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1.
The effects of different helical strake coverage on the vortex-induced vibration (VIV) of a model flexible riser were studied experimentally, with the aim of further improving the understanding of VIV responses. Uniform and linearly sheared currents were simulated to study response parameters such as non-dimensional displacement, fatigue damage, suppression efficiency, and the comprehensive evaluation is further studied. Test results of the bare model for a uniform current showed that the behavior of both the standing wave and traveling wave dominated VIV displacement. However, for a linearly sheared current, traveling wave behavior dominated VIV displacement in the high-velocity range. The results of the straked model tests indicated that the response was strongly dependent upon the amount of coverage of helical strakes. The flexible riser with 75% strake coverage gave the best comprehensive evaluation in a uniform current, and 50% strake coverage gave the best comprehensive evaluation in a linearly sheared current. 相似文献
2.
The aim of this paper is to present an analytical expression for the vertical distribution of the correlation between the horizontal (
) and vertical (
) wave velocity components. This quantity,
, which appears explicitly in the time-averaged momentum balance equations, has been shown to play an important role in the vertical distribution of wave-induced currents.The proposed formulation for
is based on an identity that relates the effective (wave) shear stress
to the effective (wave) normal stresses (
2 and
2) and to the vorticity of the oscillatory flow
gw. This general expression has been applied to simplified situations and has been shown to degenerate into other existing formulations with comparable simplifying assumptions, viz. irrotational waves in shallow water over an arbitrary bottom topography and breaking waves over a horizontal bottom.The model has also been applied to the case of waves interacting with a depth-varying current over a horizontal bottom, in which preliminary results have been obtained for a simplified situation invoking linear (small-amplitude) wave theory. 相似文献
3.
A model for the downward transfer of wind momentum is derived for growing waves. It is shown that waves, which grow due to an uneven pressure distribution on the water surface or a wave-coherent surface shear stress have horizontal velocities out of phase with the surface elevation. Further, if the waves grow in the x-direction, while the motion is perhaps time-periodic at any fixed point, the Reynolds stresses associated with the organized motion are positive. This is in agreement with several field and laboratory measurements which were previously unexplained, and the new theory successfully links measured wave growth rates and measured sub-surface Reynolds stresses. Wave coherent air pressure (and/or surface shear stress) is shown to change the speed of wave propagation as well as inducing growth or decay. From air pressure variations that are in phase with the surface elevation, the influence on the waves is simply a phase speed increase. For pressure variations out of phase with surface elevation, both growth (or decay) and phase speed changes occur. The theory is initially developed for long waves, after which the velocity potential and dispersion relation for linear waves in arbitrary depth are given. The model enables a sounder model for the transfer to storm surges or currents of momentum from breaking waves in that it does not rely entirely on ad-hoc turbulent diffusion. Future models of atmosphere-ocean exchanges should also acknowledge that momentum is transferred partly by the organized wave motion, while other species, like heat and gasses, may rely totally on turbulent diffusion. The fact that growing wind waves do in fact not generally obey the dispersion relation for free waves may need to be considered in future wind wave development models. 相似文献
4.
Numerical calculation of forces induced by short-crested waves on a vertical cylinder of arbitrary cross-section 总被引：1，自引：0，他引：1
Most off-shore oil platforms are supported by vertical cylinders extending to the ocean floor. An important problem in off-shore engineering is the calculation of the wave loading exerted on these vertical cylinders. Analytical solutions have been found for the case of plane incident waves incident on a circular cylinder by MacCamy and Fuchs [(1954), Wave forces on piles: a diffraction theory. U.S. Army Corps of Engineering, Beach Erosion Board, Technical Memorandum No. 69] and also for short-crested waves incident on a circular cylinder by Zhu [(1993), Diffraction of short-crested waves around a circular cylinder. Ocean Engng 20, 389–407]. However, for a cylinder of arbitrary cross-section, no analytic solutions currently exist. Au and Brebbia [(1983), Diffraction of water waves for vertical cylinders using boundary elements. Appl. Math. Modelling 7, 106–114] proposed an efficient numerical approach to calculate the wave loads induced by plane waves on vertical cylinders by using the boundary element method. However, wind-generated waves are better modelled by short-crested waves. Whether or not these short-crested waves can induce larger wave forces on a structure is of great concern to ocean engineers. In this paper wave loads, induced by short-crested incident waves, on a vertical cylinder of arbitrary cross-section are discussed. For a cylinder of certain cross-section, the wave loads induced by short-crested waves can be larger than those induced by plane waves with the same total wave number. 相似文献
5.
