Swimming locomotion Flashcards
What is periodic (steady or sustained) swimming?
- Cyclic repetition of the propulsive movement
- Traversing large distances at more or less constant speed
What is transient (unsteady) swimming?
- Rapid starts, escape maneuvers and turns at the last millisecond to catch prey/avoid predators
What is body and caudal fin locomotion?
body bends into a backward-moving propulsive wave
What is Medial and paired fin swimming?
Undulatory or rowing motions of the fins to create thrust
in what three ways does the transfer of momentum from the fish to the surrounding water occur while swimming?
- Lift
- Drag
- Acceleration reaction
How is lift formed with regard to swimming
Pressure differential caused by asymmetries in the flow:
–> Pressure on one side of hydrofoil is greater than on the other
What is friction drag with reguard to fish?
Friction between fish and the boundary layer of water
- -> arises as a result of viscosity in the water in areas of flow with large velocity gradients
- -> depends on wetted surface area (Sw), speed and nature of boundary layer
How is pressure drag created by fish?
Distortion of flow around solid bodies and depends on their shape
How does a fish create the pressure drag called induced drag?
Fins generate vortices unrelated to thrust. Depends largely on the shape of fin
What is the acceleration reaction regarding fish?
The inertial force associated with accelerating the water surrounding a body or appendage
What does the acceleration reaction depend on?
- Acceleration
- changing velocity
- mass of the object
- the shape of the object
What is the acceleration reaction?
F= Drag of body + Inertia of added mass
Added mass = Volume of water accelerated with the body or appendage
How does the acceleration reaction respond to increasing velocity? Decreasing velocity?
Increasing velocity = inertial forces are in the same direction as drag
Decreasing = inertial forces are in opposite direction of drag
What are the two main forces of thrust production?
- Added mass
2. Lift-based (vorticity) method
How does the lift-based (vorticity) method work?
Tail creates awake that has reverse directional rotation to that of the von karmen street vortex trail. This generates thrust as a result.
What happens at low reynolds number?
Cd is high, viscosity dominates and there are few to no vorticies
What happens at high reynold number?
Cd decreases and inertia dominates. Vorticies create pressure drag but skin friction is lowered.
What does newtons third law mean with regard to fluid moving down?
The force imparted by fluid moving down is equal and opposite to the lift force
What is froude efficiency?
Power out / Power in = (Power supplied by the jet, paddle, etc) / (Power paid by the engine to supply the increase in momentum)
= 2V1 / (V2 + V1)
What is strouhals number?
(fA) / U
f = tail beat frequency
U = average forward velocity
A = wake birth
What is a normal strouhals number?
0.25 < St < 0.35
In an anguilliform swimmer, caudal elements generate more forward thrust for what two reasons?
- Tail moves farther:
- -> moves larger amounts of fluid
- -> F= ma
- -> Newton’s third law (equal and opposite reaction) - Tail elements push more directly forward. (vector points more directly towards direction of motion)
What limits the forward speed of anguilliforms?
The body wave moves faster than the forward swimming speed so to increase forward speed they must first increase wave propagation speed.
What are the advantages of anguilliform swimming?
Very maneurverable and better at acceleration from zero velocity than Thunniforms.
- Swim slower than carrangiforms and thunniforms
Summarize Anguilliform swimming (8)
- Body undulates from head to tail
- More than one wavelength on body
- More caudal elements contribute to larger thrust forces
- Lateral forces are large and tend to cancel each other out
- Body wave travels faster (V) than forward speed (U)
- Low efficiency if defined by U/V
- Froude efficiency may be quite high
- Most certainly speed is limited
Thrust and propulsive efficiency depend on what? (4)
- Aspect ratio of the caudal fin
- -> High aspect ratio fins lead to improved efficiency because they induce less drag per unit of lift or thrust produced - Shape of the caudal fin
- -> A sharp-curved leading edge is beneficial because delays boundary layer separation for high thrust values (allows high angle of attack) - Fin stiffness
- -> A higher degree of stiffness increases thrust generation capability - Oscillatory motions of the fin
- -> Struhal number
Why do caudal elements generate more forward thrust? (2)
- Tail moves farther
- > moves larger amount of liquid
- > F=ma
- > Newton’s 3rd law (equal/opposite reaction) - Tail elements push more directly forward (vector points more directly towards direction of motion)
how do anguilliform swimmers use a drag based form of forward motion?
->Fluid is accelerated down the eel’s body as it produces thrust
-> Drag along the eel’s body removes momentum from the fluid
These two effects cancel each other out and produce no net change in downstream fluid momentum as long as the eel is swimming steadily
*As drag decreases speed of fluid moving around eels body the pressure must increase (Bernoulli) which squeezes the eel forward
Summarize Caragniform swimming: (5)
- Undulations restricted to posterior third of body
- Thrust largely from stiff caudal tail
- Generally faster swimmers than anguilliforms or sub-caragniforms
- Turning and accelerating abilities are compromised due to stiff body
- Lateral forces are concentrated at the posterior , increases recoil (waggle) of the head