Does being surrounded by water affect collisions between 2 bodies?

[skywalker09]

Imagine there is a square plate fixed at the edge on all sides, it is impacted by a body with a pre-determined kinetic energy. Now assume that this plate is submerged in water and impacted by an identical body with the same amount of kinetic energy. Will the results be different? How to determine the effects of water?

What if the water was pressurized? Would the collision in pressurized water be different from previous 2 cases?

 

[Vanadium 50]

submerged in water

Like the bottom of an ocean?

 

[DaveC426913]

Why do we have to imagine? If you are asking for factors that would affect the scenario, then give us the scenario so we can assess it. We may find factors that you haven't thought of.

 

[skywalker09]

The scenario is collisions of remotely operated underwater vehicles with subsea equipment.

 

[Vanadium 50]

I see the question is being edited. It's hard to answer a question that is changing with time.

 

[skywalker09]

I did not edit the question. I have added the info that DaveC426913 asked for.

The scenario is collisions of remotely operated underwater vehicles with subsea equipment.

If someone asks for more info, what is the protocol to provide that info?

 

[berkeman]

I see the question is being edited. It's hard to answer a question that is changing with time.

The OP was edited by a Moderator/Mentor (me) to fix up the thread title slightly and to change the thread prefix level from "A" (Advanced -- graduate school) to "I" (Intermediate -- undergrad level). The OP did not edit their first post.

If someone asks for more info, what is the protocol to provide that info?

To avoid confusion, it's best to add new info in a reply, not edit previous posts. I think you can only edit your posts for about 24 hours (I think), but if the information in a post changes, it can confuse the folks who are trying to reply in a thread. One other option is to strikeout the incorrect text in a previous post (so it's still visible), but add a note that refers to a later post with the corrected information.

Hope that helps.

And on your question, underwater ROVs don't usually travel very fast, so the collisions with other things would usually be pretty low-energy, no?

 

[skywalker09]

Thank you. The speeds are low, but mass can be quite large. The energy can vary from roughly 500 J to 5700 J. In any case, a simulation is required, but there are so many assumptions involved that it is tough to assess if the actual physics is captured or not. Being surrounded by water should affect the results - says my intuition, but I do not know for sure.

 

[berkeman]

I guess it depends a lot on the shape of the objects that are colliding. If it's 2 flat surfaces, then the water between them will provide a pretty good slowing/cushion effect. If it's 2 rounded surfaces, then I don't think it will make much difference. Probably a bigger question is whether either thing colliding is a pressure vessel (like, well, you know)...

 

[Chestermiller]

Are you familiar with the concept of viscous drag?

 

[Baluncore]

Is the fixed plate 100 m square and 1 mm thick, or 100 mm square, and 10 mm thick?

Is the water on both sides of the fixed plate, or is the plate backed by air, or by concrete?

 

[skywalker09]

Is the fixed plate 100 m square and 1 mm thick, or 100 mm square, and 10 mm thick?

Is the water on both sides of the fixed plate, or is the plate backed by air, or by concrete?

Assume a 1 m x 1 m plate, about 10 mm thick, surrounded by water on all sides. No pressure vessels are involved in the collision. The plate is an idealized target, one can think of it being welded on all sides to a more rigid structure made of I-beams.

The behavior of impactor before collision can be ignored because the kinetic energy at collision is the same. It may be assumed to be a constant, say 5000 J at the instant of collision.

 

[Baluncore]

Imagine there is a square plate fixed at the edge on all sides, it is impacted by a body with a pre-determined kinetic energy.

My imagination is continuing to expand the problem, in line with the OP instruction.
Is the impacting body a very sharp point, a sphere, or a flat plate?
Do you restrict the collision to being perpendicular to the plate, or do you also considering a diagonal impact?

 

[skywalker09]

Not a sharp point. The impactor will be 90 mm dia cylinder with a rounded (say, hemispherical) impacting surface, approaching perpendicular to the plate at its geometrical center. At the instant of the impact, the kinetic energy is 5000 J per previous reply.

This would be narrows the situation down to a very specific case where the ROV (remotely operated vehicle) docking probe of standard geometry is impacting the structure.

 

[Vanadium 50]

This thread got off to a very bad start, and probably will not improve.

1. The collision equations are exactly the same in water as in vacuum. However, the problem is underspecified: one needs p1, p2, E1 and E2 to solve an elastic collision. At the instant of the collision of course.

2.. The motion of the projectile and target both before and after the collision are modified, and this modification is complicated and depends on many factors not specified. Drag is an important element, but so is any momentum carried by the water (perhaps from the slowing of the projectile).

 

[Baluncore]

The 90 mm diameter, hemispherical-ended, impactor, will hit the 10 mm thick plate. The wet contact will not be important.

