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1.
Phase control may substantially increase the power absorption in point-absorber wave energy converters. This study deals with validation of dynamic models and latching control algorithms for an oscillating water column (OWC) inside a fixed vertical tube of small circular cross-section by small-scale testing. The paper describes experimental and numerical results for the system's dynamics, using simple and practical latching control techniques that do not require the prediction of waves or wave forces, and which will be relevant to any type of point-absorbing devices.In the experimental set-up, the upper end of the tube was equipped with an outlet duct and a shut-off valve, which could be controlled to give a latching of the inner free surface movement. The pressure drop through the open valve is used as a simplified measure of the energy extraction. The control was realized by using the real-time measurement signals for the inner and outer surface displacement.A mathematical model of the system was established and applied in numerical simulation. In the case the OWC's diameter is much smaller than the wavelength and the wave amplitude much smaller than the draft, the free surface movement inside the tube can be described as an oscillating weightless piston. For this hydrodynamic problem an analytical solution is known. In addition, the mathematical model includes the effects of viscous flow losses, the air compressibility inside the chamber and the pressure drop across the valve. Experimental results were used to calibrate some of the model parameters, and the total model was formulated as a coupled system of six non-linear, first-order differential equations. Time-domain integration was used to simulate the system in order to test the control strategies and compare with experimental results. 相似文献
2.
Oscillating Water Column (OWC) is one of the pioneer devices in harnessing wave energy; however, it is not fully commercialized perhaps due to the complicated hydrodynamic behavior. Previous studies are significantly devoted to OWC devices located in nearshore and coastal regions where incident wave energy would experience dissipation more than offshore. In this paper, a 1:15 scaled fixed offshore OWC model is tested in a large towing tank of National Iranian Marine Laboratory. Wave spectrum shape effect on the efficiency of the OWC model is addressed. Moreover, the paper investigates the effects of the geometric and hydrodynamic factors on OWC device efficiency and uncovers new points in nonlinear interaction occurring inside the chamber; i.e. sloshing. The results indicate that shape of the spectrum inside the chamber is affected by the type of incident wave spectrum, especially for long waves. Pierson–Moskowitz spectrum leaded to higher efficiency rather than JONSWAP spectrum at longer incident wave periods. According to efficiency analysis, increasing wave height may lead to air leakage from the chamber followed by vortex generation, which is a reason for decreasing the efficiency of the OWC device. Furthermore, no shift in the resonant period of the OWC model, due to wave height increase, was observed at the opening ratios equal or smaller than 1.28%. Spectral analysis of water fluctuation inside the OWC chamber illustrates two modes of sloshing. The first mode can be seen at short period waves while the second mode is visible at long period waves. The sloshing modes approximately vanish by increasing draft value. 相似文献
3.
P. Boccotti 《Ocean Engineering》1996,23(7):629-648
A long submerged horizontal circular cylinder of .90 m diameter was assembled off the beach at Reggio Calabria where the wind waves typically have significant height ranging within 0.20 and 0.40 m and dominant period within 1.8 and 2.6 s. Three ultrasonic probes recorded the waves, and two sets of pressure transducers, the first one at the cylinder and the second one in the undisturbed wave field, enabled to compare the force amplitude on the cylinder to the force amplitude on an equivalent mass of water in the undisturbed wave field (Froude-Krylov F-K force). After ten days of measurements, the experiment was repeated with a cylinder of .45 m diameter. The Keulegan-Carpenter number was within 2.5, and the wave forces proved to be inertial. The following general features emerged: (i) the force spectrum is usually very narrow even if the wave spectrum is broad; (ii) the vertical diffraction coefficient is somewhat smaller than the horizontal diffraction coefficient; (iii) the positive extremes of Fz (vertical force referred to the buoyancy force) markedly exceed the negative extremes; (iv) the pressure fluctuations induced by the highest waves at the cylinder are very similar to the measured pressure-surface displacement covariances. In each of the 580 records obtained in the course of the experiment it was found that the propagation speed reduces to about a half at the cylinder, and the amplitude of the pressure fluctuations increases of 10–15% at the upper half of the cylinder and decreases of about the same percentage at the lower half. These phenomena fully explain why the force amplitude on the cylinder is larger than the F-K force amplitude. 相似文献
4.
