Sediment Scour Model

This model predicts the advection, erosion, settling and deposition of sediment, such as sand. The scour sediment model can be used in any type of flow, though all sediment motion must take place within one fluid. The settling component of the model presumes the sediment particles to be spherical and their velocity to be small (so that viscous effects predominate in the flow around each sediment particle)–see the Drift Model for Two-Phase Flow. The settling (drift) coefficient is automatically calculated as

Here SCRDIA is the average particle diameter. RHOF is the density of the fluid, and SCRRHO is the density of the sediment particles; a typical value for sand is 2.65g/cm. The “lift” component of scour is an empirical model based on Bed-Load sediment transport models; the lift velocity

Here τ is the magnitude of the shear stresses at the interface and

is the macroscopic

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fluid density. SCRALP is an empirical parameter to adjust the rate of scour erosion for the particular application. Its default value is 1.0, based on experimental data with sand. τc is the critical shear stress. Where the shear stress is greater than τc, scour erosion occurs because the forces that cause the sediment particles to lift exceed the forces keeping the particle from lifting (gravity, cohesive forces to neighboring particles). Conversely, where the local magnitude of shear stresses is less than τc, the liquid forces are insufficient to causes particle at the surface to lift, and sediment subsequently deposits according to the settling (drift) velocity. The critical shear stress, τc, is computed from the critical Shields parameter, SCRCRT:

Here g is magnitude of gravitational acceleration.

Sediment is considered packed if the sediment volume fraction is greater than or equal to

SCRFCR; the packing density is equal to SCRFCR×SCRRHO.

Thus, regions where the sediment concentration (in units of gm/cm3 in CGS) is equal to or greater than SCRFCR×SCRRHO are packed and fluid flow does not occur – the drag imparted on the liquid by the sediment is assumed to be infinite (i.e., the inverse liquid drag quantity, the drag coefficient computed by FLOW-3D is zero). In regions where the sediment volume fraction is greater than SCRFCO, but less than SCRFCR, the drag model is activated (see Solidification Drag Model). SCRDRG in this model is equivalent to TSDRG (the default value is 1.0). When the solid fraction of sediment is less than SCRFCO, the drag model is not activated, but the viscosity is enhanced according to:

Here μ0 is the fluid viscosity and μ is the enhanced viscosity due to the presence of sediment. Both the drift and lift velocity equations can be easily customized in the routine scour_drift.f

Sediment Scour Model

该模型预测泥沙的对流、侵蚀、沉降和淤积。比如沙子。冲刷泥沙数学模型可以用在任何种类型的流动,但泥沙运动规律必须发生在一相流中。模型的沉积组成部分假设泥沙颗粒呈圆球状，运动是速度很小(这样在流动中，粘性作用起主要作用在每个泥沙颗粒四周)——可以参考“Drift Model for Two-Phase Flow”（两相流推移模型）,推移系数是自动计算由下面的公式 ：

SCRDIA是泥沙平均直径，RHOF 是液体密度，SCRRHO 是泥沙的密度，通常泥沙密度是2.65g/cm3. 冲刷的分力 “lift”（上升力） 是基于推移质运动模型的经验模型，在淤积层交界面的上升速度计算公式是：

τ是床面剪切应力，由液体密度决定。SCRALP 是经验系数，为冲淤率。缺省值是1.0。τc是临界床面剪切力，当床面剪切应力τ大于临界床面剪切力τc时，冲刷开始出现，因为使泥沙颗粒上升力大于阻止泥沙上升的力（重力、粘结力等）反之，当床面剪切应力τ小于临界床面剪切力τc时，泥沙就淤积，淤积情况根据漂移速度。临界床面剪切力τc由shield参数计算

G是重力加速度。

泥沙被认为是充满液体中的，如果泥沙体积分数高于或等于“SCRFCR”，填充密度SCRFCR等于SCRFCR×SCRRHO，SCRRHO 是泥沙的密度

因此,区域泥沙含量（单位gm /cm3 在CGS)等于或大于”SCRFCR*SCRRHO”在固液流动中不会发生——泥沙作用到液体上的拖曳力认为是无穷的(例如,逆液体拖量,阻力系数计算在FLOW-3D中归零)。泥沙地区体积分数比SCRFCO大,但是小于SCRFCR,拖曳模型被激活(见Solidification Drag Model)。在该模型中SCRDRG相当于TSDRG(缺省值是1.0)。当泥沙固态分数小于SCRFCO，拖曳模型没有被激活,但粘性度提高根据下式:

μ0 是液体粘性系数，μ是增加后的粘性系数由于泥沙的存在。“drift”和”lift”速度可提前自定义在“scour_drift.f”程序中。

Sediment concentration can be specified at inflow boundaries; simply specify a sediment concentration in the appropriate Boundaries > Boundary face > boundary dialog box or click on the ‘’ button to create time-varying data.

【含沙量可以在流入田边界条件中指定,简单定义泥沙适当含沙量，

“Boundaries > Boundary face > boundary”对话窗口，选择“sediment”创造时变数据。】

Sediment is initialized by entering the packed and/or suspended sediment concentration into Initial > fluid region > Sediment concentration. Initial fluid regions that are defined to contain a concentration of packed sediment at or above the critical solid fraction behave as solid regions impenetrable to flow.

【泥沙初始化可以是通过输入填充率，或者悬移泥沙含量在“Initial > fluid region > Sediment concentration.”中，初始化的流体区域定义了含有一定浓度的泥沙，即河床泥沙固态粒度表现得像固体地区无法穿越的流动。】