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Internal waves driven by external excitation constitute important phenomena that are often encountered in environmental fluid mechanics. In this study, a pseudospectral o-transformation model is used to simulate parametric excitation of stratified liquid in a two-layer rectangular tank. The o-transformation maps the physical domain including the liquid free surface, the interface between the liquid layers, and the bed, onto a pair of fixed rectangular computational domains corresponding to the two layers. The governing equation and boundary conditions are discreused using Chebyshev collocation formulae. The numerical model is verified for two analytical sloshing problems: horizontal excitation of constant density liquid in a rectangular tank, and vertical excitation of stratified liquid in a rectangular tank. A detailed analysis is provided of liquid motions in a shallow water tank due to excitations in the horizontal and the vertical directions. Also, the effect of pycnocline on the wave motions and patterns is studied. It is found that wave regimes and patterns are considerably influenced by the pycnocline, especially when the excitation frequency is large. The present study demonstrates that a pseudospectral o-transformation is capable to model non-linear sloshing waves in a two-layer rectangular tank. (C) 2013 Elsevier Ltd. All rights reserved.

Characterization of three-dimensional (3D) airflow remains elusive within a variety of environments and is particularly challenging over complex dune topography. Previous work examining airflow over and in the lee of dunes has been restricted to two-dimensional studies and has concentrated on dune shapes containing angle of repose lee sides only. However, the presence of vegetation in coastal dunes creates topographic differences and irregular shapes that interfere with flow separation at the crest and significantly modify lee-side airflow patterns and potential transport. This paper presents the first 3D field characterization of airflow patterns at the lee side of a subaerial dune. Flow information was obtained using an array of 3D ultrasonic anemometers deployed over a beach surface during seven offshore wind events. Data were used to measure cross-shore and alongshore lee-side airflow patterns using the three dimensions of the wind vector. Distances to re-attachment were similar to previous studies, but the range of transverse incident wind directions resulting in flow separation (0+/-35 degrees) was almost twice that previously reported (0+/-20 degrees). Airflow reversal took place with winds as slow as 1m s(-1). Transverse offshore winds generated areas of opposing wind directions both within the reversed zone and beyond re-attachment, contrary to consistent deflection in only one direction found in transverse desert dunes. Patterns of flow convergence-divergence have been reported in fluvial studies. However, while convergence was associated with weak reversal in fluvial settings, it appeared to be related to strong flow reversal here and could be produced by pressure differentials at the dune crest. UK Natural Environment Research Council. Grant Number: NE/F019483/1 peer-reviewed

On all sandy coastlines fringed by dunes, understanding localised air flow allows us to examine the potential sand transfer between the beach and dunes by wind-blown (Aeolian) action. Traditional thinking into this phenomenon had previously included only onshore winds as effective drivers of this transfer. Recent research by the authors, however, has shown that offshore air-flow too can contribute significantly, through lee-side back eddies, to the overall windblown sediment budget to coastal dunes. Under rising sea levels and increased erosion scenarios, this is an important process in any post-storm recovery of sandy beaches. Until now though, full visualisation in 3D of this newly recognised mechanism in offshore flows has not been achieved. Here we show for the first time this return flow eddy system, using 3D Computational Fluid Dynamics modelling, and reveal the presence of complex corkscrew vortices and other phenomena. The work highlights the importance of relatively small surface undulations in the dune crest which act to induce the spatial patterns of airflow (and transport) found on the adjacent beach.

Although the soil-vegetation-atmosphere exchange of momentum and heat is fairly well understood for many types of homogeneous surfaces, the disturbances created by tran sitions of one surface type to another remain to be analysed more fully. This is especially true for the impact which a large transition such as the forest edge has on the average fluxes in a small-scale heterogeneous landscape with forest. Recently acquired experimental evidence appears to some extent contradictory and at variance with conventional concepts.

Experiments were performed at the horizontal shock tube facility at Los Alamos National Laboratory to study the effect of incident shock Mach number (M) on the development of Richtmyer-Meshkov instability after a shock wave impulsively accelerates a varicose-perturbed, heavy-gas curtain. Three cases of incident shock strength were experimentally investigated: M = 1.21, 1.36, and 1.50. We discuss the state of the mixing and the mechanisms that drive the mixing at both large and small scales by examining the time evolution of 2D density fields derived from quantitative planar laser-induced fluorescence measurements. Several differences in qualitative flow features are identified as a result of Mach number variation, and differences in vortex interaction, observed using particle image velocimetry, play a critical role in the development of the flow field. Several quantities, including mixing layer width, mixing layer area, interface length, instantaneous mixing rate, the density self-correlation parameter, probability density functions of the density field, and mixing progress variables are examined as a function of time. These quantities are also examined versus time scaled with the convection velocity of the mixing layer. A higher incident Mach number yields greater mixing uniformity at a given downstream location, while a lower Mach number produces a greater amount of total mixing between the two gases, suggesting possible implications for optimization in applications with confined geometries. (C) 2013 AIP Publishing LLC.

A novel level set-based immersed-boundary method (LSIBM) is proposed for a transient computational fluid dynamics (CFD) simulation of flow across a moving immersed boundary. At various time instants, the level set method (LSM) is used to capture the moving boundary accurately, and its value is used for proper numerical implementation of boundary conditions and conservation laws in the fluid region. The method is validated on flow across a pitching airfoil and four different problems on flow across an oscillating cylinder. The boundary conditions on the moving body are applied directly in the discrete equations: The method is more physics-based as well as easier to implement numerically.

Two-dimensional direct simulation Monte Carlo (DSMC) was used to investigate the effect of wall accommodation on different modes of operation of a nozzle under high Knudsen number conditions. Substantial performance enhancement was seen at higher values of converging–diverging angles, because a reduction in the surface area-to-volume ratio decreased the wall effects. Though the normal energy accommodation had a negligible influence on the nozzle performance, it was seen to be a critical factor for accurate prediction of gas temperature along the wall. At higher Knudsen numbers, the flow tended to become one-dimensional even for the case of complete accommodation.