The conditions under which the interaction between internal waves and the rough bottom topography may be the reason for synchronous fluctuations of pressure at the bottom have been studied. Disturbances of the depth are assumed to be small compared with the mean depth of the ocean, and the Väisälä-Brunt frequency is constant. It is shown that synchronous fluctuations of pressure exist with a frequency equal to that of internal waves. The amplitude of pulsations can be approximately an order lower than the amplitude of fluctuations generated by standing surface waves. However, local maxima can exist in the low-frequency minimum of the spectrum of microseisms (in the range of 20–1000 s).Translated by Mikhail M. Trufanov. 相似文献
5.
The motion and the drift force of a floating OWC (oscillating water column) wave energy device in regular waves are studied taking account of the oscillating surface-pressure due to the pressure drop across the duct of the air chamber. The potential problem inside the chamber is formulated by making use of the Green integral equation associated with the Rankine-type Green function while the outer problem with the Kelvin-type Green function. The added mass, wave damping and excitation coefficients as well as the motion and drift force of the OWC device are calculated for various values of parameter related to the pressure drop. 相似文献
6.
In order to study the effects of coastline on wave power absorption, we describe here a linearized theory of an oscillating water column (OWC) installed on a straight coast. The sea depth is assumed to be constant and the coast is a vertical cliff. The column is a vertical circular cylinder half embedded in the cliff and open on the seaside. Forced by incident waves from any direction, the water surface inside pushes the dry air above through a Wells turbine system to generate power. Carrying out the linearized theories of radiation and diffraction analytically, we calculate the coefficients of apparent mass and radiation damping, and the chamber pressure. Optimum absorption efficiency is examined under the constraint of constant chamber volume. Results are compared with a parallel study of an OWC installed either offshore or at the tip of a thin breakwater. 相似文献
7.
完全非线性孤立波的直墙反射 总被引：2，自引：2，他引：0
报道了应用边界积分方法模拟完全非线性孤立波的传播与直墙反射,给出了波形演变过程。结果表明,本模型对计算孤立波的传播与直墙反射是有效的。三阶Boussinesq方程的孤立波解比低阶方程的孤立波解更接近完全非线性的数值解.当来波波高增大时,孤立波直墙反射的相位滞后变小。若考虑大波高孤立波的直墙反射或波——波相互作用,一阶理论预报的相位滞后往往低估实际情况。 相似文献
8.
The 1:50 scale model of a big offshore gravity platform was placed off the beach at Reggio-Calabria, where the wind waves typically have significant height ranging from 0.20 to 0.40 m and peak period from 1.8 to 2.6 sec. A first set of pressure transducers was assembled at the boundary of the platform and a second set, with the same configuration as the first one, was assembled in the undisturbed wave field, in order to compare the force amplitude on the platform to the force amplitude on an ideal equivalent mass of water (Froude-Krylov force amplitude). Along with the pressure transducers, a few wave gauges were functioning in the undisturbed wave field. It was found that the time histories of the pressure induced by the highest waves, at the columns and at the platform base, were very similar to the measured covariances of the pressure fluctuations. The statistical distribution of the forces on the platform proved to be coincident with the statistical distribution of the Froude-Krylov forces. The spectrum of the force process proved always to be narrow even in the case of the broad wave spectrum. The diffraction coefficient (ratio of the actual force amplitude to the Froude-Krylov force amplitude) was found to depend essentially on a marked reduction of the propagation speed of the pressure head waves, both at the columns and at the platform base. 相似文献
9.
