AN IMPROVED DYNAMIC SUBGRID-SCALE STRESS MODEL

1 .　INTRODUCTIONFlowsincurvedductsoccurindiverseapplicationssuchascentrifugalpumps,turbines ,aircraftintakes,riverbends ,andsoon .Acharacteristic ,whichdistin guishessuchflowsfromthoseinstraightducts ,isthegenerationofstreamwisevorticity ,orsecondarymo tion ,resultinginapressurelossandthespatialredis tributionofstreamwisevelocitywithintheduct .Sore searchesonflowsthroughbendsaremoremeaningfulforengineeringapplications .AsaresultoftherapidadvancementofcomputercapabilityandComputationalFl…     

AN IMPROVED ALGORITHM FOR DPIV CORRELATION ANALYSIS

In a Digital Particle Image Velocimetry (DPIV) system, the correlation of digital images is normally used to acquire the displacement information of particles and give estimates of the flow field. The accuracy and robustness of the correlation algorithm directly affect the validity of the analysis result. In this article, an improved algorithm for the correlation analysis was proposed which could be used to optimize the selection/determination of the correlation window, analysis area and search path. This algorithm not only reduces largely the amount of calculation, but also improves effectively the accuracy and reliability of the correlation analysis. The algorithm was demonstrated to be accurate and efficient in the measurement of the velocity field in a flocculation pool.     

NUMERICAL STUDY ON AIR FLOW AROUND AN OPENING WITH LARGE EDDY SIMULATION

Jet characteristics of air supply opening in a ventilating or an air-conditioning system is primarily decided by the folw state in the duct connected to the opening. It is valuable to study the opening jet characteristics and the flow state in a duct. In this study, the Large Eddy Simulation (LES) technique combined with the Tarlor-Galerkin Finite Element Method ( FEM) in Computational Fluid Dynamics (CFD) was applied to the problem. The 3-D flow fields in ducts around air supply opening under typical conditions were investigated by numerical simulation as well as experimental measurements. Numerical results agree well with the available experimental data. It indicates that the LES method is available under the conditions with complicated boundaries and inner accompanied by anisotropic large-scale eddies, and it is credible to predict the jet deflection characteristics around an opening.     

STUDY ON TURBULENCE FEATURES NEAR AN OSCILLATING CURVED WALL

In pursuit of possibly true turbulent characters and for exploring a change in turbulence structures near an oscillating flexible wall-curved surface, a sinusoidal oscillation mode was forced to a curved wall, whose vibrations disturbed the flow with an interacting effort between the fluid and the structure. The methodology used was the Large Eddy Simulation (LES) with fluid-structure interaction. The oscillating configuration was on a Fourier sinusoidal mode from the measurements of a Francis hydro turbine blade vibration. The effects of the vibration on the skin friction coefficient, vortices, turbulent coherent structures, and other statistical quantities were studied. The results showed that the streamwise velocity gradient along the normal direction and the normal velocity gradient along the spanwise direction were considerably increased within the viscous sublayer because of the oscillating wall, which additionally caused the low speed streaks to stay away from the wall and the high-momentum flows to be toward the wall. As a result, the streamwise vortices were much more elongated along the downstream to get an energy balance, and the wall skin friction coefficient or the wall friction velocity rose up.     

CALCULATION OF FLOW AROUND AN AIRFOIL WITH SEPARATION USING CONFORMAL MAPPING

A new method is presented to calculate the flow around an airfoil with separation.The airfoil configuration is arbitrary and is transformed to a circle by conformal mapping tech-nique.The model of separated flow,which was proposed by Jacob,is adopted here,and the po-sition of the separation point and the pressure on the airfoil are calculated by iterative coupling ofpotential flow and boundary layer.The effects of boundary layer are simulated by modificationof boundary condition.All iterative procedures converge rapidly because of using Fast FourierTransform(FFT).     

AN UNSPLIT LAGRANGIAN ADVECTION SCHEME FOR VOLUME OF FLUID METHOD

An Unsplit Lagrangian Advection(ULA) scheme for Volume Of Fluid(VOF) method is presented in this article.The ULA scheme is developed based on an algorithm of Piecewise Linear Interface Construction(PLIC).The volume fluxes between cells are calculated through solving the new equation of the linear interfaces in cells in the ULA scheme.The fluxes flowing out from one cell is the inflow fluxes for another cell.In this way the whole fluid volume is conserved strictly without using any redistribution algorithms....     