6.
《Ocean Modelling》2008,20(1):35-60
The generalized Langrangian mean theory provides exact equations for general wave–turbulence–mean flow interactions in three dimensions. For practical applications, these equations must be closed by specifying the wave forcing terms. Here an approximate closure is obtained under the hypotheses of small surface slope, weak horizontal gradients of the water depth and mean current, and weak curvature of the mean current profile. These assumptions yield analytical expressions for the mean momentum and pressure forcing terms that can be expressed in terms of the wave spectrum. A vertical change of coordinate is then applied to obtain glm2z-RANS equations with non-divergent mass transport in cartesian coordinates. To lowest order, agreement is found with Eulerian mean theories, and the present approximation provides an explicit extension of known wave-averaged equations to short-scale variations of the wave field, and vertically varying currents only limited to weak or localized profile curvatures. Further, the underlying exact equations provide a natural framework for extensions to finite wave amplitudes and any realistic situation. The accuracy of the approximations is discussed using comparisons with exact numerical solutions for linear waves over arbitrary bottom slopes, for which the equations are still exact when properly accounting for partial standing waves. For finite amplitude waves it is found that the approximate solutions are probably accurate for ocean mixed layer modelling and shoaling waves, provided that an adequate turbulent closure is designed. However, for surf zone applications the approximations are expected to give only qualitative results due to the large influence of wave nonlinearity on the vertical profiles of wave forcing terms. 相似文献
7.
Johan N. Hartnack 《Ocean Engineering》2000,27(4):831
Small amplitude water waves propagating in a medium with a steady non-uniform current are investigated. The non-uniform current is obtained by up- or downwelling through the horizontal bed. A new locally valid velocity potential correct to the second order is derived describing the combined wave–current motion. From this solution expressions for the local evolution of the wave amplitude and the wave number are extracted. These expressions are compared with the results found using the principle of wave action conservation and the linear dispersion relation, and good agreement is found at small distances compared to the wavelength. Unlike earlier works there is no restriction to deep water. The results valid for deep water are found as a special case of the general solution and agree with the solution found by Longuet-Higgins, M.S. and Stewart, R.W. (1961) The changes in amplitude of short gravity waves on steady non-uniform currents. Journal of Fluid Mechanics, 10(4), 529–549. Furthermore, it is shown that the principle of wave action conservation in fact holds for waves propagating in a medium with a steady non-uniform current maintained by up-/downwelling also on finite depth. 相似文献
8.
The boundary layer characteristics beneath waves transforming on a natural beach are affected by both waves and wave-induced
currents, and their predictability is more difficult and challenging than for those observed over a seabed of uniform depth.
In this research, a first-order boundary layer model is developed to investigate the characteristics of bottom boundary layers
in a wave–current coexisting environment beneath shoaling and breaking waves. The main difference between the present modeling
approach and previous methods is in the mathematical formulation for the mean horizontal pressure gradient term in the governing
equations for the cross-shore wave-induced currents. This term is obtained from the wave-averaged momentum equation, and its
magnitude depends on the balance between the wave excess momentum flux gradient and the hydrostatic pressure gradient due
to spatial variations in the wave field of propagating waves and mean water level fluctuations. A turbulence closure scheme
is used with a modified low Reynolds number k-ε model. The model was validated with two published experimental datasets for normally incident shoaling and breaking waves
over a sloping seabed. For shoaling waves, model results agree well with data for the instantaneous velocity profiles, oscillatory
wave amplitudes, and mean velocity profiles. For breaking waves, a good agreement is obtained between model and data for the
vertical distribution of mean shear stress. In particular, the model reproduced the local onshore mean flow near the bottom
beneath shoaling waves, and the vertically decreasing pattern of mean shear stress beneath breaking waves. These successful
demonstrations for wave–current bottom boundary layers are attributed to a novel formulation of the mean pressure gradient
incorporated in the present model. The proposed new formulation plays an important role in modeling the boundary layer characteristics
beneath shoaling and breaking waves, and ensuring that the present model is applicable to nearshore sediment transport and
morphology evolution. 相似文献
9.
