基于LES的泵站前池表面涡及液下涡流瞬态特性分析
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摘要
泵站前池或泵站进水池是泵进口前的过流部件,在不同工况的流动中存在多种旋涡,可分为自由表面涡和液下涡,这些高度不稳定的旋涡是影响水泵装置运行效率及稳定性的重要因素。为了研究泵站进水池的内部涡流,采用大涡模拟(LES)及流体体积函数(VOF)方法对泵站进水池中的三维非稳态流动进行了非定常数值计算,并进行了系统的验证。基于数值计算结果,统计了旋涡的时均特性,讨论了RANS方法和LES方法对涡流预测的区别;采用λ2等值面将旋涡结构可视化,观测到自由表面涡及附底涡周围环绕的二次涡结构;分析了不同时刻表面涡及附底涡的形态和瞬态特性,通过涡量方程得到了对流项及拉升、弯曲项对主涡涡量变化的影响。结果表明:二次涡与主涡的相互作用在一定程度上增强了主涡动量的向外耗散,并通过自旋引起主涡轴向拉升或者弯曲。
The hydraulic performance of water intake system is strongly influenced by the geometry of the pump intake and the approaching flow conditions. Undesirable vortices and the suction of air will be induced if a poor design of the system is made. An open pump intake was referred so that both the freesurface and sub-surface vortices could be taken into account under the influence of the geometry and neighboring flows. Large eddy simulation( LES) was employed to simulate the flow and the associated vortices in the pump intake and the fluctuation of free surface was resolved by means of volume of fluid model( VOF). The verification and validation of the simulated results were systematically performed. On the one hand,the mesh size near the wall was checked with y+and the LES index of quality( LES_IQ)was calculated which demonstrated the percentage of directly resolved turbulent kinetic energy in LES by using two sets of meshes with different grid quantities. On the other hand,the numerical results were compared with the well-known published experiments with respect to the transient flow feature and timeaveraged vorticity profile,where the disparities were also analyzed. Compared with most Reynolds averaged Navier-Stokes( RANS) based simulations,LES showed a better prediction for all kinds of vortices on location,shape,size of vortex core,velocity,as well as the turbulence kinetic energy inside vortex core. Based on the numerical results,time-averaged behavior of three typical vortices showed better similarities with reality that there was always a core region surrounding the axis where the azimuthal velocity stopped increasing and decreased to zero as radius went to zero. Besides,iso-surface of λ2 was adopted to visualize the vortices at different times,showing the main vortex and spirally rounding vortical structures. Transient behaviors of free surface and submerged vortices were analyzed,and the effects of advection and stretching/tilting termed on the vorticity variation were discussed via vorticity transport equation.
The hydraulic performance of water intake system is strongly influenced by the geometry of the pump intake and the approaching flow conditions. Undesirable vortices and the suction of air will be induced if a poor design of the system is made. An open pump intake was referred so that both the freesurface and sub-surface vortices could be taken into account under the influence of the geometry and neighboring flows. Large eddy simulation( LES) was employed to simulate the flow and the associated vortices in the pump intake and the fluctuation of free surface was resolved by means of volume of fluid model( VOF). The verification and validation of the simulated results were systematically performed. On the one hand,the mesh size near the wall was checked with y+and the LES index of quality( LES_IQ)was calculated which demonstrated the percentage of directly resolved turbulent kinetic energy in LES by using two sets of meshes with different grid quantities. On the other hand,the numerical results were compared with the well-known published experiments with respect to the transient flow feature and timeaveraged vorticity profile,where the disparities were also analyzed. Compared with most Reynolds averaged Navier-Stokes( RANS) based simulations,LES showed a better prediction for all kinds of vortices on location,shape,size of vortex core,velocity,as well as the turbulence kinetic energy inside vortex core. Based on the numerical results,time-averaged behavior of three typical vortices showed better similarities with reality that there was always a core region surrounding the axis where the azimuthal velocity stopped increasing and decreased to zero as radius went to zero. Besides,iso-surface of λ2 was adopted to visualize the vortices at different times,showing the main vortex and spirally rounding vortical structures. Transient behaviors of free surface and submerged vortices were analyzed,and the effects of advection and stretching/tilting termed on the vorticity variation were discussed via vorticity transport equation.
引文
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15王福军.流体机械旋转湍流计算模型研究进展[J/OL].农业机械学报,2016,47(2):1-14.http:∥www.j-csam.org/jcsam/ch/reader/view_abstract.aspx?flag=1&file_no=20160201&journal_id=jcsam.DOI:10.6041/j.issn.1000-1298.2016.02.001.WANG Fujun.Research progress of computational model for rotating turbulent flow in fluid machinery[J/OL].Transactions of the Chinese Society for Agricultural Machinery,2016,47(2):1-14.(in Chinese)
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17 TOKYAY T,CONSTANTINESCU S G.Large eddy simulation and Reynolds averaged Navier Stokes simulations of flow in a realistic pump intake:a validation study[C]∥World Water and Environmental Resources Congress,2005.
18 CELIK I B,CEHRELI Z N,YAVUZ I.Index of resolution quality for large eddy simulations[J].ASME Journal of Fluids Engineering,2005,127(5):949-958.
