Stochastic sediment transport modeling under the effects of intermittency and anisotropy of turbulent flow

Turbulent flow is a chaotic condition filled with vortex structures in the flow. Based on past studies, turbulent bursting events are composed of outward interactions (Q1), ejections (Q2), inward interactions (Q3), and sweeps (Q4). Among these events, ejections (Q2) and sweeps (Q4) significantly contribute to time occupied, momentum flux, and sediment flux from the turbulent coherent structure analysis. In addition, turbulent coherent events were assumed to occur continuously in the past. However, it is noticed that, on average, approximately 90% of the total stress was found within merely 50% of the total sampling time. Furthermore, the earlier works supposed the occurrences of these events are the same, and the change between the two states is uncorrelated. It is found that the occurrences of bursting events are a non-Markovian random process. The flow region can be divided into two parts (the near-bed region and the upper layer) by the spatial gradient of the flow velocity. The flow condition near the bed bottom tends to be anisotropic because of the turbulent structures. As mentioned above, these characteristics (intermittency, memory, and anisotropy) were not considered in past studies. This study proposes a modified Stochastic Diffusion Particle Tracking Model, a stochastic Lagrangian model to describe sediment particle movement. The proposed model integrates these characteristics, such as length-scale and time-scale of coherent events determined from the Direct Numerical Simulation dataset (DNS dataset), to reveal more details of sediment particle motion in the turbulent flow. We obtain the sediment particle trajectory from the model and analyze the anomalous diffusion in sediment transport by calculating the variance of the particle trajectory. As far as we know, extreme flow events such as floods induced by typhoons or heavy rainfalls can be regarded as highly intermittency processes. When a detailed description of the turbulent flow can be made available, we can simulate sediment particle motion more comprehensively under these extreme flow conditions.