In this chapter we review methods currently used to perform Monte Carlo calculations for quantum lattice models. A detailed exposition is given of the formalism underlying the construction of the simulation algorithms. We discuss the fundamental and technical difficulties that are encountered and give a concise survey of applications of quantum Monte Carlo methods to various quantum statistical lattice problems. New developments are illustrated and discussed in terms of two many-body problems, the Hubbard and the Heisenberg model.

Damage evolution in quasi-brittle materials is a complex process in which heterogeneity plays an important role. This heterogeneity may imply that the exact failure mode can be highly dependent upon the precise spatial distribution of initial imperfections. To model this inhomogeneity stochastic distributions of material properties must be used in numerical simulations. However, the use of a stochastic approach does not resolve the issue of the change of character of the governing differential equations during progressive damage. To avoid such a change of character higher order terms, either in space or in time, must be added to the standard continuum description (regularization techniques). A simulation technique that describes the failure process properly must incorporate both a regularization technique and a stochastic description of the disordered continuum. This statement will be substantiated here by presenting finite element analyses of direct tension tests with a standard local damage model and with a nonlocal damage model. The randomness in the damage process will be introduced by considering the initial damage threshold of the continuum damage model as a random field, characterized by a relevant distribution and autocorrelation coefficient function. The response statistics calculated by the Monte-Carlo technique will be presented for two different levels of finite element discretization. The nonlocal and random field formulations both rely on the introduction of a length parameter: the internal length scale in case of the nonlocal continuum and the correlation length for the random field. The effect of the relative variation of the correlation length and the internal length scale will also be discussed.

We determine mass and mixing angles of eta and eta' states using Nf=2+1+1 Wilson twisted mass lattice QCD. We describe how those flavour singlet states need to be treated in this lattice formulation. Results are presented for three values of the lattice spacing, a=0.061 fm, a=0.078 fm and a=0.086 fm, with light quark masses corresponding to values of the charged pion mass in a range of 230 to 500 MeV and fixed bare strange and charm quark mass values. We obtain 557(15)(45) MeV for the eta mass (first error statistical, second systematic) and 44(5) degrees for the mixing angle in the quark flavour basis, corresponding to -10(5) degrees in the octet-singlet basis., Comment: 28 pages, 9 figures, version to appear in JHEP, extended discussion of autocorrelation times and comparison to results available in the literature, added a comment for FS-effects and clarified the description of our blocking procedure

A novel approach is proposed for the use of cohesive elements in the analysis of delamination propagation in composite materials under high-cycle fatigue loading. The method is applicable to delamination propagation within the Paris-law regime and is suitable for the analysis of three-dimensional structures typical of aerospace applications. The major advantages of the proposed formulation are its complete independence of the cohesive zone length - which is a geometry-dependent parameter - and its relative insensitivity to mesh refinement. This is only possible via the introduction of three nonlocal algorithms, which provide (i) automated three-dimensional tracking of delamination fronts, (ii) an estimation of direction of crack propagation and (iii) accurate and mesh-insensitive integration of strain energy release rate. All calculations are updated at every increment of an explicit time-integration finite element solution, which models the envelopes of forces and displacements with an assumption of underlying constant cyclic loading. The method was implemented as a user-defined subroutine in the commercial finite element software LS-Dyna and supports the analysis of complex three-dimensional models. Results are presented for benchmark cases such as specimens with central cut plies and centrally-loaded circular plates. Accurate predictions of delamination growth rates are observed for different mesh topologies in agreement with the Paris-laws of the material. (C) 2012 Elsevier Ltd. All rights reserved.

In this paper, a new Lagrangian cloud model (LCM) is introduced in which the flow field is simulated by large-eddy simulation, and the droplets are treated as Lagrangian particles responding to the simulated flow field. In order to handle the extremely large number of droplets within a cloud, the concept of a super-droplet, which represents a large number of real droplets of the same size, is introduced, and the number of contributing real droplets is called the weighting factor. A novel method is developed to realize the collision/coalescence of droplets, in which the consequent variation of the droplet spectrum is represented in terms of the modification of the radius and weighting factor of super-droplets, while keeping the number of super-droplets unchanged. Using an idealized single cloud and trade wind cumuli, the LCM is shown to reproduce the general features of shallow cumulus clouds in agreement with traditional bulk models. The droplet spectrum simulated by the LCM, using collision kernels with and without the effects of turbulence, also shows a pattern consistent with the spectral bin model. Furthermore, the sensitivity of the LCM to two model parameters, the time step and the number of super-droplets, is examined.

During a storm in October 2002, wind induced ovalling vibrations were observed on several empty silos of a closely spaced group (pitch-to-diameter ratio of 1.05) consisting of 8 by 5 silos in the port of Antwerp (Belgium). Numerical simulations of the turbulent wind flow are performed to clarify the occurrence of the observed ovalling vibrations near the lee side corner of the group by studying the dynamic wind pressures on the silo surfaces and linking to the dynamic properties of the silo structures. As the orientation of the group largely affects the pressure distribution around the cylinders of the group, the influence of the angle of incidence of the wind flow on these ovalling vibrations is examined while other parameters, such as spacing ratio and Reynolds number are unchanged. To achieve results within a reasonable computation time, 2D unsteady Reynolds averaged Navier–Stokes (URANS) equations using Menter's shear stress transport turbulence model were performed. In order to elucidate the influence of the applied turbulence model and to qualitatively validate the spatial and temporal discretization of the 2D highly turbulent post-critical ( Re = 1.24 × 10 7 ) flow simulations for the silo group, single cylinder simulations were used. The geometric resemblance of the group arrangement with rectangular cylinders on the one hand and of the interstitial spaces with tube arrays (e.g. heat exchangers) on the other hand is used to qualitatively compare the observed flow phenomena. The simulations show that the silo group can be treated neither as a tube array nor as a solid bluff body. Subsequent linking of dynamic wind pressures to dynamic properties of the silo structures reveals strong narrow band frequency peaks in the turbulent pressure coefficient spectra of the silos near the lee side corners of the group that match the structural natural frequencies of the third and fourth ovalling mode shape of the silos. This match indicates a forced, resonant response which corresponds with the observed pattern of ovalling vibrations with three and four circumferential wavelengths. While the precise physical excitation mechanism is not yet fully understood, the simulations exclude discrete vortex shedding and since fluidelastic instability could not be considered, only turbulent buffeting remains which could very well give rise to the narrow band wake phenomena causing the ovalling silo wall vibrations.

