On the behavior of gravity-waves on porous slopes

displacement thickness is lower than in the pure-gas case alone. The results indicate

Short-surface waves radiated by a partially-immersed 3-dimensional oscillating body

The short-surface waves generated by a 3-D arbitrarily oscillating body floating onwater are discussed. In the far-field off the body, the phase and the amplitude functions ofthe radiated waves are determined by the ray method. An undetermined constant is includ-ed in the amplitude function. From the result of Ref. [1], the near-field boundary layersolution near the body waterline is obtained. The amplitude of this solution depends on thewhole wall shape of the body and the slope at the body waterline on the cross-sections per-pendicular to the waterline. By matching the far-field solution with the near-field bound-ary layer solution, the undetermined constant in the amplitude function of the far-fieldradiated waves is determined. For the special case of a half-submerged sphere which per-forms vertical oscillating motion, the result obtained in this paper is in agreement withthat of Ref. [ 2 ].

Two-dimensional short surface-waves of an oscillating cylinder with arbitrary shape of cross-section

The 2-D short surface waves produced by a partially submerged cylinder which performsarbitrary oscillating motion are discussed. The uniformly valid solution which is applicableto all kinds of cylinder wall cases at waterline point is obtained. It is pointed out that thesolution obtained by Holford[J] for the vertical oscillating motion of a cylinder is incomplete.The reason why his solution cannot go over to that for the case of vertical cylinder wall atwaterline point is also pointed out.

Strongly nonlinear long interfacial waves in uniform basic flows and their dispersion relationship

In this paper, long interfacial waves of finite amplitude in uniform basic flows are considered with the assumption that the aspect ratio between wavelength and water depth is small. A new model is derived using the velocities at arbitrary distances from the still water level as the velocity variables instead of the commonly used depth-averaged velocities. This significantly improves the dispersion properties and makes them applicable to a wider range of water depths. Since its derivation requires no assumption on wave amplitude, the model thus can be used to describe waves with arbitrary amplitude.

NONLINEAR GRAVITY-WAVES IN DEEP-WATER

In consideration of the problem on the boundary condition of nonlinear free water wave, coordinate transform is used to handle the free boundary. Supposing the solution form be the traveling wave, the ordinary differential equations of the one-order autonomous system with two variables are caused, then expanding the nonlinear terms at the equilibrium point with the Taylor expansion, we obtained the solution to traveling wave. The linear approximate equation near the equilibrium point is the small amplitude wave. A new nonlinear periodic traveling wave and nonlinear dispersion relation are shown when expanding to the second-order terms. A conclusion that the expansion of dispersion relation does not contain any odd-power terms of wave steepness and because of the nonlinear effort an oscillate structure is produced in the vertical direction is drawn.

Spatial structure of internal tidal waves generated on weak topographies

The generation of internal gravity waves by barotropic tidal flow passing over a two-dimensional topography is investigated. Rather than calculating the conversion of tidal energy, this study focuses on delineating the geometric characteristics of the spatial structure of the resulting internal wave fields (i.e., the configurations of the internal beams and their horizontal projections) which have usually been ignored. it is found that the various possible wave types can be demarcated by three characteristic frequencies: the tidal frequency, wo; the buoyancy frequency, N; and the vertical component of the Coriolis vector or earth's rotation.f. When different possibilities arising from the sequence of these frequencies are considered, there occur 12 kinds of wave structures in the full 3D space in contrast to the 5 kinds identified by the 2D theory. The constant wave phase lines may form as ellipses or hyperbolic lines on the horizontal plane, provided the buoyancy frequency is greater or less than the tidal frequency. The effect that stems from the consideration of the basic flow is also found, which not only serves as the reason for the occurrence of higtter harmonics but also increases the wave strength in the direction of basic flow. (C) 2009 Elsevier B.V. All rights reserved.

Determination of fractional energy loss of waves in nearshore waters using an improved high-order Boussinesq-type model

Fractional energy losses of waves due to wave breaking when passing over a submerged bar are studied systematically using a modified numerical code that is based on the high-order Boussinesq-type equations. The model is first tested by the additional experimental data, and the model's capability of simulating the wave transformation over both gentle slope and steep slope is demonstrated. Then, the model's breaking index is replaced and tested. The new breaking index, which is optimized from the several breaking indices, is not sensitive to the spatial grid length and includes the bottom slopes. Numerical tests show that the modified model with the new breaking index is more stable and efficient for the shallow-water wave breaking. Finally, the modified model is used to study the fractional energy losses for the regular waves propagating and breaking over a submerged bar. Our results have revealed that how the nonlinearity and the dispersion of the incident waves as well as the dimensionless bar height (normalized by water depth) dominate the fractional energy losses. It is also found that the bar slope (limited to gentle slopes that less than 1:10) and the dimensionless bar length (normalized by incident wave length) have negligible effects on the fractional energy losses.

