Thermomechanical and thermophysical properties of repulsive clathrates

The range of the characteristic properties of repulsive clathrates (RCs), which are new working media used for efficient energy conversion in thermomechanical systems, has been extended. The dissipation, storage, and conversion of energy by means of RCs is based on the use of the intermolecular forces acting on the interface of the system of a liquid and a nonwetting solid capillaryporous matrix and leading to ejection of the liquid from the pores of the matrix. It has been shown that it is possible to control characteristics of RCs such as compressibility, amount of the dissipated (accumulated) mechanical energy, specific heat, and thermal parameters of the compression-expansion process. The properties of RCs providing unique operation modes of power systems that are not realizable with conventional working media (gas, steam).     

黏滞阻尼器在超高层结构体系中的效率研究

250m以上的超高层结构主要有框架-核心筒、框筒-核心筒和巨型框架-核心筒等三种结构体系．由于超高层结构的高宽比通常较大（5以上）,结构的侧向变形往往成为控制因素．随着消能减震技术的不断成熟,黏滞阻尼器越来越多的应用于建筑结构领域．本文分别将黏滞阻尼器均匀布置在上述三种超高层结构体系中,对比其消能减震效果并分析原因,最后建议了黏滞阻尼器在这些结构中的较优布置．     

On the viscous dissipation modeling of thermal fluid flow in a porous medium

The problem of viscous dissipation and thermal dispersion in saturated porous medium is numerically investigated for the case of non-Darcy flow regime. The fluid is induced to flow upward by natural convection as a result of a semi-infinite vertical wall that is immersed in the porous medium and is kept at constant higher temperature. The boundary layer approximations were used to simplify the set of the governing, nonlinear partial differential equations, which were then non-dimensionalized and solved using the finite elements method. The results for the details of the governing parameters are presented and investigated. It is found that the irreversible process of transforming the kinetic energy of the moving fluid to heat energy via the viscosity of the moving fluid (i.e., viscous dissipation) resulted in insignificant generation of heat for the range of parameters considered in this study. On the other hand, thermal dispersion has shown to disperse heat energy normal to the wall more effectively compared with the normal diffusion mechanism.     

Evaluation of the drag force by integrating the energy dissipation rate in stokes flow for 2D domains using the FEM

The total drag force on the surface of a body, which is the sum of the form drag and the skin friction drag in a 2D domain, is numerically evaluated by integrating the energy dissipation rate in the whole domain for an incompressible Stokes fluid. The finite element method is used to calculate both the energy dissipation rate in the whole domain as well as the drag on the boundary of the body. The evaluation of the drag and the energy dissipation rate are post-processing operations which are carried out after the velocity field and the pressure field for the flow over a particular profile have been obtained. The results obtained for the flow over three different but constant area profiles—a circle, an ellipse and a cross-section of a prolate spheroid—with uniform inlet velocity are presented and it is shown that the total drag force times the velocity is equal to the total energy dissipation rate in the entire finite flow domain. Hence, by calculating the energy dissipation rate in the domain with unit velocity specified at the far-field boundary enclosing the domain, the drag force on the boundary of the body can be obtained.     

NUMERICAL STUDY ON CUT-AND-CONNECT OF TWO VORTEX RINGS ALONG PARALLEL AXES

A numerical study has been conducted to investigate the interaction of two viscous vortex rings along parallel axes. The generation of two vortex rings created by the ejection of a fluid through orifices and their cut-and-connect process were numerically simulated by solving the three-dimensional and time-dependent Navier-Stokes equations. Based on the quantitative velocity and pressure data obtained from the Navier-Stokes simulation, the distribution and the evolution of the vorticity, helicity density and energy dissipation function were analyzed. The helicity density and its relation with the energy dissipation function in three-dimensional flow fields were also examined. It was found that the energy dissipation plays an important role in the cancellation of the vortex circulation during the vortex tubes cutting. This energy dissipation process may be used to explain the cut-and-connect of vortex tubes in the high Reynolds number turbulent flow. The numerical solutions were compared with the experimental observations and measurements under the similar condition. The correspondence between the numerical simulation and the experimental measurement was satisfactory.     

Viscous Dissipation and Thermal Radiation Effects on Mixed Convection from a Vertical Plate in a Non-Darcy Porous Medium

The paper presents an investigation of the influence of thermal radiation and viscous dissipation on the mixed convective flow due to a vertical plate immersed in a non-Darcy porous medium saturated with a Newtonian fluid. The physical properties of the fluid are assumed to be constant. The Rosseland approximation is used to describe the radiative heat flux in the energy equation. The governing partial differential equations are transformed into a system of ordinary differential equations and solved numerically using a shooting method. The results are analyzed for the effects of various physical parameters such as viscous dissipation, thermal radiation, mixed convection parameters, and the modified Reynolds number on dynamics. The heat transfer coefficient is also tabulated for different values of the physical parameters.     