AN INVESTIGATION OF TURBULENT HEAT TRANSFER IN CHANNEL FLOWS BY LARGE EDDY SIMULATION

Large Eddy Simulation (LES) of fully developed turbulent channel flow with heat transfer was performed to investigate the effects of the Reynolds number on the turbulence behavior. In the present study, the bottom wall of the channel was cooled and the top wall was heated. The Reynolds numbers, based on the central mean-velocity and the half-width of the channel, were chosen as 4000, 6000, 104 and 2×104, and the Prandtl number as 1.0. To validate our calculations, the present results were compared with available data obtained by Direct Numerical Simulation (DNS), which proves to be in good agreement with each other. To reveal the effects of the Reynolds number, some typical quantities, including the velocity fluctuations, temperature fluctuation, heat fluxes and turbulent Prandtl number, were studied.     

EXPERIMENTAL STUDY ON GAS-LIQUID TWO-PHASE FLOWS IN AN AREATION TANK BY USING IMAGE TREATMENT METHOD

Particle Image Velocimetry （PIV） technique was employed to study experimentally gas-liquid two-phase flow in an aeration tank. In terms of the PIV principles, an algorithm of PIV based on the Fast Fourier Transformation （FFT） was worked out. The PIV program was developed and verified, and then was used to measure three kinds of states in the testing device. The program was also used to calculate and analyze the related parameters. The experimental data indicate that the bubbles in testing device have the longest resident time and stronger turbulent intensity for the gas-liquid two-phase flow in a special case （Case 3）, resulting in great increase of the oxygen transferring speed and efficiency, whereby providing the basis for the selection design of aeration tank.     

AN ENHANCED ELEMENT-FREE GALERKIN METHOD FOR DYNAMIC RESPONSE OF POROELASTIC SEABED

This study presents an effective numerical model for the dynamic response of poroelastic seabed under wave action with enhanced performance. The spatial discretization is based on the Element-Free Galerkin (EFG) method and the time integration based on the GN11 scheme. A stability strategy that adopts a smaller number of nodes for the pore water pressure compared with those for the displacements of the soil skeleton is suggested to resolve the similar difficulty as encountered in the finite element method for a problem with mixed formulation when the pore water is incompressible and the soil skeleton impervious. The accuracy of the numerical model is verified through applying it to a typical case with critical permeability. Good agreement between computational and analytical solutions is obtained.     

基于Intranet/Internet的大坝安全监测信息查询系统

大坝安全监测信息的网上发布可以让人们了解各种实时、动态信息，文 中主要介绍了在Intranet/Internet下采用ASP(active server pages)结合VBScript，JavaS cript及ActiveX技术进行大坝安全监测信息查询系统开发的方法及步骤，并结合实际系统的 开发加以说明。     

WATER ENTRY OF A WEDGE BASED ON SPH MODEL WITH AN IMPROVED BOUNDARY TREATMENT

The hydrodynamic problem of a two-dimensional wedge entering water is studied based on Smoothed Particle Hydrodynamics (SPH) model. A non-reflection boundary treatment for SPH method is proposed to reduce the reflection of sound waves. The boundary pressure is obtained using an improved coupling boundary treatment approach, which is validated by comparing the simulation results with experimental and analytical results in literature. A series of cases with different initial entering velocities are simulated. The maximum force on the wedge and the corresponding time required to reach it for the different cases of initial entering velocities of the wedge are obained and fitted into formulas against the initial entering velocity of the wedge. The maximum drag coefficients of the wedge for the different cases with Froude number greater than 2 are all near the value of 0.91.     

EXPERIMENTAL AND NUMERICAL STUDY FOR HYDRODYNAMIC CHARACTERISTICS OF AN OSCILLATING HYDROFOIL

The present article reports the experimental Particle Image Velocimetry （PIV） investigation and the corresponding numerical simulation results about the water flow over the oscillating hydrofoil and its unsteady dynamic characters. The experimental study focuses on the effect of mean angles of attack. The comparison between the PIV results and numerical prediction about the flow field using Fluent well demonstrates the capability of CFD on the simulation of the water flow around the pitching hydrofoil. The numerical results indicate that the forced oscillating frequencies have evident effects on the flow separation and vortex shedding. The simulations about the hydrodynamic drag and lift coefficients were also performed.     