Cemented granular aggregates include a broad class of geomaterials such as sedimentary rocks and some biomaterials such as the wheat endosperm. We present a 3D lattice element method for the simulation of such materials, modeled as a jammed assembly of particles bound together by a matrix partially filling the interstitial space. From extensive simulation data, we analyze the mechanical properties of aggregates subjected to tensile loading as a function of matrix volume fraction and particle-matrix adhesion. We observe a linear elastic behavior followed by a brutal failure along a fracture surface. The effective stiffness before failure increases almost linearly with the matrix volume fraction. We show that the tensile strength of the aggregates increases with both the increasing tensile strength at the particle-matrix interface and decreasing stress concentration as a function of matrix volume fraction. The proportion of broken bonds in the particle phase reveals a range of values of the particle-matrix adhesion and matrix volume fraction for which the cracks bypass the particles and hence no particle damage occurs. This limit is shown to depend on the relative toughness of the particle-matrix interface with respect to the particles.

This article reports on the inland re-intensification of tropical storm (TS) Erin (2007). In this research, the physical processes that resulted in the re-intensification of TS Erin over Oklahoma, USA, on 19 August 2007 was determined and a sensitivity study on microphysics, planetary boundary layer and convective parameterisation schemes was performed in the mesoscale modelling system, MM5. Also, we diagnosed and explained the remarkable difference between model behaviour of the original KainFritsch 1 (KF1) scheme and its revised counterpart (KF2). The numerical results showed only modest sensitivity to the selected microphysics schemes the relatively simple ‘Simple Ice’ and the advanced Reisner-Graupel. We found a relatively high sensitivity to the selected boundary layer parameterisation. Enhanced mixing in the medium range forecast (MRF) scheme leads to a relatively small convective available potential energy (CAPE), a deeper boundary layer and a lower dew point temperature, thus to a relatively stable environment. Therefore, MRF forecasts less precipitation (up to 150 mm) than the local mixing scheme, ETA. Model results appeared most sensitive to the selected convection schemes, that is, Grell, KF1 and KF2. With Grell and KF1, Erin intensifies and produces intense precipitation, but its structure remains close to a mesoscale convective system (MCS) or squall line rather than of the observed tropical cyclone. Both schemes also simulate the most intense precipitation too far south (100 km) compared to observations. On the contrary, KF2 underestimates precipitation, but the track of the convection, the precipitation and the pressure distribution are relatively close to radar and field observations. A sensitivity study reveals that the downdraft formulation is critical to modelling TS Erin’s dynamics. Within tropical cyclogenesis, the mid-level relative humidity (RH) is generally very high, resulting in very small downdrafts. KF2 generates hardly any downdrafts due to its dependence on mid-level RH. However, KF1 and Grell generate much stronger downdrafts because they both relate the downdraft mass flux (DMF) to vertical wind shear. This larger DMF then completely alters TS Erin’s dynamics. A modified version of the Grell scheme with KF2 RH-dependent downdraft formulation improved the simulation considerably, with better resemblance to observations on trajectory, radar reflectivity and system structure. Keywords: Erin, tropical storm, re-intensification, MM5, parameterisation, downdraft, convection, precipitation efficiency (Published: 14 August 2012) Citation: Tellus A 2012, 64 , 17417, http://dx.doi.org/10.3402/tellusa.v64i0.17417

International audience; The main goal of the present study is to optimise the precharge conditions such as the precharge location and dimensions that give significant effects on the mechanical performance of composite structures manufactured by the compression moulding process. As preliminary step of optimisation, we developed a manufacturing simulation program to predict the fibre volume fraction and fibre orientation. And coupled with this simulation program and a structural analysis program, a genetic algorithm (GA) is implemented to optimise the precharge conditions. The penalty function method and the repair algorithm are modified for handling constraints. The repair algorithm is applied to a symmetric structure and an arbitrary shape structure to find optimal precharge conditions. (C) 2011 Elsevier Ltd. All rights reserved.

Recently, the first prototype ovomucoid-based pressure–temperature–time indicator (pTTI) for high pressure high temperature (HPHT) processing was described. However, for temperature uniformity mapping of high pressure (HP) vessels under HPHT sterilization conditions, this prototype needs to be optimized. To this end, this work aimed at the development of an ovomucoid-based indicator with combined pressure temperature dependent inactivation kinetics and a sufficient pressure temperature stability relevant for commercial HPHT sterilization. After varying buffer type and the pH at ambient pressure and temperature (pH i ), an indicator based on 1 g/L ovomucoid in 0.1 M MES-NaOH buffer pH i 6.2 was selected. The inactivation behavior of this indicator system is characterized by pressure temperature dependent (combined Arrhenius–Eyring) first-order kinetics in the processing domain relevant for HPHT sterilization. This indicator showed good integrating properties under isobaric–isothermal and dynamic pressure temperature conditions. In a temperature uniformity study of a vertically oriented, pilot-scale HPHT vessel, pTTI readouts at different coordinates illustrated low and high temperature zones. As the inactivation of spores under HPHT is clearly positively temperature dependent, the food safety objective has to be verified in the former sampling zone.

