Laboratory testing of mathematical models for high-concentration fluid mud turbidity currents

The propagation characteristics of fluid mud turbidity currents were investigated experimentally and theoretically. Parameterizations for propagation phase transition times from slumping to self-similar and self-similar to viscous phases are proposed. Predictive capabilities of different mathematical models that fall into three different modeling approaches (force-balance, box, shallow water) were evaluated for fluid mud turbidity current propagation using our experimental observations. For the slumping and self-similar phases, the box and force-balance models showed superior predictive capabilities than the one-layer shallow water models with deep ambient condition. Fluid mud turbidity currents have a non-Newtonian rheology and their transition and propagation characteristics in the viscous phase differ vastly from the Newtonian currents. We derived and presented a viscous force-balance expression for the propagation of a non-Newtonian power-law fluid current. We then experimentally evaluated the predictive capability of this force-balance and the viscous shallow water model by Di Federico et al. (2006). Both models' predictions are observed to be in notably good agreement with the experimental data. The results of this study are expected to be useful for preliminary swift calculations of the fluid mud turbidity current propagation characteristics as well as in deciding whether more detailed calculations at the expense of complexity are required.     

Coastal current on the eastern shelf of Hidaka Bay

In order to examine seasonal variation in a coastal current and the dynamics of the current, we carried out a mooring current measurement near the coast on the eastern shelf of Hidaka Bay from December 2002 to July 2003. There seemed to be two current regimes during the observed period; one a southeastward current from December to March, and the other a northwestward current after April. Arrested topographic wave dynamics was used to understand along-shore steady momentum balance at the mooring site. It was found that the friction term was negligible during the former regime, that is, the wind stress term roughly counterbalanced the pressure gradient term. On the other hand, the contribution of each term to the momentum balance was sensitive to the resistance coefficient value during the latter regime. A numerical study showed that wind forcing alone could not reproduce the observed current velocity and momentum balance during the former regime. One possible interpretation of the observed results is superposition of the arrested topographic waves forced by the along-shore wind stress and downstream extension of the Coastal Oyashio. Numerical experiments using combined forcing of the wind stress and an inflow associated with the Coastal Oyashio supported the expected dynamics.     

不同模态下气候变化对智利竹筴鱼补充量的影响

智利竹筴鱼(Trachurus murphyi)是东南太平洋重要的经济鱼类之一,其资源量受补充量影响明显,了解补充量状况对智利竹筴鱼资源可持续利用和科学管理具有重要意义。本文基于模态分析将1971?2017年间智利竹筴鱼补充量划分不同模态,运用贝叶斯模型平均法,分析海表面温度、海表面盐度、海表面高度、厄尔尼诺和太平洋年代际振荡5个环境因子在不同模态中对补充量的解释能力,并探讨模态变动对补充量预测的影响。结果表明,第1模态(1971?1980年)更多的受捕捞因素的干扰;第2模态(1981?1990年)厄尔尼诺对补充量变动的解释概率最高;第3模态(1991?2001年)解释概率最高的环境因子为太平洋年代际振荡;第4模态(2002?2015年)厄尔尼诺为解释概率最高的环境因子。对比不考虑模态变动的分析结果,两者存在明显差异,基于不同模态的分析结果对智利竹筴鱼补充量变动的解释更为合理。研究认为,智利竹筴鱼补充量变动受到多个环境因子的影响,在不同模态时期起主导作用的环境因子也不同,推测年代际太平洋年代际振荡冷暖期交替与厄尔尼诺现象可能是诱发智利竹筴鱼补充量发生模态转变的重要因素。建议在未来智利竹筴鱼资源评估与管理中,应该考虑不同的模态变化及其影响因子。     

Bedform evolution around a submarine pipeline and its effects on wave-induced forces under regular waves