Turbulent Flow Around Fluid–Porous Interfaces Computed with a Diffusion-Jump Model for <Emphasis Type="Italic">k</Emphasis> and <Emphasis Type="Italic">ε</Emphasis> Transport Equations

Flow over vegetation and bottom of rivers can be characterized by some sort of porous structure of irregular surface through which a fluid permeates. Also, in engineering systems, one can have components that make use of a working fluid flowing over irregular layers of porous material. This article presents numerical solutions for such hybrid medium, considering here a channel partially filled with a flat porous layer saturated by a fluid flowing in turbulent regime. One unique set of transport equations is applied to both the regions. A diffusion-jump model for both the turbulent kinetic energy and its dissipation rate, across the interface, is presented and discussed upon. The discretization steps taken for numerically accommodating such model in the system of algebraic equations are presented. Numerical results show the effects of Reynolds number, porosity, and permeability on mean and turbulence fields. Results indicate that when negative values for the stress jump coefficient are applied, the peak of the turbulent kinetic energy distribution occurs at the macroscopic interface.     

Nonlinear multimodal model for TLD of irregular tank geometry and small fluid depth

Tuned liquid dampers (TLDs) utilize sloshing fluid to absorb and dissipate structural vibrational energy. TLDs of irregular or complex tank geometry may be required in practice to avoid tank interference with fixed structural or mechanical components. The literature offers few analytical models to predict the response of this type of TLD, particularly when the fluid depth is small. In this paper, a multimodal model is developed utilizing a Boussinesq-type modal theory which is valid for small TLD fluid depths. The Bateman–Luke variational principle is employed to develop a system of coupled nonlinear ordinary differential equations which describe the fluid response when the tank is subjected to base excitation. Energy dissipation is incorporated into the model from the inclusion of damping screens. The fluid model is used to describe the response of a 2D structure–TLD system when the structure is subjected to external loading and the TLD tank geometry is irregular.Shake table experiments are conducted on a rectangular and chamfered tank subjected to unidirectional base excitation. Comparisons of the experimental and predicted sloshing forces and energy dissipation per cycle indicate that the model is able to predict the fluid response at fluid depth ratios greater than h/L=0.10. Next, structure–TLD system tests are conducted and it is found that the model can predict the structural and TLD responses. The simulated and experimental results show that the TLD tank transfers energy between orthogonal structural sway modes.     

Synthesis of thermal effects in misaligned hydrodynamic journal bearings

The analysis presented herein deals with the evaluation of pressure and temperature fields which are generated in thin fluid films of varying thickness. The particular problem of a misaligned journal bearing has been studied by solving simultaneously the Reynolds and energy equations, which also include the effects of viscous dissipation and the variation of fluid viscosity with temperature. The method has been used to predict pressure and temperature fields as well as global performance parameters for a typical journal bearing operation.     

Effect of variable thermal conductivity and viscosity on single phase convective heat transfer in slip flow

For a variety of fields in which micro-mechanical systems and electronic components are used, fluid flow and heat transfer at the microscale needs to be understood and modeled with an acceptable reliability. In general, models are prepared by making some extensions to the conventional theories by including the scaling effects that become important for microscale. Some of these effects are; axial conduction, viscous dissipation, and rarefaction. In addition to these effects, temperature variable thermal conductivity and viscosity may become important in microscale gas flows due to the high temperature gradients that may exist in the fluid. For this purpose, simultaneously developing, single phase, laminar and incompressible air flow in a microtube and in the micro gap between parallel plates is numerically analyzed. Navier–Stokes and energy equations are solved and the variation of Nusselt number along the channel is presented in tabular and graphical forms as a function of Knudsen, Peclet, and Brinkman numbers, including temperature variable thermal conductivity and viscosity.     

