[HN Gopher] The swimming of a dead fish (2018)
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The swimming of a dead fish (2018)
Author : lord_sudo
Score : 139 points
Date : 2022-04-20 15:00 UTC (8 hours ago)
(HTM) web link (fyfluiddynamics.com)
(TXT) w3m dump (fyfluiddynamics.com)
| drewzero1 wrote:
| One of my favorite things about keeping aquarium fish has been
| watching the ways they interact with currents in the water. Move
| the filter output and the flow changes, and the fish adjust their
| movements around the tank accordingly. Different species have
| different shapes and different behaviors leading them to move
| through the water in a variety of ways. I think we still have a
| lot to learn from the efficiency of fish.
| gentschev wrote:
| Flow state
| Maursault wrote:
| This is the best metaphor I have ever seen for how creepy
| Christianity can be (next to the Resurrection scene in _Jesus
| Christ Superstar_ ).
|
| I'd like to build a boat from this design, a boat that moves
| against wind and current without engines, without sails, and
| without propulsion of any kind.
| thatguy0900 wrote:
| I'm very confused how this relates to christianity
| Maursault wrote:
| Simply, it is related both through the ichthys, the fish
| symbol, and the Resurrection, or reanimation of the dead.
| DrFell wrote:
| There's a whole new kind of fishing lure called a swimbait, of
| which there is a sub-type that's just a jointed fish shape. You
| cast it out, and reel it in, and it waves like a flag, looking
| exactly like a swimming fish. The only thing powering it is
| hydrodynamics.
| TheHegemon wrote:
| Not sure if I would call it "new". Swimbaits have been around
| for as long as I've been fishing (several decades).
|
| However there is definitely way more advanced ones then when I
| first started fishing.
| drewzero1 wrote:
| That's not a new idea at all, unless there's a specific
| development that I've missed. Jointed and/or flexible swimming
| lures have been around at least since I was a kid and I'm
| pretty sure the ones my dad used were old already then.
|
| My favorite lures though were the frogs, which had rubber
| bristles on the back which would pulse when pulled through the
| water to simulate a frog's legs swimming.
| soheil wrote:
| > The researchers came across this entirely by accident, and one
| of the questions that remains is how the trout is able to sense
| its surroundings well enough to intentionally take advantage of
| the effect.
|
| Should have been:
|
| The researchers came across this entirely by accident, and one of
| the questions that remains is how ignorant we are about some of
| the most basic aspects of the world.
| jjoonathan wrote:
| Hard disagree. "How is the trout able to sense..." is a good
| question and answering it will advance our understanding.
| Navel-gazing about ignorance will not.
| soheil wrote:
| Why are you conflating what scientists are doing vs what
| civilization as a whole has always thought about the subject?
| Cipater wrote:
| What a strange thing to say on a post about researchers working
| to dispel ignorance.
| soheil wrote:
| For millennia people thought fish _swam_ upstream until
| literally today. If that's not sheer ignorance I don't know
| what is.
| dr_dshiv wrote:
| Fishing in Alaska is wild when the millions of salmon go upstream
| to lay their eggs and die. They just continually swim upstream. I
| thought they just _looked_ dead (some are partially decomposed
| with pieces of flesh falling off). I just don't know how they
| stay balanced, pointing upstream, in that case.
|
| The bear love it. I counted 43 in 4 days. Lots of close
| encounters but you don't feel unsafe when there is so much food
| in the water. You can't cross the river without accidentally
| kicking fish.
| bradrn wrote:
| At first I thought this was related to the famous Dead Salmon
| study [0]. The article turned out to be completely different, and
| certainly much more amazing.
|
| [0] _Neural correlates of interspecies perspective taking in the
| post-mortem Atlantic Salmon: An argument for multiple comparisons
| correction_ -- http://www.prefrontal.org/files/posters/Bennett-
| Salmon-2009....
| soheil wrote:
| Reminds me of this Veritasium video were a wind powered car
| accelerates into the direction of the wind
| https://www.youtube.com/watch?v=jyQwgBAaBag
| JoeAltmaier wrote:
| Hm. Not a perpetual motion machine. And that fish appears to be
| tethered - when it turns sideways it re-orients just like its on
| a thread.
