NUMERICAL SIMULATION OF TURBULENT FLOW CHARACTERISTICS IN A CURVED PIPE WITH A BAFFLE

A numerical study on the characteristics of developing turbulent flow in a curved pipe with a baffle was carried out in body-fitted coordinates with the k-ε model turbulence. A curved duct of square cross-section was examined first, and the results agree very well with the experimental data. Then two kinds of pipes, a normal curved pipe and that with a baffle were studied. The computational results are presented and compared with each other to illustrate the changes of the flow after adding the baffle. The longitudinal velocity in the pipe with a baffle was characterized by outer velocity bigger than the inner one. The secondary flow was characterized by four-vortex structure with the intensity reduced, which results in the equability of the flow field of the cross-section compared with that without a baffle, and has much more significant meaning in engineering.     

VISCOUS FLOW BEHAVIOR IN A CURVED ANNULAR DUCT

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FLOW AND HEAT TRANSFER OF OLDROYD-B FLUIDS IN A ROTATING CURVED PIPE

The flow and convected heat transfer of the Oldroyd-B fluids in a rotating curved pipe with circular cross-section were investigated by employing a perturbation method. A perturbation solution up to the second order was obtained for a small curvature ratio, κ. The variations of axial velocity distribution and secondary flow structure with F, Re and We were discussed in detail in order to investigate the combined effects of the three parameters on flow structure. The combined effects of the Coriolis force, inertia force and elastic force on the temperature distribution were also analyzed, which are greater than the adding independent effects of the three forces. The variations of the flow rate and Nusselt number with the rotation, inertia and elasticity were examined as well. The results show the characteristics of the heat and mass transfer of the Oldroyd-B fluids in a rotating curved pipe.     

NUMERICAL ANALYSIS OF THE FLUID FLOW IN A ROTATING CURVED ELLIPTICAL PIPE

A numerical study was conducted for the fully developed laminar flow in rotating curved elliptical pipe. Due to the rotation, the Coriolis force can also contribute to the secondary flow. The interaction of rotation and curvature complicates the flow characteristics. The boundary fitted coordinate was adopted to study the flow characteristic in the rotating systems. The effects of rotation on the flow transition were studied in detail. The generation and mergence of vortices in rotating curved elliptical pipes were also captured for the first time. The simulation results show that the flow for the case of large aspect ratio of the cross-section is more likely to be unstable than that for smaller one.     

A Partially-Averaged Navier-Stokes model for hill and curved duct flow

Turbulent flows past hill and curved ducts exist in many engineering applications. Simulations of the turbulent flow are carried out based on a newly developed technique, the Partially-Averaged Navier-Stokes (PANS) model, including separation, recirculation, reattachment, turbulent vortex mechanism. The focus is on how to accurately predict typical separating, reattaching and secondary motion at a reasonable computational expense. The effect of the parameter, the unresolved-to-total ratio of kinetic energy (f_k), is examined with a given unresolved-to-total ratio of dissipation (f_?) for the hill flow with a much coarser grid system than required by the LES. An optimal value of f_k can be obtained to predict the separation and reattachment locations and for more accurate simulation of the resolved turbulence. In addition, the turbulent secondary motions are captured by a smaller f_k as compared with the RANS method with the same grid.     

