FINAL: Swim

Question 1

Water is much denser than air. What does this affect?

Answer 1

lift, drag, and buoyancy experienced by swimming animals

Question 2

What does the generation of lift require?

Answer 2

constant movement of air over the wings (due to the movement of the air, the movement of the animal, or both)

Question 3

What allows animals to remain suspended in water without the need to move?

Answer 3

buoyancy

Question 4

What are the forces acting on swimming animals?

Answer 4

forward: thrust
backward: drag
upward: lift, buoyancy
downward: weight

Question 4

What are the forces acting on flying animals?

Answer 4

forward: thrust
backward: drag
upward: lift
downward: weight

Question 5

Do horizontal pressures on a submerged object differ?

Answer 5

no – they are the same, therefore they cancel each other out

Question 6

Do vertical pressures on a submerged object differ?

Answer 6

yes – therefore there is a vertical pressure gradient (hydrostatic pressure increases with depth)

Question 7

Negative Buoyancy

Answer 7

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

Question 8

Neutral Buoyancy

Answer 8

• 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

Question 9

Positive Buoyancy

Answer 9

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

Question 10

What does the volume of water that must be displaced to balance the weight of the object depend on?

Answer 10

density of water

Question 11

What can affect water density? (2)

Answer 11

• salinity
• temperature

Question 12

What is the Plimsoll line?

Answer 12

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)

Question 13

Animals swimming in water may sink or float. What can they do to maintain position? (2)

Answer 13

• can expend energy to maintain position
• can achieve neutral (or near neutral) buoyancy

Question 14

What are the methods/structures for the reduction of dense substances to lower overall density (and increase buoyancy)?

Answer 14

• 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)

Question 15

What is the centre of mass (or gravity)?

Answer 15

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

Question 16

What is the centre of buoyancy?

Answer 16

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)

Question 17

centre of buoyancy below centre of mass

Answer 17

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

Question 18

centre of buoyancy above centre of mass

Answer 18

body is stable

if object is deflected, the two forces will produce a rotational force to roll it back to its stable condition

Question 19

centre of buoyancy at centre of mass

Answer 19

body is neutrally stable

Question 20

centre of buoyancy and centre of mass not in the same position along the body

Answer 20

unstable

Question 21

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?

Answer 21

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)

Question 22

What is Newton’s 3rd Law?

Answer 22

rate of momentum change of water, in the direction opposite to swimming direction, will result in a thrust force on the swimmer

Question 23

Momentum (mass x velocity) must be conserved. To go forwards, you must push something backwards. How can the momentum of water be changed?

Answer 23

• lift-based propulsion
• drag-based propulsion
• jetting

Question 24

Penguins – Smaller Wing Area

Answer 24

• wings do not need to produce lift to support weight
• weight is counteracted by buoyancy
• slightly positively buoyant
• still need to generate thrust

Question 25

Lift-based Propulsion – Wings

Does a symmetrical hydrofoil at 0º angel of attack (AoA) produce lift or drag?

Answer 25

• does NOT produce lift
• does produce drag

Question 26

Lift-based Propulsion – Wings

What is used to generate thrust?

Answer 26

downstroke and upstroke

Question 27

Lift-based Propulsion – Wings

What is the AoA mid-upstroke?

Answer 27

negative AoA –relative flow of water coming from above chord line of the wing

Question 28

Lift-based Propulsion – Wings

What is the AoA mid-downstroke?

Answer 28

positive AoA – relative flow of water coming from below chord line of the wing

Question 29

Lift-based Propulsion – Wings

Over a complete up/down wingbeat cycle, in which direction does the average force act?

Answer 29

average force is directed forward (thrust) and downward

thrust and downward-directed lift force offset buoyancy

Question 30

When does thrust = drag during swimming?

Answer 30

when swimming at constant speed

Question 31

What is aquaflying?

Answer 31

lift-based propulsion using forelimbs has independently evolved among other animals – use modified forelimbs as hydrofoils to generate thrust from lift

Question 32

Lift-based Propulsion – Tails

What do tails behave like?

Answer 32

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)

Question 33

Lift-based Propulsion – Tails

How might thrust be increased? What is the cost of doing so?

Answer 33

increasing W (downward velocity of fin)

this comes at the cost of a larger force resisting the movement of the fin

Question 34

What is the equation for buyoant force?

Answer 34

FB = pvg

Question 35

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?

Answer 35

if you want to sink, you should exhale, which will reduce your overall body density to even more lower than water density

Question 36

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?

Answer 36

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

Question 37

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)

Answer 37

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

Question 38

For a dolphin swimming with it’s tail angled below stalling angle of attack, how could it increase thrust?

Answer 38

• 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)

Question 39

During jetting locomotion, the animal can generate more thrust likely by…

Answer 39

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)