Collision with an Incompressible fluid 1

[turboengineer]

Does anybody have equations for the loads imparted on an object when it collides with water? In particular I am interested in the forces that a torpedo would see when launched from a surface vessel into the ocean while the vessel is traveling at a high speed. I’ve found a couple of papers close to what I’m looking for, but no love! Thanks a lot.

 

[Cockroach]

Hmmm, interesting subject. I have no papers but would be interested with sourcing information on the subject.

Why not think of the situation as a high energy impact and develop the argument from first principles, work-energy, dynamics, etc?

Sorry I could be of more assistance, but I also look forward to the replies. Just out of curiosity, what type of cruising velocity, estimated depth at launch and that sort of thing?

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[unclesyd]

Here are two sites that have some general information on the older torpedoes and looks like several good references. There is paper out that details how the Japanese added wooden retarding rings and dive plans that allowed them to launch aerial torpedoes in shallow waters of Pearl Harbor and not porpoise or bury in the mud. That is why there was false sense of security about a torpedo attack, because the American torpedo would hav buried in the bottom or broke in half.

The first site gives you enough information that you could calculate the impact for different torpedoes.

http://www.warships1.com/Weapons/WTUS_WWII.htm

http://www.ww2pacific.com/torpedo.html

 

[GregLocock]

It is a fairly easy modelling problem, but time consuming. I'd use a program like Michlet to model the wavemaking which is the main way the water absorbs the impact energy, and something like Working Model or ADAMS to model the rigid body motion.

Failing that

For an initial velocity of 25 m /s and a torpedo 5m long we know the collision time is of the order of 0.2 seconds, and so the decel is of the order of 125 m/s/s. This is basically a car crash, sounds about right.

The torpedo weighs say 1000 kg, so that'd be around 125kN

ish. I'd multiply it by 3 or so to get a likely peak force, and knock a bit off becasue the torpedo does not stop dead on entry into the water, in fact at higher speeds it might well plane for a while before sinking. Planing forces can be calculated using the Savitsky Brown equation.

The historical evidence is that you can design torpedoes to be launched at MUCH higher speeds than this, and of course Barnes Wallis's bouncing bomb experiments would give some estimates of the forces involved. Also you might want to look at diving shells, used by some battleships up until the end of WW2.

Cheers

Greg Locock

 

[GregLocock]

Using a variety of more or less nasty approaches I got from 4-15g, if the torpedo dives, and rather less than that if it bounces off or planes.

There is a picture at the warships1 board in a link off unclesyd's URL, showing the torpedo launched at a very low horizontal velocity, such that it basically fell out of the end of the tube, and landed flat in the water. This would be ideal from the point of view of minimising the shock to the torpedo.



Cheers

Greg Locock

 

[amorrison]

Some authors of papers in this area available at

http://imarine.mit.edu/ilearn/Web_course/imp-lec1/Introduction/node5.html