Cavitation
Lulzes
Join Date: 2017-07-25 Member: 232050Members
Join Date: 2017-07-25 Member: 232050Members
OK first of all, cavitation shouldn't happen at Ahead Slow, ever...
My question is, does the game take depth into account when modelling cavitation? i.e. can you go faster at lower depths without cavitating?
We really need to follow sub convention and have 4 speeds, Slow (1/3), 2/3, Standard (3/3) and Flank (emergency power). At Slow you should never cavitate, it's like 5kts. At 2/3, you should cavitate above about 150m, there's room for debate about the hull design/propulsion technology but that seems fair. At Standard, you should have to go below 300m to avoid cavitation, and at Flank you'd have to be quite deep indeed.
My question is, does the game take depth into account when modelling cavitation? i.e. can you go faster at lower depths without cavitating?
We really need to follow sub convention and have 4 speeds, Slow (1/3), 2/3, Standard (3/3) and Flank (emergency power). At Slow you should never cavitate, it's like 5kts. At 2/3, you should cavitate above about 150m, there's room for debate about the hull design/propulsion technology but that seems fair. At Standard, you should have to go below 300m to avoid cavitation, and at Flank you'd have to be quite deep indeed.
Comments
All I know about cavitation is from watching Cold Waters streams
I find it odd that we cavitate all the time at all depths. I wrote it up as either just a game mechanic or not yet finished. Now wouldn't it also be neat if they had thermal layering?
This one?
http://store.steampowered.com/app/2900/688I_HunterKiller/
In experimental mode it have been changed the AI only warns you when your going in flank speed only the other two speeds don't cause the AI to warn you about cavitation
Nahhh...
It's just that the Brain Coral Bubbles are so full of Ideas that they explode when hitting something.
It's metal, hollow, and underwater. What does structural integrity have to do with resonance? Or cavitation from pressure differentials caused by propeller movement through water, for that matter.
Agreed; it's kinda like saying a church bell is flimsy because a whack with a tack hammer makes a loud noise. The two aren't necessarily related. And don't underestimate just what bubbles can do - after all, it's how modern torpedoes do that voodoo they do so well. The days of a torpedo hitting the side of a ship are over and have been for a while - today, a torpedo is designed to go off under the target, letting the blast bubble break the ship in half. That a suitably large bubble could give a pretty good thump to a sub isn't beyond the realms of reality. The thinner the hull material - which is possible given advances in materials science we'd expect in the game world - the louder the sound. (Think banging on plate steel versus sheet steel.)
Or that.
As for the whole cavitation thing, the OP is right. At low speed, you need to be swimming in something other than water to cavitate - gin, maybe, but not water - or your propeller needs to be designed by an absolute imbecile. Depending on depth, speed, water characteristics, and prop geometries, cavitation is altogether possible under a variety of conditions. Granted, with a designed-by-dummies prop, you could cavitate even at low speed, but let's assume that Alterra isn't quite that stupid. So slow ahead, no bubbles. Faster than that, we can assume that the prop is designed for cost, not efficiency and/or stealth, and is thus prone to cavitation when most military props would still be silent. (As is true with civilian props in general, really.) And given how the Cyclops's prop looks in screen shots, I'd expect it to be one noisy hunk of junk.
Honestly I know nothing of what cavitation is or does, but I wanted to put my 2¢ in anyways. We already have a mechanic in place that triggers a variable condition: How much O2 is used without wearing a rebreather. That value changes at 100M and 200M, so it shouldn't be much of a stretch to apply that logic to Cyclops cavitation... At 100M depth, Cyclops speed value X triggers it, then at 200M Cyclops speed value Y triggers it. I know it's not as simple as copy/paste code, but I know having an already working template in place can make things easier to code.
Okay, the Science Signal has been shone in the night sky. Science Man has been called. For those interested in the technical, science stuff - and a possible method of how Silent Running could work - read on.
So, today's subject is cavitation.