|
| So maybe it expends _very little effort_ , but not zero.
| the_af wrote:
| Define "effort". This fish is _dead_. How can it be spending
| any non-zero effort?
| hankh18 wrote:
| They're saying that because the fish is dead it has to be
| tethered, otherwise this wouldn't last for more than a few
| seconds before the fish becomes unstable and the phenomenon
| stops. For a living untethered fish, some level of energy
| would need to be expended to keep the fish stabilized instead
| of the tether.
| the_af wrote:
| Agreed, but the tether is just to keep the fish on track,
| it's still no "effort" on the part of the dead fish in this
| experiment (effort == energy spent by the dead fish). The
| article does mention a living fish will spend energy in
| order to find the sweet spot of the current, and then the
| water flow will do the rest.
|
| Also, when the dead fish "swims forward" and hits the
| obstacle, the tether itself is playing no part. It's 100%
| the water flow and the shape/flexibility of the corpse.
| JoeAltmaier wrote:
| Add to that, some energy required to swim upstream.
| the_af wrote:
| If I understand the article correctly, the energy
| expenditure by the fish itself is zero. From TFA:
|
| > "Under just the right conditions, there's actually a
| resonance between the vortices and the fish's body that
| generates enough thrust to overcome the fish's drag. This
| means the fish can actually swim upstream without
| expending any energy of its own!"
| mojomark wrote:
| That's correct. I actually recieved a copy of this video
| about a decade ago from a grad student at the MIT tow
| tank - they were working on their Robotuna design and we
| were discussing Thunniform propulsion and this video came
| up. I've used this video in many talks because it's so
| cool.
|
| In a nutshell, fish are undulating foils. When an
| oscillating or undulating foils is submerged in a fluid,
| a trailing Karmen Vortex Street (1) is generated, which
| is a set of spatially offset vortices. One of the cool
| things about that is that as the foil "swishes" from, say
| left to right, it extracts energy from the vortex - the
| foil can propel itself forward by essentially "pushing"
| off of the vortex of spinning fluid. The result is that
| the vortex rotation slows down (that's where the energy
| to propell forward primarily comes from).
|
| Side note: This is in contrast to a single rotating
| propeller that leaves a lot of used energy in the
| swirling trailing wake. Modern profilers can use things
| like contrarotating propellers or boss cap fins to
| recover some of that energy.
|
| In any event, for this "dead fish" experiment, the Karmen
| Vortex Street (KVS) is being generated by the obstacle in
| the flow in front of the fish - this is due to the low
| pressure zone directly behind the obstacle. The flexible
| foil begins to undulate in concert with these vortices.
| If you look at the figure of the KVS, the region in the
| center line of the KVS is actually creating a flow in a
| direction that is opposite that of the vortices
| themselves. In other words, there's a flow in the center
| that's effectively sucking the fish towards the rock.
|
| Nothing magic, no free energy sadly, but definitely some
| cool science! You can absolutely use this knowledge to
| design energy harvesters (generators) from flows, like
| rivers or deep ocean currents.
|
| 1. https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_vortex_
| street
| the_af wrote:
| Awesome! I really appreciate you taking the time to write
| this explanation. Very interesting.
| pessimizer wrote:
| I'm pretty sure the tethering is for _us_. The (living) fish
| wouldn 't care about maintaining a central position in a tank.
| JoeAltmaier wrote:
| Yet it makes the fish appear to be making progress in the
| moving water. Sure it's wriggling side to side, very neat.
| But keeping up with the current? Sure, if it's held there by
| a wire.
| mizzao wrote:
| Right, but notice the part of the video where it actually
| swims forward and creates slack in the wire.
| hankh18 wrote:
| Well even without the fish moving itself it wouldn't be a
| perpetual motion machine because the running water would be an
| input to the system. Agree though that a living, untethered
| fish would need to expend energy to stay balanced and in the
| right spot.
| JoeAltmaier wrote:
| Reframe the system to the moving water. The fish is then
| swimming upstream.
|
| All frames of reference are valid. So yes, perpetual motion.
| plutonorm wrote:
| what about sailing up wind?
