the tank is a very important factor in determining the responses.
44 citations
TL;DR: In this paper, the interaction between current-free higher-order water waves with a wave-free uniform current normal to the wave crests is considered, and the combined wave-current motion resulting from the interaction is assumed stable and irrotational.
Abstract: The interaction between current-free higher-order water waves with a wave-free uniform current normal to the wave crests is considered. The combined wave-current motion resulting from the interaction is assumed stable and irrotational. The velocity potential, dispersion relation, the particle kinematics and pressure distribution up to the third order in wave amplitude are developed. The conservation of mean mass, momentum and energy, together with the dispersion relation on the free surface are used to derive a set of four nonlinear equations, through which the relationship between wave-free current, current-free wave and the combined wave-current parameters is established. Numerical results for a range of current values are also presented.
29 citations
TL;DR: In this article, the specific power of a simple irreversible closed-cycle OTEC power plant is analyzed and optimized for maximizing the per unit time and per unit total heat exchanger area production in power plants whose heat transfer area is constrained by economic consideration.
Abstract: The specific power of a simple irreversible closed-cycle OTEC power plant is analyzed and optimized. The plant uses a working fluid such as ammonia to produce power. Specific power is the net output per unit total heat exchanger surface area. Net output generated by the working fluid is obtained in terms of the rate of heat added to the working fluid from the warm surface ocean water, less the rate of heat removed from the working fluid to the cold deep ocean water. A time factor is added to iimulate the heat exchanges between the OTEC plant and its surroundings. A mathematical expression is derived for the specific power output of the irreversible OTEC heat engine. It is found that there is a bound on the specific power output. This bound provides the basis for a practical engineering effort towards maximizing the per unit time and per unit total heat exchanger area production of work in power plants whose heat transfer area is constrained by economic consideration.
27 citations
TL;DR: In this paper, it was shown that there were still inconsistencies in the prognostic equations giving overestimation of the first moment of the spectrum and consequently the significant wave height, and that variable σ a, σ b and γ must be used to achieve consistent results.
Abstract: The original prognostic equations for the JONSWAP-spectrum contained inconsistencies. A subsequent paper (Hasselmann et al ., 1976, J. phys. Oceanogr. 6 , 200–208) attempted to regularise the situation. This paper shows that there were still inconsistencies in the prognostic equations giving overestimations of the first moment of the spectrum and consequently the significant wave height. The prognostic equations are reworked systematically and results presented. It is shown that variable σ a , σ b and γ must be used to achieve consistent results
26 citations
TL;DR: In this article, the authors present a method for the calculation of hydrostatic properties of intact and damaged ship hulls and other floating structures, where the geometry of the floating structure is modelled as a set of compartments, bounded by flat panels.
Abstract: It is the purpose of this paper to present a new method for the calculation of hydrostatic properties of intact and damaged ship hulls and other floating structures. The geometry of the floating structure is modelled as a set of compartments, bounded by flat panels. Hydrostatic properties are derived by pressure integration on these panel elements. By application of Greens integration theorem the area integration is transformed into line integrals around the contour of each element. The line integrals can be expressed analytically such that the final result is that all the hydrostatic properties can be determined as a summation of easily evaluated expressions. This calculation procedure has the advantage of being well suited for computer calculation, and is easily applied to truly arbitrarily shaped floating structures. Another advantage is that it is based on a geometrical model which is equally suited for finite element strength analysis and hydrodynamic calculations based on a panel method. The application of the procedure is demonstrated by an example.
19 citations
TL;DR: In this article, a comparison between two theoretical methods for computing the second-order diffraction loads on arrays of bottom-mounted, surface-piercing vertical circular cylinders in regular waves is presented.