THE FLOW IN ROTATING CURVED CIRCULAR PIPE

1 .　ＩＮＴＲＯＤＵＣＴＩＯＮＴｈｅｂｅｈａｖｉｏｒｏｆｔｈｅｆｌｕｉｄｉｎａｒｏｔａｔｉｎｇｃｕｒｖｅｄｐｉｐｅｉｓａｆｕｎｄａｍｅｎｔａｌａｎｄｍｕｃｈ ｓｔｕｄｉｅｄｐｒｏｂｌｅｍ .Ｔｈｅｒｅａｒｅｍａｎｙｅｎｇｉｎｅｅｒｉｎｇａｐｐｌｉｃａｔｉｏｎｓｏｆｔｈｉｓｒｏｔａｔｉｎｇｓｙｓｔｅｍ ,ｓｕｃｈａｓｇａｓｔｕｒ ｂｉｎｅｓ ,ｅｌｅｃｔｒｉｃｇｅｎｅｒａｔｏｒｓａｎｄｅｌｅｃｔｒｉｃｍｏｔｏｒｓ.Ｔｈｅｓｙｓｔｅｍｃａｎａｌｓｏｂｅｆｏｕｎｄｉｎｓｅｐａｒａｔｉｏｎｐｒｏｃｅｓｓ.Ｉｔｉｓ…     

A NUMERICAL STUDY OF THE TRAIN-INDUCED UNSTEADY AIRFLOW IN A SUBWAY TUNNEL WITH NATURAL VENTILATION DUCTS USING THE DYNAMIC LAYERING METHOD

The objective of this study is to investigate numerically the characteristics of train-induced unsteady airflow in a subway tunnel with natural ventilation ducts. A three-dimensional numerical model using the dynamic layering method for the moving boundary of a train is first developed, and then it is validated against the model tunnel experimental data. With the tunnel and subway train geometries in the numerical model exactly the same as those in the model tunnel experimental test, but with the ventilation ducts being connected to the tunnel ceiling and a barrier placed at the tunnel outlet, the three-dimensional train-induced unsteady tunnel flows are numerically simulated. The computed distributions of the pressure and the air velocity in the tunnel as well as the time series of the mass flow rate at the ventilation ducts reveal the impact of the train motion on the exhaust and suction of the air through ventilation ducts and the effects of a barrier placed at the tunnel outlet on the duct ventilation performance. As the train approaches a ventilation duct, the air is pushed out of the tunnel through the duct. As the train passes the ventilation duct, the exhaust flow in the duct is changed rapidly to the suction flow. After the train passes the duct, the suction mass flow rate at the duct decreases with time since the air pressure at the opening of the duct is gradually recovered with time. A drastic change in the mass flow rate at a ventilation duct while a train passes the corresponding ventilation duct , causes a change in the exhaust mass flow rate at other ventilation ducts. Also, when a barrier is placed at the tunnel outlet, the air volume discharge rate at each ventilation duct is greatly increased, i.e., the barrier placed at the tunnel outlet can improve remarkably the ventilation performance through each duct.     

THE GAS-PARTICLE FLOW AND EROSION IN BENDS WITH A RECTANGULAR CROSS-SECTION

1.INTRODUCTIONWallerosionoccursinwidely--usedparticletransportsystems,especiallyatbends,whichconstitutesachallengingengineersproblem.Itisthereforeusefultostudyindetailthegas--particleflowandthewallerosioncausedbytheparticlesfortakingeffectivemeasurestoprotectpipesystemsfromwearing.Theexistingresearchworkfocusedoncurvedpipeswithroundcross--section.Onlyafewauthors,forexample,Mason,etal[3),paidattentiontorectangularcross--section,90"bends.Theirpaperspresentedtheexperimentalresults.Thispaper…     

Effect of vegetation on flow structure and dispersion in strongly curved channels

The effect of vegetation on the flow structure and the dispersion in a 180 o curved open channel is studied. The Micro ADV is used to measure the flow velocities both in the vegetation cases and the non-vegetation case. It is shown that the velocities in the vegetation area are much smaller than those in the non-vegetation area and a large velocity gradient is generated between the vegetation area and the non-vegetation area. The transverse and longitudinal dispersion coefficients are analyzed based on the experimental data by using the modified N- zone models. It is shown that the effect of the vegetation on the transverse dispersion coefficient is small, involving only changes of a small magnitude, however, since the primary velocities become much more inhomogeneous with the presence of the vegetation, the longitudinal dispersion coefficients are much larger than those in the non-vegetation case.     

