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Erosion by Wind: Source, Measurement, Prediction, and Control
Published in Brian D. Fath, Sven E. Jørgensen, Megan Cole, Managing Soils and Terrestrial Systems, 2020
Brenton S. Sharratt, R. Scott Van Pelt
where uz is wind velocity (m s−1) at height z (m), u* is friction velocity (m s−1), zo is aerodynamic roughness (m), and k is von Karman’s constant (0.4). Friction velocity is proportional to the shear stress exerted by wind on the surface and is typically obtained from the wind velocity profile under conditions of neutral atmospheric stability. Particle movement occurs when u* exceeds the threshold friction velocity of the particle (u*tp). The threshold velocity of a dry sand particle was originally described by Bagnold[1] as: utp*=Aσ−ρρgd
Water Pollution and its Control
Published in Danny D. Reible, Fundamentals of Environmental Engineering, 2017
This form of the equation assumes that h << w, i.e., that the hydraulic radius of the runoff is given by its depth. We shall see in a subsequent section that the ability of the flow to erode soil and sediment depends on the friction velocity or applied shear stress associated with flow. From Equation 7.4, assuming a logarithmic velocity profile in the runoff, the friction velocity is given by u* = gsh, i.e., the product of the gravitational constant, the local slope, and the depth of the runoff.
Surface water–ground Water Interactions and Modeling Applications
Published in Calvin C. Chien, Miguel A. Medina, George F. Pinder, Danny D. Reible, Brent E. Sleep, Chunmiao Zheng, and Sediment, 2003
Calvin C. Chien, Miguel A. Medina, George F. Pinder, Danny D. Reible, Brent E. Sleep, Chunmiao Zheng
Here utc is the critical threshold friction velocity; ρp and dp are the particle density and diameter, respectively; ρw is the density of water; and β is a coefficient incorporating the angle of repose of the particle (i.e., the slope of the upstream face of the sediment dune) and the partial coverage by other sediment particles. The friction velocity is related to the surface shear stress, τb, by uvc=τb/uw1/2. This relationship simply emphasizes that sediment resuspension occurs when the lift caused by the overlying flow overcomes the weight of the particle. The friction velocity is a parameter related to the surface friction that can be determined by velocity profile measurements. Raudkivi (1967) suggests that β is approximately 0.2.
A method for roughness height prediction by particle deposition and its effect on flow and heat transfer in a turbine cascade
Published in Numerical Heat Transfer, Part A: Applications, 2023
Hong Wang, Jialong Li, Haopeng Guo
The effect of surface roughness on the flow boundary layer is related to u+ which is corrected as: where fr is the roughness function which determines the velocity shift due to roughness effects. ΔB is the intercept of the roughness function, also called shift velocity, which depends on the value of dimensionless roughness height ks is the actual roughness height or equivalent sand-grain introduced by Schlichting and Gersten [29]. is the viscosity and the friction velocity based on the wall shear stress and the fluid density. Dimensionless roughness height is introduced by Nikuradse [30] to characterize the roughness effects. He divided the roughness regime into three regimes for flows. The corresponding ΔB is computed:
Numerical assessment of canopy blocking effect on partly-obstructed channel flows: from perturbations to vortices
Published in Engineering Applications of Computational Fluid Mechanics, 2022
Yuan-Yuan Jia, Zhi-Dong Yao, Huan-Feng Duan, Xie-Kang Wang, Xu-Feng Yan
The variables (u, v and h) to be solved represent spatially averaged ones. Generally, the grid size for capturing large-scale motions should be sufficiently fine. Therefore, the spatial averaging (filtering) of the original Narviar-Stokes equations leaves extra stress terms (Tx and Ty), imposing dispersive effects on the resolved flow. For a depth-averaged problem, the stress terms should incorporate the effect of both unsolved horizontal motions and subdepth scale turbulence. Basically, fine grid size is enough to capture large-scale 2D motions, particularly for horizontally large-scale-vortex-dominated shallow waters. Only the subdepth scale turbulence needs to be modeled (Hinterberger et al., 2007; Nadaoka & Yagi, 1998). The subdepth scale turbulence arises from the interaction between flow and bottom vertically, and this effect theoretically adds 3D turbulence effect in the depth-averaged modeling. Previous modeling works regarding DA-LES or 2D-URANS employed a simple formulation that correlates the eddy viscosity (vt) from the subdepth scale turbulence to the local hydrodynamics, where is the friction velocity and κ is the von Kármán constant equal to 0.4.
Removal ability of different underlying surfaces to near-surface particulate matter
Published in Environmental Technology, 2021
Zhang Yu, Yan Guoxin, Dai Liyi, Cong Ling, Wu Yanan, Zhai Jiexiu, Zhang Zhenming
The total resistance can be calculated using the following formula: where is the aerodynamic drag and is the resistance. According to the Monin–Obukhov similarity theory, the value of the aerodynamic drag caused by surface turbulence can be calculated as where the friction velocity reflects the intensity of the atmospheric turbulence, is the rough length above the plane, is the von Kármán constant (generally taken as 0.4), and is a comprehensive stability correction function. When the atmospheric conditions are stable, can be expressed as where the Monin–Obukhov length can be expressed as where is the specific heat capacity at normal pressure, is the average surface temperature, and is the sensible heat.