Abstract: This paper presents a comparison between two theoretical methods for computing the second-order diffraction loads on arrays of bottom-mounted, surface-piercing vertical circular cylinders in regular waves. One method presents a complete solution for the second-order hydrodynamic loads on the cylinder array via a numerical integration over the mean fluid free-surface. The other method is based on a large spacing approximation between the array members and involves the solution of a set of equivalent isolated body problems to obtain estimates for the second-order hydrodynamic loads. Numerical results for a pair of cylinders indicate very good agreement between the two methods at center-to-center spacing of both three and five radii, indicating that the approximate method may be sufficient to compute hydrodynamic interference effects to the second-order in many practical engineering situations.
TL;DR: In this paper, the boundary integral method is used to calculate the hydrodynamic pressure distribution on a rigid submerged cylindrical storage tank subjected to horizontal or vertical harmonic ground excitation, where the fluid is assumed to be linearly compressible and to undergo small-amplitude, irrotational motion.
Abstract: The boundary integral method is utilized to calculate the hydrodynamic pressure distribution on a rigid submerged cylindrical storage tank subjected to horizontal or vertical harmonic ground excitation. The fluid is assumed to be linearly compressible and to undergo small-amplitude, irrotational motion. The vertical axisymmetry of the structure is exploited utilizing a fluid Green's function which is separable in the cylindrical polar coordinate system and so leads in each case to a line integral equation for the fluid velocity potential on the surface of the structure. Numerical results are presented which illustrate the influence of the direction and frequency of ground motion and the body dimensions on the hydrodynamic pressure distribution. For both horizontal and vertical ground excitation, the numerical values from the boundary integral method have been verified by a comparison with those obtained by an eigenfunction expansion approach.
TL;DR: In this article, an extensive experimental program for measuring, recording and analyzing free roll decay curves for three models representative of small fishing vessels was carried out, and the results indicated that a linear plus linear angle dependent form fits best the experimental data of a heavily damped hull.
Abstract: An extensive experimental program for measuring, recording and analyzing free roll decay curves for three models representative of small fishing vessels was carried out. One of the objectives of this program is to study the form of the roll damping moment. The results indicate that a linear plus linear angle dependent form fits best the experimental data of a heavily damped hull. The linear plus quadratic velocity dependent form fits best the experimental data obtained for a lightly damped model.
TL;DR: In this article, a simplified model for the static analysis of the tethers as anchoring elements for a tension leg platform (TLP) was presented, and basic equations for calculating the gross behavior of the TLP tether/platform interaction were also provided.
Abstract: Recently [ Demirbilek, Z. 1989. Modelling and analysis of TLP tethers and responses. J. WatWay, Port, Coastal Ocean Div., Am. Soc. civ. Engrs (submitted)], a simplified model for the static analysis of the tethers as anchoring elements for a tension leg platform (TLP) was presented. Basic equations for calculating the gross behavior of the TLP tether/platform interaction were also provided. The aim of this paper is to describe the mathematical model that was developed to study the offset and set-down motions of the TLP hull. Limiting cases of the governing equations are studied and relations between motions of platform, restoring force and tether pre-tension are derived herein. The present analysis provides guidelines to designers in the evaluation of the platform performance and selection of the mooring system.
TL;DR: In this paper, the hydrodynamic pressure due to regular and random waves exerted on a large vertical cylinder in a constant water depth is investigated, and the results of the experiments are compared with the linear diffraction theory of MacCamy, R. C. and Fuchs.
Abstract: Investigations on the hydrodynamic pressures due to regular and random waves exerted on a large vertical cylinder in a constant water depth are reported in this paper. In the experimental investigation, the test cylinder embedded with diaphragm-type pressure transducers at nine different elevations was rotated about its axis to measure the dynamic pressure around its circumference. The wave field in the neighbourhood of the cylinder was measured at six different locations. The results of the experiments are compared with the linear diffraction theory of MacCamy, R. C. and Fuchs, R. A. [(1954) Wave forces on piles: a diffraction theory. U.S. Army Beach Erosion Board, Technical Memorandum No. 69]. In general, the agreement between the theoretical and experimental results is found to be satisfactory. A comparison between the regular and random wave test results is also presented and discussed.