Being a discrete-continuous process, approach to a cyclic steady state in computer simulation of Pressure Swing Adsorption is through iterative procedures and simulation itself is quite computation-intensive. Considering the fact that simulation based design itself is an iterative process, it is imperative that simulation be computationally very efficient and phenomenologically as close to the physics of adsorption-desorption as possible. Utility of lumping the components of a gas mixture into fewer pseudo-components was computationally examined in the simulation of a representative multi-step cycle of a pressure swing based adsorptive separation process applied to natural gas treatment. The actual feed had six components competing for adsorbent sites. Five different lumping alternatives were studied and compared with the simulation results for a full six-component simulation under identical equipment dimensions and operating conditions. Lumping could reduce the number of equations to be solved by more than half and the corresponding reduction in CPU time was about 90%. The six component mixture of Natural Gas was found to be sufficiently represented by two pseudo-components. The predicted recovery (in terms of Methane and Ethane) and quality (in terms of content of higher hydrocarbons) of the raffinate differed by not more than 0.8% and 0.02% respectively. The paper discusses possible heuristics for decision-making regarding appropriate lumping as verified by extensive simulation studies.

Extrudate swell is a common phenomenon observed in the polymer extrusion industry. Accurate prediction of the dimensions of an extrudate is important for appropriate design of dies for profile extrusion applications. Prediction of extrudate swell has been challenging due to (i) difficulties associated with accurate representation of the constitutive behavior of polymer melts, and (ii) difficulties associated with the simulation of free surfaces, which requires special techniques in the traditionally used Eulerian framework. In a previous work we had argued that an Arbitrary Lagrangian Eulerian (ALE) based finite element formulation may have advantages in simulating free surface deformations such as in extrudate swell. In the present work we reinforce this argument by comparing our ALE simulations with experimental data on the extrudate swell of commercial grades of linear polyethylene (LLDPE) and branched polyethylene (LOPE). Rheological behavior of the polymers was characterized in shear and uniaxial extensional deformations, and the data was modeled using either the Phan-Thien Tanner (PTT) model or the eXtended Pom-Pom (XPP) model. Additionally, flow birefringence and pressure drop measurements were done using a 10:1 contraction-expansion (CE) slit geometry in a MultiPass Rheometer. Simulated pressure drop and contours of the principal stress difference were compared with experimental data and were found to match well. This provided an independent test for the accuracy of the ALE code and the constitutive equations for simulating a processing-like flow. The polymers were extruded from long (L/D=30) and short (L/D=10) capillaries dies at 190 degrees C. ALE simulations were performed for the same extrusion conditions and the simulated extrudate swell showed good agreement with the experimental data. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.

Image-based computational modeling offers a virtual access to spatially and temporally high resolution flow and structural mechanical data in vivo. Due to inter-subject morphological variability, mesh generation represents a critical step in modeling the patient-specific geometry and is usually performed using unstructured tetrahedral meshing algorithms. Although hexahedral structured meshes are known to provide higher accuracy and reduce the computational costs both for Finite Element Analysis and Computational Fluid Dynamics, their application in computational cardiovascular studies is challenging due to the complex 3D and branching topology of vascular territories. In this study, we propose a robust procedure for structured mesh generation, tailoring the mesh structure to the subject-specific vessel topology. The proposed methodology is based on centerline-based synthetic descriptors (i.e. centerlines, radii and centerlines' normals) which are used to solve the meshing problem following a bottom-up approach. First, topologically equivalent block-structures are placed inside and outside the lumen domain. Then, a projection operation is performed, returning a parametric volume mesh which fits the original triangulated model with sub-micrometric accuracy. Additionally, a three-layered arterial wall (resembling the intima, media and adventitia) is artificially generated, with the possibility of setting variable thickness (e.g. proximal-to-distal tapering) and material anisotropy (e.g. position-dependent collagen-fibers' orientation). This new meshing procedure, implemented using open-source software packages only, is demonstrated on two challenging human cases, being an aortic arch and an abdominal aortic aneurysm. High-quality meshes are generated in both cases, according to shape-quality metrics. By increasing the computation accuracy, the developed meshing tool has the potential to further add "confidence" to the use of computational methods in vascular applications.

Suspension of solids in stirred reactor is widely used for catalytic reactions, dissolution, etc. Quality of solid suspension is an important parameter required for the reliable design, optimum performance, and scale up of the system. Quality of suspension depends on local characteristics of solid velocity and hold up profiles. The present work was focused on investigating quality of solid suspension using ultrasound velocity profiler (UVP) measurements and CFD simulations. The slip velocity measurements carried out with UVP were used to evaluate different drag correlations used in CFD simulations. Results discussed in this work would be useful for extending the applications of CFD models for simulating large stirred slurry reactors.

International audience; The behaviour of a particle bed subjected to viscous flow upstream of a weir is experimentally examined. Experiments were conducted in a rectangular flume in which oil circulated above a bed of solid particles. Within about 150 mm upstream of the weir, the bed underwent a significant change and took the shape of a dune. The temporal evolutions of dune position and dune size were determined for various discharges and weir heights. At the steady state, both parameters were independent of discharge but affected by the weir height. Results indicate that the position of the axial dune crest varies linearly with the weir height. The two mechanisms controlling the formation and evolution of the dune are the particle bed loading by the approach flow and the particle entrainment inside the separation boundary layer.

In this work, a hypothesis is proposed for two-phase flow simulation: The number of grid points required for the grid-independent solution of the flow field is lesser than that required to capture the grid-independent interface. Based on this, a dual-grid level-set(LS) method (DGLSM) is proposed, where the LS and energy equations are solved on a uniform grid which is twice finer than the continuity and momentum equations. This novel grid arrangement is aimed at improving interface resolution with only a slight increase in computational time. Qualitative and quantitative results of the DGLSM are compared against analytical, experimental, and numerical results on three stringent test problems: dam break, Rayleigh-Taylor instability, and film boiling. The results show that the DGLSM is accurate and robust, even for surface tension-dominant flows. Moreover, the hypothesis is corroborated by showing that the DGLSM is nearly as accurate as the completely refined traditional LS method at substantially less computational expense.