| steve_adams_86 wrote:
| That fish looks alive until you know it isn't. That's incredible.
|
| I've wondered many times how it's possible that fish in deep
| frozen winter rivers can survive given that they have such
| limited food and need to expend energy to stay in one spot... But
| I suppose this simplifies the equation. They can rest where the
| current allows for this phenomenon.
| locallost wrote:
| Fish can stay in one spot with little effort also because they
| have a special organ called the swim bladder. They can fill it
| up with gas and this allows them to maintain their position
| without actually swimming.
| bombcar wrote:
| Doesn't the swim bladder just help them calibrate depth in
| the water?
| locallost wrote:
| I'm not an expert, I just had to learn the basics recently
| to get my fishing permit. But the explanation given was
| that its function is allowing the fish to float / hold
| their position and save energy. Carps even have two
| chambers since they are bottom feeders and they can tilt
| their bodies mouth down more easily this way.
| krisoft wrote:
| Yes. What you are talking about is a fish keeping a fixed
| depth.
|
| Having a swim bladder enables the fish to attain neutral
| buoyancy. That way the fish doesn't need to expand energy
| to keep itself from sinking or floating up. What the
| article is talking about is position keeping against the
| current in flowing water.
|
| Swim bladder is good for up-down position keeping, the
| article's phenomenon is good for forward-backward
| position keeping.
|
| Interestingly there is a way to use a swim bladder like
| construct to propel one forward. Underwater gliders do
| this, and the process is very energy efficient.
|
| The way it works is that the glider uses its variable
| buoyancy device (an artificial swim bladder) to set a
| negative buoyancy and starts sinking. The wings of the
| glider turn this downward motion into forward speed. At
| the target depth the glider expands some energy from its
| batteries to set a positive buoyancy and keeps the
| forward momentum as it is ascending. Because of this they
| only need to use energy at two points (at the top and the
| bottom) in their saw-tooth like swim profile, and they
| can travel thousands of kilometers on a single charge.
|
| https://en.wikipedia.org/wiki/Underwater_glider
| the_af wrote:
| I don't think the parent commenter is saying the swim
| bladder is _all_ that matters. There is an "also" hidden
| in their sentence :)
| kuhewa wrote:
| Yes, although it probably does have some role in swimming
| efficiency due to obviating lift generation to get from a
| to be at the same depth.
|
| OTOH especially for laterally compressed fishes, there is a
| metabolic cost of all the paired and medial fin movement
| required just to stay oriented in the water column, and
| they may be able to save more energy by deflating and
| sitting on the bottom.
| nanidin wrote:
| I think so. Scuba divers use a BCD (buoyancy control
| device) that sounds similar - it's used to control buoyancy
| only.
| NAR8789 wrote:
| The fish is not merely holding position--it's actually
| accelerating upstream!
|
| Reminds me of this Tadashi video: [How do fish swim so
| quickly?] https://www.youtube.com/watch?v=wYDh5d9pfu8
|
| Tadashi shows that fish turn drag into thrust by swapping the
| positions of the vortices they shed. Low effort, high output.
|
| TFA takes this one step further--fish bodies are shaped to
| _effortlessly harvest_ thrust from drag. No effort, free
| energy!
| HarHarVeryFunny wrote:
| Pretty counter intuitive that it takes less energy to swim
| upstream than to move forwards in still water!
|
| I wonder if live fish swimming upriver ever do this - just
| relax and enjoy the ride - rather than putting any effort
| into it ?
| a3w wrote:
| TIL: The first law of thermodynamics has an exception for
| dead fish.
|
| (JK, don't cite me in physics class!)
| bocytron wrote:
| Reminds me of Derek's video about a vehicle which is
| powered against wind:
| https://youtube.com/watch?v=jyQwgBAaBag
| munificent wrote:
| Well, actually...
|
| In this case, the fish isn't a closed system. It's
| harvesting energy from the moving water, which requires
| external energy from the sun to keep the water cycle going.
| kuhewa wrote:
| Besides energy saving behaviour, the metabolic rate has been
| depressed due to the physicochemical effects of cold, typically
| a reduction of 2-3x per 10 degrees.