TL;DR: In this paper, the problem of the dynamics of a moored floating object under the action of regular waves is solved numerically as a boundary value problem by use of the finite-infinite element method.
Abstract: The three-dimensional problem of the dynamics of a moored floating object under the action of regular waves is solved numerically as a boundary value problem by use of the finite-infinite element method. The cross-sectional shape of the floating body and the mooring arrangements may all be arbitrary. The mathematical formulations of the problem and procedures of the numerical method are presented in this paper. A corresponding computer program WALOAD has been developed, which is capable of computing wave forces on fixed and floating structures. Numerical computations using this program could give very accurate results, even though rather coarse meshes were used. The program is easy to use and is readily applicable in many practical situations.
TL;DR: In this article, the flow field about a horizontal cylinder with its axis parallel to the crests in deep water waves is simulated by a finite-difference method based on the Navier-Stokes equation.
Abstract: The flow field about a horizontal cylinder with its axis parallel to the crests in deep water waves is simulated by a finite-difference method based on the Navier-Stokes equation. By use of ingenious techniques of implementing the conditions on the free surface and body boundaries, both wave motions and viscous motions are simultaneously simulated with a sufficient degree of accuracy in the framework of a rectangular grid system. The numerical simulations reveal that at the intermediate Keulegan-Carpenter number 1–3, vortices shed from or attached to the cylinder interact with each other in a complicated manner, render significant influences on the pressure field in the vicinity of the cylinder and consequently give nonlinear inertia forces. The simulated pressure distribution on the cylinder surface and the resultant forces show good agreement with the measured results.
TL;DR: In this article, a fourth-order solution for the mean drift induced by a steady train of waves in water of constant depth was derived, which reproduces the observed tendency of the surface drift velocity to rise in shallow water and to level-off in very deep water.
Abstract: A fourth-order solution is derived for the mean drift induced by a steady train of waves in water of constant depth. New measurements are carried out of the drift in the body of the fluid and the drift velocity gradient at the free surface. Comparison of theory and experiment shows significantly better agreement with the present fourth-order solution than with the previous second-order solution of Longuet-Higgins, M.S., 1953 [ Phil. Trans. R. Soc. 245 , 535–581]. In particular, the present solution reproduces the observed tendency of the surface drift velocity to rise in shallow water and to level-off in very deep water.
TL;DR: In this article, a finite element model of the dynamic viscoelastic response of marine cables was developed using an updated Lagrangian description of the principle of virtual work and a three-parameter linear visco-elastic constitutive equation for synthetic cables.
Abstract: A finite element model of the dynamic viscoelastic response of marine cables was developed using an updated Lagrangian description of the principle of virtual work and a three-parameter linear viscoelastic constitutive equation for synthetic cables. This model can be used for dynamic analysis of complexly connected and curved systems of cables in the ocean. Two simple examples were considered: a point load on a changing cable which gave comparisons of two different numerical solution methods to an exact solution, and the periodic response to an initial transverse disturbance of a straight segment, which was used to demonstrate and validate the finite element model.
Journal Article•
TL;DR: Based on the theoretical solution of second order nonlinear random waves, a relationship between the second-order nonlinear spectra and the linear spectra is presented in this paper, and a model which separats the linear spectrum from the target spectra was set up by using nonlinear constrained optimization method.
Abstract: Based on the theoretical solution of second order nonlinear random waves, a relationship between the second order nonlinear spectra and the linear spectra is presented in this paper, and a model which separats the linear spectra from the target spectra is set up by using nonlinear constrained optimization method.A theoretical model for digital simulation of nonlinear random waves is also given using Fast Fouries Transform algorithm for nonlinear random wave computation with a greatly shortened. According to the method which determines the amplitudes by separating the linear spectra from the target spectra, the disagreement between simulated spectra and target spectra is solved. The estimated spectra and statistic characteristics of simulated random waves are, also preliminarily analysed.
TL;DR: In this article, an approach based on the influence of transverse velocity was proposed to account for the apparent increase in the drag coefficient of a submerged cylinder undergoing vibration as a result of hydrodynamic forces.
