Study On Large Eddy Simulation Of Density Currents Under Water

Density currents refers to the stratified movement that occurs between two fluids with different densities but can be mixed with each other.When the environmental fluid is water,it is called density currents under water,it is one of the most classical types among the numerous density currents,which generally exists in many natural water bodies and artificial water bodies,such as stratification in reservoirs,sand sinks,cooling pools,estuaries and harbors.To master the formation mechanism and movement rules of the density currents under water can provide theoretical support for the study of sediment deposition,pollutant diffusion transportation,and water mixing at the mouth of rivers and lakes.The density currents under water is a fluid moving in other environmental fluids that can be mixed.The shear force generated by the relative moving between the fluids makes the turbulence in the area near the fluid contact surface become stronger and the vortex structure increase sharply,which plays an extremely important role in the formation and development of underwater density currents.Because of the particularity of the underwater density currents,a method that can capture the vortex structure accurately and meticulously is needed to describe the motion characteristics of the density currents.Large eddy simulation(LES)is an important method to study density currents under water.It classifies the turbulent eddy structure into solvable large-scale eddies and unsolvable small-scale eddies by specific filtering methods.The unsolvable small-scale vortex can obtain the effect of this part of the vortex structure on the whole turbulence by establishing a closed-equation model,while the unsolvable large-scale vortex can directly calculate the incompressible Navier-Stokes equation,So it was retained the information of the large-scale vortex structure and accurately reflect the fluid characteristics within a certain scale of the turbulence.three-dimensional large eddy simulation method is used to calculate the classical density currents test by Islan et al.The main conclusions are as follows:1.Three-dimensional large eddy numerical simulation method is used to calculate three models of different size mesh.When the number of mesh in this model reaches two millions,the simulation results can meet the accuracy requirements,as the grid density increases,the results obtained do not change much,so the number of optimal grids for this model is 2 million.In addition,the calculation results of the optimal grid model are in good agreement with the experimental data,indicating the applicability of the three-dimensional large eddy simulation method to simulate the underwater density currents.2.The stable velocity field was extracted,obtained the turbulent energy data were by fast Fourier transform(FFT)and plotted the turbulent energy spectrum.The figure shows that the turbulent energy curve of density currents under water conforms to the general rule of classical turbulence.When the wave number k is between 25 and 50 m-1 conforms to-5/3 power law.That is the inertia sub-region of the vortex scale.3.Three-dimensional large eddy simulation can show two different large-scale turbulent mass formation mechanisms,It's Including viscous and non-viscous mechanisms.4.Vortex junction is captured by Q criterion.At the entrance,the density currents is still in the development stage,and the number of vortex structures is very small,but the size and strength are large.From X=1m to X=5m,the vortex structure has matured,the vortex structure becomes broken and its strength decreases.Due to the influence of gravity,the speed is gradually accelerated,the number and strength of the vortex structure also increase.The hydroptically similar jump occurs when X=5m,and the strength,number and scale of vortex structure increase rapidly.At the end of this phenomenon near X=8m,the strength and number of vortex structure gradually weakened.5..Large eddy simulation calculations were carried out on six case with different environmental fluid density and main fluid specific gravity,and the evolution process and velocity vector field of underwater gravity flow were analyzed and discussed.It is found that the movement of the density currents is mainly related to the specific gravity of the main fluid and the environmental fluid,but not to the single factor of the environmental fluid density.