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 tUb/h ≈ 100. After that, the sand ripples grew up gradually and an significant coalescence was observed during tUb=/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.
2015, 47(4): 613-623.