在层结和非层结环境下植被群对异重流运动的影响

Effect of Vegetation Patch on Gravity Current Movement in Stratified and Non-Stratified Environments

异重流在自然环境界及工程应用中广泛存在。本文于线性层结及非层结盐水中开展了一系列实验,对开闸式异重流通过植被群的发展和演变特性进行了研究。采用高速相机对异重流头部位置、头部速度和掺混系数等进行了分析,结果表明:异重流在层结水体中头部速度明显减小;层结水体和植被对异重流掺混均有抑制作用;当层结水体和植被同时存在时,异重流的掺混程度与植被高度和水深之比α相关,当α=0.43时,异重流的掺混最为剧烈。本研究也利用粒子图像测速仪(particle image velocimetry,PIV)对异重流局部区域的速度场和涡度场进行研究,结果显示,层结环境抑制异重流正负涡度场发展,但当α=0.20时,即一部分异重流爬上植被顶部时,层结水体与植被的协同效应对跃上植被顶端的异重流下界面涡度有强化作用。

Gravity currents exist widely in nature and engineering.In this study,a series of laboratory experiments were conducted to investigate lock-release gravity currents through vegetation patches into different ambient environments.The head position,front velocity and entrainment coefficient of gravity currents were analyzed by using the images acquired by a high-speed camera.The results show that the front velocity in linearly stratification environments is smaller than that in constant-density ambient.Both stratified environment and vegetation patches can inhibit the entrainment between gravity current and ambient water.As stratified environment and vegetation patches exist simultaneously,the mixing and entrainment of gravity currents are related to the ratioαof vegetation height to water depth.When α=0.43,the entrainment and mixing between gravity current and ambient water are the strongest.The particle image velocimetry(PIV)technique was applied to study the velocity and vorticity fields in detail.The results show that the stratification environment can restrain the development of the vorticity both at the upper and lower interfaces between current and ambient water.However,whenα=0.20,i.e.some gravity currents climbs to the top of vegetation patches,the combined effects of the stratified water and the vegetation patch strengthen the vorticity near the top of vegetation patches.