ESPL lecture 1
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- Created on: 28-05-22 19:18
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- ESPL
- aeolian multiple choice
- lecture 1
- winds - and trade winds
- winds are thermally driven by the heat budget
- the differences of heat latitudinally create differences in heat distribution nd budget globally
- this difference creased moving air - rising or sinking based on pressure
- air moves from high to lower pressure through the differential creating WIND
- PGF – pressure gradient force – the force of moving from high to low pressure
- air moves from high to lower pressure through the differential creating WIND
- this difference creased moving air - rising or sinking based on pressure
- the differences of heat latitudinally create differences in heat distribution nd budget globally
- the differences of heat latitudinally create differences in heat distribution nd budget globally
- this difference creased moving air - rising or sinking based on pressure
- air moves from high to lower pressure through the differential creating WIND
- PGF – pressure gradient force – the force of moving from high to low pressure
- air moves from high to lower pressure through the differential creating WIND
- this difference creased moving air - rising or sinking based on pressure
- cells
- the Hadley cell drives wind through circulation - moves air in pockets globally through ascending and descending air
- the crioulos effect determines the direction of winds
- the Hadley cell drives wind through circulation - moves air in pockets globally through ascending and descending air
- winds are thermally driven by the heat budget
- Large-scale airflow patterns - coriolous
- the spinning of the earth causes wind to move in different directions in each hemisphere
- deflect to the right in the N hemisphere and the right in the southern hemisphere
- The
spinning gives the winds a secondary direction of force and the overall
direction acts as a function of the total forces acting upon it (see green
arrows)
- Coriolis and PFG can cancel each other out leading to a deviation and horizontal movement
- The
spinning gives the winds a secondary direction of force and the overall
direction acts as a function of the total forces acting upon it (see green
arrows)
- deflect to the right in the N hemisphere and the right in the southern hemisphere
- Wind
has momentum the mass if the wind X the velocity
Part
of the momentum is based on the spinning of the earth à this is greatest near the equator – see
circular motion – furthest from the axis of rotation
- At
its widest part it is rotating fastest – higher rotational force
Momentum
in conserved
- Rotational speed becomes less of a contribution their relational speed is higher compared to the speed of rotation Higher speeds of wind at higher latitudes
- WIND and MOMENTUM
- Wind
has momentum the mass if the wind X the velocity
Part
of the momentum is based on the spinning of the earth à this is greatest near the equator – see
circular motion – furthest from the axis of rotation
- At
its widest part it is rotating fastest – higher rotational force
Momentum
in conserved
- Rotational speed becomes less of a contribution their relational speed is higher compared to the speed of rotation Higher speeds of wind at higher latitudes
- WIND and MOMENTUM
- wind has momentum =mass X velocity
- mass based on qualities of the atmosphere
- wind has momentum =mass X velocity
- At
its widest part it is rotating fastest – higher rotational force
Momentum
in conserved
- wind has momentum =mass X velocity
- mass based on qualities of the atmosphere
- Wind
has momentum the mass if the wind X the velocity
Part
of the momentum is based on the spinning of the earth à this is greatest near the equator – see
circular motion – furthest from the axis of rotation
- At
its widest part it is rotating fastest – higher rotational force
Momentum
in conserved
- the spinning of the earth causes wind to move in different directions in each hemisphere
- large scale airflow patterns-continentality
- the global circulation is affected by the distribution of land and ocean
- this also impacts wind speed
- local topography like mountains can impact wind speeds
- smoothness and roughness also impacts wind speeds - smoother surfaces = faster wind
- Westerlies
in N hemisphere a weather systems from other latitudes
impact winds in their surroundings
- eddies and turbulent flows disrupt air - low pressure system (30-60 degrees N/S)
- Westerlies
in N hemisphere a weather systems from other latitudes
impact winds in their surroundings
- this also impacts wind speed
- the global circulation is affected by the distribution of land and ocean
- global distribution of windspeeds
- higher over the seas = lmore smooth
- slower over land - westerlies are quicker than trade winds - 60 degrees N/S
- higher over the seas = lmore smooth
- BOUNDARY LAYER INTERFERENCE
- biggest interference with the surface
- air region above the surface where airflow is influenced by ground’s
friction
Dependent
on the roughness of the surface
- full atmospheric boundary layer - 1-2km
- near ground boundary layer - 10cm
- velocity profile - change in speed with height
- velocity profile determined by air flow type
- laminar flow - air moves in layers and do not mix - not common in natural flow
- turbulent flow - more common eddies in flow mix and deforms the air
- irregular gusts with fastest air moving at bottom due ot the compression of air
- requires negligible friction constant velocity at a height
- turbulent flow - more common eddies in flow mix and deforms the air
- laminar flow - air moves in layers and do not mix - not common in natural flow
- velocity profile determined by air flow type
- full atmospheric boundary layer - 1-2km
- air region above the surface where airflow is influenced by ground’s
friction
Dependent
on the roughness of the surface
- Profile
measurements with height – velocity profile
Profile
shape (plot) will allow the winds behaviour to be quantified and the movement
caused by the wind to be assessed and predicted.