Assessment of the predictive capability of RANS models in simulating meandering open channel flows

The predictive capability of Reynolds-averaged numerical simulation(RANS) models is investigated by simulating the flow in meandering open channel flumes and comparing the obtained results with the measured data. The flow structures of the two experiments are much different in order to get better insights. Two eddy viscosity turbulence models and different wall treatment methods are tested. Comparisons show that no essential difference exists among the predictions. The difference of turbulence models has a limited effect, and the near wall refinement improves the predictions slightly. Results show that, while the longitudinal velocities are generally well predicted, the predictive capability of the secondary flow is largely determined by the complexity of the flow structure. In Case 1 of a simple flow structure, the secondary flow velocity is reasonably predicted. In Case 2, consisting of sharp curved consecutive reverse bends, the flow structure becomes complex after the first bend, and the complex flow structure leads to the poor prediction of the secondary flow. The analysis shows that the high level of turbulence anisotropy is related with the boundary layer separation, but not with the flow structure complexity in the central area which definitely causes the poor prediction of RANS models. The turbulence model modifications and the wall treatment methods barely improve the predictive capability of RANS models in simulating complex flow structures.     

FLUID FLOW IN ROTATING HELICAL SQUARE DUCTS

1 .　INTRODUCTIONTherotatinghelicalsquareducthasextensive lyapplicationsinengineeringforenhancingtheratesofheatandmasstransfer ,suchascoolingsystemsforconductorsofelectric generatormo tors ,gasturbines ,separation processes .Becauseoftherotation ,theCoriolisforcecanalsocontrib utetothe generationofsecondaryflow ,whichcomplicatestheflowcharacteristics .To gethighperformanceofthesedevices,itisimportanttofindouttheflowcharacteristicsinrotatinghelicalsquareducts .Duetothecomplexityoftheproble…     

CURVED OPEN CHANNEL FLOW ON VEGETATION ROUGHENED INNER BANK

A RNG numerical model together with a laboratory measurement with Micro ADV are adopted to investigate the flow through a 180o curved open channel(a 4 m straight inflow section,a 180o curved section,and a 4m straight outflow section)partially covered with rigid vegetations on its inner bank.Under the combined action of the vegetation and the bend flow,the flow structure is complex.The stream-wise velocities in the vegetation region are much smaller than those in the non-vegetation region due to the retardation caused by the vegetation.For the same reason,no clear circulation is found in the vegetated region,while in the non-vegetation region,a slight counter-rotating circulation is found near the outer bank at both 90o and downstream curved cross-sections.A comparison between the numerical prediction and the laboratory measurement shows that the RNG model can well predict the flow structure of the bend flow with vegetation.Furthermore,the shear stress is analyzed based on the numerical prediction.The much smaller value in the inner vegetated region indicates that the vegetation can effectively protect the river bank from scouring and erosion,in other words,the sediment is more likely to be deposited in the vegetation region.     

子格（SGS）模型在内流湍流中的大涡模拟

本文将子格模型应用于大涡模拟，对直方管，弯方管和弯曲槽道内充分展湍流，进行了数值模拟，并将所得的结果与实验数据作比较，计算表明，应用于子格模型的大涡模拟可以用来模拟比较复杂的内流，研究湍流运动的结构，检验湍流模型。     

表孔门槽的空化特性分析

表孔门槽与二维凹槽在水力特性上有很大的差异，这决定了它们在空化特性上也有所不同.由于流动形式和堰面曲率的影响，表孔水流往往在堰面上压力低、流速高；在门槽底部，由于压差的作用，槽内旋涡常常随绕流从下游角隅绕出槽外.堰面、涡心以及绕流区的交汇，使压力剧烈下降，极易在门槽下游边墙底部发生混合型空化，其初生空化数远大于水洞中二维凹槽的初生空化数，应在设计中引起重视。     

