Strongly coupling of partitioned fluid–solid interaction solvers using reduced-order models

In this work a powerful technique is described which allows the implicit coupling of partitioned solvers in fluid–structure interaction (FSI) problems. The flow under consideration is governed by the Navier–Stokes equations for incompressible viscous fluids and modeled with the finite volume method. The structure is represented by a finite element formulation. The method allows the use of a black box fluid and structural solver because it builds up a reduced order model of the fluid and structural problem during the coupling process. Each solution of the fluid/structural solver in the coupling process can be seen as a sensitivity response of an applied displacement/pressure mode. The applied modes and their responses are used to build up a reduced-order model. The proposed model is used to predict the unsteady flow fields of a particular flow-induced vibrational phenomenon – a fixed cubic rigid body is submerged in an incompressible fluid flow (water), an elastic plate is attached to the rigid body in the centre of the downstream face, and the vortices, which separate from the corners of the rigid body upstream, generate lift forces which excite continuous oscillations of the elastic plate downstream. The computational results show that a fairly good convergence solution is achieved by using the reduced-order model that is based on only a few displacement and stress modes, which largely reduces the computational cost, compared with traditional approaches. At the same time, comparison of the numerical results of the model with available experimental data validates the methodology and assesses its accuracy.     

Effect of pressure gradient on coherent structures in a turbulent boundary layer

By using the idea of resonant triad of the theory of hydrodynamic stability, the effect of pressure gradient on coherent structures in a turbulent boundary layer is investigated. The favorable pressure gradient suppresses the generation of the coherent structure, while the adverse pressure gradient has the opposite effect. The scale, form, as well as the propagation speed of the coherent structures are different from those with zero pressure gradient. The theoretical results are, in general, in agreement with those found from experiments. From the calculated probability density curve of the circulation differences of the nearly streamwise vortex pairs, it is found that the adverse pressure gradient makes the vortex pair more symmetric. Project supported by the National Natural Science Foundation of China.     

Hydrodynamic interaction in suspended sediment distribution of open channel turbulent flow

In the present study, a theoretical model is proposed to predict the vertical distribution of suspended sediment in open channel turbulent flow. We derive our mathematical model based on the six prominent hydrodynamic mechanisms, which are upward sediment flux due to the turbulent diffusion, downward gravitational settling of the sediment, hindered settling phenomenon, secondary current, fluid induced lift force on the suspended particles and the gradient of Reynolds normal stress of the suspended sediment. The importance of the hydrodynamics is described by the changes of suspended sediment concentration profile in terms of the particle-flow interaction caused by those mechanisms. We also address the significance of the co-existence of those mechanisms for estimating the suspended sediment concentration profile. The present model agrees satisfactorily with a wide spectrum of experimental data in available literature. Unlike the previous researchers, we select a broad range of previously published models of vertical sediment concentration distribution for comparison analysis, and the proposed model shows better prediction accuracy which is confirmed by error analysis.     

Forms of conversation and problem structuring methods: a conceptual development

Problem structuring methods (PSMs) are a family of participatory and interactive methods whose purpose is to assist groups of diverse composition tackle a complex problematic situation of common interest. This is achieved through modelling and facilitation, with a view to generating consensus on problem structure, and agreement on initial commitments. Despite the apparent success of PSMs reported in the literature, little progress has been made towards the development of theoretical models that integrate these experiences and guide PSM practitioners and academics in developing and implementing effective PSM interventions. In particular, no theoretical models have been presented concerning how conversational processes within a group are affected by PSMs. This paper develops a theoretical model of conversation intended to provide a means to identify a specific role for the analytical assistance provided through PSMs, and for evaluating their effectiveness. The hypothesis articulated from this model is that PSMs have the potential to improve the quality of the conversation in which actors participate. PSMs generate this effect through facilitating the structuring and sense-making activities embedded within a conversation. Improvement in the quality of conversation should tend to help actors engage in dialogue as a particular form of conversation, achieve shared understanding, and increase the actors’ ownership of the commitments achieved during the conversation. The paper concludes with a discussion of the implications of the model for research and practice.     

