全部书籍

1 Primary & Middle School Mathematics1–9 年级在线 2 High School Mathematics10–12 年级在线 3 University Mathematics — Year 1本科一年级在线 4 University Mathematics — Year 2本科二年级在线 5 University Mathematics — Year 3本科三年级在线

1 Primary & Middle School Physics1–9 年级在线 2 High School Physics10–12 年级在线 3 University Physics — Year 1本科一年级在线 4 University Physics — Year 2本科二年级在线 5 University Physics — Year 3本科三年级在线

1 School Chemistry — Grades 1 to 121–12 年级在线 2 University Chemistry — Year 1本科一年级在线 3 University Chemistry — Year 2本科二年级在线 4 University Chemistry — Year 3本科三年级在线

1 Primary & Middle School Biology1–9 年级在线 2 High School Biology10–12 年级在线 3 University Biology — Year 1本科一年级在线 4 University Biology — Year 2本科二年级在线 5 University Biology — Year 3本科三年级在线

1 Primary & Middle School Earth and Space Sciences1–9 年级在线 2 High School Earth and Space Sciences10–12 年级在线 3 University Earth and Space Sciences — Year 1本科一年级在线 4 University Earth and Space Sciences — Year 2本科二年级在线 5 University Earth and Space Sciences — Year 3本科三年级在线

1 Markets I: The Ecosystem and Exchange-Traded Markets市场在线 2 Markets II: Rates, FX and Credit市场在线 3 Markets III: Commodities, Energy and Crypto市场在线 4 Quantitative Methods方法在线 5 Derivatives and Volatility衍生品在线 6 Rates, Credit, XVA and Risk利率、信用与风险在线 7 Research Craft: Predictors, Backtests, Measurement, Portfolios研究在线 8 Strategies I: Equities and Futures策略在线 9 Strategies II: Volatility, Relative Value, Macro and the Bank Desks策略在线 10 Microstructure and Execution交易执行在线 11 Market Making and High-Frequency Trading做市在线 12 Machine Learning for Markets机器学习在线 13 Low-Latency Software技术在线 14 Networks, Hardware and Trading Infrastructure技术在线 15 Research, Data and Risk Platforms技术在线 16 The Desk and the Firm公司运营在线 17 The Industry: Firms, Roles and Careers职业发展在线 18 Interviews I: Maths, Probability and Speed职业发展在线 19 Interviews II: Markets, Trading Games and the Final Round职业发展在线 20 Interviews III: Code职业发展在线

全部游戏 Math Fuse Word Reactor Atlas Dash

应用关于

Coach

English Français Español Nederlands Português हिन्दी العربية Bahasa Indonesia 简体中文 বাংলা

登录开始阅读

书籍暂未推出简体中文版——当前显示英文版。

Earth and Space Sciences · 术语表

什么是 Shields parameter, threshold of motion？

又称： Shields parameter · threshold of motion

Definition 20.9 University Earth and Space Sciences — Year 1 · 第 20 章 — Erosion and Transport

The Shields parameter of a flow of bed shear stress $\tau_b$ over grains of diameter $d$ and density $\rho_s$ is

\theta = \frac{\tau_b}{(\rho_s - \rho)\,g\,d}.

The threshold of motion of the grains is the bed shear stress, or the value $\theta_c$ of the Shields parameter, at which they begin to move.

The threshold of motion of quartz grains in water at 20\, C, from the curve of Soulsby and Whitehouse, computed from the fine-sand boundary (0.0625\, mm) to the size of a cobble. (a) The critical Shields parameter is nearly constant for gravel and rises for fine sand. (b) The critical bed shear stress grows almost in proportion to the diameter for gravel. — The threshold of motion of quartz grains in water at $20\,{}^{\circ}\mathrm{C}$ , from the curve of Soulsby and Whitehouse, computed from the fine-sand boundary ( $0.0625\,\mathrm{mm}$ ) to the size of a cobble. (a) The critical Shields parameter is nearly constant for gravel and rises for fine sand. (b) The critical bed shear stress grows almost in proportion to the diameter for gravel.

示例

Example 20.11 (What the river moves)

In the river $2\,\mathrm{m}$ deep on a slope of $0.001$ , $\tau_b = 19.6\,\mathrm{Pa}$ . For grains of $20\,\mathrm{mm}$ , $\theta = 19.6/(1651.8 \times 9.81 \times 0.020) = 0.060$ , just above their threshold of $0.055$ : they move. For grains of $40\,\mathrm{mm}$ , $\theta = 0.030$ , half the threshold: they stay. The river moves its bed up to coarse gravel; for sand of $1\,\mathrm{mm}$ , $\theta = 1.21$ , nearly forty times the threshold of $0.031$ .

在原文中阅读 →