Abstract: Numerous flexible underwater structures (towing and mooring lines, trawl lines, oil platform risers, submarine periscopes, etc.) are used in navigation and exploitation of sea resources. Current technologies require a knowledge of the hydrodynamic forces exerted on these structures. All theoretical calculations employ hydrodynamic coefficients, in particular the drag coefficient for which a review of the literature is provided. An original scheme is proposed, based on the influence of transverse velocity, which can account for the apparent increase in the drag coefficient of a submerged cylinder undergoing vibration as a result of hydrodynamic forces. Several applications are presented which show that the use of empiric values for the drag coefficient of a cylinder vibrating on the order of 1.8 is still perfectly justified.
TL;DR: In this article, a scheme for utilizing ocean wave energy to generate a high power laser was examined, which can be used for unguided communication under water or through the atmosphere, and in spacecraft propulsion.
Abstract: A possible scheme for utilizing ocean wave energy to generate a high power laser is examined. Such a laser can be used (i) unguided communication under water or through the atmosphere, and (ii) in spacecraft propulsion. The natural frequencies and the frequency response of the proposed two degree-of-freedom primary energy converter are obtained by using Lagrange's equations. Further, it is shown that the basic principle used in primary energy conversion can be developed into a mechanism for roll stabilization of marine vessels.
TL;DR: In this paper, the performance of two cable fairings is compared at low Reynolds numbers for flow normal to the leading edge and with the fairings swept, and the cross-flow model, or independence principle, appears to work surprisingly well.
Abstract: The performance of two cable fairings is compared at low Reynolds numbers for flow normal to the leading edge and with the fairings swept. An important feature of the flow is the laminar separation bubble; flow visualization is used to observe its behaviour. Commonly used cable loading functions are shown to over-predict the swept cable drag whereas the cross-flow model, or independence principle, appears to work surprisingly well.
TL;DR: In this article, the lateral stability of a submarine hoseline in a slowly varying current is investigated, where the hose segment is assumed to slide on the sea bottom without twisting and the stability is evaluated in terms of lateral deflections, hose tensions, and anchor loads.
Abstract: The lateral stability of a submarine hoseline in a slowly varying current is investigated. If the current force overcomes the sea bottom resistance, the hose segment is assumed to slide on the sea bottom without twisting. The stability is evaluated in terms of lateral deflections, hose tensions, and anchor loads. The behavior of a hoseline in a variable current is simulated based on nonlinear cable-like response to lift and Morison-type drag forces. Principles and the numerical algorithm of the simulation model are briefly summarized. A parametric analysis is conducted to study the influence on the hose response of the physical parameters considered in the simulation model. The results indicate that, for a practical hoseline, the most critical parameters are: the segment length-to-span ratio, the axial rigidity of the hose, the hose size, and the current velocity. The sea bottom resistance is negligible from a design point of view.
TL;DR: In this paper, the authors outline a procedure for reliability estimation of structural members in extreme stress and fatigue limit states, using a spectral approach for the extreme response statistics with stochastic loading, and the fatigue reliability is estimated with a narrowbanded stress assumption with discrete, but significant sea states within the life of the structure.
Abstract: Since offshore towers are high-cost, high-risk structures, reliability analysis is of great importance in their design. This paper presents a possible practical approach to certify a design through selective critical member reliability estimates. After a brief review of current research in this field, the authors outline a procedure for reliability estimation of structural members in extreme stress and fatigue limit states. A spectral approach for the extreme response statistics with stochastic loading is described. The reliabilities are computed by the Level II first-order second moment (advanced) method. The fatigue reliability is estimated with a narrow-banded stress assumption with discrete, but significant sea states within the life of the structure. Two numerical examples, a three shallow water model and a two-dimensional deep water model are presented along with the influences of stochastic variables (sea state, current, tubular member diameter) on reliabilities (extreme stress and fatigue damage).