A 3D hydrodynamic numerical model of the Río de la Plata and Montevideo’s coastal zone

The 3D hydrodynamic numerical model MOHID was applied in the Río de la Plata and Montevideo coastal zone in order to represent the main dynamics and to study its complex circulation pattern. The hydrodynamic model was calibrated and validated considering the following main forces: fresh water flow, astronomical and meteorological tides in the oceanic boundary, and wind acting on the water surface. A series of water levels measured at six coastal stations and vertical profiles of current velocity measured at four different locations in the estuarine zone of the Río de la Plata were used for calibrating and validating the hydrodynamic model. The calibration process was carried out in two steps. First the astronomical waves propagation was calibrated comparing harmonic constants of observed and computed sea surface elevation data. Next, both the astronomical and meteorological wave propagation was calibrated. Direct comparison of scatter plot and root-mean square errors of model results and field data were used when evaluating the calibration quality. The calibrated model shows good agreement with the measured water surface level in the entire domain with mean error values being minor than 20% of the measured data and correlation factors higher than 0.74. Also, the intensity and velocity direction observed in the currents data are well represented by the model in both bottom and surface levels with errors similar to 30% of the currents data components. Using the 3D calibrated model the bottom and surface residual circulation for a four month period of time was analyzed.     

Experimental investigation of a moored floating system

Floating structures are generally excited by a more or less irregular sea state. Therefore the structure undergoes a nonlinear dynamical behaviour which results from hydrodynamic effects or the kinematic coupling of different components. The motions of these structures have been analysed intensively with numerical techniques which predict a wide range of nonlinear effects. On the other hand, experiments are still important when it comes to verifying these theoretical findings. Investigating such a floating structure experimentally requires a complex setup: A wave generator needs to be driven in an appropriate way to yield waves with the required characteristics at the structure. The structure itself is usually designed similar to a real–world system and it has to allow for a comparison with numerical analyses. Sensors which measure the tracks of the individual components not only have to reach a prescribed precision but also need to ensure that the motion is not perturbed as a result of the measurements. Lastly, unwanted disturbances have to be avoided. For experiments in a wave tank this includes that reflections have to be minimized. This talk addresses the development of an experimental setup for the investigation of a floating structure. It highlights components which are found to be critical for the obtained accuracy and proposes techniques to reduce experimental errors. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Recent progresses in studies on wave-current loads on foundation structure with piles and slab?

The foundation structure with piles and slab is widely used in o?shore wind farm construction in shallow water. Experimental studies on the hydrodynamic loads acting on the piles and slab under irregular waves and currents are summarized with discussion on the e?ects of pile grouping on the wave forces and wave impact loads on the slab locating near the free surface. By applying the theoretical solution of the wave di?racted by the slab and using the Morison equation to evaluate the wave force on the piles, the e?ects of the slab on the wave forces acting on the piles are analyzed. Based on the Reynolds-averaged Navier-Stokes (RANS) equations and the volume of ?uid (VOF) method, a numerical wave basin is developed to simulate the wave-structure interaction. The computed maximum wave force on the foundation structure with piles and slab agrees well with the measured data. The violent deformation, breaking, and run-up of the wave around the structure are presented and discussed. Further work on the turbulent ?ow structures and large deformation of the free surface due to interaction of the waves and foundation structures of o?shore wind farms needs more e?cient approaches for evaluating hydrodynamic loads under the e?ects of nonlinear waves and currents.     

Flasque Model Structures for Simplicial Presheaves

It is well known that there are two useful families of model structures on presheaves: the injective and projective. In fact, there is at least one more: the flasque. For some purposes, both the projective and the injective structure run into technical and annoying (but surmountable) difficulties for different reasons. The flasque model structure, which possesses a combination of the convenient properties of both structures, sometimes avoids these difficulties. (Received: February 2006)     

Numerical modeling of 3-D flow on porous broad crested weirs

Gabion weirs with optional design as a broad crested weirs are suitable structures to reduce flash flood with a minimal negative impact on the water environment. In the present study, the 3-D flow was simulated around gabion weirs with respect to free-surface water. The Reynolds-averaged Navier–Stokes equations are solved to predict water surface over the gabion weir. The VOF method with the geometric reconstruction scheme was applied to treat the complex free-surface flow. Simulations were performed using three variants of the k–ε and the RSM models to find the water level and velocity distribution profile and results are compared with several experimental data available in the literature. The structured mesh was used for all domains with high dense mesh near the solid region. A comparison between experimental data and simulations indicates that the k–ε model can be used to predict the complex flow and water level with high accuracy.     

基于三类模型的四大银行股票收益率预测研究

运用时间序列分析的预测方法,对四大银行的股票日对数收益率序列进行拟合与预测分析,分别构建ARMA模型、GARCH模型以及ARMA-GARCH组合模型,通过模型比较,实证分析表明:在拟合效果上,ARMA-GARCH模型的拟合优度优于ARMA模型和GARCH模型;在预测效果上,ARMA模型的预测效果最优,ARMA-GARCH模型次之.     