Functionally, cavitation is the development of voids (or bubbles) in a liquid, caused by a decrease in pressure. A propeller in water shares a lot of the same characteristics of a propeller in air, including the creation of high- and low-pressure zones. The faster the prop spins, the more intense these pressure zones become. When the pressure drops below what's called the saturation pressure of the liquid, the liquid undergoes a phase change - it transitions from liquid to vapor. It's similar to boiling, but a different process. The functional effect, though, is that voids form in the liquid where there ordinarily wouldn't be any. Since these bubbles exist only because of an artificial low-pressure zone, they're unable to exist when the ambient pressure returns to normal and collapse.
So, the process is:
1. Propeller cuts through water, creating low pressure zones on the trail faces and tips of each blade.
2. As the pressure drops, it passes below the saturation pressure, which allows water in the zone to undergo a phase change from liquid to gas. (Bubbles are made.)
3. These bubbles then end up in water at the normal pressure for the depth - in which they cannot exist - and implode.
Functionally, it's the implosion of these bubbles - not their creation - that makes the noise subs would rather avoid.
And make no mistake, most propellers cavitate. Civilian props, such as on outboard motors, cavitate at anything faster than the speed of smell. They're just not designed with pressure management and cavitation in mind. Larger, broader props, such as on cargo or passenger ships, will also cavitate at high enough speed. Military props have their geometries tuned to minimize the low-pressure zones that give rise to cavitation, meaning they're able to operate at higher revolutions before they cavitate, but cavitate they will at a high enough speed. (Military sub props are weird looking things, shaped as they are not because someone had a seizure when they were designing it, but because of cavitation geometry.)
Cavitation can occur even without props - all that needs to happen is a sufficient decrease in pressure relative to ambient. Remember the Oroville Dam emergency this year? The spillway designed to allow excess water to be drained off essentially failed; a big hole was ripped in the concrete and dug out part of the dam face. Bad luck? Nope, that's cavitation, kiddies, and it's not the first time. In 1983, Glen Canyon Dam almost failed because floodwaters had to be vented through the spillways. Except the spillways kept barfing up big chunks of concrete and rock. When the gates were shut to find out what was going on, they discovered that cavitation was busily chewing away the concrete liner and the rock underneath. Unable to vent water, they had to get creative to avoid losing the dam. On smaller scales, cavitation will chew up pump impellers, ship propellers, and pretty much anything that produces adequately low pressure as to allow cavitation.
So, let's now turn our attention to the Silent Running mode. It's important that we note what it does before we discuss it: it suppresses all noise coming from the sub. There's no mention of cavitation. Cavitation is one source of noise for a sub - particularly one that's hustling - but there are many others: engine noise, transformer hum, electronics, air handlers, and more. Silent Running Mode might be similar to the WWII method of shutting everything down and going to batteries - in our case, shutting off that noisy engine that makes the aft compartment such a headache, killing the air systems, cycling down transformers, thus eliminating most of the emitted noise.
But let's assume that it's cavitation the system is shutting up. The Cyclops has a ducted propeller, which although more efficient at low speed (up to about ten knots or so), are substantially noisier at high speeds and prone to cavitation. So that's an explanation of why the Cyclops is a noisy pig. So what about SR Mode? The easy solution would be a hydraulic or pneumatic ram system that opens up the duct, alleviating the pressure problem. But why would that be timed? It shouldn't; it should just increase power consumption.
It could re-pitch the prop blades to reduce pressure differential at higher speeds, but that, too, should be able to be done indefinitely.
What are we left with? Manipulating the water itself. If the shroud is a refrigeration system, it could be chilling the water going into the duct - lower temperature means higher density, which means the cavitation threshold speed goes up. Or it could be gravitationally-based, artificially pumping up the pressure. Either way would reduce cavitation (although the gravity method should be pulling in nearby fish, plants, and debris, turning your sub into a giant mobile Cuisinart). Both options would be highly energy inefficient and excessively complex...which sounds like Alterra's speed, really. ("Sure, we could just design a more efficient prop, but my nephew's in charge of propeller sales and my wife would kill me if I got him fired, so come up with something else.")