Domains composed of a porous part and an adjacent free-flow region are of special interest in many fields of application. So far, the coupling of free flow with porous-media flow has been considered only for single-phase systems. Here we extend this classical concept to two-component nonisothermal flow with two phases inside the porous medium and one phase in the free-flow region. The mathematical modeling of flow and transport phenomena in porous media is often based on Darcy's law, whereas in free-flow regions the (Navier-) -Stokes equations are used. In this paper, we give a detailed description of the employed subdomain models. The main contribution is the developed coupling concept, which is able to deal with compositional (miscible) flow and a two-phase system in the porous medium. It is based on the continuity of fluxes and the assumption of thermodynamic equilibrium, and uses the Beavers-Joseph-Saffman condition. The phenomenological explanations leading to a simple, solvable model, which accounts for the physics at the interface, are laid out in detail. Our model can account for evaporation and condensation processes at the interface and is used to model evaporation from soil influenced by a wind field in a first numerical example

We perform a numerical analysis of the decelerated swirling flow into the discharge cone of a model Francis turbine operated at variable discharge and constant head, using an axisymmetric turbulent swirling flow model and a corresponding simplified computational domain. Inlet boundary conditions correspond to velocity and turbulent kinetic energy profiles measured downstream the Francis runner. Our numerical results are validated against experimental data on a survey section further downstream in the cone, showing that the Reynolds stress turbulence model with a quadratic pressure-strain term correctly captures the flow field. It is shown that the diffuser performance quickly deteriorates as the turbine discharge decreases, due to the occurrence and development of vortex breakdown, with a central quasistagnant region. We investigate a novel flow control technique, which uses a water jet injected from the runner crown tip along the axis. It is shown that the jet discharge can be optimized for minimum overall losses, while the vortex breakdown is eliminated. This flow control method is useful for mitigating the Francis turbine flow instabilities when operating at partial discharge.

A cohesive zone interface element degradation law is presented for analyzing delamination crack propagation under cyclic loading. Development of the law is based on a detailed study of the numerical cohesive zone and the extraction of strain energy release rate from this zone, enabling a direct link with experimental Paris Law data. The law is implemented using three-dimensional interface elements within the explicit finite element code LS-Dyna. Validation is presented by way of application to composite material fatigue fracture toughness tests; Double Cantilever Beam for Mode I, End Notch Flexure for mode II and Mixed Mode Bending for the mixed mode case. In all cases good agreement with experimental data available in the open literature and/or theoretical solutions was obtained. A cohesive zone interface element degradation law is presented for analyzing delamination crack propagation under cyclic loading. Development of the law is based on a detailed study of the numerical cohesive zone and the extraction of strain energy release rate from this zone, enabling a direct link with experimental Paris Law data. The law is implemented using three-dimensional interface elements within the explicit finite element code LS-Dyna. Validation is presented by way of application to composite material fatigue fracture toughness tests; Double Cantilever Beam for Mode I, End Notch Flexure for mode II and Mixed Mode Bending for the mixed mode case. In all cases good agreement with experimental data available in the open literature and/or theoretical solutions was obtained.

Ventilation processes inside the greenhouse strongly affect air renewal and internal climatic conditions, which themselves interact with the growth and homogeneity of the crop. Natural ventilation is often chosen since it is the most economic method available. Studies of internal distributed climate induced by ventilation have been taking place for the past 25 years. Experimental studies have pointed out the impact of vent configurations on airflow pattern, particularly when the wind is the main driving force. However, the development of computational fluid dynamics (CFDs) has only recently provided the opportunity to simulate the climate inside greenhouses for known vent configurations, and to test a wide range of geometries with different vent combinations under different climatic conditions. In this article, the main factors governing air movements inside the greenhouse are first analysed. The characteristics of the laboratory scale models and field experiments are reviewed, with particular focus on the technologies implemented. The principles of CFD, the main modelling approach, together with its adaptations to greenhouse climate simulation, are then described in detail. Conclusions of studies concerning ventilation efficiency inside greenhouses are reviewed with respect to greenhouse geometry and opening arrangements. Other parameters affecting ventilation, such as wind speed and direction, the addition of insect-proof or shading screens, and interactions with the crop, are also discussed.

A major computational challenge for a multiscale modeling is the coupling of disparate length and timescales between molecular mechanics and macroscopic transport, spanning the spatial and temporal scales characterizing the complex processes taking place in flow-induced blood clotting. Flow and pressure effects on a cell-like platelet can be well represented by a continuum mechanics model down to the order of the micrometer level. However, the molecular effects of adhesion/aggregation bonds are on the order of nanometer. A successful multiscale model of platelet response to flow stresses in devices and the ensuing clotting responses should be able to characterize the clotting reactions and their interactions with the flow. This paper attempts to describe a few of the computational methods that were developed in recent years and became available to researchers in the field. They differ from traditional approaches that dominate the field by expanding on prevailing continuum-based approaches, or by completely departing from them, yielding an expanding toolkit that may facilitate further elucidation of the underlying mechanisms of blood flow and the cellular response to it. We offer a paradigm shift by adopting a multidisciplinary approach with fluid dynamics simulations coupled to biophysical and biochemical transport. Ann Biomed Eng

The two fluid model is used to simulate upward gas-liquid bubbly flow through a vertical conduit. Coalescence and breakup of bubbles have been accounted for by embedding the population balance technique in the two fluid model. The simulation enables one to track the axial development of the voidage pattern and the distribution of the bubbles. Thereby it has been possible to propose a new criterion for the transition from bubbly to slug flow regime. The transition criteria depend on (i) the breakage and coalescence frequency, (ii) the bubble volume count below and above the bubble size introduced at the inlet, and (iii) the bubble count histogram. The prediction based on the present criteria exhibits excellent agreement with the experimental data. It has also been possible to simulate the transition front bubbly to dispersed bubbly flow at a high liquid flow rate using the same model. [DOI: 10.1115/1.3203205]