SIMULATION OF FLUID-SOLID INTERACTION ON WATER DITCHING OF AN AIRPLANE BY ALE METHOD

Ditching is considered as one of the important aspects of safety performances of airplanes.It is related primarily with the fluid-solid interaction,whose studies mainly depend on experiments at the present time.Numerical and analytical methods for fluid-solid interaction by using 3-D full scale airplane’s model will reduce the dependence on the expensive model tests.Numerical studies can be used to estimate the safety of ditching and provide a reference for the crashworthiness design.This article proposes a 3-D dynamical structural model after the real shape of an airplane and an Arbitrary Lagrange-Euler (ALE) fluid-field model,to simulate the fluid-solid interactions caused by low speed ditching.The simulation is based on interaction computational methods,within LS-DYNA nonlinear finite-element code.The results of pressure distributions and accelerating time histories of the airplane’s subfloor are discussed in the context of the safety of ditching,and the simulation results and the analytical methods are verified.     

AN ANALYSIS OF THE OXYGEN BALANCE IN AN ANOXIC TIDAL RIVER

ＡＮＡＮＡＬＹＳＩＳＯＦＴＨＥＯＸＹＧＥＮＢＡＬＡＮＣＥＩＮＡＮＡＮＯＸＩＣＴＩＤＡＬＲＩＶＥＲＪｏｓｅｐｈＨ．Ｗ．Ｌｅｅ，Ｋ．Ｗ．Ｃｈｏｉ（ＤｅｐａｒｔｍｅｎｔｏｆＣｉｖｉｌａｎｄＳｔｒｕｃｔｕｒａｌＥｎｇｉｎｅｅｒｉｎｇ，Ｕｎｉｖｅｒｓｉｔｙｏｆ...     

LARGE-EDDY SIMULATION OF THREE-DIMENSIONAL VORTICAL STRUCTURES FOR AN IMPINGING TRANSVERSE JET IN THE NEAR REGION

The three-dimensional vortical structures for an impinging transverse jet in the near region were numerically investigated by means of Large-Eddy Simulation (LES). The LES results reproduced the skewed jet shear layer vortices close to the jet nozzle and the scarf vortex in the near-wall zone, in good agreement with the experimental observations. Different vortical modes in the skewed jet shear layer close to the jet nozzle were identified depending upon the velocity ratio between jet and crossflow, namely changing from an approximately axisymmetric mode to a helical one with the velocity ratios varying from 20 to 8. Moreover, the scarf vortex wrapped around the impinging jet in the near-wall zone showed distinct asymmetry with regard to its bilateral spiral legs within the near region. And the entrainment of the ambient crossflow fluids by the scarf vortex in the near-wall zone was appreciably influenced by its asymmetry and in a large part occurred on the surface of the spiral roller structures in the course of spreading downstream.     

AN INVADER WITHIN AN ALTERED LANDSCAPE: ONE CATFISH,TWO RIVERS AND AN INTER‐BASIN WATER TRANSFER SCHEME

African sharptooth catfish Clarias gariepinus has become established as a non‐native invasive species in Eastern Cape, South Africa, where it was translocated primarily through an inter‐basin water transfer scheme into the Great Fish and Sundays rivers. This study examined the patterns in catfish distribution and abundance, and compared trophic niches in relation to the ichthyofauna of the two rivers. Correspondence analysis revealed upstream to downstream gradients associated with the spatial distribution in species richness for most species within the mainstream and mainstream to tributary gradients that were associated mostly with the spatial distribution of native minnows in both rivers. Catfish was predicted to occur widely within the mainstream habitats and to decrease progressively from mainstreams to tributaries. Based on classification and regression trees, the physico‐chemical environment was found to be a good proxy for predicting the occurrence and abundance of catfish. Although non‐significant relationships were observed between catfish and other native fish species abundances, the study suggests potential impact due to predation and interference in habitats where the invader co‐occurs with other fishes. Comparisons of trophic niches indicated higher trophic diversity for the mainstream ichthyofauna than the tributary communities in both rivers, suggesting an upstream to downstream continuum in community structure and resource availability. Catfish within the invaded mainstream had comparable trophic niches and similar dispersion patterns among individuals for both rivers, but indicated differences in shapes of scatter. This suggests that the catfish exhibited a differential response, probably in relation to resource availability, that may be indicative of its dietary plasticity. The study suggests the proliferation of catfish and its probable impact within the mainstream flow‐altered habitats where invasion resistance was possibly reduced. Comparisons of trophic niches provided information on its probable impact at different scales and the potential risk of invasion of tributaries inhabited by native minnow species. Copyright © 2012 John Wiley & Sons, Ltd.     

