- Количество слайдов: 20
Particle Settling Velocity Put particle in a still fluid… what happens? Fd Fg Speed at which particle settles depends on: particle properties: D, ρs, shape fluid properties: ρf, μ, Re
STOKES Settling Velocity Assumes: spherical particle laminar settling Gravity: Drag:
At terminal velocity, Fg = Fd Solve for Ws, Stokes settling velocity: Remember Assumptions! Stokes Region: Ws. D/ < 1 Laminar spherical non-flocculated
Assumptions: Shape close to a sphere SF = 1 SF < 1 Laminar – What if turbulent? Cd = f (ν, D, ρ) - turbulent Cd = 24/Re - laminar
Turbulent part of the curve: Gibbs formulation – (cgs units) use for spheres 0. 0063 cm < D < 1. 0 cm
Laminar (Stokes) vs. Turbulent (Gibbs) settling Stokes Gibbs
For a 0. 01 mm particle: How long to settle through 10 m of water column? Size (D) Ws (Stokes) Time(S) Time(G) Very coarse sand 1 mm 64 cm/s 16 sec 1. 1 min Fine to very fine sand 0. 1 mm 0. 64 cm/s 26 min 28. 2 min Silt 0. 01 mm 0. 0064 cm/s 43 hrs Clay 0. 001 mm 0. 000064 cm/s 180 days
What we see: “Observations of suspended sediment concentration collected around the mouths of rivers around the globe provide clear support for the hypothesis that mud and sand both sink rapidly from discharge plumes” Hill et al, in press “After a large flood, more than 80% of the flood sediment could be accounted for in water depths of less than 50 m at distances less than 20 km from the river mouths. Given that currents typically fall in the range of 10 -20 cm/s, these observations suggest that particles must have been sinking at speeds of approximately 0. 1 mm/s which is typical of medium silts and exceeds settling velocity of clay particles by an order of magnitude” Drake et al. , 1972
Settling Camera : Particles have a tendency to form aggregates: Larger particles (settle faster) Lower density (settle slower)
According to: Physio-chemical factors in particle aggregation, Johnson et al. Aggregation applies to the general process of formation of larger particles from the collision of smaller particles. Flocculation refers to aggregation when the bonding agent that holds particles together is a high molecular weight polymer that operates through inter-particle bridging. Coagulation describes the process of aggregation in which primary particles are destabilized by charge neutralization through double layer compression. But … in Oceanography the terms are used interchangeably to mean the formation of larger particles from smaller.
Aggregation of particles: • physio-chemical processes • biological processes Physio-chemical processes + + Electrostatic forces – All particles are charged. . Van der Waals force – attraction of one molecules nuclei with another’s electrons. Born Repulsion – once close enough, electrons of one repulse electrons of another.
All these forces require particles to be close together How do they get together and cause collisions? • Brownian motion • Shearing mechanisms • Differential settling Smaller grain sizes – of similar size of different size Larger grain sizes Brownian motion Shear of similar size Differential settling of different size Shear
Flocculation - less common in rivers (ionic strength) - reversible process Limits to size Biological Processes • fecal pellets – settle at high rates • mucous “stringers”
When aggregation occurs, • Aggregates can grow O(102) larger than compound particles. • “card-house” structure with much water in interstices. • Floc density typically ranges 1. 27 – 1. 07 g/cm 3 primary aggregates – 1. 16 -1. 27 g/cm 3 secondary aggregates – 1. 06 – 1. 07 g/cm 3 • Floc settling velocity is substantially higher than the settling velocity of the component grains.
Hindered Settling At increasing concentrations, flocs interact hydrodynamically • Particles cause an upward flow of the fluid they displace
At ~ 10 – 20 g/l (10 - 20 ppm of flocculated sediment) hindered settling occurs