The paper presents an experimental investigation of seabed evolution behavior around a submarine pipeline and the hydrodynamic forces on the pipeline under regular waves. Unlike the previous flume tests that have largely used beds with median sands, this study focuses on fine sediments such as sandy silt and silt. The primary objective of the study was to investigate: (i) the scour process under different wave conditions and with different sediments and (ii) the influence of the bedform evolution on the hydrodynamic forces experienced by the pipeline. In terms of scour and ripple formation, four distinct regimes of the near-field bed evolution behavior are identified which are: (I) no scour, (II) scour without ripples, (III) scour with small ripples and (IV) scour with large ripples. The influence of bedform evolution on wave forces was found to vary significantly in different regimes. In regime I, the wave forces were quite stable; in regime II and III, the wave forces underwent a gradual reduction before reaching their equilibrium values at fairly early stages of the scour process; in regime IV, the wave forces were significantly affected by the migrating ripples and can be rather unsteady throughout the testing period.     

Hydrodynamics of the Semi-Immersed Cylinder by Forced Oscillation Model Testing

In this paper, the hydrodynamic coefficients of a horizontal semi-immersed cylinder in steady current and oscillatory flow combining with constant current are obtained via forced oscillation experiments in a towing tank. Three non-dimensional parameters (Re, KC and Fr) are introduced to investigate their effects on the hydrodynamic coefficients. The experimental results show that overtopping is evident and dominates when the Reynolds number exceeds 5×10⁵ in the experiment. Under steady current condition, overtopping increases the drag coefficient significantly at high Reynolds numbers. Under oscillatory flow with constant current condition, the added mass coefficient can even reach a maximum value about 3.5 due to overtopping while the influence of overtopping on the drag coefficient is minor.     

Numerical simulation of a partially buried pipeline in a permeable seabed subject to combined oscillatory flow and steady current

Hydrodynamic forces exerting on a pipeline partially buried in a permeable seabed subjected to combined oscillatory flow and steady current are investigated numerically. Two-dimensional Reynolds-Averaged Navier-Stokes equations with a k−ω turbulent model closure are solved to simulate the flow around the pipeline. The Laplace equation is solved to calculate the pore pressure below the seabed with the simulated seabed hydrodynamic pressure as boundary conditions. The numerical model is validated against the experimental data of a fully exposed pipeline resting on a plane boundary under various flow conditions. Then the flow with different embedment depths, steady current ratios and KC numbers is simulated. The amplitude of seepage velocity is much smaller than the amplitude of free stream velocity as expected. The normalized Morison inertia, drag and lift coefficients based on the corresponding force coefficients of a fully exposed pipeline are investigated. The normalized Morison force coefficients reduce almost linearly with the increase of embedment depth and that the KC only has minor effect on the normalized Morison coefficients. It is also found that the permeable seabed condition causes a slight increase on the inline force and has a little effect on the lift force, compared with corresponding conditions in an impermeable bed.     

应用数值模拟方法探讨河口最大浑浊带若干机理

利用平面二维潮流方程结合悬沙输运方程模拟了河口最大浑浊带现象。并且对河口边界进行进一步概化 ,比较了恒定流与非恒定流、稳定源与非稳定源、矩形河口与线性河口等不同条件下河口悬沙浓度的平面分布特点。结果表明河口地形边界和非恒定潮流作用对河口最大浑浊带的悬沙富集有重要贡献。     

水流作用下渔网养殖空间变化的计算方法

渔网的动力分析是养殖网箱设计和优化的基础。通过把一系列杆相互铰接在一起,建立网的模型,模拟网在来流作用下的变形。作用在杆上的水动力可以通过Morison公式来计算,采用有限单元的离散方法,并利用网目合并的方法来减少总的自由度数目。模型的计算结果同试验成果做了对比,计算与试验在柔性网的变形方面,结果吻合良好。     

Steady drift force on vertical cylinder - viscous vs. potential

The contribution of the steady drift force on a floating structure may arise from waves, wind and current. The component of the wave drift force may be due to the second-order diffraction theory or potential effect and may be due to the velocity squared force or viscous effect. The presence of current in waves increases the effect of the viscous force. The expressions for these terms for a vertical cylinder are derived and their relative importance is investigated. Plots are presented showing the regions where the viscous or potential drift force predominates. Experiments were conducted with both small and large diameter cylinders. The mean drift forces obtained in these tests are compared with the theory.     