- Relationship between surface profile and air flow.
- Velocity
profile
Closest
to ground = more friction = slower winds
Turbulent
mixing is effective at moving velocity away from the surface
Most
exchange happens near the surface
- Turbulent mixing, creates a rapid increase of u near bed
- If height (z)
plotted on log
scale …
over flat, un-vegetated ground, without strong heating… the velocity profile
will plot as a straight line (semi-log plot)
- the law of the wall
- Height
in LOG
Intercepts
on y axis
the law of the wall –
1.Uses
a log fit, velocity profile bever reaches surface (y axis intercept) by
assuming a log fit
- 2.Assumes that above the surface what exists in reality is a )ms-1 dead air zone with no wind speed because of friction
- Z0 –
aerodynamic roughness length – property of air flow
Measure - take height/30
Rougher
surfaces = increased Z0
Protection
form erosion by rougher surfaces
- Surfaces should always have the same Z0 at any different wind speed
- profile where LoW holds
- u/u_? =1/k Ln z/z_0
- u
= velocity (at height z)
z0 = aerodynamic roughness length
k = von Karman’s constant (~0.4)
u* = shear velocity
- LofW allows key parameters (z0 & u*) to be derived from u & z
- u
= velocity (at height z)
z0 = aerodynamic roughness length
k = von Karman’s constant (~0.4)
u* = shear velocity
- u/u_? =1/k Ln z/z_0
- Height
in LOG
Intercepts
on y axis
the law of the wall –
1.Uses
a log fit, velocity profile bever reaches surface (y axis intercept) by
assuming a log fit
- the law of the wall
- biggest interference with the surface
- winds - and trade winds
- lecture 1
- Shear velocity is proportional to gradient of velocity profile
- •Drag
of airflow over ground exerts shear stress
at surface (?0)
- •This stress represents ability of wind to move sediment …BUT we cannot directly measure ?0
- •Instead, we can calculate shear velocity (u*), which is proportional to the slope (y/x) of the velocity profile
- Law of the wall
- Faster
wind speed = greater shear stress
Ability
to entrain particles - erosivity
power
and potential to move matter = erosivity
- •So, u* is proportional to the slope (y/x) of the profile •i.e. faster wind at z results in a steeper profile gradient, therefore greater drag and greater u*
- Remember…
•Assumes windspeed is zero just
above surface
•Applies only when velocity profile
is log straight
•Law of the Wall does not hold:
- -Airflow is topographically-altered (e.g. accelerated) X -In non-neutral atmospheric conditions X -Vegetated surfaces X (Needs adjustment…)
- Vegetated surfaces - on average may give a Z0 at a greater height
- Faster
wind speed = greater shear stress
Ability
to entrain particles - erosivity
power
and potential to move matter = erosivity
- Law of the wall
- •Instead, we can calculate shear velocity (u*), which is proportional to the slope (y/x) of the velocity profile
- •This stress represents ability of wind to move sediment …BUT we cannot directly measure ?0
- sediment types
- percentages of rocks 20% carbonates 15% sandstone 60% clay minerals and quatrz
- quarts dominates aeolian deposits because it is heavy and chemically stable
- Abundant
(60%), highly resistant
When
in transport it isn’t broken down physically or chemically – accumulates
- Chemically stable – requires highly alkaline solutions to dissolve - pHs not found naturally
- Abundant
(60%), highly resistant
When
in transport it isn’t broken down physically or chemically – accumulates
- quarts dominates aeolian deposits because it is heavy and chemically stable
