Physics of swimming

• not all fish swim• not all swimmers are fast or efficient• but - in order to swim fast, all fish have the same constraints

due to physics - thus fast fish tend to look similar

Physics of swimming

Properties of water as medium in which to move• density - 830x greater than air• viscosity – 70x greater than air

Physics of swimming

Properties of water as medium in which to move• density - 830x greater than air• viscosity – 70x greater than air• lift – force exerted on object perpendicular to direction of flow (or movement) - proportional to the area over which the pressure difference acts

Physics of swimming

lift

Properties of water as medium in which to move• density - 830x greater than air• viscosity – 70x greater than air• lift • drag – 830x greater than in air

- increases with speed of object or current - due to separation of flow from object into turbulent flow

Physics of swimming

Properties of water as medium in which to move• density - 830x greater than air• viscosity – 70x greater than air• lift • drag – 830x greater than in air

- increases with speed of object or current - due to separation of flow from object into turbulent flow

• boundary layer - laminar or turbulent

Physics of swimming

Reynolds number (Re): ratio of inertial forces to viscous forcesRe = LVr/m

L = length of objectV = velocity of objectr = density of fluidm = viscosity of fluid

Physics of swimming

Reynolds number (Re): ratio of inertial forces to viscous forcesRe = LVr/m

L = length of objectV = velocity of objectr = density of fluidm = viscosity of fluid

flow changes to turbulent at Re ~ 2,000

turbulent flow is a consequence of increasing speedincreasing length(decreasing viscosity)(increasing density of liquid)

boundary layer changes to turbulent as Re goes from 5x105-5x106

Physics of swimming

Reynolds number (Re): ratio of inertial forces to viscous forcesRe = LVr/m

L = length of objectV = velocity of objectr = density of fluidm = viscosity of fluid

Examples of Re:animal speed Rewhale 10 m/s 300,000,000tuna 10 m/s 30,000,000copepod 20 cm/s 300sea urchin sperm 0.2 mm/s 0.03

Physics of swimming

for efficient swimmingavoid separation of boundary layer from surfacemaximize laminar flow in boundary layer

Physics of swimming

for efficient swimmingavoid separation of boundary layer from surfacemaximize laminar flow in boundary layerminimize turbulent flow in wake

Physics of swimming

solutions: streamline body (tapering):

aspect ratio of about 0.25 maximum thickness of body 1/3 from front (head)

Physics of swimming

a

b

Aspect ratio = a/b

solutions: streamline body (tapering):

aspect ratio of about 0.25 maximum thickness of body 1/3 from front (head)

drag reduction - keep body rigid

Physics of swimming

solutions: streamline body (tapering):

aspect ratio of about 0.25 maximum thickness of body 1/3 from front (head)

drag reduction - keep body rigid slime layer to reduce frictional drag rough surface (cteni) keeps boundary layer attached?

Physics of swimming

Australian Museum

Swimming modes“kick and glide” active - sustained for hours or daysburst - only for up to 30 secs

large fishes have greater difference between burst and active than small fishes

Physics of swimming

Swimming modes“kick and glide” active - sustained for hours or daysburst - only for up to 30 secs

large fishes have greater difference between burst and active than small fishes

Physics of swimming

active swimming accomplished using red muscle along sides of fish - high myoglobin and mitochondrial enzymes

burst swimming with white muscle - great contractile speeds, low endurance

Physics of swimming

BCF - body/caudal fin propulsion

anguilliform entire body undulateslaterally flattened, elongated body

inefficient

Anguilliformes – moray eel

Perciformes – snake mackerel, etc.

subcarangiformswim with posterior portion of body, less than one wavelengthtend toward truncate, rounded, or emarginate tailshead still yaws with motion of swimmingaspect ratio of tail ~1.5-2

cods, basses, trout, many others

Salmoniformes – rainbow trout

carangiform

less that half to one third of body flexes

generally narrow peduncle, flared and strongly forked or lunate tail

high aspect ratio tail (square of span/surface area) ~3.5

herrings, jacks, some scombrids

Perciformes – jacks (Carangidae)

thunniformextremely stiff body, narrow peduncle, high aspect ratio tail (4-10)large tendons to support muscular energy transmission to tail; stiffened tailtunas, marlins, sailfishes, some sharks

Perciformes – tuna (Thunnidae)

ostraciiformonly moves tail, rest of body rigid boxfishes, porcupine fish

Tetraodontiformes – boxfish (Tetraodontidae)

MPF - median/paired fin propulsion

rajiiform undulate pectoral fins from front to back, with wing-like ‘flapping’

Rajiiformes – Rajidae(manta ray)

Tetraodontiformes - pufferfish (Ostracidae)

didontiform undulate pectorals for sculling and maneuvering

labriform oscillate pectorals for sculling and maneuvering

Perciformes – parrotfish (Labridae)

amiiform/gymnotiformuse undulatory waves of dorsal (Amia) or anal (Gymnotids) finsalso seahorses, with narrow-base dorsal

Gymnotiformes - knifefish

Amiiformes - bowfin

balistiform use simultaneous motion of dorsal and anal fins - triggerfish (used to some extent in eels, percids, flatfish)

Tetraodontiformes – triggerfish (Balistidae)

tetraodontiform both dorsal and anal finsmove together to each side

Tetraodontiformes – ocean sunfish (mola)

NON-SWIMMING LOCOMOTIONgliding above water - flying fishes

add to take-off propulsion by using tail lobe in water like propellermay fly up to 400 m, as high up as 5 mmay add pelvic fins as secondary gliding surfaces

Scorpaeniformes – flying gunard

Other forms of nonswimming locomotion:

burrowing - eels, gobies, flatfish, rayswriggling - eelshitchhiking - remoras, lampreypush-and-hold - gobies using pelvic disk; lamprey using oral disk

Other forms of nonswimming locomotion:

‘walking’ or climbing on pectorals - walking catfish, mudskippers

walking on bottom – sea robins using pectoral rays; batfish and relatives walk on modified pelvics

Lophiiformes - batfishSiluriformes – walking catfish

Perciiformes (Gobiidae)

- mudskipper

Other forms of nonswimming locomotion:

‘walking’ or climbing on pectorals - walking catfish, mudskippers

walking on bottom - searobins using pectoral rays; batfish and relatives walk on modified pelvics

Aulopiformes - tripod fish

Other forms of nonswimming locomotion:

leaping - mullets, tuna, sailfish - also salmonids moving upstream

Other forms of nonswimming locomotion:

jet propulsion - by forcible ejection of water out of gills by operculum used as ‘assist’ to fast take-off by some percids, sculpins

passive drift - larvae, sargassum fish

frogfish