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Aeolian Processes I. Aeolian Processes I Entrainment of particles – settling timescales Threshold friction speeds Suspension vs. saltation vs. reptation vs. creep Dependences on gravity, densities of particle/air Aeolian Processes II Migration rates Dune types Dunefield pattern formation - PowerPoint PPT Presentation
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PTYS 554
Evolution of Planetary Surfaces
Aeolian Processes IAeolian Processes I
PYTS 554 – Aeolian Processes I 2
Aeolian Processes I Entrainment of particles – settling timescales Threshold friction speeds Suspension vs. saltation vs. reptation vs. creep Dependences on gravity, densities of particle/air
Aeolian Processes II Migration rates Dune types Dunefield pattern formation Ripples vs. dunes Ventifact, yardang erosion Dust-devils and wind streaks
PYTS 554 – Aeolian Processes I 3
Suspension vs saltation
PYTS 554 – Aeolian Processes I 4
Suspension All particles eventually settle out of a quiescent atmosphere Reynolds number quantifies whether an atmosphere is quiescent
Re > 10s means turbulent flow (viscosity doesn’t damp eddies) High velocity flows are more turbulent Low viscosity fluids are more turbulent
Consider laminar flow around a falling sphere Drag from sphere affects air within a cylinder ~2d wide
Downward force from weight – buoyancy
Upward force from viscous drag Stress ~ viscosity x strain rate Area affected is curved wall of cylinder …and ignoring some numerical factors
Equating the two gives the terminal velocity
Stokes’ law
v
d
3d
d
PYTS 554 – Aeolian Processes I 5
Turbulent flow As before downward force from weight – buoyancy
Falling particle is opposed by ram pressure
Equating these to find the settling velocity – not very sensitive to particle size
v
d
Low pressure
High pressure
PYTS 554 – Aeolian Processes I 6
Turbulent eddies have speeds ~0.2 the mean windspeed
For suspension: For dust sized particles: Mars, Venus and Titan are effective at suspending particles …but Venus (and Titan?) probably doesn’t have high near-surface winds
PYTS 554 – Aeolian Processes I 7
In a planetary boundary layer Drag of wind on surface produces a shear stress Measured with drag plates
We define a ‘shear velocity’ u*
Just another way to quantify the shear stress
For a Newtonian fluid (like air):
In a thin laminar sub-layer η is constant and a property of the fluid (and temperature)
Above this layer, turbulence dominates, η is a property of the flow and varies with height and u Empirically – law of the wall… (κ is Von Karman’s constant ~ 0.41)
PYTS 554 – Aeolian Processes I 8
Z0 is the equivalent roughness height 1/30th of the grain size for quiescent
situations Otherwise it’s empirically determined from
several wind measurements at different heights
Mediumsand
Greeley, 1985
PYTS 554 – Aeolian Processes I 9
Two regimes
Small particles hide within the laminar zone, larger particles stick up into the turbulent zone Balance shear stresses with weight – buoyancy of particles
At the threshold velocity, some component of drag force balances the particle weight
Transition at: D ~ 0.7 δNeither approach works well in the transition zone
Anderson and Anderson 2010
or A2 often called θA~0.1
PYTS 554 – Aeolian Processes I 10
More detailed, gets you within a factor of 2 of deriving A
Anderson and Anderson 2010
PYTS 554 – Aeolian Processes I 11
Define the frictional Reynolds number A varies with this value
A
Re*~3.5
where n >>>1
Small particles in laminar zone
Large particles in turbulent zone
Recall:
Turbulent zone:
Laminar zone:
uT
d
?
PYTS 554 – Aeolian Processes I 12
‘A’ should be constant in the fully-turbulent case Instead is depends on the fluid/particle density ratio A cautionary tale in using ‘dimensionless’ scaling from one planet to another…
Quartz in
water
Quartz on Earth
Iversen et al.1987
Ice on Titan
Basalt on Venus
Basalt on Mars
PYTS 554 – Aeolian Processes I 13
Minimum exists when Re ~ 3.5
Easiest particles to move depends on Atm. viscosity Atm. density Particle weight (density and gravity) Buoyancy effects minor (until we get to the fluvial processes lectures)
uT
d
?
~225 microns for Earth
PYTS 554 – Aeolian Processes I 14
Easiest particles to move are sand-sized
1mm 1cm0.1 mm
Sand-sizedDust Gravel
Saltation threshold increases with particle size
Particles classified by Udden-Wentworth scalemmD 2
Greeley, 1985
PYTS 554 – Aeolian Processes I 15
Necessary wind speed depends on atmospheric density
PYTS 554 – Aeolian Processes I 16
Easy to move but not easy to suspend Particles are launched off the surface, but re-impact a short time later – saltation!
Greeley, 1985
PYTS 554 – Aeolian Processes I 17
Impact vs fluid threshold It’s easier to keep saltation going than start it Impact threshold is ~0.8 times the fluid threshold
for Earth …but ~0.1 times the fluid threshold for Mars
This is what makes martian saltation possible
Kok, 2010
Kansas State University
Grains travel by saltation Impacting grains can dislodge new particles (reptation) Impacting grains can push larger particles (creep) Impacting grains knock finer particles into suspension
Fluid Mars
Impact Mars
Impact Earth
PYTS 554 – Aeolian Processes I 18
Saltation length scales ~cm
Greeley, 1985
PYTS 554 – Aeolian Processes I 19
Bagnold’s description of momentum loss Mass flux per unit length – q Momentum change of grains mass x (u2-u1) over a distance L, with u2>>u1
Stress is:
Avg. horizontal velocity ~ 0.5 u2
Time of flight is 2w1/g
L = u2 w1/g
so: u2/L = g/w1
Stress is also And w1 ~ u* (ignoring factors ~1)
L
v1
v2
Sand flux per unit length is proportional to shear velocity cubed
Bagnold’s experimental work showed particle size is also a factor
v1w1
u1
PYTS 554 – Aeolian Processes I 20
There are many variations fit to empirical data
Greeley, 1985
PYTS 554 – Aeolian Processes I 21
Titan
95%
Zero
Zero
Zero
5% methane
Density Kg m-3 71.92 1.27 0.027 5.3
Gravity (m s-2) 8.9 9.8 3.7 1.35
Dune material Basalt Quartz Basalt Organics(lower density)
Dune Potential(All else being equal)
Venus
Titan
Earth
Mars
PYTS 554 – Aeolian Processes I 22
As usual – all else is not equal
Venus has very few dunes (two fields known) Lack of weathering into small particles Detectability of dunes ? Low surface winds
Mars has extensive dunefields Very high wind speeds Lots of active weathering breaking up rocks
Dune Potential(All else being equal)
Venus
Titan
Earth
MarsFortuna-Meshkenet fieldWeitz et al. 1994
PYTS 554 – Aeolian Processes I 23
Aeolian Processes I Entrainment of particles – settling timescales Threshold friction speeds Suspension vs. saltation vs. reptation vs. creep Dependences on gravity, densities of particle/air
Aeolian Processes II Migration rates Dune types Dunefield pattern formation Ripples vs. dunes Ventifact, yardang erosion Dust-devils and wind streaks