Section 1: High Re, surface movement, fluid-sediment interactions Flashcards
What Re world do copepods live in
Both high and low
What are the mechanoreceptory setae on copepod antennae used for
Detect change in water flow and streamline, detect food source or predators,
When do copepods enter high Re world
When jumping forwards with swimming legs to catch prey. In high Re (inertia) world can temporarily ignore viscosity
How does an airfoil create lift
Increases the angle of attack, the velocity of airflow is higher on the upper surface, lower pressure on the top of the foil means lift created
When does stall occur
Maximum lift without stall is 16 degrees. Separation point jumps up (separated flow expands), so no lift
How to prevent stall
Stop propagation of flow separation up surface- use eddy flaps. Flaps trap flow separation, stops it moving up the air foil, allows higher angle of attack
Adaptation of whales to overcome drag
Tubercules on the fins stops stalling, allows higher angle of attack for quick manoeuvres. Lower drag coefficient. Leading edge tubercules generate vortices that delay separation thus preventing stall. Vortices hug the surface until the end of the air foil, delaying separation, allows increased speed and higher angle of attack.
Adaptation of scallops
Propelled by jets as shell is clamped shut, riblet on shell optimised to prevent formation of hairpin vortices, keep vortices channelled streamwise, stops vortices crossing in the flow
Adaptations of fish tails
Counter-rotating vortex pair formed with central jet of high velocity flow that generates propulsion. Highspeed around vortex, thrust generated in the center
How does burrowing help with food collection
Using flow separation vortices to trap food particles that are slowed by the burrow
What happens when exopolymer substances (EPS) are exuded
EPS (mucus) exuded in phytoplankton blooms increase seawater viscosity. Can be so viscous that flow in fish gills retarded/stop, can kill the fish, suffocate
Becoming more common with eutrophication and warming
What is a hydraulic jump
Example
A sudden change in water level, from subcritical (tranquil) flow to supercritical(shooting) flow. Increase in flow depth, loss in flow power, generation of turbulence.
Often occurs at changes in slope that results in loss of flow power
eg Taum Sauk dam break
What is the Froude number that standing waves are found at
Fr = 1 (stable)
can form in Fr range 0.8-1.77
If less than 1 will start to move upstream, eventually washed away because wave speed faster than the current
What must boats do to go faster
Escape own bow wave to go faster than hull length (wave length)
Hull speed is highest efficient speed of vessel
How to leave hull speed
Leave bow wave
Lift hull out of the water to increase speed
Boat will bounce (planning)
How is surface swimming speed limited
By body/hull length (hard if small)
Fluid forces that act on a grain
Lift component (up)
Drag component (same direction as flow)
Resultant fluid force (in between the two)
Gravity force (down)
What happens in accordance with the increased strength of threshold stress
Increasing flow power of the fluid
Fine grain carried in suspension
Coarser grain are the bedload transport
Low energy bedload, high suspended
What happens in accordance with the increased strength of threshold stress
Increasing flow power of the fluid
Fine grain carried in suspension
Coarser grain are the bedload transport
Low energy bedload, high suspended
Why are clays sticky
Subject to electrochemical inter-particle forces (van der Waals bonds)
Cohesive behaviour applies for muds & sand/mud mixtures with clay contents >15-20%
Stabilising biotic factors
Biofilms (EPS) or filamentous networks
Compaction by burrowing macrofauna increases density
Sediment armouring: organisms cover surface, shielding sediment
Biofiltration & bio-deposition: filter feeding removes suspended sediment
Destabilising biotic factors
Pelletisation enhances erodibility
Grazing reduces stabilising influence of microphytobenthos
Burrow cleaning results in ejected material
Blistering: bubble formation under biofilms results in suspension
What is saltation trajectory
If fluid force is sufficient, the grain will take off in an impulse
Turbulence can cause grain0grain collisions
Major components of hydraulic sorting in relation to particle size distribution
