I Does being surrounded by water affect collisions between 2 bodies?

[jrmichler]

there’s no testing planned. But, we’re hoping to improve simulation by accounting for surrounding fluid.

I would be very surprised if anybody, anywhere, with the best software available could create a simulation capable of analyzing this problem with reasonable accuracy. That is because it is multiphysics - impact forces coupled with hydrodynamics. The situation is even more difficult if the impact is sufficient to cause yielding or complete failure of any solid component. My experience with nonlinear dynamic analysis is that all analyses need to be compared to experimental results, and that those experiments need to be sufficiently similar to the analysis.

Is it possible to write kinematic or energy/momentum equations for such collision in air? How to consider effect of fluid?

There are analysis techniques that supposedly work in simple cases. What little I have read on the subject compared kinetic energy to the energy to plastically yield a simple shape. I do not recall anything for your case of low velocity with high momentum. Water will probably have a large effect because the mass of water affected by movement of the plate is much larger than the mass of the plate. Unless the impact is a such low velocity that inertial effects are minimal.

All of which sent me down a rabbit hole when I searched for, and found, videos of low speed impacts in water using search terms ship hitting dock.

In your case, an analysis good enough to get usable results would almost certainly cost more than an experiment. The experiment would be needed to confirm the analysis. And if you did the experiment, you might not need the analysis.

 

[skywalker09]

Who is the customer for these "simulations" and "calculations"? Who is their business insurance company? Will that insurance company accept a risk analysis done by an amateur using advice from an Internet forum?

Can you please come clean about what this is for?

How is this relevant? My goal is to improve the methods my organization uses currently. The effect of water is ignored currently for design/analysis. I am hoping that the experts here can point to some equations and/or literature for further study.

 

[skywalker09]

I would be very surprised if anybody, anywhere, with the best software available could create a simulation capable of analyzing this problem with reasonable accuracy. That is because it is multiphysics - impact forces coupled with hydrodynamics. The situation is even more difficult if the impact is sufficient to cause yielding or complete failure of any solid component. My experience with nonlinear dynamic analysis is that all analyses need to be compared to experimental results, and that those experiments need to be sufficiently similar to the analysis.

There are analysis techniques that supposedly work in simple cases. What little I have read on the subject compared kinetic energy to the energy to plastically yield a simple shape. I do not recall anything for your case of low velocity with high momentum. Water will probably have a large effect because the mass of water affected by movement of the plate is much larger than the mass of the plate. Unless the impact is a such low velocity that inertial effects are minimal.

All of which sent me down a rabbit hole when I searched for, and found, videos of low speed impacts in water using search terms ship hitting dock.

In your case, an analysis good enough to get usable results would almost certainly cost more than an experiment. The experiment would be needed to confirm the analysis. And if you did the experiment, you might not need the analysis.

Wow, thanks! Is it even possible to express the situation in terms of equations?

 

[DaveC426913]

How is this relevant?

You would be surprised at how often one's own assumptions and preconceptions are the root one's own troubles. Often, the problem one is trying to solve is not the problem one should be trying to solve. You're asking for help with your 'known unknowns' but you likely would benefit from wisdom about your 'unknown unknowns'.

For example:

Wow, thanks! Is it even possible to express the situation in terms of equations?

Bringing to bear similar problems and solutions you might not have thought of.



(More cynically, there may be a small suspicion this is related the OceanGate disaster. We should know that going in.)

.

 

[skywalker09]

You would be surprised at how often one's own assumptions and preconceptions are the root one's own troubles. Often, the problem one is trying to solve is not the problem one should be trying to solve. You're asking for help with your 'known unknowns' but you likely would benefit from wisdom about your 'unknown unknowns'.

For example:

Bringing to bear similar problems and solutions you might not have thought of.



(More cynically, there may be a small suspicion this is related the OceanGate disaster. We should know that going in.)

.

OceanGate - not at all, in the slightest. What are the unknown unknowns in this context?

 

[DaveC426913]

What are the unknown unknowns in this context?

That ...
... is the $64,000 question. It is what these good people are trying to assess. But to do that, they need to know the application.For all we know, you're going to tell us '... for installation at the bottom of Lake Michigan' or '...for a military landing scaffold'.
and the experts here will say ... 'Ohhhh. No no no. You haven't factored in the whozitz of fresh water' or 'Wait, what grade of military steel are you cleared for?'

Or somesuch thing. Obvs these are facetious examples; the point is, they can't advise you as long as they can only see the scenario through a keyhole of your devising. The unknown unknowns are your unknowns.

That's kind of the PF way.

 

[berkeman]

One thing that is still an unknown for me is what-all this probe can bump into. I know @skywalker09 initially has been asking about impacting a flat plate, but what about other things in that environment like other ROV propellers, or cables, or batteries, or...