Zai-Jin You 《Ocean Engineering》1996,23(3):225-242
The bed roughness ks and current velocity profiles in the presence of waves with an arbitrary angle θ to currents are studied. It is found that the movable bed roughness is affected by both the wave and the current and only slightly by the angle θ between the wave propagation and the current, and that existing formulae derived in purely oscillatory flows generally fail to predict ks. In the present study, a new formula which takes account the effect of the wave and the current on the bed roughness is suggested to calculate ks in combined wave-current flows. With the present formula, the current profiles calculated by the model of You agree satisfactorily with the laboratory data of van Kampen and Nap and Havinga, and the field measurements of Grant and Williams and Drake et al. 相似文献
10.
The dynamic feature of the Modaomen Estuary (ME) in the Pearl River Delta in southern China has been the subject of extensive research. In previous studies, wave–current interaction (WCI) was often neglected due to its complexity. This study uses a coupled hydrodynamic module TELEMAC-2D and wave propagation module TOMAWAC in the TELEMAC-Mascaret modeling system to quantify the effects of WCI on the hydrodynamics in the ME. The coupled wave and current modeling system was well validated against the field measurements at selected locations. The model results show that WCI varies with the seasonal change in runoff in the ME. The effect of waves on the currents is insignificant during the wet season with a current change of no more than 0.07 m/s; but, in contrast, the currents have a noticeable effect on waves. However, during the dry season, the interactions of waves and currents on each other are found to be equally significant. When wave model and current model are coupled, the velocity could increase up to a maximum of 0.30 m/s and decrease up to a maximum of 0.17 m/s. WCI is greatly affected by the propagation directions of wave and current in both seasons. Generally, wave height decreases and current increases for a following wave and current; wave height increases and current decreases for an opposing wave and current. The effects of waves on currents change with the tide. Changes are larger during neap tide than during spring tide, and stronger during ebb tide than during flood tide. 相似文献
11.
A fully nonlinear Boussinessq-type model with several free coefficients is considered as a departure point. The model is monolayer and low order so as to simplify numerical solvability. The coefficients of the model are here considered functions of the local water depth. In doing so, we allow to improve the dispersive and shoaling properties for narrow banded wave trains in very deep waters. In particular, for monochromatic waves the dispersion and shoaling errors are bounded by ~ 2.8% up to kh = 100, being k the wave number and h the water depth. The proposed model is fully nonlinear in weakly dispersive conditions, so that nonlinear wave decomposition in shallower waters is well reproduced. The model equations are numerically solved using a fourth order scheme and tested against analytical solutions and experimental data. 相似文献
12.
A Boussinesq model for simulating wave and current interaction 总被引：1，自引：0，他引：1
A new formulation of a pair of Boussinesq equations for three-dimensional nonlinear dispersive shallow-water waves is presented. This set of model equations permits spatial and temporal variations of the bottom topography and the presence of uniform currents. The newly derived equations are used to simulate the propagation of cnoidal waves and their interactions with a uniform current in a wave channel. The modified Euler's predictor-corrector algorithm for time advancing and a central difference representation for the space derivatives are applied to the computation of the basic equations. A set of open boundary conditions is developed to effectively transmit the cnoidal waves out of the computational domain. It is found that, as expected, the wave length decreases with an opposing current and increases with a following current. The wave height increases in magnitude with an opposing current and decreases with a following current. The Mach reflection due to oblique cnoidal waves propagating into an open channel with an opposing current is also investigated. Due to the opposing current, the wave patterns are compressed into smaller saddle-like regions in comparison with the Mach reflection without current effect. 相似文献
13.