|
| In terms of having enough energy, energy stores accrued during
| summer and autumn go a long way, but many fish are indeed a
| negative energy flux state over winter. In fact, over-winter
| starvation in the first year or three is a common ecological
| bottleneck, where even if fish are capable of reproducing and
| adults are fine and can survive the first year, the 0+ age
| class may not have had sufficient time to store enough energy
| before winter to survive until spring.
| franciscop wrote:
| Alternatively, I learned in fluid dynamics that there's a
| gradient of velocity of the water circulating through any pipe;
| closer to the walls/floor it's speed is effectively 0, closer
| to the center it's at the maximum speed (in an ideal
| pipe/laminar fluid, that is barring turbulences/rocks/etc). In
| the real world there's definitely pockets of water within the
| river were the water speed is insignificant/still.
|
| https://accendoreliability.com/fluid-flows-pipes/
|
| https://en.wikipedia.org/wiki/Laminar_flow
| steve_adams_86 wrote:
| That's a great observation too. I wonder if areas like these
| would cause fish to congregate, which might explain why
| animals like minks can readily find fish under the ice.
| There's so much I don't know, haha.
| munificent wrote:
| _> I wonder if areas like these would cause fish to
| congregate_
|
| Any fisherman, especially one who fishes rivers, has a
| _wealth_ of intuitive understanding of what kinds of water
| features lead to greater concentrations of fish and indeed
| a lot of it has to do with depth and turbulence. You 'll
| often hear them talking about whether a patch of water
| looks "fishy" or not, and that often has to do with how
| still or turbulent the surface is (along with many other
| factors).
| Symmetry wrote:
| It's not just fish. Our hands naturally tend to conform to the
| objects we grasp without our needing to think about it. And
| robotic grasping with hands designed to do the same is much, much
| easier than the other way around.
| HarHarVeryFunny wrote:
| Dead fish swimming upstream .. Reminds me a bit of this: a wind-
| powered vehicle than can accelerate directly into the wind...
|
| https://www.youtube.com/watch?v=yCsgoLc_fzI
| atx42 wrote:
| Actually, it's going with the wind, but faster than the wind.
| Veritasium also has a video of the dead fish going upstream
| though.
| carapace wrote:
| I can't find links at the moment (what's up with search engines
| these days?) but there's a similar phenomenon where dead whales
| move forward due to the action of waves and the natural shape and
| movement of their bodies and flukes. There were folks building a
| kind of boat propulsion system shaped like whale tails.
| karmakaze wrote:
| > one of the questions that remains is how the trout is able to
| sense its surroundings well enough to intentionally take
| advantage of the effect
|
| Doesn't a fish have the capability to try swimming at different
| frequencies to find a resonant one that minimizes effort (or
| maximizes forward motion)? Swimming might not be the right word,
| maybe flexibilities. I obviously don't do research in the field
| so wouldn't know if this is actually a dumb question.
| sjducb wrote:
| With my former biologist hat on I would ask the authors to define
| "dead"
|
| Lots of cells and tissues remain alive for months after death.
| It's easily possible that the nerves and muscles of this fish are
| alive enough to trigger basic autonomic swimming responses that
| are powering it upstream.
|
| I want to see an artificial fish model that shows this behaviour.
| kuhewa wrote:
| I reckon efficient swimming is too complex a process for
| spontaneous innervation to be responsible.
|
| A fresh dead and pithed fish might flop, and might even flop
| several times in a row but that's about as far as it goes.
|
| E.g. Just a change in temperature can make an efficiently
| swimming fish's red muscles' duty cycles so maladaptive they
| are doing negative work, swimming requires really specific
| firing patterns.
|
| Also consider -- chances are they were moving an anesthetised
| fish into the experimental apparatus without realising it
| overdosed, in which case innervation would have been
| negligible.
| natosaichek wrote:
| Where does the energy for muscle flexing come from in a dead
| fish? Without a heart circulating blood (and ATP), My
| expectation would be that the muscles would quickly run out of
| power.
|
| If you do that test where you apply electricity to a dead
| frog's leg to get it to kick, it will only work a few times
| before it's out of juice.
| mizzao wrote:
| "months after death"? Can you share a citation here?
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