We perform three-dimensional numerical and experimental study of the dynamic contact angle using volume of fluid (VOF) method applied to microfluidic channels with integrated pillars. Initially, we evaluated different dynamic contact angle models (hydrodynamic, molecular kinetic and empirical) for capillary filling of a two-dimensional microchannel using analytical formulation. Further, the models which require a minimum prescription of adjustable parameters are only used for the study of capillary filling of microchannels with integrated pillars using different working fluids such as DI water, ethanol and isopropyl alcohol. Different microchannel geometry with varying diameter/height/spacing were Studied for circular pillars. Effect of square pillars and changing the overall number of pillars on the capillary phenomena were also simulated. Our Study demonstrated that the dynamic contact angle models modifies the transient response of the meniscus displacement and also the observed trends are model specific for the various microchannel geometries and working fluids. However, the different models have minimal effect on the meniscus profile. Different inlet boundary conditions were applied to observe the effect of grid resolution selected for numerical study on the capillary filling time. A grid dependent dynamic contact angle model which incorporates effective slip in the model was also used to observe the grid convergence of the numerical results. The grid independence was shown to improve marginally by applying the grid dependent dynamic contact angle model. Further we did numerical experiments of capillary filling considering variable Surface wettability on the top and bottom walls of the microchannel with alternate hydrophilic-hydrophobic patterns. The meniscus front pinning was noticed for a high wetting contrast between the patterns. Non uniform streamline patterns indicated mixing of the fluid when using patterned walls. Such a microfluidic device with variable surface properties with integrated pillars may be useful for carrying out biological operations that often require effective separation and mixing of the fluids. (C) 2009

The level set (LS) method is one of the most popular and recent methods for two-phase flow simulation. However, its origin and governing equations rely heavily on mathematical sources. The present work attempts mass and volume conservation law-based derivation of the LS advection equation and the continuity equation, respectively, for two-phase flow with phase change. Physical interpretation of the Heaviside function and the Dirac delta function, used in the LS method, is done here in a novel way and the resulting expressions are used in the derivations. The diffused interface, used in the LS method, is shown to introduce negligible error on a sufficiently fine grid.

We derive an analytic expression for the matric flux potential (M) for van Genuchten-Mualem (VGM) type soils which can also be written in terms of a converging infinite series. Considering the first four terms of this series, the accuracy of the approximation was verified by comparing it to values of M estimated by numerical finite difference integration. Using values of the parameters for three soils from different texture classes, the proposed four-term approximation showed an almost perfect match with the numerical solution, except for effective saturations higher than 0.9. Including more terms reduced the discrepancy but also increased the complexity of the equation. The four-term equation can be used for most applications. Cases with special interest in nearly saturated soils should include more terms from the infinite series. A transpiration reduction function for use with the VGM equations is derived by combining the derived expression for M with a root water extraction model. The shape of the resulting reduction function and its dependency on the derivative of the soil hydraulic diffusivity D with respect to the soil water content theta is discussed. Positive and negative values of dD/d theta yield concave and convex or S-shaped reduction functions, respectively. On the basis of three data sets, the hydraulic properties of virtually all soils yield concave reduction curves. Such curves based solely on soil hydraulic properties do not account for the complex interactions between shoot growth, root growth, and water availability

Udgivelsesdato: 15. april Birdsong has developed into one of the important models for motor control of learned behaviour and shows many parallels with speech acquisition in humans. However, there are several experimental limitations to studying the vocal organ - the syrinx - in vivo. The multidisciplinary approach of combining experimental data and mathematical modelling has greatly improved the understanding of neural control and peripheral motor dynamics of sound generation in birds. Here, we present a simple mechanical model of the syrinx that facilitates detailed study of vibrations and sound production. Our model resembles the ‘starling resistor', a collapsible tube model, and consists of a tube with a single membrane in its casing, suspended in an external pressure chamber and driven by various pressure patterns. With this design, we can separately control ‘bronchial' pressure and tension in the oscillating membrane and generate a wide variety of ‘syllables' with simple sweeps of the control parameters. We show that the membrane exhibits high frequency, self-sustained oscillations in the audio range (>600 Hz fundamental frequency) using laser Doppler vibrometry, and systematically explore the conditions for sound production of the model in its control space. The fundamental frequency of the sound increases with tension in three membranes with different stiffness and mass. The lowerbound fundamental frequency increases with membrane mass. The membrane vibrations are strongly coupled to the resonance properties of the distal tube, most likely because of its reflective properties to sound waves. Our model is a gross simplification of the complex morphology found in birds, and more closely resembles mathematical models of the syrinx. Our results confirm several assumptions underlying existing mathematical models in a complex geometry.

In this study, we compared microchannel droplet formation in a microfluidics device with a two phase lattice Boltzmann simulation. The droplet formation was found to be qualitatively described, with a slightly smaller droplet in the simulation. This was due to the finite thickness of the interface in the simulations. Dependence on dispersed flow rate could be very nicely predicted by the model, while a better insight was obtained on the internal pressures and flow velocities during droplet formation. These were found to be well described by simple relations; (1) the pressure inside the dispersed phase was predicted very well by the Laplace pressure while (2) the flow rate through the neck could be estimated by the flow through an orifice. These insights simplify the development of design rules for new microchannel devices.

Average shock spacing and average standing wavelength are estimated from computed near-field of axisymmetric underexpanded screeching jets at low supersonic Mach numbers. These are used to predict screech frequencies based on existing formulations which rely either on shock spacing or on standing wavelength. Results obtained suggest that shock spacing and standing wavelength may respectively be dominant length scales for A1 and A2 axisymmetric screech modes observed for axisymmetric jets at low supersonic Mach numbers. This implies that presence of two axisymmetric screech modes in supersonic jet screech could be attributed to the two co-existing length scales in the jet near-field.

In this paper we review simulation methods, which address suspension flows at different length scales. Taking the multiscale problem of confined suspension flow in fractionation and sorting applications as a guiding example, we discuss these numerical methods for use in a multiscale simulation framework. We have classified the methods in terms of dimensionless numbers, governing the physics of confined suspension flows, and the resolution of particles on the computation grid. From our literature review, we conclude, that for suspension flows at Peclet numbers larger than unity, Lattice Boltzmann is a good method to implement for all levels of the multiscale simulation framework.

