NUMERICAL SIMULATION OF EROSION AND TRANSPORT OF FINE SEDIMENTS BY LARGE EDDY SIMULATION
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摘要: 在传统水沙输移数值模拟研究中一般采用雷诺时均模拟技术（Reynolds-averaged simulation，RANS）.与RANS相比，大涡模拟技术（large eddy simulation，LES）能够更加精确反映细部流动结构，计算机的发展使得采用LES探讨水流和泥沙运动规律成为可能.本文尝试给出净冲刷条件下悬沙计算的边界条件，采用动态亚格子模式对循环槽道和长槽道中的水流运动和泥沙输移进行了三维大涡模拟研究.利用直接数值模拟（directnumerical simulation，DNS）结果对LES模型进行了率定，计算结果符合良好，在此基础上初步探讨了泥沙浓度、湍动强度和湍动通量等的分布特征.结果表明，净冲刷条件下输沙平衡时泥沙浓度符合Rouse公式分布，单向流动中泥沙浓度沿着流向逐渐增大.泥沙浓度湍动强度和湍动通量都在近底部达到最大值，沿着垂向迅速减小.湍动黏性系数和扩散系数基本上在水深中间处达到最大.湍动Schmidt数沿着水深方向不是常数，在近底部和自由水面附近较大，在水深中间处较小.Abstract: In general Reynolds-averaged simulation (RANS) is used in the traditional numerical simulation of water flow and sediment transport.Large eddy simulation (LES) can reflect flow structures more accurately and give more details of water flow compared with RANS.The development of computing technology makes it possible to study the rules of water flow and sediment transport by an LES model.In this paper, we tried to introduce boundary conditions for suspended sediment transport for the LES model under the net-erosion condition.Water flow and sediment transport in a cyclic case and a one-way flow case were calculated via the LES model with a dynamic sub-grid stresses module and a suspended sediment calculation module in the paper.Direct numerical simulation (DNS) results were used to calibrate the LES model and the results from LES showed good agreements with the DNS results.The distribution characteristics of sediment concentration, turbulence intensity and turbulent fluxes of sediment were explored in the paper.Under the neterosion condition, the equilibrium sediment concentration profile was coincident with the line of the Rouse equation.It showed that the turbulence intensity and turbulent fluxes of sediment had peak values near the bottom and then decreased rapidly along the vertical direction.The turbulent viscosity and diffusion coefficients were calculated and their peak values were or near the mid-depth of water.The turbulent Schmidt number was not constant along the vertical direction, and it was larger near the free surface and the bottom while it was smaller near the mid-depth of water flow.
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Key words:
- fine sediment /
- net erosion /
- LES /
- sediment transport
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图 3 算例1中LES和DNS流向速度和湍动强度比较
Figure 3. Comparisons of the stream wise velocity and turbulence intensities between LES and DNS in case 1
图 4 无量纲切应力计算结果对比图
Figure 4. The comparison of the dimensionless shear-stresses between LES and DNS
图 5 算例2中不同位置上流向速度和湍动强度分布
Figure 5. The profiles of the stream-wise velocities and turbulence intensities at different locations in case 2
图 6 算例2中无量纲切应力的分布
Figure 6. The profiles of the dimensionless shear stresses at different locations in case 2
图 8 算例2中断面平均的泥沙浓度沿程分布
Figure 8. Distribution of the cross-section averaged sediment concentration in case 2
图 9 算例1中泥沙浓度湍动强度和湍动通量的分布
Figure 9. The turbulence intensity of sediment concentration and vertical sediment turbulence flux in case 1
图 10 湍动黏性系数和扩散系数分布图
Figure 10. The turbulence viscosity coefficient and the turbulence diffusion coefficient in two cases
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