PIV MEASUREMENTS OF THE NEAR-WAKE FLOW OF AN AIRFOIL ABOVE A FREE SURFACE

The near-wake flow of a NACA0012 airfoils mounted above a water surface were experimentally studied in a wind/wave tunnel. The main objective of this study is to investigate the influence of the free surface on the structure of the airfoil trailing wake. The flow structure was measured with different ride heights between the airfoil and free surface using a Particle Image Velocimetry (PIV) system. The Reynolds number based on the chord length of the airfoil was about 3.5×103. For each experimental condition, large amount of instantaneous velocity fields were captured and ensemble-averaged to get the spatial distributions of mean velocity and mean vorticity, as well as turbulence statistics. The results show that the flow structures of the airfoil wake varies remarkably with the change in the ride height.     

NUMERICAL SIMULATION OF HYDRODYNAMIC CHARACTERISTICS ON AN ARC CROWN WALL USING VOLUME OF FLUID METHOD BASED ON BFC

In the present study, a new algorithm based on the Volume Of Fluid (VOF) method is developed to simulate the hydrodynamic characteristics on an arc crown wall. Structured grids are generated by the coordinate transform method in an arbitrary complex region. The Navier-Stokes equations for two-dimensional incompressible viscous flows are discretized in the Body Fitted Coordinate (BFC) system. The transformed SIMPLE algorithm is proposed to modify the pressure-velocity field and a transformed VOF method is used to trace the free surface. Hydrodynamic characteristics on an arc crown wall are obtained by the improved numerical model based on the BFC system (BFC model). The velocity field, the pressure field and the time profiles of the water surface near the arc crown wall obtained by using the BFC model and the Cartesian model are compared. The BFC model is verified by experimental results.     

NUMERICAL STUDY OF FLOW PAST AN AXISYMMETRIC BODY WITH A RING WING AT VARIOUS ATTACK ANGLES

1 .　INTRODUCTIONFortraditionalbody fittedgridsystemusedtocomputetheflow pasttheaxisymmetricbody ,thesingularityonthelongitudinalaxisisalongstand ingobstacle .Rationalsolutionishardtobeachievedevenwithfinemeshesgeneratedwithinthevicinityofthelongitudinalaxis .Hence ,appro priategridsystemisreguiredtoavoidthesingulari ty .Extensivenumericalexperimentsareconductedtoobtainthedesired gridsysteminthis paper .BasedontheideaofDDM[1] ,theoriginalflowfieldofthebodyisdividedinto 8sub domainsac cor…     

AN OPERATOR-SPLITTING ALGORITHM FOR ADVECTION-DIFFUSION-REACTION EQUATION

An operator-splitting algorithm for three-dimensional advection-diffusion-reactionequation is presented.The method of characteristics is adopted for the pure advection operator,the explicit difference scheme is used for diffusion,and a prediction-correction scheme is em-ployed for reaction.The condition for stability of the algorithm is analysed.Severall inear andnonlinear examples are illustrated to test the convergence and accuracy of the numerical proce-dure,and satisfactory agreements between computed and analytical solutions are achieved.Dueto its simplicity,stability,and validity for both one-and two-dimensional problems,the success-ful algorithm can be used to numerical simulations of viscous fluid flows,the transport of pollu-tants and sedimentations in reservoirs,lakes,rivers,estuaries and other environments,cooling-problems in heat or nuclear power plants,etc.