低柱体雷诺数下柱体上游薄层水流马蹄涡特征研究
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摘要
马蹄涡是水下柱体基部产生局部侵蚀的主要动力,目前对大雷诺数深水条件的马蹄涡特征研究广泛,而在低柱体雷诺数浅薄层水流条件下因技术条件限制,试验手段难以准确捕获马蹄涡。为准确掌握在该水流条件下马蹄涡的特征,本研究构建了高分辨率高频的粒子图像测速系统,系统测量了6组浅薄层水流条件下柱体前端的瞬时流场。通过分析平均流场特征提取出流动分离点,采用旋转强度的方法识别马蹄涡进而提取马蹄涡位置及强度,使用Oseen涡和纯剪切的控制方程叠加模拟马蹄涡进而计算出马蹄涡的半径。结果表明:在低柱体雷诺数水流条件下(Re D<5 000),随着柱体雷诺数的增加流动分离点急剧向下游移动,同时马蹄涡急剧向柱体端和床面靠近,其半径减小而旋转强度增加。在浅薄层水流条件下,当柱体直径一定时,随着水深的增加,流动分离点向上游运动,同时马蹄涡向远离柱体端和朝水面运动,马蹄涡的半径增加,且其各项参数显著大于明渠水流条件下的参数。随后,结合已有工作,归纳出分离点、马蹄涡特征随柱体雷诺数增加而经历的不同阶段:当5 0008 000,流动分离点向下游缓慢移动,马蹄涡的各项参数仍维持稳定。研究结果可为柱体基部科学布设防冲设施提供依据和参考。
The horseshoe vortex(HV) is formed at the upstream of a vertical cylinder when flow passes the cylinder and it is responsible for the local scouring at the base of the cylinder. Extensive works had been carried out to investigate the characteristics of HV in open channel flow with high Reynold number and large flow depth. However, it was difficult to measure HV experimentally in low Reynold number and shallow flow depth, in view of limitation of experimental technology. To capture HV accurately in shallow water flow, a high resolution and high frequency particle image velocimetry(HR-PIV) was employed in present study. Subsequently, the flow fields upstream of the cylinder were captured by HR-PIV in 6 experimental groups with shallow flow depth. The separation points of each groups were obtained by analyzing the characteristics of time-averaged flow fields. The HV was calculated by λci criterion where λci represented the swirling strength of vortex. Then the locations of HV were obtained in accordance with the maximal swirling strength point. In addition, the radius of HV was calculated by superposition of Oseen vortex and pure shear model. The results showed that within a low cylinder Reynolds number(Re D) where Re D<5 000, as the increase of Re D, the location of separation point and HV were rapidly approaching the cylinder simultaneously, whereas the HV moved rapidly towards the flume bed,while the radius of HV decreased and the swirling strength increased. Under shallow water flow conditions and the cylinder diameter keeping constant, as the increase of flow depth, the locations of separation point moved towards upstream; HV moved towards upstream and free surface, simultaneously, while the radius of HV increased. Theses HV parameters in the present flow conditions were larger than those in open channel flows.Derived from previous works, it was found that the separation point and HV would display a different manner as Re D became lager. When 5 0008 000, the separation point was slowly moving downstream and HV still remained stable. The research results can provide a basis and reference for engineering design for preventing local scouring at the base of cylinder.
The horseshoe vortex(HV) is formed at the upstream of a vertical cylinder when flow passes the cylinder and it is responsible for the local scouring at the base of the cylinder. Extensive works had been carried out to investigate the characteristics of HV in open channel flow with high Reynold number and large flow depth. However, it was difficult to measure HV experimentally in low Reynold number and shallow flow depth, in view of limitation of experimental technology. To capture HV accurately in shallow water flow, a high resolution and high frequency particle image velocimetry(HR-PIV) was employed in present study. Subsequently, the flow fields upstream of the cylinder were captured by HR-PIV in 6 experimental groups with shallow flow depth. The separation points of each groups were obtained by analyzing the characteristics of time-averaged flow fields. The HV was calculated by λci criterion where λci represented the swirling strength of vortex. Then the locations of HV were obtained in accordance with the maximal swirling strength point. In addition, the radius of HV was calculated by superposition of Oseen vortex and pure shear model. The results showed that within a low cylinder Reynolds number(Re D) where Re D<5 000, as the increase of Re D, the location of separation point and HV were rapidly approaching the cylinder simultaneously, whereas the HV moved rapidly towards the flume bed,while the radius of HV decreased and the swirling strength increased. Under shallow water flow conditions and the cylinder diameter keeping constant, as the increase of flow depth, the locations of separation point moved towards upstream; HV moved towards upstream and free surface, simultaneously, while the radius of HV increased. Theses HV parameters in the present flow conditions were larger than those in open channel flows.Derived from previous works, it was found that the separation point and HV would display a different manner as Re D became lager. When 5 000
引文
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[21]Qi Meilan.Riverbed scouring around bridge piers in river section with sand pits[J].Journal of Hydraulic Engineering,2005,36(7):835-839.[齐梅兰.采沙河床桥墩冲刷研究[J].水利学报,2005,36(7):835-839.]
[22]Unger J,Hager W H.Down-flow and horseshoe vortex characteristics of sediment embedded bridge piers[J].Experiments in Fluids,2007,42(1):1-19.
