基于颗粒离散元的沙纹演变大涡模拟研究

应用大涡模拟、"点球" 浸入边界法和基于"事件驱动" 模型和"弹簧-阻尼" 模型的颗粒离散元法, 数值模拟了明渠湍流中沙纹的演变. 通过对不同谢尔兹数下无沙纹床面的推移质输沙率进行计算, 并与经典输沙率公式进行对比, 验证了模型的精度和可靠性. 随后, 对明渠湍流中沙纹床面的演变过程进行了数值模拟, 计算了有沙纹床面的推移质输沙率、沙纹长度和高度、等效床面高度的最大值、最小值和平均值、沙纹形状阻力、体积流速随时间的变化曲线. 研究发现:初始平整的床面在较短的时间内(tU_b=h≈100) 发展出数条沙纹, 随后沙纹逐渐发展, 在tU_b=h为1 600~2 000 时, 沙纹发生合并. 在沙纹数量不变的条件下, 沙纹高度随时间近似呈线性增长, 而沙纹的长度的平均值却保持恒定. 随着沙纹高度的增大, 输沙率和体积流速逐渐降低, 沙纹形状阻力则逐渐增大;当沙纹发生合并时, 沙纹高度快速增加, 输沙率、体积流速和沙纹形状阻力也出现了大幅跳跃. 在同等的水流强度条件下, 有沙纹床面的输沙率小于平整床面的输沙率. 

LARGE EDDY SIMULATION OF SAND RIPPLE EVOLUTION USING DISCRETE PARTICLE METHOD

This paper numerically simulates the evolution of sand ripples in a turbulent open channel flow by using a methodology in which three numerical technologies were combined, i.e. the large eddy simulation, the "point-particle" immersed boundary method and the discrete particle method with coupled "event-driven" and "spring-dashpot" models. The accuracy and reliability of the numerical model were verified by comparing the numerical results of bed-load transport rate of a featureless sand bed with the results from empirical formulas at different Shields numbers. After that, the numerical model was applied in simulating the evolution process of sand ripples in a turbulent open channel flow. The time variations of the sediment transport rate, the length and height of the sand ripples, the maximum, averaged and minimum values of the effective bed location, the shape drag of the sand ripples and the bulk velocity were investigated. It was found that several sand ripples were developed from an initially flat sand bed in a short time period with tU_b/h ≈ 100. After that, the sand ripples grew up gradually and an significant coalescence was observed during tU_b=/h from 1600 ～ 2000. The sand ripples' height increases approximately linearly when their number is constant, while the mean sand ripples' length is invariant. With the increasing sand ripple height, the sediment transport rate and the bulk velocity decreases gradually, while the shape drag of sand ripples increases gradually. However, when the sand ripples merge, a rapid increasing pattern is observed in the curve of their height, together with large jumps observed in the curves of the transport rate, the bulk velocity and the shape drag of sand ripples. Under the same flow conditions, the sediment transport rate of a featured sand bed is lower than that of a featureless sand bed.