Assuming the plate is dented and not punctured, where the surfaces meet, there will be transmission of energy to the plate, and some reflection of energy, back into the impactor, due to the geometry of the contact being an impedance mismatch. The impactor will bounce off the plate with that reflected energy.

The plate is a drum membrane, with a conical deflection, that couples well to the water on the other side. Most energy will be transmitted from the plate to the water, as a ramped-impulse by the plate, that will radiate forwards.

At the same time, water on the impact side will be pushed toward the departing plate by hydrostatic pressure. There may be a momentary cavitation, as a depression wave is launched by the drum membrane, radiating back along the line of impact. If cavitation occurs, the reflected depression wave will have a steeper ramp than the forward pressure wave.

The drum plate will not ring for long after the impact, since the pressure waves in the water will quickly remove the energy from the plate. It will sound more like a thud than a rumble.

 

[DaveC426913]

OK, maybe this clarifying diagram is coming a bit late and a bit redundant. Things were changing pretty fast there for a bit...

Anything else I can add?

 

[Baluncore]

Anything else I can add?

I thought that, not just the corners, but all the edges of the plate were kept co-planar, supported by RSJs = I-beams, arranged parallel to the edges.

 

[DaveC426913]

Ok, yeah. Updated.

 

[berkeman]

This would be narrows the situation down to a very specific case where the ROV (remotely operated vehicle) docking probe of standard geometry is impacting the structure.

Like this one?

https://www.f-e-t.com/wp-content/uploads/2019/10/iso-docking-latch.pdf

 

[Baluncore]

The technical term for an impactor, with a hemispherical end, is a "moyle". Imagine a bald headed man, ...

 

[skywalker09]

This thread got off to a very bad start, and probably will not improve.

1. The collision equations are exactly the same in water as in vacuum. However, the problem is underspecified: one needs p1, p2, E1 and E2 to solve an elastic collision. At the instant of the collision of course.

2.. The motion of the projectile and target both before and after the collision are modified, and this modification is complicated and depends on many factors not specified. Drag is an important element, but so is any momentum carried by the water (perhaps from the slowing of the projectile).

What could I have done differently for a better thread?

 

[skywalker09]

Like this one?

View attachment 332584
https://www.f-e-t.com/wp-content/uploads/2019/10/iso-docking-latch.pdf

Yes, it is close

 

[skywalker09]

Ok, yeah. Updated.

View attachment 332582

This is spot on. Much better than I could have drawn.

 

[skywalker09]

The 90 mm diameter, hemispherical-ended, impactor, will hit the 10 mm thick plate. The wet contact will not be important.

Assuming the plate is dented and not punctured, where the surfaces meet, there will be transmission of energy to the plate, and some reflection of energy, back into the impactor, due to the geometry of the contact being an impedance mismatch. The impactor will bounce off the plate with that reflected energy.

The plate is a drum membrane, with a conical deflection, that couples well to the water on the other side. Most energy will be transmitted from the plate to the water, as a ramped-impulse by the plate, that will radiate forwards.

At the same time, water on the impact side will be pushed toward the departing plate by hydrostatic pressure. There may be a momentary cavitation, as a depression wave is launched by the drum membrane, radiating back along the line of impact. If cavitation occurs, the reflected depression wave will have a steeper ramp than the forward pressure wave.

The drum plate will not ring for long after the impact, since the pressure waves in the water will quickly remove the energy from the plate. It will sound more like a thud than a rumble.

Yes, this is a very clear qualitative description of what may be observed.

How will the absence of water affect the results (collision in air) or "damage"? For collision in water, will the ambient hydrostatic (head) pressure change the results or "damage"? For context, many of my co-workers believe that effect of surrounding water are negligible and can be ignored.

Is there a way to write simplified energy or momentum equations for these scenarios - collision in air and collision in water?

 

[berkeman]

Are you asking about the air case so you can more easily do collision testing out of the water?

 

[skywalker09]

Are you asking about the air case so you can more easily do collision testing out of the water?

I am asking about the case in air because all the work done to date ignores effects of surrounding water. Also, to understand the formulations to be used in air and then how to modify for surrounding water. No testing is intended.

Past work assumes impactor(s) to be idealized rigid body with a known mass and 'velocity at impact'.

 

[berkeman]

It seems like the good news is that the testing in air should be the same or a little more harsh compared to in the water, so it only increases your safety factor.

 

[skywalker09]

It probably will, there’s no testing planned. But, we’re hoping to improve simulation by accounting for surrounding fluid. Is it possible to write kinematic or energy/momentum equations for such collision in air? How to consider effect of fluid?

 

[berkeman]

It probably will, there’s no testing planned. But, we’re hoping to improve simulation by accounting for surrounding fluid. Is it possible to write kinematic or energy/momentum equations for such collision in air? How to consider effect of fluid?

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?

 

[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?