多洞联合泄洪通风补气系统运行特性

结合水-气分层流理论,构建了如美泄洪洞与溢洪洞联合泄洪通风补气系统理论计算模型,研究了如美多条泄洪隧洞的通风补气特性,绘制并总结了如美各条泄洪隧洞的负压、补气竖井风速及通风量随泄洪洞洞顶余幅高度及其补气竖井截面尺寸的变化规律。结果表明：泄洪洞负压出现极小值和极大值并存的3分区,而溢洪洞负压无明显变化;泄洪洞补气竖井风速和通风量出现极大值,溢洪洞补气竖井通风量反之出现极小值。综合考虑各条隧洞通气参数的变化,可为联合泄洪系统中泄洪洞及其补气竖井尺寸的选择提供参考依据。     

排水系统入海口新型消能工

通过实体模型试验研究了某排水系统入海口水工建筑物的水力性能,设计提出一种新型的墩栅涡流室消能工。该新型消能工的水力特点是:流出排水隧洞的高速水流大部分通过进口墩栅空隙流入涡流室,小部分在进口墩栅前发生水跃后扎入涡流室中产生强烈的顺时针漩涡,通过水跃、墩栅的阻力和涡流室漩涡的共同作用消能,并将来流的动能转换为势能,之后,涡流室中的大部分水流在重力作用下通过出口墩栅空隙流向下游,其余水流从墩栅顶部溢流。这种新型的墩栅涡流室不仅消能效果良好而且出流均匀,并容许各种杂物如石块、泥沙等自由通过,避免涡流室中发生严重的淤积。     

桂花水电站掺气分流墩与消力池联合应用消能效果的研究

通过试验研究了桂花水电站拱坝掺气分流墩的体型、布置和水流流态,消力池和消力池中T型墩的体型和布置,掺气分流墩和消力池联合应用的水流流态和消能效果。试验表明,掺气分流墩与消力池联合应用明显改变了水流流态,具有显著的消能效果。     

THREE-DIMENSIONAL NUMERICAL MODELING OF SECONDARY FLOWS IN A WIDE CURVED CHANNEL

Most natural rivers are curved channels, where the turbulent flows have a complex helical pattern, as has been extensively studied both numerically and experimentally. The helical flow structure in curved channels has an important bearing on sediment transport, riverbed evolution, and pollutant transport study. In this article, different turbulence closure schemes i.e., the mixing-length model and the k – ? model with different pressure solution techniques i. e., hydrostatic assumptions and dynamic pressure treatments are applied to study the helical secondary flows in an experiment curved channel. The agreements of vertically-averaged velocities between the simulated results obtained by using different turbulence models with different pressure solution techniques and the measured data are satisfactory. Their discrepancies with respect to surface elevations, superelevations and secondary flow patterns are discussed.     

Simulation of subcritical flow pattern in 180° uniform and convergent open-channel bends using SSIIM 3-D model

In meandering rivers, the flow pattern is highly complex, with specific characteristics at bends that are not observed along straight paths. A numerical model can be effectively used to predict such flow fields. Since river bends are not uniform-some are divergent and others convergent-in this study, after the SSIIM 3-D model was calibrated using the result of measurements along a uniform 180° bend with a width of 0.6 m, a similar but convergent 180° bend, 0.6 m to 0.45 m wide, was simulated using the SSIIM...     

Flow imbalance in MHD flows in splitting ducts with locally different electric conductivities of the walls

In this study, three-dimensional liquid-metal magnetohydrodynamic splitting flows in a duct with one inflow channel and two outflow channels with locally different wall electric conductivities(different conductance ratios) are computationally examined. The numerical calculation is achieved with the use of CFX code, with a structured grid system chosen after a series of mesh independence tests. The detailed features of the MHD flows are depicted in a case where the electric conductivities of the two outflow ducts are quite different. Also, the interdependency among the current, fluid velocity, pressure and electric potential is elucidated in order to describe the electromagnetic characteristics of the liquid-metal flows. The result shows that the imbalance of the mass flow rate in the two outflow channels strongly depends on the wall conductivity(i.e., conductance ratio) of each outflow channel.