A 127-year-old physics riddle regarding the Kelvinangle in boat wakes solved | Hacker News Reader

A 127-year-old physics riddle regarding the Kelvinangle in boat wakes solved

phys.org

102 pointsErrorcod343 comments

> "Fuel consumption can double if the vessel is traveling downstream compared to upstream," Ellingsen said.

This is counterintuitive. The fact that it's easier to make waves traveling upstream must outweigh the fact that if you're going downstream the water, you know, carries you.

Fuel consumption in boats is usually measured at some speed relative to the waters surface, not the earth.

Or relative to time (litres or kilograms per hour).

prob a journalistic typo.

Time on it’s own is not enough as the faster the boat goes the more fuel it consumes per hour.

That said, at each specific speed you get a rate as x gallons per hour.

That doesn't make consumption double for "traveling upstream" vs "traveling downstream". The "traveling" part means actually getting somewhere, not treading water. If you turn off your engine entirely, you will still travel downstream and consume zero fuel. The article is about the necessary specific context for the statement to be accurate.

There's a point about fuel consumption when travelling downstream though: you can't just switch off your engine and drift downstream, because you'll run aground fairly soon. The ship needs steering, and steering needs engine power and relative speed to water.

This relies on the surface of the water being static. In most rivers the surface flows. Indeed, there's a shear layer under the surface, so in a river these effects happen in the opposite direction.

has to be a typo

I'm not sure if this is a typo, but you need a certain speed to be able to steer the ship. Going too slow downstream is like stalling a airplane, you lose control over it.

Can someone please explain to me why this is so groundbreaking? If I blow a smoke ring and there is no wind my smoke ring stays circular. If there is wind blowing from the side my smoke ring will become distorted. Likewise if there is a current in water then I expect that current to disturb the propagation of waves moving through the water, not so? The medium in which the wave is propagating is itself moving.

Surely this has not remained a mystery for 127 years.

The news is that someone worked out the maths behind it and wrote it down - then proved they were right.

It's a bit like the fact that everyone knows the shower curtain will billow towards them in the shower but no one has managed to prove why that happens.

Great answer, cheers

Because the hot air inside rises and cooler air pushes inward. It's called convection, and I don't thing anyone needs to run a simulation and publish a paper solving the "shower curtain problem" just because it hasn't been "done" before.

If the water is air-temperature or colder, then curtain does or doesn't blow inward? Care to make make a prediction based on your theory?

Straight from the first paragraph of the Hypotheses section, emphasis mine.

>Also called Chimney effect or Stack effect, observes that warm air (from the hot shower) rises out over the shower curtain as cooler air (near the floor) pushes in under the curtain to replace the rising air. By pushing the curtain in towards the shower, the (short range) vortex and Coandă effects become more significant. However, the shower-curtain effect persists when cold water is used, implying that this cannot be the only mechanism at work.

Scientists investigate and prove things that are "common sense" things all the time because the "common sense" answer turns out to be wrong.

scientists have been arguing about the exact physical forces in this problem for a long time (similarly true for airplane wings). We like to actually confirm our theories with experimental data, and often times, these sorts of papers help the field in unexpected ways in the long term.

It has been done before. See the physics winner for 2001: https://improbable.com/ig/ig-pastwinners.html

I like that it took him two weeks of compute time and it's only a "partial solution"

That's fluid dynamics for you. :-/

Indeed. There are a few hypotheses about it [1]

[1] https://en.wikipedia.org/wiki/Shower-curtain_effect

I can definitely imagine this being really hard to work out in an equation, but I'm somewhat surprised that this implies that it has been difficult to come up with experiments that would give data that might help figure this out. Surely we have ways of measuring air vortices and pressure that would clarify a lot of what is happening to the air flow?

Funny coincidence! I thought about this when I showered this morning. The water probably sucks air down the drain with it, causing slightly negative pressure causing the curtain to bellow inwards.

Down the drain? I might buy the water sucking down air, but down the drain?

Isn't the point of traps largely to prevent air from traveling either direction through the drain?

I'd love to be on the grants committee when someone requests funding to set up a shower in their lab.

The thing that makes this more interesting is that if you have a large shower (eg a wheelchair wetroom) then the shower curtain behaves differently depending on where you are inside it. Try walking up and down your bathtub next time you have a shower.

The explanation for it might have consequences for a lot of other models though, no?

The shower thing is explained by the coanda (Bernoulli) effect.

"The problem of identifying the cause of this effect has been featured in Scientific American magazine, with several theories given to explain the phenomenon but no definite conclusion."

According to a quick googling, not really.

According to you. Another answer says it's convection. So not so intuitive after all ;)

> the coanda (Bernoulli) effect

Those are two different effects.

Because physics is about formalising intuition.

If you don't write the algorithm predicting the empirical consequences - you haven't solved the problem.

The mystery is in the absence of an algorithmic solution.

Think: Reverse Mathematics. https://en.wikipedia.org/wiki/Reverse_mathematics

Ok, but simulations were already showing the correct results.

So this is not so much a fundamental insight, as it is a tool for engineers to do back of the envelope computations.

Which at the end of the day is kind of useless until you have fundamental insight.

Maybe it can help theorist to more efficiently identify what is fundamental insight.

The link to reverse engineering is great. I wonder how well it has worked with fission and making a star in my backyard.

Techniques aren’t to be dogmatic or pragmatic. They are to be explored and reinvented. Whoever came up with don’t reinvent the wheel just wants to soak up as much profit from the current paradigm as possible.

Think of Tesla, Elon said something like x > 50%(don’t remember it wasn’t like 97% I believe) of pets used had to be redesigned and were not factory standard or template.

Veribility in the back envelope may give false positives which can hinder progress is all I’m saying :)

Intuition also tells you the correct result for many a complex computations.

Because intuition is "simulation" after all. Gedankenexperiment.

You know the correct result for dropping a glass on the floor.

Insight comes from knowing why and how the glass shatters, not that it shatters.

A simulation gives about as much insight as a series of matrix transformations optimized to approximate a call result result

Wait

Real life effect of this is that now people will know how to draw more fuel-efficient routes for ships, tankers etc, so they will consume less fuel which can lead to cheaper logistics, less CO2 release and less negative impact on the environment.

So I think it’s OK to call this groundbreaking :)

but how can you know subsurface currents? Doppler sonar?

Perhaps the strength of the currents can now be mathematically inferred from the deformed shape of the wake. Look at recent satellite images of ship wakes and plot a better course for your ship.

Other interesting physics riddles solved after some time:

https://en.wikipedia.org/wiki/Feynman_sprinkler

https://en.wikipedia.org/wiki/Blackbird_(land_yacht)

And the infamous plane on a treadmill..

I hate the way this article is presented.

It takes an interesting and intuitive effect that we couldn't explain mathematically and now can, and tries as hard as possible to shove every fucking clickbait word in as possible.

Kudos to the author for presenting this material in possibly the least consumable way imaginable. I hate it.

I understand that there is now proof of the usual V-shaped wake angle not always being ~39 degrees depending on subsurface currents.

I'm a bit confused about the ring waves part of this article. How is the boat leaving a ring wave when it moves? Or is the boat just being lowered into the water to form a ripple? You can't tell this from the top-down view.

For some reason, the way that the article was written made it seem that a boat moving through the water could leave an off-center ring wave wake, which makes no sense.

A wake is essentially a ring wave which is being continuously generated by a moving point. If a ring wave can be generated off-center, the wake will also be off-center.

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