异重流在不同粗糙底床上运动特性的实验研究
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摘要
进行一系列开闸式异重流水槽实验,考虑三种不同底床粗糙度对异重流运动特性的影响.通过高速摄像机拍摄异重流的运动过程,并利用粒子图像测速技术(PIV)记录局部流场结构.结果表明:异重流运动过程分为滑塌阶段和自相似阶段,底床粗糙度可以改变异重流的运动状态,尤其是通过底床附加阻力降低异重流头部速度并迫使其更早进入自相似阶段,但是对滑塌阶段速度影响不显著.异重流的弗劳德数在滑塌阶段变化不显著,而在自相似阶段呈现递减趋势;掺混系数随头部位置和理查德森数增大均呈现递减趋势,而粗糙底床可以加强掺混,增大掺混系数.异重流上界面与环境水体掺混形成正涡度带,下界面由于底床无滑移条件和底床流场结构的多方向性形成正负涡度带交错的现象,并且异重流剖面速度峰值会出现抬升现象.
A series of lock-exchange experiments were carried out to study the effect of three different bed roughness on gravity currents dynamics.A high-speed camera was applied to acquire the evolution process of the gravity currents,and a PIV system was used to investigate the microstructures of gravity currents.Results show that movement process of gravity currents is divided into slumping phase and self-similar phase. Bed roughness can significantly influence the motion of gravity currents, and especially decrease the front velocity by increasing the resistance,causing gravity currents into the self-similar phase at an early stage,but could not significant influence velocity of gravity current at slumping phase. The Froude number of gravity currents stays unchanged significantly at slumping phase, but decreases with the front position at self-similar phase. The entrainment coefficient shows a decreasing trend with the increase of the front position and the Richardson number,while bed roughness could strengthen entrainment and increase the entrainment coefficient.The upper interface of gravity currents keeps mixing with the environment,inducing the positive vorticity,and the lower interface forms the positive and negative vorticity due to the no-slip bottom boundary condition and the complexity of the flow structure at the bed.
A series of lock-exchange experiments were carried out to study the effect of three different bed roughness on gravity currents dynamics.A high-speed camera was applied to acquire the evolution process of the gravity currents,and a PIV system was used to investigate the microstructures of gravity currents.Results show that movement process of gravity currents is divided into slumping phase and self-similar phase. Bed roughness can significantly influence the motion of gravity currents, and especially decrease the front velocity by increasing the resistance,causing gravity currents into the self-similar phase at an early stage,but could not significant influence velocity of gravity current at slumping phase. The Froude number of gravity currents stays unchanged significantly at slumping phase, but decreases with the front position at self-similar phase. The entrainment coefficient shows a decreasing trend with the increase of the front position and the Richardson number,while bed roughness could strengthen entrainment and increase the entrainment coefficient.The upper interface of gravity currents keeps mixing with the environment,inducing the positive vorticity,and the lower interface forms the positive and negative vorticity due to the no-slip bottom boundary condition and the complexity of the flow structure at the bed.
引文
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[2] HE Z G,LV Y F,YUAN Y P,et al.Front propagation of gravity currents on inclined bottoms in linearly stratified fluids[J].Environmental Fluid Mechanics,2018,22:1-18.
[3] TOKYAY T E,GARC A M H.Effect of initial excess density and discharge on constant flux gravity currents propagating on a slope[J].Environmental Fluid Mechanics,2014,14(2):409-29.
[4] BRADFORD S F,KATOPODES N D.Hydrodynamics of turbid underflows,I:formulation and numerical analysis[J].Journal of Hydraulic Engineering,1999,125(10):1006-1015.
[5] VARJAVAND P,GORGIJ A D,FARSADIZADEH D,et al.Experimental observation of saline underflows and turbidity currents,flowing over rough beds[J].Canadian Journal of Civil Engineering,2015,42(11):834-844.
[6] ZHOU J,CENEDESE C,WILLAMS T,et al.On the propagation of gravity currents over and through a submerged array of circular cylinders[J]. Journal of Fluid Mechanics,2017,831:394-417.
[7] NOGUEIRA H I S, ADDUCE C, ALVES E,et al.Analysis of lock-exchange gravity currents over smooth and rough beds[J].Journal of Hydraulic Research,2013,51(4):417-431.
[8] DAI A.Experiments on gravity currents propagating on different bottom slopes[J].Journal of Fluid Mechanics,2013,731:117-141.
[9] JACOBSON M R,TESTIK F Y.Turbulent entrainment into fluid mud gravity currents[J]. Environmental Fluid Mechanics,2014,14(2):541-563.
[10]贺治国,林挺,赵亮,等.异重流在层结与非层结水体中沿斜坡运动的实验研究[J].中国科学:技术科学,2016,46(6):570-578.
[11] FERNANDO H J S,PRINCEVAC M.Turbulent entrainment into natural gravity-driven flows[J].Journal of Fluid Mechanics,2005,533:259-268.
[12] CANTERO M I,BALACHANDAR S,GARC A M H,et al.Turbulent structures in planar gravity currents and their influence on the flow dynamics[J].Journal of Geophysical Research Oceans,2008,113(C8):1-22.
[13]彭明.开闸式异重流的流动结构和颗粒输运的实验研究[D].北京:北京大学图书馆,2013.
[14] ALTINAKAR M S,GRAF W H,HOPFINGER E J.Flow structure in turbidity currents[J]. Journal of Hydraulic Research,1996,34(5):713-718.
[15] NOURMOHAMMADI Z,AFSHIN H,FIROOZABADI B. Experimental observation of the flow structure of turbidity currents[J]. Journal of Hydraulic Reasearch,2011,49(2):168-177.
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