Carreau fluid flow due to nonlinearly stretching sheet with thermal radiation,heat flux,and variable conductivity

A sophisticated theoretical and mathematical model is proposed. It is verified that this model can estimate and monitor the detailed behavior for the steady Carreau fluid flow past a nonlinear stretching surface and the predicted phenomena due to the presence of heat flux, thermal radiation, and viscous dissipation. Despite the fact that some properties of the fluid do not depend on the temperature, the fluid thermal conductivity is assumed to depend on the temperature. Based on accelerating the fluid elements, some of the kinetic energy for the fluid can be turned to the internal heating energy in the form of viscous dissipation phenomena. The contribution in this study is that a similar solution is obtained, in spite of the high nonlinearity of the Carreau model,especially, with the heat flux, variable conductivity, and viscous dissipation phenomena.Some of the major significant findings of this study can be observed from the reduction in the fluid velocity with enhancing the Weissenberg number. Likewise, the increase in the sheet temperature is noted with increasing the Eckert number while the reverse behavior is observed for increasing both the radiation parameter and the conductivity parameter.Finally, the accuracy and trust in the proposed numerical method are validated after benchmarking for our data onto the earlier results.     

基于格子-波尔兹曼法的流体能耗求解

格子-波尔兹曼法是近年来新兴的一种计算流体力学数值方法。随着这种方法的不断发展,人们将它用于流体的仿真、优化等不同场合。与此同时,一些与流场流速和压强相关的物理量(如能耗)的求解也成为关注的焦点。本文介绍了能耗这一流体宏观量的格子-波尔兹曼法求解及其实现。与传统的有限差分法不同,本文在求解有关的速度梯度时使用了格子-波尔兹曼-矩法,这种方法不但能够避免有限差分法在边界处失效的缺点,而且计算简单,算法局部性好,适合大规模并行计算。本文在分析其数值解精度的基础上,使用这种方法进行了以能耗极小为目标的直通道内椭圆挡块的参数优化。这些分析和算例分别定量和定性地说明了本文算法的准确性。     

Superquadric DEM-SPH coupling method for interaction between non-spherical granular materials and fluids

The research on the coupling method of non-spherical granular materials and fluids aims to predict the particle–fluid interaction in this study. A coupling method based on superquadric elements is developed to describe the interaction between non-spherical solid particles and fluids. The discrete element method (DEM) and the smoothed particle hydrodynamics (SPH) are adopted to simulate granular materials and fluids. The repulsive force model is adopted to calculate the coupling force and then a contact detection method is established for the interaction between the superquadric element and the fluid particle. The contact detection method captures the shape of superquadric element and calculates the distance from the fluid particle to the surface of superquadric element. Simulation cases focusing on the coupling force model, energy transfer, and large-scale calculations have been implemented to verify the validity of the proposed coupling method. The coupling force model accurately represents the water entry process of a spherical solid particle, and reasonably reflects the difference of solid particles with different shapes. In the water entry process of multiple solid particles, the total energy of the water entry process of multiple solid particles tends to be stable. The collapse process of the partially submerged granular column is simulated and analyzed under different parameters. Therefore, this coupling method is suitable to simulate fluid–particle systems containing solid particles with multiple shapes.     

On the propagation of waves through porous solids

This note reexamines Biot's model for the propagation of acoustic waves in a material such as cohensionless sand, infused with a fluid, within the context of mixture theory. Instead of the standard entropy equation that is used in mixture theory, an inequality for the viscous dissipation is employed here due to a conceptual difficulty that one encounters in applying the standard equation to a mixture of sand and a fluid. The wave equations are reformulated by taking the velocity field, instead of the displacement, for the fluid as a primary quantity. By recognizing and thereby exploiting the dependence of the stored energy of the sand on the pore fluid pressure and choosing an appropriate form for the rate of dissipation, a set of governing equations are obtained which are equivalent to those derived by Biot [J. Acoust. Soc. Am. 28(1956) 168, 179; J. Appl. Phys. 33(1962) 1482]. A differential equation for the pore fluid pressure is derived and the effects of drag and virtual mass are dealt with in a unified fashion. The procedure allows us to develop generalizations to Biot's equations in a rational manner.     

Volumetric methods for evaluating energy loss and heat transfer in cavity flows

Methods have been developed for calculating irreversible energy losses and rates of heat transfer from computational fluid dynamics solutions using volume integrations of energy dissipation or entropy production functions. These methods contrast with the more usual approach of performing first law energy balances over the boundaries of a flow domain. Advantages of the volumetric approach are that the estimates involve the whole flow domain and are hence based on more information than would otherwise be used, and that the energy dissipation or entropy production functions allow for detailed assessment of the mechanisms and regions of energy loss or entropy production. Volume integrations are applied to the calculation of viscous losses in a lid‐driven cavity flow, and to the viscous losses and heat transfer due to natural convection in a side‐heated cavity. In the convection problem comparison with the entropy increase across a stationary heat conducting layer leads to a novel volume integral expression for the Nusselt number. The predictions using this method compare well with traditional surface integrals and benchmark results. Copyright © 2007 John Wiley & Sons, Ltd.     

