FINAL: Swim Flashcards
Water is much denser than air. What does this affect?
lift, drag, and buoyancy experienced by swimming animals
What does the generation of lift require?
constant movement of air over the wings (due to the movement of the air, the movement of the animal, or both)
What allows animals to remain suspended in water without the need to move?
buoyancy
What are the forces acting on swimming animals?
forward: thrust
backward: drag
upward: lift, buoyancy
downward: weight
What are the forces acting on flying animals?
forward: thrust
backward: drag
upward: lift
downward: weight
Do horizontal pressures on a submerged object differ?
no – they are the same, therefore they cancel each other out
Do vertical pressures on a submerged object differ?
yes – therefore there is a vertical pressure gradient (hydrostatic pressure increases with depth)
Negative Buoyancy
sink
- water volume = object volume
- density of surrounding fluid < density of submerged object
- mass of displaced water < mass of submerged object
- apparent mass of object = mass of object - mass of displaced water
Neutral Buoyancy
- water volume = object volume
- mass of displaced water = mass of submerged object
- buoyant force is directly proportional to volume and mass of displaced water
- apparent mass of object = mass of object - mass of displaced water = 0
Positive Buoyancy
float
- water volume = object volume
- mass of displaced water > mass of submerged object
- buoyant force is greater than weight
- apparent mass of object = mass of object - mass of displaced water
What does the volume of water that must be displaced to balance the weight of the object depend on?
density of water
What can affect water density? (2)
- salinity
- temperature
What is the Plimsoll line?
line painted on side of ship’s hull that marks the depth to which it can be safely loaded in different waters
line indicates height of water level on the side of the hull – equivalent to hull’s displacement (volume submerged)
Animals swimming in water may sink or float. What can they do to maintain position? (2)
- can expend energy to maintain position
- can achieve neutral (or near neutral) buoyancy
What are the methods/structures for the reduction of dense substances to lower overall density (and increase buoyancy)?
- make skeletons from cartilage instead of ossified bone
- loss of protective shells
- fatty livers that are mostly oil (lower density than water)
- fatty blubber
- modify ionic composition to include less heavy ions (but keep body fluid at same osmolarity as seawater)
- rigid gas-filled structures (ie. shell)
- flexible gas-filled structures (ie. swim bladder)
What is the centre of mass (or gravity)?
unique point in an object/system which can be used to describe the system’s response to external forces and torques
- point where the weighted relative position of the distributed mass sums to zero
- or the point where (if force is applied to it) it moves in the direction of the force without rotating
- location where all the mass of the object could be concentrated at a single point
What is the centre of buoyancy?
equal to centre of mass of water displaced by the object
located where centre of mass of object would be if it had uniform density (always in the middle)
centre of buoyancy below centre of mass
body is unstable
opposing buoyant (up) and weight (down) forces will produce a rotational force (pair of ‘turning moments’) that will roll the object to establish a stable condition
centre of buoyancy above centre of mass
body is stable
if object is deflected, the two forces will produce a rotational force to roll it back to its stable condition
centre of buoyancy at centre of mass
body is neutrally stable
centre of buoyancy and centre of mass not in the same position along the body
unstable
The centre of buoyancy and centre of mass is not in the same position along the bodies of sharks. What tends to happen to sharks?
shark pitches forward/down until buoyancy and weight are acting along same position along the body
- but horizontal position of shark is maintained by upwards-directed force generated by the fin (A)
- weight of shark is supported by sum of buoyant force and lift force (A)
What is Newton’s 3rd Law?
rate of momentum change of water, in the direction opposite to swimming direction, will result in a thrust force on the swimmer
Momentum (mass x velocity) must be conserved. To go forwards, you must push something backwards. How can the momentum of water be changed?
- lift-based propulsion
- drag-based propulsion
- jetting
Penguins – Smaller Wing Area
- wings do not need to produce lift to support weight
- weight is counteracted by buoyancy
- slightly positively buoyant
- still need to generate thrust
Lift-based Propulsion – Wings
Does a symmetrical hydrofoil at 0º angel of attack (AoA) produce lift or drag?
- does NOT produce lift
- does produce drag
Lift-based Propulsion – Wings
What is used to generate thrust?
downstroke and upstroke
Lift-based Propulsion – Wings
What is the AoA mid-upstroke?
negative AoA –relative flow of water coming from above chord line of the wing
Lift-based Propulsion – Wings
What is the AoA mid-downstroke?
positive AoA – relative flow of water coming from below chord line of the wing
Lift-based Propulsion – Wings
Over a complete up/down wingbeat cycle, in which direction does the average force act?
average force is directed forward (thrust) and downward
thrust and downward-directed lift force offset buoyancy
When does thrust = drag during swimming?
when swimming at constant speed
What is aquaflying?
lift-based propulsion using forelimbs has independently evolved among other animals – use modified forelimbs as hydrofoils to generate thrust from lift
Lift-based Propulsion – Tails
What do tails behave like?
lift-producing hydrofoil
like a propeller but instead of rotating around a central point, tail fin oscillates up/down (dolphin, whale) or side/side (tuna, shark)
Lift-based Propulsion – Tails
How might thrust be increased? What is the cost of doing so?
increasing W (downward velocity of fin)
this comes at the cost of a larger force resisting the movement of the fin
What is the equation for buyoant force?
FB = pvg
If you want to dive deeper and remain there with minimal effort, should you inhale and hold a lungful of air, or exhale as much as possible?
if you want to sink, you should exhale, which will reduce your overall body density to even more lower than water density
QUIZ 9, Q9: An airfoil starts moving on the upstroke has the following (rough) streamline pattern. According to Kelvin’s theorem, what would you expect the direction of the first vortex shed to be?
the streamline profile means that there is high velocity under the airfoil and low velocity above the airfoil
- this creates a shearing resulting in a non-zero vorticity
- direction of the shearing and the flow means there will be a counterclockwise “vortex” around the airfoil
- according to Kelvin’s theorem, in an enclosed fluid, the circulation must total to zero, so the first vortex shed when the airfoil starts moving upwards will be clockwise
QUIZ 9, Q10: A reverse von Karman street is formed behind a fish. If a second fish were to follow this fish, which position is the least efficient:
a) closer to fish, but not directly behind it – beside only one vortex street (outside von Karman street)
b) farther from fish, but directly behind it – between vortices on left and right (in von Karman street)
reverse von Karman street will push water backwards through the middle so it will be more difficult for another fish to swim behind the fish
vortices will also have flow forwards on the outside, so it will be easier for a fish to ride the forwards flow velocity outside the reverse von Karman street
it would be least efficient to swim where the red fish is
For a dolphin swimming with it’s tail angled below stalling angle of attack, how could it increase thrust?
- increase amplitude of tail movement
- increase frequency of tail movement
because we are operating below the max performance angle of attack, any increase in angle of attack or velocity will increase lift more than drag
increasing oscillation amplitude or frequency (with everything else constant), this will increase both angle of attack and velocity
lift increase will tilt the resultant vector forward, and therefore increase the forward component (thrust)
During jetting locomotion, the animal can generate more thrust likely by…
increase the velocity of the fluid being pushed back
- animal will not have much variation in the amount of fluid it is moving backwards since there is a finite amount of space in the body (ie. cephalopod mantle)