Purpose Application of a comprehensive, user-friendly, digital computer circulatory model to estimate hemodynamic and ventricular variables. Methods The closed-loop lumped parameter circulatory model represents the circulation at the level of large vessels. A variable elastance model reproduces ventricular ejection. The circulatory model has been modified embedding an algorithm able to adjust the model parameters reproducing specific circulatory conditions. The algorithm reads input variables: heart rate, aortic pressure, cardiac output, and left atrial pressure. After a preliminary estimate of circulatory parameters and ventricular elastance, it adjusts the amount of circulating blood, the value of the systemic peripheral resistance, left ventricular elastance, and ventricular rest volume. Input variables and the corresponding calculated variables are recursively compared: the procedure is stopped if the difference between input and calculated variables is within the set tolerance. At the procedure end, the model produces an estimate of ventricular volumes and Emaxl along with systemic and pulmonary pressures (output variables). The procedure has been tested using 4 sets of experimental data including left ventricular assist device assistance. Results The algorithm allows the reproduction of the circulatory conditions defined by all input variable sets, giving as well an estimate of output variables. Conclusions The algorithm permits application of the model in environments where the simplicity of use and velocity of execution are of primary importance. Due to its modular structure, the model can be modified adding new circulatory districts or changing the existing ones. The model could also be applied in educational applications.

The growing transport of LNG in partially filled tanks raises the demand to have accurate methods to predict the fluid behaviour in these sloshing tanks and the effect of the sloshing fluid on the tanker motion. To examine the motion of the sloshing fluid, model experiments have been carried out on a scale of 1:10. Different tank filling ratios and types of motion have been tested to study the sloshing fluid behaviour for various sea states. The model experiments have been carried out to provide extensive validation material for the numerical method COM-FLOW. The details of this improved Volume Of Fluid (iVOF) method are presented in the paper The method resolves the governing equations in both fluids, one of them being compressible. The compressibility of the second phase is especially important for more violent flow conditions, when two-phase phenomena such as air entrapment and air entrainment occur frequently Particular attention in the numerical method has been paid to the treatment of the flow variables around the interface, especially the density. The fluid is convected by means of a first- or second-order upwind scheme. The behaviour of the sloshing fluid strongly depends upon the regularity of the tank motion and the filling ratio of the tank. Video frames, wave probes and pressure transducers have been used to compare the fluid flow of simulation and experiment. Two-phase effects such as air entrapment are more common for increasing tank filling ratios and for more irregular tank motion. A realistic simulation of these effects is possible by modeling two-phase flow, especially when using a relatively fine grid and applying the less-dissipative second-order upwind scheme. Compared to the earlier paper on the numerical simulation of sloshing in LNG tanks [8], where the numerical method was validated for regular sway motion, more extensive attention is paid to the accuracy of the applied discretisation schemes in space and time. The results of different schemes are now evaluated for both regular and irregular sway and roll motion of LNG tanks.

Hydrological models featuring root water uptake usually do not include compensation mechanisms such that reductions in uptake from dry layers are compensated by an increase in uptake from wetter layers. We developed a physically based root water uptake model with an implicit compensation mechanism.Hydrological models featuring root water uptake usually do not include compensation mechanisms such that reductions in uptake from dry layers are compensated by an increase in uptake from wetter layers. We developed a physically based root water uptake model with an implicit compensation mechanism. Based on an expression for the matric flux potential (M) as a function of the distance to the root, and assuming a depth-independent value of M at the root surface, uptake per layer is shown to be a function of layer bulk M, root surface M, and a weighting factor that depends on root length density and root radius. Actual transpiration can be calculated from the sum of layer uptake rates. The proposed reduction function (PRF) was built into the SWAP model, and predictions were compared to those made with the Feddes reduction function (FRF). Simulation results were tested against data from Canada (continuous spring wheat [(Triticum aestivum L.]) and Germany (spring wheat, winter barley [Hordeum vulgare L.], sugarbeet [Beta vulgaris L.], winter wheat rotation). For the Canadian data, the root mean square error of prediction (RMSEP) for water content in the upper soil layers was very similar for FRF and PRF; for the deeper layers, RMSEP was smaller for PRF. For the German data, RMSEP was lower for PRF in the upper layers and was similar for both models in the deeper layers. In conclusion, but dependent on the properties of the data sets available for testing,the incorporation of the new reduction function into SWAP was successful, providing new capabilities for simulating compensated root water uptake without increasing the number of input parameters or degrading model performance.

The growing transport of LNG in partially filled tanks raises the demand to have accurate methods to predict the fluid behaviour in these sloshing tanks and the effect of the sloshing fluid on the tanker motion. To examine the motion of the sloshing fluid, model experiments have been carried out on a scale of 1:10. Different tank filling ratios and types of motion have been tested to study the sloshing fluid behaviour for various sea states. The model experiments have been carried out to provide extensive validation material for the numerical method ComFLOW . The details of this improved Volume Of Fluid (iVOF) method are presented in the paper. The method resolves the governing equations in both fluids, one of them being compressible. The compressibility of the second phase is especially important for more violent flow conditions, when two-phase phenomena such as air entrapment and air entrainment occur frequently. Particular attention in the numerical method has been paid to the treatment of the flow variables around the interface, especially the density. The fluid is convected by means of a first-or second-order upwind scheme. The behaviour of the sloshing fluid strongly depends upon the regularity of the tank motion and the filling ratio of the tank. Video frames, wave probes and pressure transducers have been used to compare the fluid flow of simulation and experiment. Two-phase effects such as air entrapment are more common for increasing tank filling ratios and for more irregular tank motion. A realistic simulation of these effects is possible by modeling two-phase flow, especially when using a relatively fine grid and applying the less-dissipative second-order upwind scheme. Compared to the earlier paper on the numerical simulation of sloshing in LNG tanks [8], where the numerical method was validated for regular sway motion, more extensive attention is paid to the accuracy of the applied discretisation schemes in space and time. The results of different schemes are now evaluated for both regular and irregular sway and roll motion of LNG tanks.Copyright © 2008 by ASME