End of the day, SR would appear to be a highly complicated workaround to a relatively straightforward problem. (Simpler solution would be to just cut the prop duct off, really.) But for the sake of gameplay, even if it doesn't make scads of sense, it does serve a useful function.
Oh, and @Obraxis -- if you were looking for technical fluff RE: Cyclops always cavitating even at low speeds and the Silent Running mode, there's a few explanations in the previous post that you guys could throw in there if you wanted.
Nope, steam wasn't around for the one I am talking about.
This one:
I used to think the adeptus mechanicus were the worst engineers ever, but I would much rather drive around a cathedral covered in skulls than the cyclops. At least things they built can't be destroyed by sharks.
The incredibly, mind-bogglingly terrible equipment is a huge immersion breaker. This is supposed to be the future, but people make equipment more effective than most of ours in their garage. Yet another thing that I am angry about in this games downward spiral.
If that panel were secured with just some normal, household screws, our character would probably have been shaken to death before it came off.
Normal re-entry for a Soyuz is 4.5 Gs, ballistic re-entry if something necessitates it is ~8Gs http://www.chron.com/news/nation-world/article/U-S-astronaut-recalls-terrifying-Soyuz-descent-1759455.php
Most fighters are only rated for 9Gs IIRC?
I think this is also about the same 8G descent (and, BTW, they had smoke in the cabin for that one):
Holy moly kudos for typing all that out
And did, in any of these descents, a massive metal panel come off the wall and fly around? I maintain that a few good screws driven into the metal would easily have kept that thing in place.
I disagree with your foundation point that it is metal, and I base my disagreement on the fact that the Brain Coral bubbles cause the mighty roar that they do now. Don't try to tell me the Cyclops is made out of Titanium and thus must be metal because I will dismiss that out of hand--again based on the noise of the Brain Coral bubbles. Nope. By some miracle of science, I will insist until my dying breath the Cyclops is made out of cardboard. So is the propeller. Now, personally, I don't believe a propeller made out of cardboard propelling a craft made of cardboard can achieve anywhere near the speeds necessary to cause cavitation at any depth!
In other words, I wrote my post before I watched the video explaining what cavitation is... So, yeah, your point is well made. But my very nature demands that I argue even when I'm wrong--even with you.
Probably got a solid whack from the fire extinguisher first, loosening it up?
Then what's the fire extinguisher doing not being secured? However you slice it, that lifepod is a deathtrap.
Yes, even by the looks, the fire extinguisher shouldn't be strapped in like that. Probably something along the lines of they haven't had an accident requiring lifepod ejection in uncontrolled descent conditions in who knows how long, so they've forgotten how to stow things like that (should have been stowed inside the hull with a breakable glass panel in front, or similar).
Cuts costs, does it not
At the end of the day, that's the practical answer to most of these complaints. Why wasn't the access panel screwed down? Because four screws are more expensive than one pressure-fit spring - which is why panels in your car are no longer screwed down, either. Why is the extinguisher not strapped securely in? "Well, we did some safety simulations and found out that it'll only pop loose like 3% of the time, and in most of those cases the occupant dies anyway, so we don't see justification for the extra expense." We don't need armor on a research sub, so we're not going to put it on - just make the hull barely as thick as it needs to be to survive the depth on the label. Sure, it could endanger a crew, but that's their fault for not buying a better model from us; maybe their leadership will decide to buy one of our fine military machines after the Cyclops gets whacked.
Everything we see in-game points to Alterra being the quintessential megacorporation: profits before all. Which means unnecessary expenses won't be made - and the definition of "unnecessary" is highly distorted. Three cents in screws versus one cent for a spring - use the spring. Doesn't matter that screws are better; they're more expensive.