非悬沙在风成和河流环境中输移的停止阈值
The Cessation Threshold of Nonsuspended Sediment Transport Across Aeolian and Fluvial Environments
摘要：
使用颗粒尺度模拟的非悬浮泥沙运输大范围的牛頓流体驱动的运输，包括空气和水，我们确定散装运输停止阈值$Θ^r_t$外推的运输负荷作为一个函数的无量纲流体剪切应力（“盾数”）$Θ$的消失运输限制。在此极限下，连续输送的模拟恒定状态可由简单的解析模式方程描述，该方程将平均输送层特性与壁面流速分布的规律相关联。我们使用这个模型来计算任意环境下的$Θ^r_t$，并推导出一个一般的Shields的阈值图，其中的Stokes数代替粒子雷诺数。尽管我们的流体动力学描述的简单，预测的停止阈值，无论是从模拟和解析模式，定量同意在空气中的运输和粘性和湍流液体的测量，尽管不适合这些测量。我们解释的解析模式作为一个连续的反弹运动的输送颗粒的描述，因此$Θ^r_t$作为最小的流体剪切应力需要补偿输送颗粒的平均能量损失在床面的平均反弹。这种解释得到了$Θ^r_t$附近模拟的支持，这意味着需要夹带机制来维持$Θ^r_t$以上的运输。虽然夹带湍流事件维持间歇性运输，夹带颗粒床的影响维持连续运输。结合Valyrakis等人提出的起动临界能量准则，提出了泥沙输移阻力的新概念。
英文摘要：
Using particle-scale simulations of non-suspended sediment transport for a large range of Newtonian fluids driving transport, including air and water, we determine the bulk transport cessation threshold $Θ^r_t$ by extrapolating the transport load as a function of the dimensionless fluid shear stress (`Shields number') $Θ$ to the vanishing transport limit. In this limit, the simulated steady states of continuous transport can be described by simple analytical model equations relating the average transport layer properties to the law of the wall flow velocity profile. We use this model to calculate $Θ^r_t$ for arbitrary environments and derive a general Shields-like threshold diagram in which a Stokes-like number replaces the particle Reynolds number. Despite the simplicity of our hydrodynamic description, the predicted cessation threshold, both from the simulations and analytical model, quantitatively agrees with measurements for transport in air and viscous and turbulent liquids despite not being fitted to these measurements. We interpret the analytical model as a description of a continuous rebound motion of transported particles and thus $Θ^r_t$ as the minimal fluid shear stress needed to compensate the average energy loss of transported particles during an average rebound at the bed surface. This interpretation, supported by simulations near $Θ^r_t$, implies that entrainment mechanisms are needed to sustain transport above $Θ^r_t$. While entrainment by turbulent events sustains intermittent transport, entrainment by particle-bed impacts sustains continuous transport. Combining our interpretations with the critical energy criterion for incipient motion by Valyrakis and coworkers, we put forward a new conceptual picture of sediment transport intermittency.
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