开闸式异重流3维运动现象及不确定度研究
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摘要
异重流在自然界和工程界非常普遍,对人类工业生产及生活具有重要的影响。利用两台高速摄相机从侧视及顶视记录开闸式异重流3维运动形态,通过重复性试验分析异重流头部位置、速度、高度及角度的不确定度(采用变异系数衡量),研究与异重流3维运动的关联性。由试验顶视图结果可知,异重流头部位置存在横向差异,异重流运动确实存在3维现象。异重流横向扩散不均匀和水槽边壁摩擦阻力的差异是造成异重流存在3维运动现象的根本原因,不确定度正是因为异重流3维运动而存在,这与试验观测到的异重流头部平面形态由波瓣和沟裂组成的现象一致。试验表明:在所研究时刻及盐度下,头部位置、速度、高度和角度均服从正态分布。当异重流处于加速阶段,头部位置和速度的变异系数随时间迅速递减,减速阶段则维持在定值附近,且变异系数均小于6%;头部高度和角度的变异系数在运动过程中始终维持在定值附近。由试验结果可知,异重流自身3维运动现象所导致的头部位置或速度的不确定度,约等于平均头部位置或速度的5%～6%,可作为日后进行单次异重流试验量测时不确定度的参考依据。
Gravity currents encountered in both natural environment and engineering applications have important influence on industrial production and life. In this paper, two high-speed digital cameras(one is from the top view and the other one is from the side view) were employed to obtain the three-dimensional motions of lock-exchange gravity current through a large number of repetitive experiments, i.e. these experiments were carried out under the same conditions. The front location, speed, height, and angles of the gravity current and their uncertainties were analyzed,and the effects of three-dimensional current motions on these uncertainties were investigated. The images from the top-view show that the density current head positions across the flume are different, indicating the presence of the three-dimensional motions in gravity current. The results show that the gravity current is composed of lobes and clefs, and the three-dimensional patterns of current motions from the lateral diffusion and the friction from the wall may be the cause of the uncertainty for current motions. The front location, speed, height, and angles of gravity current follow normal distributions at the selected time. The uncertainties of these parameters were measured by the coefficient of variation(COV). For the front locations, the COV values decreases rapidly in the initial stage, and decreases below 6% slowly in the subsequent stage. The COV values for the front speed also rapidly reduces and then maintains a near constant value, approximately equal to 5%. Based upon the experimental results, the uncertainty for the front location or speed of gravity currents due to the three-dimensional gravity current motions will be approximately 5%～6%of the averaged front location or current speed, which can be used as a reference value for future individual measurement.
Gravity currents encountered in both natural environment and engineering applications have important influence on industrial production and life. In this paper, two high-speed digital cameras(one is from the top view and the other one is from the side view) were employed to obtain the three-dimensional motions of lock-exchange gravity current through a large number of repetitive experiments, i.e. these experiments were carried out under the same conditions. The front location, speed, height, and angles of the gravity current and their uncertainties were analyzed,and the effects of three-dimensional current motions on these uncertainties were investigated. The images from the top-view show that the density current head positions across the flume are different, indicating the presence of the three-dimensional motions in gravity current. The results show that the gravity current is composed of lobes and clefs, and the three-dimensional patterns of current motions from the lateral diffusion and the friction from the wall may be the cause of the uncertainty for current motions. The front location, speed, height, and angles of gravity current follow normal distributions at the selected time. The uncertainties of these parameters were measured by the coefficient of variation(COV). For the front locations, the COV values decreases rapidly in the initial stage, and decreases below 6% slowly in the subsequent stage. The COV values for the front speed also rapidly reduces and then maintains a near constant value, approximately equal to 5%. Based upon the experimental results, the uncertainty for the front location or speed of gravity currents due to the three-dimensional gravity current motions will be approximately 5%～6%of the averaged front location or current speed, which can be used as a reference value for future individual measurement.
引文
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[5]Wilson R I,Friedrich H,Stevens C,et al.Turbulent entrainment in sediment-laden flows interacting with an obstacle[J].Physics of Fluids,2017,29(3):1-11.
[6]Soler M,Colomer J,Serra T,et al.Sediment deposition from turbidity currents in simulated aquatic vegetation canopies[J].Sedimentology,2017,64(4):1132-1146.
[7]Thomas L P,Dalziel S B,Marino B M.The structure of the head of an inertial gravity current determined by particletracking velocimetry[J].Experiments in Fluids,2003,34(6):708-716.