An analysis of horizontal dispersion due to the drift current with an Ekman layer

An analytical method for describing horizontal matter dispersion in shear currents is presented using a tensor expression from the point of view that matter dispersion due to the shear effect should be one of the principal mixing dilution processes. Although the behavior of horizontal dispersion is considerably more complicated than common longitudinal dispersion, the present study elucidates the vertical structure of dispersion and the dispersing process from the initial to the stationary stage, besides the usual depth-averaged dispersion coefficient at the stationary stage. As one of the typical applications of horizontal dispersion, dispersion due to the pure drift current with an Ekman layer is examined theoretically using the present method. This examination reveals that the displacement of the centroid and the major axis of dispersion are twisted in the vertical direction more than the direction of the current vector forming the Ekman spiral; that the variance increases in proportion to the third power of the elapsed time; and that the dispersion coefficient at the stationary stage remains constant, independent of the depth normalized by an Ekman layer thickness. Such dependence of the dispersion coefficient in the steady current is shown to be different from that in the oscillatory current, which is inversely proportional to the depth normalized by a Stokes layer thickness. This is considered to be induced by the difference of the vertical profiles of the first order moment in both currents, that is, the shear region of the first order moment is restricted around the floor by the alternation of the current shear in the oscillatory current while it is diffused in the whole depth in the steady current.     

Drag and inertia coefficients for a circular cylinder in steady plus low-amplitude oscillatory flows

Computer simulations of steady plus low-amplitude oscillatory flow about a circular cylinder are reported at a fixed Reynolds number of 150 based on the steady component. The conventional Keleugan–Carpenter number based on the oscillatory component is fixed at π/5. The oscillation frequency is varied so as to study a wide spectrum of flows where inertial forces dominate at one end and viscous drag forces at the other as a function of the modified Keleugan–Carpenter number. The hydrodynamic force on the cylinder in-line with the flow direction is represented by Morison's equation and an extended version with three terms. The drag and inertia coefficients in Morison's equation are determined by least-squares fits to data directly computed from integration of skin friction and pressure distributions around the periphery of the cylinder. The root-mean-square value of the residue of reconstructed minus directly-computed forces varies between 2 and 41% depending on the flow parameters. Comparable results can be obtained with a semi-theoretical approach using inviscid inertia and quasi-steady viscous drag terms. Physical explanations for the variation of the force coefficients are provided and implications for pertinent flow–structure interactions are discussed.     

Viscous drift forces in regular and irregular waves

A method to compute wave- and current-induced viscous drift forces and moments on floating platforms in regular and random waves is presented. The relative velocity drag term of Morison's equation is used in conjunction with frequency domain first-order motion transfer functions to compute the drift forces and moments. Mean viscous drift forces and moments in regular waves in all six degrees-of-freedom of a tension leg platform are computed. The relative importance of the free-surface force integration, steady current, wave-current interaction and platform motions on the computed drift forces and moments are discussed. The results from this method, in the frequency domain, are used to compute the drift forces and responses in irregular waves using existing methods developed for potential drift computations. Comparisons with results from time-domain computations are also presented and good agreement between the frequency-domain and time-domain results is found. Some comparisons with experimental data are also made. The frequency-domain method is found to be an efficient and useful tool for the analysis of semi-submersible and tension leg platforms during the preliminary design stage in which extensive parametric studies need to be undertaken.     