《Coastal Engineering》1987,11(2):115-129
The continuity equation for mean longshore current velocity, V = gmT sin 2θb, agrees with selected field and laboratory data covering a wide range of conditions. Agreement between continuity equation and data is improved by eliminating those laboratory data which imply deep-water wave crests at angles near or greater than 90 degrees to the shoreline. Agreement between continuity equation and data is further improved by adjusting breaker angles to account for convection of the breaker point by the longshore current. Breaker point convection increases breaker angle by an amount predictable from the analysis developed here. This increase in angle is significant in those laboratory experiments with breaking wave crests at high angles to the shoreline.In the continuity equation, m is bottom slope, T is wave period, and θb is breaker angle, but breaker height does not appear. According to radiation stress theory, mean velocity does depend on breaker height, but only weakly. Consistency between the two approaches would require a dimensionless velocity, Cb/gT, to be relatively constant, which it is. (The same dimensionless velocity appears in the analyses of breaker point convection.) The continuity equation is functionally independent of friction and mixing, in keeping with its derivation from simple conservation of mass considerations. The equation has no adjustable coefficients. The degress of agreement with data and the internal consistency of the analysis suggests that it is a good predictor of mean velocity in ordinary longshore currents. 相似文献
14.
Breaking wave loads on coastal structures depend primarily on the type of wave breaking at the instant of impact. When a wave breaks on a vertical wall with an almost vertical front face called the “perfect breaking”, the greatest impact forces are produced. The correct prediction of impact forces from perfect breaking of waves on seawalls and breakwaters is closely dependent on the accurate determination of their configurations at breaking. The present study is concerned with the determination of the geometrical properties of perfect breaking waves on composite-type breakwaters by employing artificial neural networks. Using a set of laboratory data, the breaker crest height, hb, breaker height, Hb, and water depth in front of the wall, dw, from perfect breaking of waves on composite breakwaters are predicted using the artificial neural network technique and the results are compared with those obtained from linear and multi-linear regression models. The comparisons of the predicted results from the present models with measured data show that the hb, Hb and dw values, which represent the geometry of waves breaking directly on composite breakwaters, can be predicted more accurately by artificial neural networks compared to linear and multi-linear regressions. 相似文献
15.
16.
Graham J.M. Copeland 《Coastal Engineering》1985,9(3):195-219
Numerical models of combined surface gravity wave refraction, diffraction and reflection can be solved conveniently in terms of the water surface displacement, η, and a vertically integrated, wave-induced, water particle velocity, Q. However, the normal formulation for the radiation stress components, expressed in terms of the wave energy, wave number and water depth, is correct only for linear progressive waves.This paper describes a method of calculating the radiation stress for a linear progressive wave plus an arbitrary reflected or back-scattered wave in terms of variables η and Q. The calculations are related to a finite-difference scheme. Correction factors are given which compensate for the errors introduced by the use of finite differentials in the calculation of certain elements of radiation stress.The theory upon which the analytical method is based is only exact for water of uniform depth. However, results are presented which show that the errors are not significant (typical error less than ± 2%) for bed slopes of less than 1:3. 相似文献
17.
Based mainly on TOGA COARE data, that is, the CI''D data from R/V Xiangyanghong No.5 (Pu et al.,1993),the temperature and current data from the Woods Hole mooring and other deep current data, the layered numerical profiles of buoyancy frequency and mean current components are figured out.A numerical method calculating internal wave dispersion relation without background shear current, used by Fliegel and Hunkins (1975),is improved to be fit for the internal wave equation with mean currents and their second derivatives.The dispersion relations and wave functions of the long crested internal wave progressing in any direction can be calculated inveniently by using the improved method.A comparison between the calculated dispersion relation in the paper and the dispersion relation in GM spectral model of ocean internal waves (Garret and Munk, 1972) is performed.It shows that the mean currents are important to the dispersion relation of internal waves in the western equatorial Pacific Ocean and that the currents make the wave progressing co-directional with (against) the currents stretched (shrink).The influence of the mean currents on dispersion relation is much stronger than that of their second derivatives, but that on wave function is less than that of their second derivatives.The influences on wave functions result in the change of vertical wavenumber, that is, making the wave function stretch or shrink.There exists obvious turning depth but no significant critical layer absorption is found. 相似文献
18.