[23]Roulund A,Sumer B M,Freds?e J,et al.Numerical and experimental investigation of flow and scour around a circular pile[J].Journal of Fluid Mechanics,2005,534(534):351-401.
[24]Wei Q D,Chen G,Du X D.An experimental study on the structure of juncture flows[J].Journal of Visualization,2001,3(4):341-348.
[2]Kairouz K A,Rahai H R.Turbulent junction flow with an upstream ribbed surface[J].International Journal of Heat&Fluid Flow,2005,26(5):771-779.
[3]Kirkil G,Constantinescu G.A numerical study of the laminar necklace vortex system and its effect on the wake for a circular cylinder[J].Physics of Fluids,2012,24(7):415-443.
[4]Dargahi B.The turbulent flow field around a circular cylinder[J].Experiments in Fluids,1989,8(1):1-12.
[5]Graf W H,Yulistiyanto B.Experiments on flow around a cylinder;the velocity and vorticity fields[J].Journal of Hydraulic Research,1998,36(4):637-654.
[6]Ozturk N A,Akkoca A,Sahin B.Flow details of a circular cylinder mounted on a flat plate[J].Journal of Hydraulic Research,2008,46(3):344-355.
[7]Chen Qigang,Qi Meilan,Li Jinzhao,et al.Study on the features of approaching flow upstream of a circular cylinder inopen channel flows based on PIV measurement[J].Journal of Hydraulic Engineering,2015,46(8):967-973.[陈启刚,齐梅兰,李金钊,等.基于粒子图像测速技术的明渠圆柱上游行近流特征研究[J].水利学报,2015,46(8):967-973.]
[8]Chen Qigang,Qi Meilan,Li Jinzhao.Kinematic characteristics of horseshoe vortex upstreamof circular cylinders in open channel flow[J].Journal of Hydraulic Engineering,2016,47(2):158-164.[陈启刚,齐梅兰,李金钊.明渠柱体上游马蹄涡的运动学特征研究[J].水利学报,2016,47(2):158-164.]
[9]Chen Qigang,Qi Meilan,Zhang Qiang,et al.Experimental study on the multimodal dynamics of the turbulent horseshoe vortex system around a circular cylinder[J].Physics of Fluids,2017,29(1):015106.
[10]Akilli H,Rockwell D.Vortex formation from a cylinder in shallow water[J].Physics of Fluids,2002,14(9):2957-2967.
[11]Fu H,Rockwell D.Shallow flow past a cylinder:Transition phenomena at low Reynolds number[J].Journal of Fluid Mechanics,2005,540:75-97.
[12]Nezu I.Open-channel flow turbulence and its research prospect in the 21st century[J].Journal of Hydraulic Engineering,2005,131(4):229-246.
[13]Zhong Qiang,Wang Xingkui,Miao Wei,et al.High resolution PTV system and its application in the measurement inviscous sub layer in smooth open channel flow[J].Journal of Hydraulic Engineering,2014,45(5):513-520.[钟强,王兴奎,苗蔚,等.高分辨率粒子示踪测速技术在光滑明渠紊流黏性底层测量中的应用[J].水利学报,2014,45(5):513-520.]
[14]Zhou J,Adrian R J,Balachandar S,et al.Mechanisms for generating coherent packets of hairpin vortices in channel flow[J].Journal of Fluid Mechanics,1999,387(10):353-396.
[15]Gao Q,Ortiz-Due?as C,Longmire E K.Analysis of vortex populations in turbulent wall-bounded flows[J].Journal of Fluid Mechanics,2011,678:87-123.
[16]Tomkins C D,Adrian R J.Spanwise structure and scale growth in turbulent boundary layers[J].Journal of Fluid Mechanics,2003,490:37-74.
[17]Zhong Qiang,Chen Qigang,Li Danxun,et al.The scale and eirculation characteristics of spanwise vortexes in open channel flows[J].Journal of Sichuan University(Engineering Science Edition),2013,45(Supp2):66-70.[钟强,陈启刚,李丹勋,等.明渠湍流横向涡旋的尺度与环量特征[J].四川大学学报(工程科学版),2013,45(增刊2):66-70.]
[18]Varun A V,Balasubramanian K,Sujith R I.An automated vortex detection scheme using the wavelet transform of the d2 field[J].Experiments in Fluids,2008,45(5):857-868.
[19]Carlier J,Stanislas M.Experimental study of eddy structures in a turbulent boundary layer using particle image velocimetry[J].Journal of Fluid Mechanics,2005,535(535):143-188.
[20]Herpin S,Stanislas M,Soria J.The organization of near-wall turbulence:A comparison between boundary layer SPIVdata and channel flow DNS data[J].Journal of Turbulence,2010,11(47):1-30.
[21]Qi Meilan.Riverbed scouring around bridge piers in river section with sand pits[J].Journal of Hydraulic Engineering,2005,36(7):835-839.[齐梅兰.采沙河床桥墩冲刷研究[J].水利学报,2005,36(7):835-839.]
[22]Unger J,Hager W H.Down-flow and horseshoe vortex characteristics of sediment embedded bridge piers[J].Experiments in Fluids,2007,42(1):1-19.
[23]Roulund A,Sumer B M,Freds?e J,et al.Numerical and experimental investigation of flow and scour around a circular pile[J].Journal of Fluid Mechanics,2005,534(534):351-401.
[24]Wei Q D,Chen G,Du X D.An experimental study on the structure of juncture flows[J].Journal of Visualization,2001,3(4):341-348.
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