A general theoretical model for the magnetohydrodynamic flow of micropolar magnetic fluids. Application to Stokes flow

Many practical applications, which have an inherent interest of physical and mathematical nature, involve the hydrodynamic flow in the presence of a magnetic field. Magnetic fluids comprise a novel class of engineering materials, where the coexistence of liquid and magnetic properties provides us with the opportunity to solve problems with high mathematical and technical complexity. Here, our purpose is to examine the micropolar magnetohydrodynamic flow of magnetic fluids by considering a colloidal suspension of ferromagnetic material (usually non‐conductive) in a carrier magnetic liquid, which is in general electrically conductive. In this case, the ferromagnetic particles behave as rigid magnetic dipoles. Thus, the application of an external magnetic field, apart from the creation of an induced magnetic field of minor significance, will prevent the rotation of each particle, increasing the effective viscosity of the fluid and will cause the appearance of an additional magnetic pressure. Despite the fact that the general consideration consists of rigid particles of arbitrary shape, the assumption of spherical geometry is a very good approximation as a consequence of their small size. Our goal is to develop a general three‐dimensional theoretical model that conforms to physical reality and at the same time permits the analytical investigation of the partial differential equations, which govern the micropolar hydrodynamic flow in such magnetic liquids. Furthermore, in the aim of establishing the consistency of our proposed model with the principles of both ferrohydrodynamics and magnetohydrodynamics, we take into account both magnetization and electrical conductivity of the fluid, respectively. Under this consideration, we perform an analytical treatment of these equations in order to obtain the three‐dimensional effective viscosity and total pressure in terms of the velocity field, the total (applied and induced) magnetic field and the hydrodynamic and magnetic properties of the fluid, independently of the geometry of the flow. Moreover, we demonstrate the usefulness of our analytical approach by assuming a degenerate case of the aforementioned method, which is based on the reduction of the partial differential equations to a simpler shape that is similar to Stokes flow for the creeping motion of magnetic fluids. In view of this aim, we use the potential representation theory to construct a new complete and unique differential representation of magnetic Stokes flow, valid for non‐axisymmetric geometries, which provides the velocity and total pressure fields in terms of easy‐to‐find potentials, via an analytical fashion. Copyright © 2009 John Wiley & Sons, Ltd.     

Cluster model calculation of N near K-edge energy-loss fine structures in hexagonal GaN crystal

A cluster model is used to calculate electron energy-loss fine structures in crystal. The multiple-scattering self-consistent-field method is employed in the calculation. Our theoretical results of N near K-edge energy loss fine structures in hexagonal GaN crystal are in good agreement with the experimental spectra. Future possible experiments in energy-filtered transmission electron microscopy (EFTEM) are discussed and proposed because our theoretical work can provide clear assignments for transmitted electrons with different energy losses.     

An equivalent mechanical model for fluid sloshing in a rigid cylindrical tank equipped with a rigid annular baffle

A mass-spring mechanical model for linear sloshing of fluid in a rigid cylindrical tank with a rigid annual baffle is developed. By means of the subdomain method, the complicated fluid domain is divided into several subdomains with pure boundary conditions and interface conditions. Combined with the continuity conditions of velocity and pressure on interfaces, the velocity potential of fluid in each subdomain is analytically solved. The mass-spring model for fluid sloshing is established by generating the same hydrodynamic shear force and overturning moment as those from the exact solutions when the tank is subjected to horizontal excitations. The present model gives all the mechanical parameters including the convective mass-spring oscillators and the impulsive mass as well as their positions. Comparative studies between the present solutions and the available results verify the correctness of the mechanical model. Based on the normalized equivalent masses and spring stiffnesses, the dynamic responses of fluid in tanks are discussed with respect to the fluid height, the baffle position and the inner radius of the baffle, respectively. The present model is especially suitable for dealing with complicated liquid-structure systems. The motivation and novelty of this study are confirmed by an application of the present model to a multi-tank system.     