TL;DR: In this paper, the dynamic response of a damaged, twin pontoon, four column semisubmersible at a scale of 1:100 is discussed, with the model oriented in head, beam and quartering directions for both intact and damage conditions.
Abstract: A brief review of the stability requirements of semisubmersibles is presented. Results of the dynamic response of a damaged, twin pontoon, four column semisubmersible at a scale of 1:100 are discussed. The tests involved both regular and irregular waves with the model oriented in head, beam and quartering directions for both intact as well as damage conditions. Four damage conditions representing partial damage to one column were simulated: two in windward (positive) direction and two in leeward (negative). The “light damage” condition represented about 9% flooding of the damaged column, while for the “moderate damage” the flooding was equivalent to about 18%. For moderate damage conditions, regular wave studies showed that the motions are essentially nonlinear, although for light damage conditions this cannot be said with certainty. Model tests showed certain asymmetry in the motions of the damage semisubmersible with respect to the position of the damaged column. Moderate damage conditions seemed to produce significant subharmonic response of the vessel in a frequency range which is twice the natural frequency of the vessel in heave. These observations were confirmed from the results of irregular wave studies. Irregular wave studies showed that the quartering sea pitch and roll motions in windward damage conditions are as significant as those in the leeward conditions which was not the case for regular wave studies. The energy due to the motion of the semisubmersible was concentrated in the frequency of 0.7–1.3 Hz, which corresponds to the energetic range of the normal sea state. The natural frequencies of the vessel in damaged condition in pitch, roll and heave are higher than the corresponding frequencies in the intact condition of the vessel. The natural frequency in heave, for both intact and damage conditions, is higher than either those of pitch and roll in similar conditions. These natural frequencies in pitch and roll begin to approach that of the natural frequency in heave as the damage condition increases. This is true irrespective of the position of the damaged column or the sea state. The value of the natural frequency in heave itself increased much more slowly with increase in damage condition. It was inferred that the nonlinear wave pressure term played only a minor role in the asymmetry of motions of the vessel, while the mooring characteristics had a more dominant influence.
TL;DR: In this article, it was shown that a planing plate only has fluid on one side (the high pressure side) of the plate, and that the fluid in contact with the plate travels more slowly as the plate trim angle (and therefore static pressure) increases.
Abstract: Added mass theory has been shown to give excellent agreement with experimental measurements on planing surfaces at normal planing angles [e.g. Payne, P.R. (1982, Ocean Engng 9 , 515–545; 1988, Design of High-Speed Boats, Volume 1: Planning . Fishergate. Inc., Annapolis, Maryland)] and to agree exactly with more complex conformal transformations where such a comparison is possible. But at large trim angles, it predicts non-transient pressures that are greater than the free-stream dynamic pressure and so cannot be correct. In this paper, I suggest that the reason is because, unlike a body or a wing in an infinite fluid, a planing plate only has fluid on one side—the “high pressure” side. So the fluid in contact with the plate travels more slowly as the plate trim angle (and therefore static pressure) increases. This results in lower added mass forces than Munk, M. (1924) The Aerodynamic Forces on Airship Hulls (NACA TR-184) and Jones, R. T. (1946) Properties of Low-Aspect-Ratio Pointed Wings at Speeds Below and Above the Speed of Sound (NACA TR-835) originally calculated for wings and other bodies in an infinite fluid. For simplicity of presentation, I have initially considered the example of a triangular (vertex forward) planning plate. This makes the integration of elemental force very simple and so the various points are made without much trouble. But the penalty is that there seem to be no experimental data for such a configuration; at least none that I have been able to discover. But at least the equations obtained in the limits of zero and infinite aspect ratio, small trim angles (τ) and τ = 90° all agree with established concepts and the variation of normal force with trim angle looks like what we would expect from our knowledge of how delta wings behave in air. I then employed the new equation to calculate the force on a rectangular planing surface at a trim angle τ, having a constant horizontal velocity u o and a vertical impact velocity of z. This happens to have been explored experimentally by Smiley, R. F. [(1951) An Experimental Study of Water Pressure-Distributions During Landings and Planing of a Heavily Loaded Rectangular Flat Plate Model (NACA TM 2453)] up to trim angles of τ = 45°, and so a comparison between theory and experiment is possible. The results of this comparison are encouraging, as is also a comparison with the large trim angle planing plate measurements of Shuford, C. L. [(1958) A Theoretical and Experimental Study of Planing Surfaces Including Effects of Cross Section and Plan Form (NACA Report)]. As two practical applications, I first employed the new equations to calculate the “design pressures” needed to size the plating of a transom bow on a high-speed “Wavestrider” hull. The resulting pressures were significantly different to those obtained using semi-empirical design rules in the literature. Then I used the theory to critically review data obtained from tank tests of a SES bow section during water impact to identify how the “real world” of resilient deck plating diverged from the “model world” of extreme structural rigidity.