On the large-scale variability of the circulation and heat state of the ocean and atmosphere in the North Atlantic

A conclusion about two extreme regimes existing in the large-scale circulation in the North Atlantic has been drawn based on an analysis of the inter-annual variability of the analogue to the Rossby index, as well as that of the heat and dynamic characteristics in separate areas of the north subtropical circulation. The former is defined by a high level of circulation both in the atmosphere and in the subtropical water circulation. In the current century this regime was realized mainly in the years pertaining to the middle and end of a 22-year solar activity cycle (a 22-year cycle). The relatively low level of atmospheric circulation and the slackened water mass transport are typical of the second regime. It dominated mainly during the years relevant to the beginning and second half of a 22-year solar activity cycle.Translated by Mikhail M. Trufanov.     

Wave-induced surface drift of an inextensible thin film

In this study, the surface drift of an inextensible film due to a series of progressive gravity waves is investigated both analytically and experimentally. A second approximation of the conformal mapping that transforms the progressive sinusoidal surface to the horizontal axis is applied, thus allowing the analysis to formally accommodate a boundary layer thickness that is much less than the wave height. By computing the stream function to the third order that is an order higher than the past analysis, a pressure component in phase with the wave slope is revealed. The pressure force generated is comparable to the bottom shear on the thin film induced by the moving fluid and thus cannot be ignored. Based on the combined forces on the surface firm, a mean drift is estimated by assuming that the opposing force is due to the viscous drag induced by the drift motion. The experimental results show that the computed drift velocity based on the present study mostly underestimates the experimental observations, but it is in closer agreement than the classical Phillips' 7/4 estimate.     

Ideal shocks in 2‐layer flow Part II: Under a passive layer

In this second part of the study, ideal shock theory in two‐layer stratified flow is extended to include a third passive layer (i.e., a two and a half layer system). With the presence of a passive layer, two linear wave modes and "viscous tail modes" exist, complicating the solubility conditions and uniqueness proofs for two layer shocks. It is found however, that shocks can be unambiguously classified as external or internal based on the states of criticality that they connect. The steepening condition, while still necessary, provides a less restrictive constraint than it did with a rigid lid. Thus, we have to rely more on solutions to the full viscous shock equations to establish shock existence. The detailed structure, momentum exchange, and Bernoulli loss in a viscous shock are examined using an analytical weak shock solution and a set of numerical solutions for shocks with finite amplitudes. A shock regime diagram ( F ₁ by F ₂) is constructed based on the numerical integration of the full viscous shock equations. For strong external jumps, a cusp region (i.e., in the sense of catastrophe theory) is identified on the regime diagram. For pre‐shock states within the cusp, three end states are possible and two of these are realizable. The cusp has several physical implications. It indicates that an equal distribution of dissipation between the two layers in shocks is mathematically possible but physically inaccessible. It also allows hysteresis in time varying flows, and promotes the occurrence of double shocks (i.e., closely spaced shocks of different character). The results are compared with classical shock solutions and a set of time dependent numerical experiments.     

Experimental investigation of forces and geometry of a net cage in uniform flow

A scale model of a flexible circular net with different weights attached to the bottom was tested in a flume tank. Global forces and net deformation were measured for different steady current velocities. Three different sizes of bottom weights were used in the tests. The results from these tests are presented and discussed with the emphasis on the dependency between the forces and the geometry. Comparison is also made to empirical based formulas for calculation of drag and lift forces on net structures. Findings show that i) the forces on, and deformation of a flexible net structure are mutually highly dependent on each other; ii) estimates of global forces on a flexible net structure calculated using simple drag formulas derived from stiff net panel experiments give large errors when compared to experimental measurements; iii) numerical models taking into account the dependency between force and deformation should be used to obtain accurate estimates of forces on flexible net structures; and iv) the forces on a flexible net structure are dependent on Reynolds number, and their dependency are similar to that of a regular cylinder.     

