Swimming Of A Dead Fish: An Informational Article For The Curious And Informed

The swimming of a dead fish is a phenomenon that may be seen when some types of fish die.

Opisthotonus, or "opisthotonic posturing," is another name for this phenomenon.

It happens when the fish's muscles tighten after death, creating rigor mortis and stiffening of the fish's body.

The fins and tail are often extended, and the mouth is open.

Anger or fear is not the right emotion to feel when dying.

This is just a normal reaction.

All of the dead fish's swimming is driven totally by the energy in the flowing water.

Of course, evolution being what it is, genuine marine species have evolved to benefit from the energy boost provided by flowing water, not only while they're dead.

The majority of fish have a "lateral line."

Why Does Dead Fish Swim?

The obstruction creates a wake of alternating vortices in the flow.

When the fish is appropriately positioned in that wake, the eddies bend its body, causing its head and tail to point in opposite directions.

Under the right circumstances, a resonance between the vortices and the fish's body provides enough thrust to overcome the fish's drag.

This implies the fish can travel upstream without wasting any of its energy!

The researchers found this by accident, and one of the mysteries is how the trout can sense its surroundings so well that it can take advantage of the impact.

This study was conducted and reported on by D. N. Beal and M. S. Triantafyllou of the Massachusetts Institute of Technology, Cambridge, and Harvard University, Cambridge, in the United States.

The findings were published in the Journal of Fluid Mechanics.

According to the author, when he was a kid, his father would take him trout fishing, and he'd spend hours on the riverbank marveling at the trout's seemingly effortless ability to hold their position in the fast-moving water.

As it turns out, those trout were swimming effortlessly in the manner shown above.

The fish you see swimming behind the obstacle is no longer alive.

For it to move, its body has to be able to get the energy from the flow around it.

Is It True That Only Dead Fish Swim With The Stream?

To prevent being washed downstream, a fish must expend energy and swim against the river in nature.

The fish in the aquarium travel in the same direction as the moving card, which may be considered a stationary item in the stream.

Swimming with a dead fish, on the other hand, is incredible!

To extend this conclusion from the dead fish experiments to mechanical devices and to demonstrate the applicability of these results in an engineering context, a flexibly mounted two-dimensional rigid foil is capable of producing net thrust using only fluid-induced motion and negative mechanical power input, i.e., all of the required mechanical power is extracted from the flow.

Placing a light object in this forward-flowing, stagnant fluid would cause it to move forward toward the object, just like the dead fish did in the stream.

The mirrored permanent eddies right behind the object and the trail of mirrored wake vortices would push the object forward.

For those who dispute the force of this stagnant forward flow, the Magnus effect explains what happens when this flow is directed preferentially beneath one surface of a revolving cylinder, creating lift on the cylinder.

This is how some ships with rotors use wind power. It works the same way that an angled wing lifts by diverting sluggish flow to one side.

Conclusion

A dead fish is driven upstream as its flexible body resonates with impending vortices created in the wake of a bluff cylinder, despite being far beyond the cylinder's suction area. In this technique of passive propulsion, the fish's body takes enough energy from the approaching vortices to generate enough push to overcome its drag.

A passively attached high-aspect-ratio foil is demonstrated to push itself upstream in a comparable turbulent wake and at nearly the same distance behind a bluff cylinder, using a similar flow energy extraction process.

In this situation, mechanical energy is removed from the flow simultaneously as thrusts are created.

These findings demonstrate experimentally that, given the right circumstances, a body may trail or even catch up to another upstream body without using its ene

This is very important for developing low-drag energy collecting systems and fish energetics that live in moving water and swim through wake-forming barriers.