Particle Settling Velocity Put particle in a still fluid… what happens? Speed at which particle settles depends on: particle properties: D, ρ s , shape - PowerPoint PPT Presentation

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Particle Settling Velocity Put particle in a still fluid… what happens? Speed at which particle settles depends on: particle properties: D, ρ s , shape fluid properties: ρ f , μ, Re. F d. F g. STOKES Settling Velocity Assumes: spherical particle laminar settling Gravity:

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Particle Settling Velocity Put particle in a still fluid… what happens? Speed at which particle settles depends on: particle properties: D, ρ s , shape

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Particle Settling Velocity

Put particle in a still fluid… what happens?

Speed at which particle settles depends on:

particle properties: D, ρs, shape

fluid properties: ρf, μ, Re

Fd

Fg

STOKES Settling Velocity

Assumes: spherical particle

laminar settling

Gravity:

Drag:

At terminal velocity,

Fg = Fd

Solve for Ws,

Stokessettling velocity:

Remember Assumptions!

Stokes Region: WsD/ < 1 Laminar

spherical

non-flocculated

Assumptions:

Shape close to a sphere

Laminar –

What if turbulent?

Cd = f (ν, D, ρ) - turbulent

Cd = 24/Re - laminar

SF = 1

SF < 1

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

sand1 mm64 cm/s16 sec 1.1 min

Fine to

very fine sand0.1 mm0.64 cm/s26 min 28.2 min

Silt0.01 mm0.0064 cm/s43 hrs

Clay0.001 mm0.000064 cm/s180 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 Brownian motion

• of different sizeShear

• Larger grain sizes of similar size Differential settling

• of different sizeShear

• 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/cm3

• primary aggregates – 1.16-1.27 g/cm3

• secondary aggregates – 1.06 – 1.07 g/cm3

• Floc settling velocity is substantially higher than the settling velocity of the component grains.

Hindered SettlingAt 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