Vectors that suck
• Gravity
• Drag1. Skin friction• Depends on total exposed surface
2. Pressure drag• Depends on shape and Reynolds number
Ratio of inertial and viscous forcesRe = Fi = ρlv
Fv Bacteria swimming Re = 0.000001Fruit fly flying Re = 100Large whale swimming Re = 200 000 000
At low Re, streamlining does no good• decreases in pressure drag are offset by total
exposed surface area
At high Re, streamlining can be very effective• decreases drag by up to a full order of magnitude
Reynolds Number
Vectors that are good: LiftLower pressure, higher velocity above,Higher pressure, lower velocity below
(Bernoulli Principle)
- Caused by asymmetry, inclinations, or both
- Force is created perpendicular to the direction of flow of the overall fluid
• Thrust is used to transfer momentum to liquid– Drag based
• Pulling yourself through water
– Lift based• Pushing water back
How do fish make thrust?
Vortices
• What is a vortex?– Translates about jet of fluid formed by
airfoil
• What do vortices mean?– Force imparted to fluid thrust or drag– Directionality
Vortex shedding• Vortices formed at the trailing edge of the wing are
shed as the shear forces become too great to maintain flow entrainment
Swimming: 2 main ways
• Drag based ‘paddlers’– Usually paired-fin swimming– Better at acceleration
• Lift based ‘flyers’– Rotating or folding wing/fin– tail = hydrofoil – Better at maintaining inertia– Some paired-fin swimmers
How it’s studied
• Videography2 cam (3-D)
High speed
• Digital Particle
Image Velocimetry
(DPIV)
• EMGs with sonomicrometry
Sharks: function of the body
Rising: 22°Holding: 4-11 °Sinking: -11°
Body orientation adjusted to induce appropriate lift
Sharks: function of the caudal finGenerates both thrust and lift by moving water
posteriorly and ventrally.
• Provide negligible lift
• Pectoral fins held so that flow speed & pressure are equal on dorsal & ventral surfaces
• Fins are not actively held in any particular position
Sharks: pectoral fins in horizontal swimming
Biomechanics of benthic station-holding or…sitting
Experience strong
currents &/or heavy flow• Face current • Flat against substrate
to reduce drag• (-) lift with P1 =
increased friction with substrate
Batoid locomotionPectoral fins
1. Undulatory-drag basedPass waves down fins (ant to post)
2. Oscillatory- lift basedFlap fins up and down
3. Axial-undulatory-lift based
Undulate pec fins, but also pass waves down axial skeleton (ant to post)
Batoid locomotionPelvic fins – Punting
- Skates
QuickTime™ and aSorenson Video decompressorare needed to see this picture.
QuickTime™ and aSorenson Video decompressorare needed to see this picture.
fieldcaptive
Holocephalan locomotion
Pectoral fins - combination of:1. Undulatory
• Pass waves down fins (anterior to posterior)
2. Oscillatory• Flap fins up and down
Narrow caudal peduncle
Body form & fin shapeSharks Type 1 Fast-swimming pelagics: Carcharodon, Isurus
Conical head
Large pec fins Large, deep body
Reduced for streamlining
Highheterocercal angle
Externally symmetric
Body form & fin shapeSharks Type 2 Generalized, continental swimmers
Ex: Alopius, Carcharhinus, Negaprion, Sphyrna, Mustelus
Less deep body
Flattened ventral head & body surface
Large pec fins
Moderately sized pelvic, 2nd dorsal, anal fins
Lower heterocercaltail angle
Body form & fin shapeSharks Type 3 Slow swimming, epibenthic, benthic, &
demersal sharks Ex: Ginglymostoma, Galeus, Hexanchiformes
Blunt snout
Large head More post. 1st dorsal fin
Sm./no hypochordal& subterminal lobe
More ant. Pelvic fins
Low HC tail angle
Body form & fin shapeSharks Type 4 Most are deepsea
– Ex: Only squalean or dogfish sharks
Many body shapes
Higher pec fin insertionLack anal fin
Large epicaudal lobe
Body form & fin shape
Batoids Type 5 Benthic, but includes some pelagics
Dorsoventrallyflattened body
Enlarged pec fins
Reduced caudal half
Body form & fin shape
Holocephalans / chimeras Type 6
Laterally compressed
Leptocercal (long & tapering)to heterocercal tail
Conclusions
• Current literature discusses only a small number of taxa
• Studies carried out in controlled lab settings
• Little information on biomechanics in natural conditions
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