All this serious oceanography research reminds me of something a little less serious but in the same scientific arena:
1) "And this is why Lochtefeld may soon be hailed as some kind of prophet, for he has created one technology, and champions another, that has the potential to generate artificial waves even a surfer could love."
"The archdruid of surf has undergrad degrees in math and geophysics. His master's thesis - written in a house overlooking Sunset Beach on the North Shore of Oahu - is a 240-page work titled "Wave Transformation Over Coral Reefs." His PhD thesis is even thicker: a two-volume tome called "Sediment Transport and Inland Tidal Inlet Hydraulics."
...
Black has cracked the code of the world's great breaks; they now lie dissected, quantified, and digitized in his computer, ready to be reconstituted anywhere - especially indoors. With the push of a button, his patented pool floors will morph into an almost infinite variety of shapes to produce steep 8-foot-high tubes that peel for 100 yards, or gentle 3-foot rollers that peak after 40, or waves that break to the left or right every 12 seconds, all under a transparent dome and available for a few bucks a ride."
Scale models of nautical craft have always been limited by some unrealistic assumptions -- assumptions that are recognized but not well-adjusted for.
One important aspect of a scale-model ship/boat is that as you change the size of a model but keep the relative dimensions constant, its surface area changes as the square of its length, but its mass and volume change as the cube. If you halve the size of a model, its surface area is 1/4 the original, but its volume and mass will be 1/8 the original. Simply put, this means a scale model won't behave very much like the full-size craft.
As models become more sophisticated, these unavoidable limitations of scale modeling begin to argue for computer modeling, where you can model the full-size vessel using a carefully designed numerical simulation, including many factors that can't be realistically modeled in a laboratory tank experiment.
You can scale the geometry and accommodate dynamic and kinematic similarity by matching Froude, Reynolds, Drag and Pressure coefficients - or what non-dimensional parameters you have based on the identified variables (http://en.wikipedia.org/wiki/Buckingham_%CF%80_theorem)
I would expect that the surface tension would have negligible impact on the physics of the boat in open ocean, maybe unless you are interested on the behaviour of the spray across the decks.
> Does the volume matter? Can the weight be offset?
There are remedies for some of these scaling issues, but they're accommodations, not cures. It's not at all uncommon for a craft based on a scale model to behave substantially differently when it's finally built.
> Simply put, this means a scale model won't behave very much like the full-size craft.
This is a basic fact that is known for decades in the field of science and engineering. This isn't pose any problem in modelling.
Sometimes, it's far better to study scale models instead of CFD models because in spite of the scale differences, it's possible to reproduce fluid dynamics results in reducd models while computational models fail to even account for them.
And for some cases you actually model at over scale >1:1 as this give better results its screamingly expensive so only tends to get done for flows inside nuclear rectors and so on.
I've always wondered about how insects can survive large falls (e.g. from desks or ceilings) mostly unharmed, yet for humans a fall from a similar height, relatively to the body size, would be certain death.
Yes -- there's a folk tale about how a bumblebee is an aerodynamic impossibility. What that actually means is if a bumblebee is scaled up to the size of a bird, say, it can't stay aloft because its wing area is only increasing as the square, but its volume and mass as the cube, of the size change.
The same applies to scaling up helicopters. It is much easier to scale up a plane than to scale up a chopper. A nice example of this is the little 3/4/6/hex copters that you see RC models of.
I don't think buoyancy would be much of an issue with scaling, because that's just related to density and shape, neither of which change when scaling the model (at least until you get small enough that surface tension becomes noticeable, which I presume doesn't happen for ship modeling). I suspect the bigger issue would be the structural integrity of the boat. A 10 foot steel ship is going to be a heck of a lot stronger than the same ship scaled up to 100 feet.
> I don't think buoyancy would be much of an issue with scaling, because that's just related to density and shape, neither of which change when scaling the model ...
