层结环境下刚性植被群对异重流影响的数值模拟

Numerical simulations of the effect of rigid vegetation patch on motions of gravity current in linearly stratified environments

  • 摘要: 利用FLUENT软件模拟了层结环境下开闸式异重流通过刚性植被群的演变特性,使用二维大涡模拟,并结合用户自定义函数加入植被阻力效应,对异重流的头部速度、瞬时卷吸常数、势能转变,动能剖面变化等与环境水体层结度、植被群高度的关系进行相关分析。结果表明:层结环境会使异重流头部后期产生中性层入侵,但植被的存在会抑制此现象;在运动初期,异重流在低层结度环境时的头部速度比在高层结度环境的大,经过转变点,此趋势相反,而植被的存在会延迟这个转变点的出现,且转变点的出现随植被高度的增加而滞后;层结环境和植被都会抑制异重流的卷吸,但当这两种因素同时存在时,植被对异重流掺混抑制的影响起主要作用。异重流运动过程中,势能早期主要用于维持异重流前进,经过转变点后,大部分势能主要被不可逆掺混消耗,且植被和层结环境会将这个转变点提前。相同层结环境下,植被越高,异重流与环境流体间不可逆掺混引起的背景势能越不占优。层结环境会抑制断面处动能的多次波峰现象,但植被会促进该现象的发生。

     

    Abstract: In this study, the FLUENT software was used to simulate the evolution characteristics of the lock-released gravity currents flow through rigid vegetation regions in stratified environments. The two-dimensional large eddy simulation (LES) was adopted, and the vegetation resistance was added using the user-defined function (UDF) to analyze the relationship between instantaneous entrainment coefficients, potential energy transition, and kinetic energy variation with environment stratification degree as well as vegetation height. The results show that the stratified environment will cause the neutral layer to intrude in the later stage of the density currents, but the vegetation will inhibit this phenomenon. At the beginning of the movement, the head velocity of the gravity current in the weak stratified environment is greater than that in the strong stratified environment. After a transition point, the trend is opposite, i.e. the existence of vegetation patches will delay the appearance of this transition point, and the appearance of the transition point lags as the height of vegetation patches increases. Both stratified environment and vegetation can reduce curent entrainment with ambient fluids, but when these two factors coexist, vegetation plays a major role in decreasing the mixing of density currents. During current propagation, the potential energy is mainly used to maintain the currents flow in the early stage. After a transition point, the potential energy is then consumed by irreversible mixing. In addition, the vegetation and stratified environment will advance this transition point. Under the same stratification environment, the higher the vegetation is, the smaller the proportion of background potential energy to the total potential energy during the evolution of density currents is, indicating that the irreversible mixing of density currents and environmental fluid is less dominant in the transformation of total potential energy. Also, the stratified environment will restrain the multiple peaks of kinetic energy at some characteristic sections, but vegetation will promote this phenomenon.

     

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