(least frequent) Traction: always in contact with the bed, big
(most frequent) Intermittent suspension, mid size
(least frequent) Continuous suspension, silk fine particles
Sequence of bedform development in shallow flows with increasing flow power
1: flat plane bed
2: typical ripple patter, laminations/foresets - dip down current
3 (weak boil): dunes with ripples superposed
4 (boil, flow surface not in phase with bed): dunes, laminations/foresets - dip down current
Upper stage plane bed: washed-out dunes or transition
The result of deposition during Upper stage plane bed regime
Parallel laminated sands
Often enhanced by micas giving flag stones
What is a graded bed
Coarsest material deposited first
Loss of flow power, heaviest first
What are antidunes
Flow surface in phase with the bed
Air-water interface - standing waves
Can steepen and break due to turbulence
Antidunes in deeper tidal creeks
Zones of increasing erosion
Decreasing sheer stress, deposition
Laminations dip up current
Ripple and dune nomenclature - morphology
Half height: trough or crest
Stoss side: sediment moved up
Lee side: sediment grain rolls down, protected from water or wind (air)
Summit: particles reach top then fall down slip face
Brink point: start of slip face
In large dunes the summit and brink point don’t align
Flow over ripple bedforms
Flow past a blunt object or boundary of flow, diverts from direction of flow - flow separation
Occursat break point
Detaches and reverse flow
Separation bubble, weak eddying flow, promotes erosion in the trough, enhances the amplitude of the bedform
Define the buffer layer
A zone of intense interaction between turbulent and viscous forces
Also known as turbulence generation layer
Structure of boundary layer from surface up
Laminar/viscous sublayer (may not exist)
Buffer layer (turbulence generation layer)
Fully turbulent outer layer
What is a burst process
Low velocity fluid eject to higher up the flow
Outward leap of low velocity fluid
What is a sweep process
Turbulence eddy comes down towards the bed
Inward motion of high velocity fluid
Where is more turbulent in a boundary layer
Close to the bed
Spanwise velocity component (motion): right angle to the flow
What is shark skin made of
Dermal denticles
Riblet structures formed of crests and troughs
How do riblet work
Riblets are within viscous sublayer of the flow
In viscous flow, ribbed surface appears as a smooth surface located at a virtual origin
But location of virtual origin, depending on flow direction - higher for cross flow
Cross flow width reduced by ribs: results in 10% decrease in drag
Vortices always displaced further away from boundary- riblets move vortices away from skin
Origin of ripples
Dependent on a viscous sublayer, need a burst or sweep process to generate ripples
Sweep of high velocity turbulent fluid suspends grain
Grain settles to form piles
Flow separation occurs causing further erosion
Ripples form along bed
Existence of laminar sublayer
Pre-requisite for ripple formation
When D 0.7mm- laminar sublayer destroyed
Medium sand, grain size gets bigger than the maximum thickness of the sublayer
Disrupts viscous sublayer, ceases to exist
Range of ripple formation
limited to grain sizes <600μm - 700μm
Coincides with upper limit of stability of laminar/viscous sublayer
Increase strength of flow, strength of sheer stress increase
Wavelength of ripple depends on grain size
Migration rate: 1-2mm per minute
Range of dune formation
(mega-ripples)
Development time 10 minutes to 10 days
Migration rate: River sand bar 1m/hour, Tidal estuary 0.3m/day
Bigger bedform so take longer to form
What are starved ripples
Insufficient sand/silt to form full ripple bedform
Wave celerity
Wave orbits become compressed with depth
Often consists of forwards and reverse stroke - sometimes one is more prominent
Vortex development
Sediment is entrained in the lee of the rippled during strong onshore flow
Entrained sediment in coherent vortices in lee of the ripple as flow strength increases
Flow reverses
Symmetrical or asymmetrical ripples
Oscillation of wave motion in shallow water can generate symmetrical ripples
But asymmetrical if forwards or reverse stroke is stronger
Interference ripple bedforms
Wave interference generates these
May also occur with wave and unidirectional (eg tidal) currents