You Yage
YuZhi Associated Professor Guangzhou Institute of Energy Conversion Chinese Academy of Sciences Guangzhou
. Professor Guangzhou Institute of Energy Conversion Chinese Academy of Sciences Guangzhou 《中国海洋工程》1995,(4)
In this paper, the extreme wave loads on an on-shore wave power device are investigated. First, boundary element method is applied to solve the three dimensional potential problem based on the small amplitude wave assumption. Then the motion of the Oscillating Water Column (OWC) inside the device and its laods on the device are calculated in time domain. Several protective techniques often applied are simulated by changing the constraint of the upper end of the chamber of the device. Numerical results are used to judge the effectiveness of these techniques. The investigation shows that damping can not effectively restrain the motion of OWC when the period of incident wave is long, which may cause dangerous loads on the structure. The shut chamber can effectively restrain the motion of OWC, but alternatively cause high pressure in the chamber. A Contracting opening with a Taper (CT) can exhaust a great amount of kinetic energy of OWC, and significantly decrease the loads. It is a promising protective tec 相似文献
10.
11.
Understanding the hydrodynamic interactions between ocean waves and the oscillating water column (OWC) wave energy converter is crucial for improving the device performance. Most previous relevant studies have focused on testing onshore and offshore OWCs using 2D models and wave flumes. Conversely, this paper provides experimental results for a 3D offshore stationary OWC device subjected to regular waves of different heights and periods under a constant power take–off (PTO) damping simulated by an orifice plate of fixed diameter. In addition, a 3D computational fluid dynamics (CFD) model based on the RANS equations and volume of fluid (VOF) surface capturing scheme was developed and validated against the experimental data. Following the validation stage, an extensive campaign of computational tests was performed to (1) discover the impact of testing such an offshore OWC in a 2D domain or a wave flume on device efficiency and (2) investigate the correlation between the incoming wave height and the OWC front wall draught for a maximum efficiency via testing several front lip draughts for two different rear lip draughts under two wave heights and a constant PTO damping. It is found that the 2D and wave flume modelling of an offshore OWC significantly overestimate the overall power extraction efficiency, especially for wave frequencies higher than the chamber resonant frequency. Furthermore, a front lip submergence equal to the wave amplitude affords maximum efficiency whilst preventing air leakage, hence it is recommended that the front lip draught is minimized. 相似文献
12.
海洋蒸发波导是重要的军事敏感要素,严重影响着海上电磁波的传播。风向、风速是决定蒸发波导强弱的重要因素,为精确模拟蒸发波导条件下的电磁波传播,准确评估预报雷达、通信等电子设备的性能,文中利用抛物线方程模型,结合粗糙度参数和蒸发波导条件下大气修正折射指数廓线模型来求解电磁波波动方程,基于电磁波传播的数值模拟结果,重点研究蒸发波导条件下风速和风向对电磁波传播的影响。并得到如下结论:在同一传播距离处,风速越大,海面粗糙度越大,海面电磁波传播损耗就越大;当电磁波传播方向与风向夹角为0°时,其传播路径损耗远远大于在45°或90°条件下,即顺风条件下电磁波传播损耗大于侧风。 相似文献
13.
《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
14.
Free inertia-gravity internal waves are considered in a two-dimensional vertically nonuniform flow in the Boussinesq approximation. The equation for vertical velocity amplitude includes complex factors caused by the gradient of the flow velocity component transverse to the wave-propagation direction; therefore, the eigenfunction and wave frequency are complex. It is shown that the decrement of damping (imaginary correction to the frequency) of 15-min internal waves is two orders of magnitude smaller than the wave frequency; i.e., the waves weakly damp. Vertical wave fluxes of heat and salt are nonzero due to the phase shift between fluctuations of the vertical velocity and temperature (salinity) different from π 2. The vertical component of the Stokes drift speed is also nonzero and contributed into the vertical transport. 相似文献
15.