中高层大气对低层大气声重波响应的分析

Acoustic Gravity waves (AGW) play an important role in balancing the atmospheric energy and momentum budget. Propagation of gravity wave in the atmosphere is one of the important factors of changing middle and upper atmosphere and ionosphere. The purpose of this dissertation is to study the propagation of gravity wave in a compression atmosphere whit means of numerical simulation and to analyze the response of middle and upper atmosphere to pulse disturbance from lower atmosphere. This work begins with the establishment of 2-D fully nonlinear compressible atmospheric dynamic model in polar coordinate, which is used ton numerically study gravity wave propagation. Then the propagation characteristics of acoustic gravity wave packets are investigated and discussed. We also simulate the response of middle and upper atmosphere to pulse disturbance of lower atmosphere in background winds or without background winds by using this model and analyze the data we obtained by using Fourier Transform (FT), Short-time Fourier Transform (STFT) and Empirical Mode Decomposition (EMD) method which is an important part of Hilbert-Huang Transform (HHT). The research content is summarized in the following: 1. By using a two-dimensional full-implicit-continuous-Eulerian (FICE) scheme and taking the atmospheric basic motion equations as the governing equations, a numerical model for nonlinear propagation of acoustic gravity wave disturbance in two-dimensional polar coordinates is solved. 2. Then the propagation characteristics of acoustic gravity wave packets are investigated and discussed. Results of numerical simulation show that the acoustic gravity wave packets propagate steadily upward and keep its shape well after several periods. 3. We simulate the response of middle and upper atmosphere to pulse disturbance of lower atmosphere in background winds or without background winds by using this model, and obtain the distribution of a certain physical quantity in time and space from earth’s surface to 300km above. The results reveal that the response of ionosphere occurs at a large horizontal distance from the source and the disturbance becomes greater with increasing of height. The situation when the direction of the background wind is opposite to or the same as the direction of disturbed velocity of gravity-wave is studied. The results show that gravity wave propagating against winds is easier than those propagating along winds and the background wind can accelerate gravity wave propagation. Just upon the source, an acoustic wave component with period of 6 min can be found. These images of simulation are similar to observations of the total electron content (TEC) disturbances caused by the great Sumatra-Andaman earthquake on December 26 in 2004. 4. Using the EMD method the disturbed velocity data of a certain physical quantity in time and space can be decomposed into a series of intrinsic mode function (IMF) and a trend mode respectively. The results of EMD reveal impact of the gravity wave frequency under the background winds.

Air flow over a mountain and the convective boundary-layer

In this paper, the analytical model coupling the convective boundary layer (CBL) with the free atmosphere developed by Qi and Fu (1992) is improved. And by this improved model, the interaction between airflow over a mountain and the CBL is further discussed. The conclusions demonstrate: (1) The perturbation potential temperatures in the free atmosphere can counteract the effect of orographic thermal forcing through entraining and mixing in the CBL. If u(M)BAR > u(F)BAR, the feedback of the perturbation potential temperatures in the free atmosphere is more important than orographic thermal forcing, which promotes the effect of interfacial waves. If u(M)BAR < u(F)BAR, orographic thermal forcing is more important, which makes the interfacial height and the topographic height identical in phase, and the horizontal speeds are a maximum at the top of the mountain. (2) The internal gravity waves propagating vertically in the free atmosphere cause a strong downslope wind to become established above the lee slope in the CBL and result in the hydraulic jump at the top of the CBL. (3) With the CBL deepening, the interfacial gravity waves induced by the potential temperature jump at the top of the CBL cause the airflow in the CBL to be subcritical.

Application of the characteristic CIP method to a shallow water model on the sphere

Semi-implicit algorithms are popularly used to deal with the gravitational term in numerical models. In this paper, we adopt the method of characteristics to compute the solutions for gravity waves on a sphere directly using a semi-Lagrangian advection scheme instead of the semi-implicit method in a shallow water model, to avoid expensive matrix inversions. Adoption of the semi-Lagrangian scheme renders the numerical model always stable for any Courant number, and which saves CPU time. To illustrate the efficiency of the characteristic constrained interpolation profile (CIP) method, some numerical results are shown for idealized test cases on a sphere in the Yin-Yang grid system.