- sediment sizes
- •Sand
= 63 to 2000 mm
diameter particles
- •Silt + clay < 63 mm diameter (often referred to as ‘dust’ in aeolian processes)
- •Sand
= 63 to 2000 mm
diameter particles
- percentages of rocks 20% carbonates 15% sandstone 60% clay minerals and quatrz
- particle entrainment
- lift and drag promote entrainment
- gravity (particle size, mass) prevent
- Cohesion
– attraction – electrostatic forces, interparticle forces, friction,
Small
pressure systems within the grain
- system Pressure tries to move the particle up into the velocity gradient - pressure difference
- If
enough shear velocity is applied, it will be entrained
This
is the CSV U*c
Threshold
to entrainment
- For
winds to help the landscape the CSV needs to be understood
Function
of…
particle
size,
Gravity,
Atmospheric
density,
Moisture,
- Not a
factor of the wind but sediment and environment
If
the shear velocity is enough to cause entrainment
- Wind represent erosivity – ability to erode Surface represents erodibility – ability to be eroded
- u*c
is the u* at which aeolian transport occurs
= threshold of entrainment
- u*c = f(particle size, gravity, atmospheric density) u*c is related to the sediment & environment
- u*c
is the u* at which aeolian transport occurs
= threshold of entrainment
- Wind represent erosivity – ability to erode Surface represents erodibility – ability to be eroded
- Not a
factor of the wind but sediment and environment
If
the shear velocity is enough to cause entrainment
- For
winds to help the landscape the CSV needs to be understood
Function
of…
particle
size,
Gravity,
Atmospheric
density,
Moisture,
- If
enough shear velocity is applied, it will be entrained
This
is the CSV U*c
Threshold
to entrainment
- system Pressure tries to move the particle up into the velocity gradient - pressure difference
- bagnolds equation
- Relates
to sediment and the environment
Denser
sediment = more shear velocity to move
Air
density – usually a constant
g –
gravity
D –
greater diameter = bigger grains are harder to move
- A - coefficient of flow (value depends on if fluid or impact threshold is relevant) Usually has 1 of 2 values – Fluid - just air flow increasing in velocity - energy transfer à the impact of other sand grains = impact threshold - lower than fluid
- Relates
to sediment and the environment
Denser
sediment = more shear velocity to move
Air
density – usually a constant
g –
gravity
D –
greater diameter = bigger grains are harder to move
- creep
- •Rolling of large (>500 mm) grains along surface/bed
- •Bagnold (1941) 20-25% sand moves by creep (up to 40%)
- saltation
- Most
dynamic process
Moves
a wide range of particle sizes
Biggest
size fraction of sand (70-500mm)
- Incoming particles hit the surface with a shallow rebound (6-16°) and then a steeper rebound (50-90°) from the bed Asymmetric movement
- Ration H x12-15 = hop distance
- Some
energy lost to the bed
50-60%
retained
10%
lost into the dislodged particles
- reptation - splash
- Most relocating grain occur down wind Some do go back into the flow This breaks cohesion - breaking bonds freeing them for other forms of transportsed
- reptation - splash
- Most
dynamic process
Moves
a wide range of particle sizes
Biggest
size fraction of sand (70-500mm)
- saltation
- •Bagnold (1941) 20-25% sand moves by creep (up to 40%)
- •Rolling of large (>500 mm) grains along surface/bed
- lecture 1
- aeolian multiple choice
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