An Oscillating Water Column (OWC) device can output energy through reciprocating or unidirectional airflow. The unidirectional airflow is helpful to utilize a simple and high-efficiency unidirectional air turbine. The pentagonal BBDB proposed by us based on OWC principle can be regarded as a floating Oscillating Body and its Power Take-Off (PTO) consists of a chamber, a water column, a turbine and a generator. The Capture Width Ratio (CWR) of the pentagonal BBDB model with the reciprocating and unidirectional airflow was studied in this paper. The wave flume test results indicate the mean CWR of the pentagonal BBDB model with reciprocating airflow can reach up to 121.91% and the mean CWR of the model with unidirectional airflow could reach 100.94% during the whole wave cycle in regular waves. For irregular waves, the mean CWR of the model with the unidirectional airflow is as high as 62.83% during the whole wave cycle. Hopefully, the combination of the pentagonal BBDB with the check valve to output power during the air exhalation and conventional high-efficiency unidirectional turbine will improve the total efficiency of the BBDB. 相似文献
16.
Current records obtained in the inshore region along the Fukushima coast are analyzed. The existence of periodical current fluctuations whose period is about 100 hours and whose amplitude is as large as 15–25cm s–1 is recognized. Auto-spectral analyses are made also for sea level, atmospheric pressure and wind records. Each spectrum has significant peaks at the similar period to the current spectrum. The wind spectrum has a broad peak compared with the current. The periodical current fluctuations propagate southward with speed of 3–5 km h–1. These propagation speeds seem to correspond to those of the second-and third-mode shelf waves. 相似文献
17.
Noriyuki Iwata 《Journal of Oceanography》1971,27(3):102-108
Two-dimensional ocean wave spectrum developing under the atmospheric surface pressure fluctuations is linearly correlated with that of wind pressure itself, so that angular distribution of energy of ocean surface waves can be determined by directional properties of surface pressure fluctuations with the same frequency to the surface wave.From empirically determined spectral formula of the atmospheric surface pressure fluctuations the coefficients of Fourier series expanded around mean direction of wind are analytically integrated, from which r.m.s. angular distribution, spectral peakedness and long-crestedness are calculated, compared with previously proposed empirical formulae and observations carried out by ultrasonic current meter. 相似文献
18.
A total variation diminishing Lax–Wendroff scheme has been applied to numerically solve the Boussinesq-type equations. The runup processes on a vertical wall and on a uniform slope by various waves, including solitary waves, leading-depression N-waves and leading-elevation N-waves, have been investigated using the developed numerical model. The results agree well with the runup laws derived analytically by other researchers for non-breaking waves. The predictions with respect to breaking solitary waves generally follow the empirical runup relationship established from laboratory experiments, although some degree of over-prediction on the runup heights has been manifested. Such an over-prediction can be attributed to the exaggeration of the short waves in the front of the breaking waves. The study revealed that the leading-depression N-wave produced a higher runup than the solitary wave of the same amplitude, whereas the leading-elevation N-wave produced a slightly lower runup than the solitary wave of the same amplitude. For the runup on a vertical wall, this trend becomes prominent when the wave height-to-depth ratio exceeds 0.01. For the runup on a slope, this trend is prominent before the strong wave breaking occurs. 相似文献
19.
Kuniaki Okuda 《Journal of Oceanography》1983,38(6):331-338
For wind waves generated in a wind-wave tunnel, the surface pressure and also the pressure distribution along the internal streamlines were calculated from the measured internal velocity field. In distinct waves, with wave height comparable with or larger than the mean, the surface pressure is found to vary drastically in a narrow region around the crest, showing a dominant minimum near the crest. On the other hand, the pressure distribution along the streamline shows systematic variations that are nearly in phase with the streamline profile. It is shown that the occurrence of the pressure in phase with the streamline profile is linked with the internal vorticity distribution, especially with the presence of a high vorticity region below the crest described in Part I of this study. As a result of the occurrence of such pressure variations, the dispersion relation is modified by about 10% from that for linear irrotational waves. It is argued from the present measurements that the dispersion relation and also the energy transfer from wind into wind waves are strongly affected by the internal vortical structure so that the assumption of irrotational gravity waves cannot be applied to the wind waves being studied. 相似文献