Ocean wave imaging mechanism by imaging radar

Analytical representations of the high frequency spectra of ocean wave and its variation due to the variation of ocean surface current are derived from the wave-number spectrum balance equation. The ocean surface imaging formulation of real aperture radar (RAR) is given using electromagnetic wave backscattering theory of ocean surface and the modulations of ocean surface winds, currents and their variations to RAR are described. A general representation of the phase modulation induced by the ocean surface motion is derived according to standard synthetic aperture radar (SAR) imaging theory. The detectability of ocean current and sea bottom topography by imaging radar is discussed. The results constitute the theoretical basis for detecting ocean wave fields, ocean surface winds, ocean surface current fields, sea bottom topography, internal wave and so on.

Effect of Stokes drift on upper ocean mixing

Stokes drift is the main source of vertical vorticity in the ocean mixed layer. In the ways of Coriolis - Stokes forcing and Langmuir circulations, Stokes drift can substantially affect the whole mixed layer. A modified Mellor-Yamada 2.5 level turbulence closure model is used to parameterize its effect on upper ocean mixing conventionally. Results show that comparing surface heating with wave breaking, Stokes drift plays the most important role in the entire ocean mixed layer, especially in the subsurface layer. As expected, Stokes drift elevates both the dissipation rate and the turbulence energy in the upper ocean mixing. Also, influence of the surface heating, wave breaking and wind speed on Stokes drift is investigated respectively. Research shows that it is significant and important to assessing the Stokes drift into ocean mixed layer studying. The laboratory observations are supporting numerical experiments quantitatively.

澳大利亚阿德莱德地区大气重力波的气辉观测研究

气辉是研究中高层大气及电离层高度的重力波的传播特性的一种重要手段。本文利用澳大利亚阿德莱德地区一个三镜头气辉仪7年的OH和OI气辉观测数据，结合同一地区中频雷达所测日平均风场数据，采用最大熵谱分析方法，分析研究了该地区上空中层顶区域重力波的统计传播特性，并研究了重力波在中层顶区域的垂直传播特征。结果表明: （1）在中层顶气辉观测区，通过OH和OI分别探测到49个和28个周期大于40分钟的重力波事件。统计结果表明，在OH和OI两个高度上观测到的较大尺度重力波主要集中在周期为68分钟，水平相速度为36米/秒。同时这些重力波接近水平方向传播，具有东南向的优势传播方向。OH仰角平均为10度，OI为8度。表明传播到中层顶气辉层区域的这些重力波可能产生于相距观测点很远的地方，并且受到背景风场的方向滤波和拉伸的作用。 （2）在1999年1月12日晚上的气辉观测中， 我们通过OH和OI气辉在两个不同高度上观测到了同一次重力波事件，根据两个高度上观测到的扰动时序相位差， 我们判断该重力波为上行波。然而，我们所得到的大部分事例中，符合在两个高度上观测到的重力波事例之间具有明显相关性的极少。分析表明，由于该气辉仪及其观测方法本身存在固有的误差，同时也因为重力波在中层顶高度受到强烈的风剪切作用而导致传播相位的改变，使得我们很少能在两个气辉探测高度上观测到同一个重力波事件。