Dissipation matrix and artificial heat conduction for Godunov‐type schemes of compressible fluid flows

This paper aims to reassess the Riemann solver for compressible fluid flows in Lagrangian frame from the viewpoint of modified equation approach and provides a theoretical insight into dissipation mechanism. It is observed that numerical dissipation vanishes uniformly for the Godunov‐type schemes in the sense that associated dissipation matrix has zero determinant if an exact or approximate Riemann solver is used to construct numerical fluxes in the Lagrangian frame. This fact connects to some numerical defects such as the wall‐heating phenomenon and start‐up errors. To cure these numerical defects, a traditional numerical viscosity is added, as well as the artificial heat conduction is introduced via a simple passage of the Lax–Friedrichs type discretization of internal energy. Copyright © 2016 John Wiley & Sons, Ltd.     

Amortissement visqueux d'un résonateur mécanique en environnement gazeuxViscous damping of a mechanical resonator in a gaseous environment

We study dissipation phenomena due to the presence of a gaseous environment, leading to the damping of the oscillations in vibrating systems such as mechanical resonators. In the so-called “viscous” pressure region (p ranging between 10^?3 mbar and 1 bar), we suggest a simple model allowing an order of magnitude analysis of the dissipation mechanisms. This model, based on the classical form of the energy conservation equation in fluid mechanics, leads to a p^1/2 variation scale for the dissipation. In addition, we present experimental results that are found to be in good agreement with the predictions of the model. To cite this article: D. Perret et al., C. R. Mecanique 331 (2003).     

Experimental investigation of helicity in turbulent swirling jet using dual-plane dye laser PIV technique

This paper reports a new method of generating two light sheets using a dye laser system and the use of this dual-plane dye laser system to analyse average helicity and energy dissipation in a turbulent swirling flow. The dual-plane PIV system that was used in this study consisted of three cameras and a single frequency Nd:YAG laser, which was used to generate two parallel light sheet planes with differing wavelengths(colour). The method of generating two different light sheet wavelengths using a single laser source is an innovative and new technique. Stereoscopic PIV measurements were obtained in one plane with the use of two CCD cameras, and standard PIV measurements were obtained in the other plane with the use of one CCD camera. The light scattered by the particles on two different light sheets were separated using appropriate optical filters. The measurements obtained were used to estimate the components of the velocity gradient tensor. The tensor components were then used to determine the average vorticity components and helicity quantities of the fluid that was investigated. To determine the average turbulent kinetic energy dissipation, the continuity equation was used to infer the out-of-plane gradient of the out-of-plane velocity. From the analysis of the results, it was found that regions with high helicity were correlated with regions of high turbulent kinetic energy dissipation.     

饱和黏弹性多孔介质中的平面波及能量耗散

研究了流体饱和不可压黏弹性多孔介质中的非均匀平面波及其能量流和能量耗散规律. 在流 相和固相物质微观不可压、固相骨架宏观服从积分型本构关系和小变形的假定下，利用 Helmholtz分解，得到了饱和黏弹性多孔介质中非均匀平面波的一般解以及纵波、横波相速 度和衰减率等的解析表达式，分析了平面波传播矢量和衰减矢量之间的关系. 数值结果表明 孔隙流体与固相骨架间的相互作用以及固相骨架的黏性对波的相速度、衰减率等有着显著的 影响. 同时，得到了饱和黏弹性多孔介质的能量方程，给出了能量流矢量和能量耗散率. 对 非均匀平面纵波和横波，推导了平均能量流矢量和平均能量耗散率的解析表达式.     

截锥形弹体在液体介质中运动速度衰减规律分析

水面舰船被动防护体系中液舱的主要功能之一是阻止高速弹体（爆炸破片）对内部重要结构、设备和人员的威胁,高速弹体打击液舱的过程包含着复杂的能量传递与耗散。为了分析弹体形状对其在液体介质中运动速度衰减的影响, 开展了一系列不同头形因数的截锥形弹体在不同入水速度下弹体垂直侵彻液体介质过程的数值模拟,得到了垂直侵彻液体介质时弹体速度衰减特性,发现高速弹体在液体介质中运动的阻力因数与弹体形状和无量纲速度有关。基于对系列数值模拟计算结果的拟合分析,提出了计及头形因数的截锥形弹体 垂直侵彻液体介质时的速度衰减经验公式,通过开展数值算例分析验证了公式计算结果的可靠性。本文中提出的经验公式可实现对高速弹体在液体介质中速度衰减的准确快速计算,为舰船防护液舱结构设计提供一定的参考。