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