[8]Dai A.Experiments on gravity currents propagating on different bottom slopes[J].Journal of Fluid Mechanics,2013,731:117-141.
[9]Garc A M H,Parsons J D.Mixing at the front of gravity currents[J].Dynamics of Atmospheres&Oceans,1996,24(1/2/3/4):197-205.
[10]Benjamin T B.Gravity currents and related phenomena[J].Journal of Fluid Mechanics,1968,31(2):209-248.
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[13]李子奈,潘文卿.计量经济学[M].北京:高等教育出版社,2010.
[14]Adduce C,Sciortino G,Proietti S.Gravity currents produced by lock exchanges:Experiments and simulations with a twolayer shallow-water model with entrainment[J].Journal of Hydraulic Engineering,2012,138(2):111-121.
[15]Anjum H J,Mcelwaine J N,Caulfield C P.The instantaneous Froude number and depth of unsteady gravity currents[J].Journal of Hydraulic Research,2013,51(4):432-445.
[16]Guan Helan.The uncertainty analysis in physical experiment of college[J].Physical Experiment of College,2007,20(1):75-77.[管荷兰.大学物理试验中的不确定度分析[J].大学物理试验,2007,20(1):75-77.]
[17]Reed G F,Lynn F,Meade B D.Use of coefficient of variation in assessing variability of quantitative assays[J].Clinical&Diagnostic Laboratory Immunology,2002,9(6):1235-1239.
[18]魏和清,罗良清.实用统计学[M].北京:中国财政经济出版社,2011.
[2]He Zhiguo,Lin Ting,Zhao Liang,et al.Experiments on gravity currents down a ramp in unstratified and linearly stratified salt water environments[J].Scientia Sinica Technologica,2016,46(6):570-578.[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试验研究[J].中国科学(技术科学),2016,46(6):570-578.]
[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):409-429.
[4]Dorrell R M,Darby S E,Peakall J,et al.The critical role of stratification in submarine channels:Implications for channelization and long runout of flows[J].Journal of Geophysical Research Oceans,2014,119(4):2620-2641.
[5]Wilson R I,Friedrich H,Stevens C,et al.Turbulent entrainment in sediment-laden flows interacting with an obstacle[J].Physics of Fluids,2017,29(3):1-11.
[6]Soler M,Colomer J,Serra T,et al.Sediment deposition from turbidity currents in simulated aquatic vegetation canopies[J].Sedimentology,2017,64(4):1132-1146.
[7]Thomas L P,Dalziel S B,Marino B M.The structure of the head of an inertial gravity current determined by particletracking velocimetry[J].Experiments in Fluids,2003,34(6):708-716.
[8]Dai A.Experiments on gravity currents propagating on different bottom slopes[J].Journal of Fluid Mechanics,2013,731:117-141.
[9]Garc A M H,Parsons J D.Mixing at the front of gravity currents[J].Dynamics of Atmospheres&Oceans,1996,24(1/2/3/4):197-205.
[10]Benjamin T B.Gravity currents and related phenomena[J].Journal of Fluid Mechanics,1968,31(2):209-248.
[11]Simpson J E.Gravity currents in the laboratory,atmosphere,and ocean[J].Annual Review of Fluid Mechanics,1982,14:213-234.
[12]Parsons J D,Garcia M H.Similarity of gravity current fronts[J].Physics of Fluids,1998,10(12):3209-3213.
[13]李子奈,潘文卿.计量经济学[M].北京:高等教育出版社,2010.
[14]Adduce C,Sciortino G,Proietti S.Gravity currents produced by lock exchanges:Experiments and simulations with a twolayer shallow-water model with entrainment[J].Journal of Hydraulic Engineering,2012,138(2):111-121.
[15]Anjum H J,Mcelwaine J N,Caulfield C P.The instantaneous Froude number and depth of unsteady gravity currents[J].Journal of Hydraulic Research,2013,51(4):432-445.
[16]Guan Helan.The uncertainty analysis in physical experiment of college[J].Physical Experiment of College,2007,20(1):75-77.[管荷兰.大学物理试验中的不确定度分析[J].大学物理试验,2007,20(1):75-77.]
[17]Reed G F,Lynn F,Meade B D.Use of coefficient of variation in assessing variability of quantitative assays[J].Clinical&Diagnostic Laboratory Immunology,2002,9(6):1235-1239.
[18]魏和清,罗良清.实用统计学[M].北京:中国财政经济出版社,2011.
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