But that's false. The hull wetted area of the model changes as the square, while the volume and mass change as the cube, of the size change. If corrective measures aren't taken, the model will displace more water proportional to its wetted area as it becomes larger, as a result of which it will gradually sit lower in the water as its size increases.
There are a number of ways to deal with these issues, but it's not true that one can scale a model without considering them in detail, and carefully ballasting the model to force it into an approximation of full-size reality.
Interesting. That doesn't match my initial intuition, but it sounds like you're right. Can you point to a source, or perhaps explain this a little more? I'm trying to imagine a pathological shape that would obviously float at one size but not when scaled up.
> I'm trying to imagine a pathological shape that would obviously float at one size but not when scaled up.
I wouldn't go that far. I know models sit at different heights for different scales, all else being equal, but I don't think you will ever see a non-pathological object sink at one scale but remain afloat at another. The reason I think this is true is that, if I take two objects having the same overall density and connect them together, this cannot change their position in the water, their buoyancy. If I think of the two objects as a single model, the same should be true.
But the difference between connecting two models, and a proportional scaled-up model, is that the ratio of surface area to volume is different for the scaled-up model compared to the two independent models. So the comparison isn't perfect.
(pause for thought ...)
For a solid object sitting on a table, making the object larger and noting the previously described square-versus-cube rule, the table loading should increase for each square unit of table area as the model's size increases, by a unit rule, meaning if you double one of the three dimensions, the table loading (per unit of area) doubles also. But because a boat model sinks into the water as its mass increases, and because that sinking is across curved surfaces, it's more like a three-dimensional area increase than a two-dimensional one, so it can't be compared to an object with a flat bottom sitting on a table.
The tl;dr: the more I think about this, the more I think I was wrong to say it the way I did -- and I say this because the shape of the boat hull means the wetted area can increase as fast as the boat's mass, i.e. as the cube of a dimensional change.
Expressed another way, even though a larger boat model sits lower in the water, with some care the waterline position on the hull can be made to stay the same.
If the water line can be made to change when scaling the boat up, it should be trivial to make a pathological object that will sink when scaled. Just make the shape of a box without a lid, weighted so that the water line is just at the top of the walls. If scaling the shape raises the waterline, then water will flow into the box and it will sink. I still can't intuitively convince myself that the waterline would shift when the model scales.
I should have made clear what I meant by "sits lower in the water", and I'm responsible for the confusion.
Because the boat is a three-dimensional object, and because the subsurface part contacts the water in three dimensions, if it is scaled up with all else the same, it should still have the same waterline, but (obviously) its keel is deeper in the water.
> I still can't intuitively convince myself that the waterline would shift when the model scales.
Your instincts are serving you well, and the answer is simple -- I wasn't sufficiently careful in how I described it, and I let an error creep into yesterday's conversation. In fact, a model whose dimensions are held constant but is scaled up, should show the same waterline at all scales.
Again, I apologize for sounding so sure of myself when I wasn't.
Okay, so you were talking about the keel of the larger model being deeper underwater in absolute terms. I suppose that would effect buoyancy, because pressure increases as depth increases. This might be significant enough to warrant adjustments, at least when scaling very large ships down to very small models.
I already mentioned that obviously surface tension becomes noticeable at very small scales, but that model boats are presumably nowhere near that small.
> but if its done in a uniform fashion things should work out ok right?
I wouldn't be so certain of this. If the "uniform distribution" of the added weight results in the walls being (proportionately) thicker or denser than in the original, the model may be an invalid one.
That's true, but it doesn't contradict what I said above. And I wouldn't call them "wrong" results. Obviously all modeling efforts are approximations, efforts to produce an estimate of the final craft's behavior. No one expects perfect modeling, except of course by the finished craft.
So, incredibly off topic - but do facilities like this scare anyone else? I've always had this incredibly irrational fear of really large things like this when they're connected to water. And anyone ever see pictures of water sinkholes? Seriously gives me the creeps.
