Action; reaction...

[Externet]

Hello all.

Squeezing with a fixed force a water filled bladder, water exiting trough a orifice causing underwater propulsion as a squid/octopus does.
If the orifice is small, the jet exit velocity is faster and the time to deplete the bladder is longer.  
A larger jet exit orifice produces a slower jet velocity of larger mass for a shorter time until the bladder depletes.
Which orifice provides more propulsion (as to reach further, not faster ?)

How does nature selected the optimal orifice size for the octopus need ?  Proportionally to the size/mass of the growing octopus ?

[swansont]

6 hours ago, Externet said:

Hello all.

Squeezing with a fixed force a water filled bladder, water exiting trough a orifice causing underwater propulsion as a squid/octopus does.
If the orifice is small, the jet exit velocity is faster and the time to deplete the bladder is longer.  
A larger jet exit orifice produces a slower jet velocity of larger mass for a shorter time until the bladder depletes.
Which orifice provides more propulsion (as to reach further, not faster ?)

A given mass at a higher speed, and thus more energy and momentum (and can be applied to any unit of mass ejected), will go a greater distance in an open space. But that's not the same as "more propulsion" which depends on the momentum of all of the ejected fluid.

You have to de-couple the two in asking your question. What is it you want to optimize?

Quote

How does nature selected the optimal orifice size for the octopus need ?  Proportionally to the size/mass of the growing octopus ?

That's biology, not physics.

[Externet]

Thanks.  Mass times velocity to get the most distance. OK.

On the wrong-here octopus biology, I guess the increasing muscular force and body mass of a growing specimen governs the jet orifice size to obtain greater propulsion.  As a grown octopus does not keep the small jet youth orifice.

[MigL]

If your criteria is efficiency ( the least amount of energy expended ), then best results are obtained by closely matching the velocity of the jet efflux to the speed of the squid through the water. At slow squid speed, a slow jet efflux is desirable; as squid speed increases, so should jet efflux speed for maximizing efficiency.

You will notice propeller aircraft are most efficient below about 400 mph; very low speed of air accelerated backwards.
Airliners with large turbofans are more efficient in the 400 to 600 mph range; large fans with slow moving jet efflux.
Military aircraft are low bypass or straight turbojet, and are efficient supersonically up to about M3-4; small fans with fast jet efflux.
Rockets or Scramjets ( no moving parts to slow airflow ) can be more efficient from M5 up; really fast jet efflux.

[swansont]

On 1/31/2020 at 3:27 PM, Externet said:

Thanks.  Mass times velocity to get the most distance. OK.

Just to clarify, your question and my answer were for the jet, not the octopus.