Probability distribution of random wave–current forces

Based on the second-order solutions obtained for the three-dimensional weakly nonlinear random waves propagating over a steady uniform current in finite water depth, the joint statistical distribution of the velocity and acceleration of the fluid particle in the current direction is derived using the characteristic function expansion method. From the joint distribution and the Morison equation, the theoretical distributions of drag forces, inertia forces and total random forces caused by waves propagating over a steady uniform current are determined. The distribution of inertia forces is Gaussian as that derived using the linear wave model, whereas the distributions of drag forces and total random forces deviate slightly from those derived utilizing the linear wave model. The distributions presented can be determined by the wave number spectrum of ocean waves, current speed and the second order wave–wave and wave–current interactions. As an illustrative example, for fully developed deep ocean waves, the parameters appeared in the distributions near still water level are calculated for various wind speeds and current speeds by using Donelan–Pierson–Banner spectrum and the effects of the current and the nonlinearity of ocean waves on the distribution are studied.     

张力腿平台内孤立波作用特性数值模拟

依据三类内孤立波理论KdV、eK dV和MCC的适用性条件,采用Navier-Stokes方程为流场控制方程,以内孤立波诱导上下层深度平均水平速度作为入口边界条件,建立了两层流体中内孤立波对张力腿平台强非线性作用的数值模拟方法。结果表明,数值模拟所得内孤立波波形及其振幅与相应理论和实验结果一致,并且在内孤立波作用下张力腿平台水平力、垂向力及力矩数值模拟结果与实验结果吻合。研究同时表明,张力腿平台内孤立波载荷由波浪压差力、粘性压差力和摩擦力构成,其中摩擦力很小,可以忽略;水平力的主要成分为波浪压差力和粘性压差力,粘性压差力与波浪压差力相比较小却不可忽略,流体粘性的影响较小;垂向力中粘性压差力很小,流体粘性影响可以忽略。     

A practical approach to predicting cross-flow and in-line VIV response for deepwater risers

In this study, a practical model is proposed to predict cross-flow (CF) and in-line (IL) vortex-induced vibrations of a flexible riser in time domain. The hydrodynamic force as a function of non-dimensional amplitude and frequency is obtained from the forced vibration experimental data of a two-dimensional cylinder. An empirical nonlinear damping model is used to simulate the hydrodynamic damping outside the experiment's range. Coupling effect of CF and IL-VIV is taken into account by implanting a magnification model for the IL hydrodynamic force associated with CF amplitude, and by increasing the non-dimensional amplitude corresponding to the IL hydrodynamic coefficient in the second excitation region. The experimental models of flexible riser under the uniform and sheared current are simulated to validate the proposed model. The predicted displacement, curvatures, excited modes and fatigue damage show reasonable agreement with the measured data.     

Numerical Modeling of the Stability of Horizontal Multidrain Oil Wells

The stability of the horizontal multidrain wells is a crucial issue and several factors are involved in this matter, including in-situ stresses, magnitude and distribution as well as the mainbore trajectories. In this paper, this issue is evaluated by assuming different circumstances for the above mentioned factors, based on finite difference three-dimensional modeling by using the finite difference numerical software, FLAC3D. The stability of the mainbore and lateral branches is analyzed based on the Normalized Yielded Zone Area (NYZA) criterion, i.e. the ratio of the surrounding yielded cross-sectional area to the initial area of the well. Optimum mud pressures are obtained in the mainbore and lateral branches in different mainbore trajectories under three in-situ stress regimes. In addition, the stability of the junction where the lateral branches are bifurcated from the mainbore is assessed in those situations. The optimum trajectory of the mainbore, in which the junction has obtained the most stable condition, is selected in each stress regime. It was concluded that in the Normal Faulting (NF) stress regime, the mainbore and junction stability varies in relation to the mainbore trajectories, inversely. However, in the other two stress regimes, i.e. Strike Slip (SS) and Reverse Faulting (RF), the variations of the mainbore and junction stability are in the same trend with respect to the mainbore trajectory deviations.