电离层行进式扰动的GPS台网监测研究

In this dissertation, we investigated two types of traveling ionospheric disturbances (TIDs)/gravity waves (GWs) triggered separately by auroral energy input during super geomagnetic storms and solar terminator (ST) under quiet geomagnetic conditions (kp<3+) using TEC measurements from the global network of GPS receivers. Research into the generation and propagation of TIDs/GWs during storms greatly enhance our understandings on the evolution processes of energy transportation from the high-latitude’s magnetosphere to the low-latitude ionosphere and the conjugated effect of TIDs propagation between the northern and southern hemispheres. Our results revealed that the conjugacy of propagation direction between the northern and southern hemispheres was subject to the influence of Coriolis force. We also figure out the evolution processes of ionospheric disturbances at the global scale. These are important topics that had not been well addressed previously. In addition, we also obtained thee wave structures of medium scale TIDs excited by the solar terminator (ST) moving over the northern America and physical mechanisms involved. Our observations confirm that the ST is a stable and repetitive source of ionospheric wave disturbances and the evidence of solar terminator generated disturbances has been demonstrated experimentally via the GPS TEC measurement. The main researches and results of this dissertation are as follows. First, the global traveling ionospheric disturbances (TIDs) during the drastic magnetic storms of October 29–31, 2003 were analyzed using the Global Position System (GPS) total electron content (TEC) data observed in the Asian-Australian, European and North American sectors. We collected the most comprehensive set of the TEC data from more than 900 GPS stations on the International GNSS Services (IGS) website and introduce here a strategy that combines polynomial fitting and multi-channel maximum entropy spectral analysis to obtain TID parameters. Moreover, in collaboration with my thesis advisor, I have developed an imaging technique of 2-dimensional map of TIDs structures to obtain spatial and temporal maps of large scale traveling ionospheric disturbances (LSTIDs). The clear structures of TEC perturbations map during the passage of TIDs were displayed. The results of our study are summarized as follows: (1) Large-scale TIDs (LSTIDs) and medium-scale TIDs (MSTIDs) were detected in all three sectors after the sudden commencement (SC) of the magnetic storm, and their features showed longitudinal and latitudinal dependences. The duration of TIDs was longer at higher latitudes than at middle latitudes, with a maximum of about 16 h. The TEC variation amplitude of LSTIDs was larger in the North American sector than in the two other sectors. At the lower latitudes, the ionospheric perturbations were more complicated, and their duration and amplitude were relatively longer and larger. (2) The periods and phase speeds of TIDs were different in these three sectors. In Europe, the TIDs propagated southward; in North America and Asia, the TIDs propagated southwestward; in the near-equator region, the disturbances propagated with the azimuth (the angle of the propagation direction of the LSTIDs measured clockwise from due north with 0°) of 210° showing the influence of Coriolis force; in the Southern Hemisphere, the LSTIDs propagated conjugatedly northwestward. Both the southwestward and northeastward propagating LSTIDs are found in the equatorial region. These results mean that the Coriolis effect cannot be ignored for the wave propagation of LSTIDs and that the propagation direction is correlated with the polar magnetic activity. (3) The day (day of year: 301) before the SC (sudden commencement) of magnetic storm, we observed a sudden TEC skip disturbances (±10 TECU). It should be a response for the high flux of proton during the solar flare event, but not the magnetic storms. Next, the most comprehensive and dense GPS network’s data from North-America region were used in this paper to analyze the medium scale traveling ionospheric disturbances (MSTIDs) which were generated by the moving solar terminator during the quiet days in 2005. We applied the multi-channel maximum entropy spectral analysis to calculated TID parameters, and found that the occurrence of ST-MSTIDs depends on the seasonal variations. The results of our study are summarized as follows: (1) MSTIDs stimulated by the moving ST (ST-MSTIDs) are detected at mid-latitudes after the passage of the solar terminator with the life time of 2～3 hours and the variation amplitude of 0.2～0.8 TECU. Spectral analysis indicated that the horizontal wavelength, average period, horizontal phase velocity of the MSTIDs are around 300±150 km，150±80 m/s and 25±15 min, respectively. In addition, ST-MSTIDs have wave fronts elongating the moving ST direction and almost parallel to ST. (2) The statistical results demonstrate that the dusk MSTIDs stimulated by ST is more obvious than the dawn MSTIDs in summer. On the contrary, the more-pronounced dawn MSTIDs occurs in winter. (3) Further analysis indicates that the seasonal variations of ST-MSTIDs occurrence frequency are most probably related to the seasonal differences of the variations of EUV flux in the ionosphere region and recombination process during sunrise and sunset period at mid-latitudes. Statistical study of occurrence characteristics of TIDs using the GPS network in North-American and European during solar maximum, In conclusion, statistical studies of the propagation characteristics of TIDs, which excited by the two common origins including geomagnetic storms and moving solar terminator, were involved with global GPS TEC databasein this thesis. We employed the multichannel maximum entropy spectral analysis method to diagnose the characteristics of propagation and evolvement of ionospheric disturbances, also, the characteristics of their regional distribution and climatological variations were revealed by the statistic analysis. The results of these studies can improve our knowledge about the energy transfer in the solar-terrestrial system and the coupling process between upper and lower atmosphere (thermosphere-ionosphere-mesosphere). On the other hand, our results of the investigation on TIDs generated by particular linear origin such as ST are important for developing ionospheric irregularity physics and modeling the transionosphere radio wave propagation. Besides, the GPS TEC representation of the ST-generated ionospheric structure suggests a better possibility for investigating this phenomenon. Subsequently, there are scientific meaning of the result of this dissertation to deeply discuss the energy transfer and coupling in the ionosphere, as well as realistic value to space weather forecast in the ionosphere region.

Short-Term Nonlinear Response of Tension Leg Platform in Random Sea Waves

Most of the existing mathematical models for analyzing the dynamic response of TLP are based on explicit or implicit assumptions that motions (translations and rotations) are small magnitude. However, when TLP works in severe adverse conditions, the a priori assumption on small displacements may be inadequate. In such situation, the motions should be regarded as finite magnitude. This paper will study stochastic nonlinear dynamic responses of TLP with finite displacements in random waves. The nonlinearities considered are: large amplitude motions, coupling the six degrees-of-freedom, instantaneous position, instantaneous wet surface, free surface effects and viscous drag force. The nonlinear dynamic responses are calculated by using numerical integration procedure in the time domain. After the time histories of the dynamic responses are obtained, we carry out cycle counting of the stress histories of the tethers with rain-flow counting method to get the stress range distribution.