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

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.