TL;DR: In this paper, the in-line and transverse forces on a smooth circular cylinder placed horizontally near a plane boundary are analyzed in the time domain, and the time histories of intantaneous water surface elevation and the corresponding in-lines and transversal forces are Fourier decomposed into various frequency components.
Abstract: The random wave induced in-line and transverse forces on a smooth circular cylinder, placed horizontally near a plane boundary, are analyzed in the time domain. The time histories of intantaneous water surface elevation and the corresponding in-line and transverse forces are Fourier decomposed into various frequency components. The time histories of water particle kinematics for different frequency components are generated using the linear numerical transforms. Each frequency component of the in-line force is correlated with the corresponding wave component, and the in-line hydrodynamic coefficients of inertia and drag are evaluated through the use of the Morison equation and least-squares method. In addition, the in-line root mean square (r.m.s.) coefficients are also obtained. The component transverse forces are analyzed in terms of maximum and r.m.s. transverse force coefficients. All the in-line and transverse force transfer coefficients are correlated with the component Keulegan-Carpenter number or period parameter, gap ratio of the cylinder from the plane boundary and depth parameter. The component force transfer coefficients are also compared with those obtained from regular wave force experiments, and the coefficients obtained from the time domain analysis of the entire length of the instanteneous random water surface elevation and the corresponding force records. It is found that the component force transfer coefficients compare quite well with the coefficients determined from the regular wave force experiments, and show the same trend with all the parameters mentioned above as the regular wave force results under the same cylinder and water depth conditions.
TL;DR: In this paper, the authors describe the design of a seawater supply system installed at the University of Connecticut's Noank laboratory on Long Island Sound, which eliminates biofouling by uninterrupted rapid flow (> 2.5 m/sec).
Abstract: Seawater supply systems serving coastal laboratories typically are affected by two maladies: reduced flow caused by biofouling, and gas supersaturation of the influent stream. The first results in unpredictable supplies and extended maintenance; the second increases the mortality of the experimental animals. We describe the design of a system installed at the University of Connecticut's Noank laboratory on Long Island Sound. Biofouling has been eliminated by uninterrupted rapid flow (> 2.5 m/sec); simple degassing units seem to have solved the gas supersaturation problem. Engineering features of the system are described in detail. Potential problems and future improvements are discussed.
TL;DR: In this article, an autoregressive wave force model is developed which is capable of accounting for localized flow history effects, and the optimal length of the filter and its sensitivity are illustrated using data from small-scale wave tank tests.
Abstract: An autoregressive wave force model is developed which is capable of accounting for localized flow history effects. It was developed in conjunction with the analysis of a series of experiments performed to study the wave-induced forces acting on a free-to-surge vertical cylinder in random waves. The wave force model incorporates a relative motion form of the Morison equation. The formulation presented in this study is quite general, but the filter coefficients in the model must be uniquely determined for each data set. The optimal length of the filter and its sensitivity are illustrated using data from small-scale wave tank tests. A high frequency wave force component observed in the experimental data is reproduced using this model. Lastly, the autoregressive wave force model is used to predict the response of a tension-leg platform to a wave train. A comparison of the results obtained both with and without the filter model are presented.