19 MARSHALL J S,BENINATI M L.External turbulence interaction with a columnar vortex[J].Journal of Fluid Mechanics,2005,540:221.
20 JI B,LUO X,ARNDT R E A,et al.Numerical simulation of three dimensional cavitation shedding dynamics with special emphasis on cavitation-vortex interaction[J].Ocean Engineering,2014,87:64-77.
21 JI B,LUO X,ARNDT R E A,et al.Large eddy simulation and theoretical investigations of the transient cavitating vortical flow structure around a NACA66 hydrofoil[J].International Journal of Multiphase Flow,2015,68:121-134.
2 ANSAR M,NAKATO T.Experimental study of 3D pump-intake flows with and without cross flow[J].Journal of Hydraulic Engineering,2001,127(10):825-834.
3 LI Y,WU Y,MANSA K,et al.The flow research in an open type pump sump by PIV experiments[C]∥ASME 2004 Heat Transfer/Fluids Engineering Summer Conference,2004:111-119.
4 MANSA K,ZHANG B,LI X,et al.PIV experimental investigation on the flow in a model of closed pump sump[J].Tsinghua Science and Technology,2003,8(6):681-686.
5 SUERICH-GULICK F,GASKIN S J,VILLENEUVE M,et al.Free surface intake vortices:theoretical model and measurements[J].Journal of Hydraulic Research,2014,52(4):502-512.
6 SUERICH-GULICK F,GASKIN S J,VILLENEUVE M,et al.Free surface intake vortices:scale effects due to surface tension and viscosity[J].Journal of Hydraulic Research,2014,52(4):513-522.
7 CONSTANTINESCU G S,PATEL V C.Numerical model for simulation of pump-intake flow and vortices[J].Journal of Hydraulic Engineering,1998,124(2):123-134.
8 RAJENDRAN V P,CONSTANTINESCU S G,PATEL V C.Experimental validation of numerical model of flow in pump-intake bays[J].Journal of Hydraulic Engineering,1999,125(11):1119-1125.
9 TOKYAY T E,CONSTANTINESCU S G.Validation of a large-eddy simulation model to simulate flow in pump intakes of realistic geometry[J].Journal of Hydraulic Engineering,2006,132(12):1303-1315.
10 RAJENDRAN V P,PATEL V C.Measurement of vortices in model pump-intake bay by PIV[J].Journal of Hydraulic Engineering,2000,126(5):322-334.
11 OKAMURA T,KAMEMOTO K,MATSUI J.CFD prediction and model experiment on suction vortices in pump sump[C]∥The9th Asian International Conference on Fluid Machinery,2007:1-10.
12 CHUANG W L,HSIAO S C.Three-dimensional numerical simulation of intake model with cross flow[J].Journal of Hydrodynamics,Ser.B,2011,23(3):314-324.
13 LUCINO C,GONZALO DUR S.Vortex detection in pump sumps by means of CFD[C]∥XXIV Latin American Congress on Hydraulics,2010:21-25.
14 AKIHIKO N,NOBUYUKI H.Large eddy simulation of vortex flow in intake channel of hydropower facility[J].Journal of Hydraulic Research,2010,48(4):415-427.
15王福军.流体机械旋转湍流计算模型研究进展[J/OL].农业机械学报,2016,47(2):1-14.http:∥www.j-csam.org/jcsam/ch/reader/view_abstract.aspx?flag=1&file_no=20160201&journal_id=jcsam.DOI:10.6041/j.issn.1000-1298.2016.02.001.WANG Fujun.Research progress of computational model for rotating turbulent flow in fluid machinery[J/OL].Transactions of the Chinese Society for Agricultural Machinery,2016,47(2):1-14.(in Chinese)
16宋希杰,刘超,杨帆,等.水泵进水池底部压力脉动特性实验[J/OL].农业机械学报,2017,48(11):196-203.http:∥www.j-csam.org/jcsam/ch/reader/view_abstract.aspx?flag=1&file_no=20171124&journal_id=jcsam.DOI:10.6041/j.issn.1000-1298.2017.11.024.SONG Xijie,LIU Chao,YANG Fan,et al.Experiment on characteristics of pressure fluctuationat bottom of pumping suction passage[J/OL].Transactions of the Chinese Society for Agricultural Machinery,2017,48(11):196-203.(in Chinese)
17 TOKYAY T,CONSTANTINESCU S G.Large eddy simulation and Reynolds averaged Navier Stokes simulations of flow in a realistic pump intake:a validation study[C]∥World Water and Environmental Resources Congress,2005.
18 CELIK I B,CEHRELI Z N,YAVUZ I.Index of resolution quality for large eddy simulations[J].ASME Journal of Fluids Engineering,2005,127(5):949-958.
19 MARSHALL J S,BENINATI M L.External turbulence interaction with a columnar vortex[J].Journal of Fluid Mechanics,2005,540:221.
20 JI B,LUO X,ARNDT R E A,et al.Numerical simulation of three dimensional cavitation shedding dynamics with special emphasis on cavitation-vortex interaction[J].Ocean Engineering,2014,87:64-77.
21 JI B,LUO X,ARNDT R E A,et al.Large eddy simulation and theoretical investigations of the transient cavitating vortical flow structure around a NACA66 hydrofoil[J].International Journal of Multiphase Flow,2015,68:121-134.