For the type of stuff they do at Carderock, I'd say experiments are way better. Maneuver dynamics is really, really hard to do, as the time scales of the problem are long (ships turn on the order of a minute), something very challenging for CFD to accurately represent. Also, accurate free surface liquid dynamics is really damn hard to do, and I'd be skeptical of of most model's accuracy. I certainly wouldn't make billion dollar decisions on it.
In aerodynamics, which has had much more time and money invested, CFD can almost completely replace the wind tunnel, but only for aerodynamics problems we understand (ie wings, fairings, nacelles, turbine blades, backward facing steps, etc). As soon as you leave the validated/understood world, CFD becomes a tool used to augment the designer's intuitions, with the wind tunnel providing an essential ground truth.
Fluid dynamics is the business of predicting complex nonlinearities to a few %, and imo, complex nonlinear models are only useful when validated.
It's telling that when they debated building the DDG1000 they built a 1/4 scale ship:
I'm hoping someone with recent experience answers your question. I worked at Carderock in the early 90s, and I can tell you then, computer models worked only for the simplest shapes (only underwater vehicles, nothing on the surface) and were used to narrow down the number of physical models, but the computer models were nowhere close enough to real world to be used in place of extensive scale models.
of course, the phone I am writing this on probably has more processing power than the cray I was using back then!
I'll add that it is not just processing power that is the issue. The Navier Stokes equations for fluid flow (and the related equations) do not have closed form solutions. And even if they did, there are observed flow phenomena that cant be captured. CFD is an area where our mathematical understanding of the problem needs to advance further before we can accurately model in silicon. Some incredibly fascinating problems there - I sometimes wish I had stayed in the field.
I can say, with some exposure to the issue through my own work, that as time passes CFD modeling, mostly in the form of numerical differential equations played out in a supercomputer simulation, is rapidly replacing physical models, partly because computer modeling is becoming less expensive, and partly because one can try more design variations in a shorter time than one can with real models.
CFD is improving at great speed, along with the increase of computer power, but isn't yet in the level required to replace all scale model tests and they can still be very computer intensive. I guess that nowadays the state of the art are these new Smoothed-particle hydrodynamics models. I've worked with basins like that to test harbour and breakwaters designs and I can tell that it is common and good practice to test breakwater designs in scale models before being built, there are too many variables and hydrodynamic process (like turbulence, porous media flow, wave breaking, over-topping etc..) that cannot yet be reliably modelled using CFD, specially when they cannot be simplified to a 2D domain. Nonetheless there are types of scale models that are increasingly rare these days because they were replaced by CFD modelling, like sediment transport, although they were/are pretty cool to watch. Also scale models are very useful to calibrate CFD models.
The article doesn't do a great job explaining it, but this is the premiere testing facility. Carderock Naval Base is home to several facilities, including the David Taylor Model Basin. I toured the space for an interview once and it was very impressive.
1) "And this is why Lochtefeld may soon be hailed as some kind of prophet, for he has created one technology, and champions another, that has the potential to generate artificial waves even a surfer could love."
http://archive.wired.com/wired/archive/7.08/kahuna.html
2)
"The archdruid of surf has undergrad degrees in math and geophysics. His master's thesis - written in a house overlooking Sunset Beach on the North Shore of Oahu - is a 240-page work titled "Wave Transformation Over Coral Reefs." His PhD thesis is even thicker: a two-volume tome called "Sediment Transport and Inland Tidal Inlet Hydraulics." ...
Black has cracked the code of the world's great breaks; they now lie dissected, quantified, and digitized in his computer, ready to be reconstituted anywhere - especially indoors. With the push of a button, his patented pool floors will morph into an almost infinite variety of shapes to produce steep 8-foot-high tubes that peel for 100 yards, or gentle 3-foot rollers that peak after 40, or waves that break to the left or right every 12 seconds, all under a transparent dome and available for a few bucks a ride."
"Endless Summer (on Demand)" http://archive.wired.com/wired/archive/12.05/surfing.html