'Fluid cloak' to help submarines leave no wake | Hacker News Reader

'Fluid cloak' to help submarines leave no wake

newscientist.com

32 pointsColinWright18 comments

I'm a former Navy submarine officer.

While this idea is nice, it's a marginal risk factor. It's difficult to detect a wake, and you can only detect them when a submarine is near the surface.

What really matters is being quiet. Noise transients can be heard for miles, and whether a submarine is shallow or deep. And that's why 50+ years of research has focused on making and keeping submarines quiet.

So while this technology is interesting, it's not very useful for Navy submarines.

Well, the turbulent flow in the wake generates some noise.

If your average sea creature could read, they would be ecstatic about any development reducing engine/wake noise... it's getting too noisy out there.

I'm not sure how this would be practical in any way. They're only hiding the effects of the body moving through fluids, not the effects of what's moving the body in the first place.

In the case of submarines, the only way to create propulsion is to push water out the backside. I'm pretty sure this alone would create a visible wake: see what it looks like by running a boat's engine without it actually moving.

My general feeling is that if this were really practical, there's no way we'd be hearing about it. The technology, and its author, would be quietly vanishing into the Naval Research Laboratory.

I wonder what does happen in those situations, actually. If I, as a university academic, invented something that could make a submarine undetectable, I'd be willing to consider any cash offers to keep it secret that the US Navy might be willing to make. But how do I approach 'em?

I've never heard of this kind of thing happening... but then again, I wouldn't, would I?

My general feeling is that if this were really practical, there's no way we'd be hearing about it.

It is not practical.

From the article, a sphere 1 cm across could only stay wake and drag free at speeds of no more than 1 cm/second. If the object is bigger, the limiting speed goes down even more.

I'd be willing to consider any cash offers to keep it secret that the US Navy might be willing to make. But how do I approach 'em?

They approach you.

My general feeling is that if this were really practical, there's no way we'd be hearing about it

I figure it's either so far from being realized in any capacity it doesn't matter, or the Navy has already had it (or something) better for years/decades.

Remember that when a wave moves through a volume of fluid, say air or water for example, its the energy of the wave that moves and not so much the medium. This is the classic example given in high school when talking about waves at the beach. (I always thought it was disingenuous because water is obviously moving when the waves break. The power of analogies sometimes breaks down.)

Anyway, if you can imagine a sub moving forward through water it pushes its way through the medium. Water molecules get jostled by all the external jiggers and bits on the vehicle. So if you look at a volume of water say 20m x 20m x 3m (basically a big flat square). Most of the water in that 3m wide space stays in that space as the sub moves through the square. However, all the jiggery bits have imparted energy to the molecues in the space. So as the sub finishes passing through, there's a lot of excess energy left in its wake. That's your noise and turbulence and such.

So if you can put something around your sub that helps to return the water molecules to the same or similar state they were in before the vehicle passes through a particular volume, then effectively its like the sub was never there. You get to move through the water much more efficiently because you aren't imparting as much energy to it, and its a lot quieter.

This even got me thinking about airplanes. So we all (should) know that its not the speed of air moving over a wing that gives it lift. Its pressure differentials created by a wing moving through a volume of air. Air can be perfectly still and powered flight works just fine.

So imagine you could put one of these grid things around a plane. If you can effectively reduce the turbulence created by moving through air (a fluid), then you get better fuel efficiency, less noise, and you can go faster. This is why some designs for newer planes are considering tiny dimples in the surfaces like a golf ball. It helps reduce turbulence.

Edit See my addendum comment below.

"Air can be perfectly still and powered flight works just fine."

No it can't. "For every action, there is an equal and opposite reaction"; if the plane is experiencing lift a.k.a. an upward force (in the fully Newtonian sense of the term) from the air, the plane must be imparting an equal and opposite force downward on something else, in this case the air, which will cause the air to be accelerated downward. A plane flying through the air does not merely cut through the air, it actually causes a downwards net acceleration. Nothing else is possible. (That's not to say it doesn't do so by some clever work in the details, including on the leading edge of the wing where some air is briefly pushed upwards, but I'm talking about the net force on the air considered as a whole.)

There's an amazing amount of misinformation in this area and a lot of people have managed to talk themselves into positions where somehow forces are distinguished from the masses and accelerations in question, or some other similarly gibberish splitting of unsplittable concepts. ("F = ma" is a mathematical statement saying precisely that there is no difference between a force and an accelerating mass, there's nowhere to wedge the two apart, they are fundamentally, deeply, profoundly the same thing. The school system does an incredibly poor job of teaching what equality truly is and a surprising amount of subsequent failure to teach math stems directly from that simple oversight... but I digress.) A vehicle accelerating upward must be doing so by pushing something else downward, be that rocket exhaust, electromagnetic radiation (theoretically possible but impractical on Earth), or, yes, air. (Or more generally, any vehicle that is accelerating with less that the acceleration dictated by gravity towards Earth, but I think it's easier to see if we postulate an aircraft actually accelerating upwards.)

People have trouble understanding the gravity = acceleration bit. However, I find if you walk someone though the idea of ground effects vs high altitude flight they can quickly grasp the downdraft issue.

AKA ground effects means the air is acting like a pole and supporting the craft due to an increase in pressure on the bottom of the craft AND and increase in pressure on the ground.

High altitude flight means there is no direct increase in pressure below the craft just a downdraft of air under it. Granted that motion eventually bounces off the earth but it can take a while and and the path need not be directly down (ex: crosswind) and it need not be the same width as the airplane, but it does flow from the airplane all the way down to the ground.

Rather than "powered flight", it might be better to talk about "heavier-than-air flight". If you used this concept to create a shell around a neutrally bouyant blimp, then you could (in theory) move it through the air without at all disturbing the prevailing air currents in the area it was moving through.

You would generate some turbulence as you were getting up to speed or changing course, but when you're not doing that you wouldn't need to exert any net force on the air.

Sorry, let me tighten up what I said. When I said "air can be perfectly still" I didn't mean to imply no downward force was generated. That was obvious to me so I didn't add that bit. What I mean is that the air that a wing traverses through doesn't have to be moving already, aka wind or air current. The power of the engine pushes the wing through the medium, and the action of this of courses causes downward air movement and thus elevates the wing.

While I do think these scientists are persuing a worthwhile endeavour, it does feel like the complexity of this approach is so great that it would never be useful under normal operational scenarios. Perhaps some HN submariners would like to comment on the utility of this approach?

Years ago there was a lot of buzz about MHD (Magnetohydrodynamics) but I guess it's still just science fiction but it sounds like it would help a lot with reducing turbulence.

I suspect this is of marginal utility nowadays, because submarines are visible from the satellites, right?

Submarines (submerged) are not visible by satellite. Satellites also orbit (reconaissance satellites would rarely be geosynchronous), so even if they could see submarines, they would only see them when their courses intersected.

I find it remarkable that within a matter of only a few years, such advancements are being in various fields generally under the umbrella of "invisibility". Ranging from electromagnetic wave propagation over nano-scale materials to controlling fluid dynamics to create visual invisibility to sound cloaks and now something that works similarly in water.

Talk about living in the age of wonders.

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