The present study is carried out to investigate the influence of surface roughness in combined electro-osmotic and pressure-driven flow in microchannel. Two-dimensional theoretical model is developed to predict the behavior of velocity profiles in rough microchannel. The concept of surface roughness-viscosity model is used to account the effect of surface roughness. The pluglike velocity profile for electro-osmotic flow and the parabolic velocity profile for pressure-driven flow with delay in attaining the centerline velocity are observed. It is found that for electro-osmotic flow, the deviation in velocity profile from a flow in a smooth channel occurs near the wall, whereas in pressure-driven flow, such deviation is dominant in the core region. A superposition of pluglike and parabolic velocity profiles is found in combined electro-osmotic and pressure-driven flow. It is also observed that in the case of combined flow, the deviation in velocity profile from the smooth channel case reduces gradually with the distance from the wall.

The growing transport of LNG in partially filled tanks raises the demand to have accurate methods to predict the fluid behaviour in these sloshing tanks and the effect of the sloshing fluid on the tanker motion. To examine the motion of the sloshing fluid, model experiments have been carried out on a scale of 1:10. Different tank filling ratios and types of motion have been tested to study the sloshing fluid behaviour for various sea states.The model experiments have been carried out to provide extensive validation material for the numerical method COM-FLOW. The details of this improved Volume Of Fluid (iVOF) method are presented in the paper The method resolves the governing equations in both fluids, one of them being compressible. The compressibility of the second phase is especially important for more violent flow conditions, when two-phase phenomena such as air entrapment and air entrainment occur frequently Particular attention in the numerical method has been paid to the treatment of the flow variables around the interface, especially the density. The fluid is convected by means of a first- or second-order upwind scheme.The behaviour of the sloshing fluid strongly depends upon the regularity of the tank motion and the filling ratio of the tank. Video frames, wave probes and pressure transducers have been used to compare the fluid flow of simulation and experiment. Two-phase effects such as air entrapment are more common for increasing tank filling ratios and for more irregular tank motion. A realistic simulation of these effects is possible by modeling two-phase flow, especially when using a relatively fine grid and applying the less-dissipative second-order upwind scheme.Compared to the earlier paper on the numerical simulation of sloshing in LNG tanks [8], where the numerical method was validated for regular sway motion, more extensive attention is paid to the accuracy of the applied discretisation schemes in space and time. The results of different schemes are now evaluated for both regular and irregular sway and roll motion of LNG tanks.

1424-8220; International audience; Is it possible to derive accurately Total Suspended Matter concentration or its proxy, turbidity, from remote sensing data in tropical coastal lagoon waters? To investigate this question, hyperspectral remote sensing reflectance, turbidity and chlorophyll pigment concentration were measured in three coral reef lagoons. The three sites enabled us to get data over very diverse environments: oligotrophic and sediment-poor waters in the southwest lagoon of New Caledonia, eutrophic waters in the Cienfuegos Bay (Cuba), and sediment-rich waters in the Laucala Bay (Fiji). In this paper, optical algorithms for turbidity are presented per site based on 113 stations in New Caledonia, 24 stations in Cuba and 56 stations in Fiji. Empirical algorithms are tested at satellite wavebands useful to coastal applications. Global algorithms are also derived for the merged data set (193 stations). The performances of global and local regression algorithms are compared. The best one-band algorithms on all the measurements are obtained at 681 nm using either a polynomial or a power model. The best two-band algorithms are obtained with R412/R620, R443/R670 and R510/R681. Two three-band algorithms based on Rrs620.Rrs681/Rrs412 and Rrs620.Rrs681/Rrs510 also give fair regression statistics. Finally, we propose a global algorithm based on one or three bands: turbidity is first calculated from Rrs681 and then, if < 1 FTU, it is recalculated using an algorithm based on Rrs620.Rrs681/Rrs412. On our data set, this algorithm is suitable for the 0.2-25 FTU turbidity range and for the three sites sampled (mean bias: 3.6 %, rms: 35%, mean quadratic error: 1.4 FTU). This shows that defining global empirical turbidity algorithms in tropical coastal waters is at reach.

The occupational health and safety of the workers in chemical plants are of great importance because they are exposed to concentrations of toxic substances that may exceed institutional limits. This work presents a methodology of assessing the indoor dispersion of the toxic chemical substance Vinyl Chloride Monomer (VCM) in a Polyvinyl Chloride (PVC) chemical plant using Computational Fluid Dynamics (CFD) techniques. A mathematical model using the CFD Code PHOENICS based on solving the full 3-D Navier-Stokes and scalar conservation equations together with turbulence modelling is used to predict the toxic VCM dispersion in a geometrically complex industrial area. The source emissions, as well as the effect of the geometrical details of the building structure were also studied. The results showed that the use of a CFD is a promising technique to study the occupational exposure in the known carcinogen VCM and to design the proper ventilation system to reduce the consequences of an accidental release of VCM in a workplace. (C) 2007 Elsevier Ltd. All rights reserved. Environmental Modelling & Software

The defogging process on a windshield is one of the important concerns in the automotive industry for driver safety and certification. Therefore, the simulation of the windshield defogging process provides a very effective and useful tool for the designers to obtain a more efficient and rapid defogging process. In this paper, an experimental and numerical study is presented for the windshield defogging process of an automobile. A computational fluid dynamics (CFDs) tool was used to perform the relevant three dimensional (3-D) transient simulations. Both droplet and wall film models are employed for comparison of the demisted area on a windshield in the numerical calculations. The 3-D fluid flow, temperature distribution and heat transfer characteristics of the interior surfaces of the automobile cabin are also considered. It is shown that the results of the simulations are in a good agreement with the experimental data presented in this study.