Locomotion of Articulated Bodies in a Perfect Fluid

This paper is concerned with modeling the dynamics of N articulated solid bodies submerged in an ideal fluid. The model is used to analyze the locomotion of aquatic animals due to the coupling between their shape changes and the fluid dynamics in their environment. The equations of motion are obtained by making use of a two-stage reduction process which leads to significant mathematical and computational simplifications. The first reduction exploits particle relabeling symmetry: that is, the symmetry associated with the conservation of circulation for ideal, incompressible fluids. As a result, the equations of motion for the submerged solid bodies can be formulated without explicitly incorporating the fluid variables. This reduction by the fluid variables is a key difference with earlier methods, and it is appropriate since one is mainly interested in the location of the bodies, not the fluid particles. The second reduction is associated with the invariance of the dynamics under superimposed rigid motions. This invariance corresponds to the conservation of total momentum of the solid-fluid system. Due to this symmetry, the net locomotion of the solid system is realized as the sum of geometric and dynamic phases over the shape space consisting of allowable relative motions, or deformations, of the solids. In particular, reconstruction equations that govern the net locomotion at zero momentum, that is, the geometric phases, are obtained. As an illustrative example, a planar three-link mechanism is shown to propel and steer itself at zero momentum by periodically changing its shape. Two solutions are presented: one corresponds to a hydrodynamically decoupled mechanism and one is based on accurately computing the added inertias using a boundary element method. The hydrodynamically decoupled model produces smaller net motion than the more accurate model, indicating that it is important to consider the hydrodynamic interaction of the links.     

Global dynamics and control of malicious signal transmission in wireless sensor networks

This paper studies the global dynamics of a discontinuous delayed model of malicious signal transmission in wireless sensor networks under the framework of differential inclusion. The local stability of two types of steady states are investigated for the discontinuous system by studying the corresponding characteristic equation. The sufficient conditions for the existence of two types of globally asymptotically stable steady states are obtained for the discontinuous system by using the comparison arguments method. Furthermore, the optimal control of the discontinuous system is investigated by using Pontryagin’s maximum principle. Numerical simulations of two examples are carried out to illustrate the main theoretical results. The obtained results can help us to better control and predict the spread of malicious signal transmission in wireless sensor networks.     

主动冷却点阵夹层防热结构温度响应计算模型北大核心CSCD

针对点阵夹层结构主动热防护问题,建立了夹层结构面板和芯体导热与冷却剂对流耦合的非稳态传热理论模型,利用有限体积法离散控制方程并在MATLAB中进行了迭代求解.模型首次考虑了面板与夹芯杆之间的收缩热阻,并利用分离变量法得到了收缩热阻的近似解析解.基于单胞模型和周期性边界条件,模拟得到了模型所需的表面对流传热系数h_(b)和h_(fin).最后,选取多单胞计算工况进行数值模拟和理论模型对比,并讨论了收缩热阻对模型预测精度的影响.结果表明:理论模型能够准确预测夹层结构及内部流体的温度变化,理论与仿真之间的最大误差不超过1%;随着外加热流密度不断增大,忽略收缩热阻使得计算结果造成的误差不断增大;与数值模拟相比,理论模型可显著地减少计算时间并节省计算资源,尤其适用于非均匀、非稳态复杂热载荷下点阵夹层结构的温度响应计算.     

Nonlinear creep of thick-walled polymer tubes under internal pressure

The process of creep in a thick-walled tube of rigid homogeneous epoxide-type polymer under under the action of a steady internal pressure is studied. A theoretical analysis has been made on the basis of a non-linear generalized Maxwell equation, taking account of two terms of the spectrum of polymer relaxation times. A comparison is given with the results of a calculation with a single term of the spectrum and according to linear theory. The theoretical results are compared with those obtained experimentally in a specially constructed apparatus. The possibility is shown of evaluating the long-term strength of a thick-walled polymer tube under the action of a steady internal pressure.Institute of Chemical Physics, Academy of Sciences of the USSR, Moscow. Translated from Mekhanika Polimerov, No. 3, pp. 493–501, May–June, 1970.     

Modelling and characterization of a pneumatically actuated peristaltic micropump

There is an emerging class of microfluidic bioreactors which possess long-term, closed circuit perfusion under sterile conditions with in vivo-like flow parameters. Integrated into microfluidics, peristaltic-like pneumatically actuated displacement micropumps are able to meet these requirements. We present both a theoretical and experimental characterization of such pumps. In order to examine volume flow rate, we have developed a mathematical model describing membrane motion under external pressure. The viscoelasticity of the membrane and hydrodynamic resistance of the microfluidic channel have been taken into account. Unlike other models, the developed model includes only the physical parameters of the pump and allows the estimation of their impact on the resulting flow. The model has been validated experimentally.