This paper describes a simple method for determining the wavelength of small amplitude waves under laboratory conditions where reflected wave components are present both with and without a mean current flow superimposed. It assumes a locally horizontal bed but requires no a priori assumption concerning the form of the dispersion relation with a coexisting current. Synchronous measurements of the water surface recorded along any straight line are analysed to yield Fourier coefficients at each location. It is then shown that for all practical conditions excluding a perfect standing wave, the average rate of change of wave phase in the chosen direction can be related directly to the component of incident wave number in that direction, irrespective of reflection coefficient or relative current strength. The technique has been applied to regular and bichromatic waves in a flume with an absorbing wave generator, and can also be applied in 3-D wave basins where waves and currents intersect at arbitrary angles. In combined wave–current experiments, by assuming the linear dispersion relation, it is also possible to estimate the effective current velocity. 相似文献
19.
Jin-Bao Song 《Deep Sea Research Part I: Oceanographic Research Papers》2009,56(5):659-671
The response of near-surface current profiles to wind and random surface waves are studied based on the approach of Jenkins [1989. The use of a wave prediction model for driving a near surface current model. Dtsch. Hydrogr. Z. 42, 134–149] and Tang et al. [2007. Observation and modeling of surface currents on the Grand Banks: a study of the wave effects on surface currents. J. Geophys. Res. 112, C10025, doi:10.1029/2006JC004028]. Analytic steady solutions are presented for wave-modified Ekman equations resulting from Stokes drift, wind input and wave dissipation for a depth-independent constant eddy viscosity coefficient and one that varies linearly with depth. The parameters involved in the solutions can be determined by the two-dimensional wavenumber spectrum of ocean waves, wind speed, the Coriolis parameter and the densities of air and water, and the solutions reduce to those of Lewis and Belcher [2004. Time-dependent, coupled, Ekman boundary layer solutions incorporating Stokes drift. Dyn. Atmos. Oceans. 37, 313–351] when only the effects of Stokes drift are included. As illustrative examples, for a fully developed wind-generated sea with different wind speeds, wave-modified current profiles are calculated and compared with the classical Ekman theory and Lewis and Belcher's [2004. Time-dependent, coupled, Ekman boundary layer solutions incorporating Stokes drift. Dyn. Atmos. Oceans 37, 313–351] modification by using the Donelan and Pierson [1987. Radar scattering and equilibrium ranges in wind-generated waves with application to scatterometry. J. Geophys. Res. 92, 4971–5029] wavenumber spectrum, the WAM wave model formulation for wind input energy to waves, and wave energy dissipation converted to currents. Illustrative examples for a fully developed sea and the comparisons between observations and the theoretical predictions demonstrate that the effects of the random surface waves on the classical Ekman current are important, as they change qualitatively the nature of the Ekman layer. But the effects of the wind input and wave dissipation on surface current are small, relative to the impact of the Stokes drift. 相似文献
20.
Numerical Simulation of Spatial Lag Between Wave Breaking Point and Location of Maximum Wave-Induced Current 总被引：2，自引：0，他引：2
A quasi three-dimensional numerical model of wave-driven coastal currents with the effects of surface rollers is developed for the study of the spatial lag between the location of the maximum wave-induced current and the wave breaking point.The governing equations are derived from Navier-Stokes equations and solved by the hybrid method combining the fractional step finite different method in the horizontal plane with a Galerkin finite element method in the vertical direction.The surface rollers effects are considered through incorporating the creation and evolution of the roller area into the free surface shear stress.An energy equation facilitates the computation process which transfers the wave breaking energy dissipation to the surface roller energy.The wave driver model is a phase-averaged wave model based on the wave action balance equation.Two sets of laboratory experiments producing breaking waves that generated longshore currents on a planar beach are used to evaluate the model's performance.The present wave-driven coastal current model with the roller effect in the surface shear stress term can produce satisfactory results by increasing the wave-induced nearshore current velocity inside the surf zone and shifting the location of the maximum longshore current velocity landward. 相似文献