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Physical Models In an Age of Computers

timothy posted more than 2 years ago | from the bah-all-models-should-be-life-sized dept.

Earth 78

Harperdog points out this article "about the Bay Model in Sausalito, California, which was built in 1959 to study a (terrible) plan to dam up San Francisco Bay. The model was at the forefront of research and testing on water issues that affected all of California; its research contributions have been rendered obsolete by computer testing, but there are many who think it could contribute still. Now used for education and tourism, the model is over 1 1/2 acres and replicates a 24-hour tidal cycle in just 14 minutes. Good stuff."

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Been there (3, Interesting)

Anonymous Coward | more than 2 years ago | (#38283036)

My wife and I stumbled across the Bay Model while on our honeymoon.

We stopped in and walked around for a while.

Very cool place!!

Re:Been there (4, Interesting)

Z00L00K (682162) | more than 2 years ago | (#38283196)

What's interesting is that real life models sometimes still yields a better result than the computer simulated, but both have advantages and disadvantages.

Especially streaming water is a tricky thing to model entirely right and you can get surprises when you have a complex situation and try it in a physical model compared to a computer simulation.

Re:Been there (4, Interesting)

jd (1658) | more than 2 years ago | (#38283726)

Any model that is inherently chaotic (read: almost all of them) cannot be simulated on a computer accurately. There will be a cumulative error which will grow extremely rapidly (this is known as the "Butterfly Effect" after a well-meaning but ultimately damaging title of a research paper on the subject). Fluid dynamics is a major headache when it comes to chaotic systems, since the equations are violently unstable at most points most of the time. The only reason anyone can get any work done at all with CFD is that most research groups simplify the equations to one very narrow, very specific range of conditions. Even then, CFD is treated as a first approximation. A physical model of some sort is almost always built, somewhere along the line.

Indeed, one of the unsolved Millenium Challenges is to find out just how bad the Navier-Stokes equations really are. I'd have thought a more interesting challenge would be to find an alternative way to model chaotic systems. (Chaos means that the system is deterministic but not predictable. Which is no different from saying that you have a superposition of probablistic outcomes where the actual outcome is only knowable by observing the system. A field that is of interest to all kinds of people, including physicists and sci-fi writers. A standardized way of dealing with such systems that can yield any additional information at all - doesn't matter what or how little - would have a lot of appeal.)

Re:Been there (4, Interesting)

LateArthurDent (1403947) | more than 2 years ago | (#38283956)

Any model that is inherently chaotic (read: almost all of them) cannot be simulated on a computer accurately...Indeed, one of the unsolved Millenium Challenges is to find out just how bad the Navier-Stokes equations really are.

Is that really a chaotic system problem, or an incomplete model problem? Taking the Navier-Stokes equations as an example, they actually are surprisingly accurate at describing turbulence, even though, as you've mention, we haven't proven that they describe turbulent flow properly in all cases. So typically problems with fluid flow models come from making too many simplifications, having incorrect initial conditions, or making bad assumptions, not a problem with the Navier-Stokes equations per se.

In fact, as far as chaotic systems go, wouldn't a physical model have the same problems as well? The challenge there is a very high sensitivity to certain variables, so your physical model of the system may turn out to not scale at all.

Re:Been there (4, Informative)

jd (1658) | more than 2 years ago | (#38285198)

The Navier-Stokes equations are definitely chaotic, but turbulence is itself chaotic. It's actually a wonderful example of it.

A physical model would have the same problems, yes, since you can't scale atoms and the sensitivity to initial conditions means that given a long enough run (which is going to depend on the exact nature of the system) the cumulative error will swamp the system. The advantage of physical systems (for now) is that both the step size and the particle size under consideration are considerably smaller and modeled to a far greater level of precision. Physical models are still imperfect and can lead to all kinds of false assumptions if relied upon too heavily, but as the step before the full-scale system, it's the best we currently have.

A 1:1000 model essentially treats a block of 1000 molecules on the full scale as being the same as 1 molecule on the small scale. This means that at 0'C and 1 atmosphere, such a model would consider 2.687 x 10^22 (remember, you're considering 1000 at a time) [wikipedia.org] molecules of an ideal gas per cubic metre of gas under consideration. In comparison, the world's fastest supercomputer can perform around 1 x 10^16 FLOPS and the cartesian form of Navier-Stokes [wikipedia.org] looks to me like you're going to need to perform around 24-25 floating point operations per iteration per molecule. So you're looking at around 600 million seconds per cubic meter of simulated gas flow to get CFD equal to a physical model, if no simplifying assumptions can be made, and that's only true if thee world's fastest supercomputer's FLOPS rating is with a level of floating-point precision great enough not to introduce rounding errors within those 600 million seconds.

Re:Been there (2)

LateArthurDent (1403947) | more than 2 years ago | (#38285346)

Acceptable simulation performance is the part of the whole thing I hadn't considered, I understand your points now.

Thanks for taking the time to draft that example, made the whole thing a lot clearer for me.

Re:Been there (3, Informative)

wagnerrp (1305589) | more than 2 years ago | (#38285422)

Neither. It's a computational capacity problem. The complexity and non-linearity of the equations means they cannot be feasibly solved in continuum for anything but the most simple of cases. That means you're left with an iterative, discretized version of them. In order to solve a flow directly using discrete Navier-Stokes, your computational grid must be sufficiently fine to resolve the smallest scales of turbulent mixing. Even with several decades of exponential growth in performance, we are still nowhere near what is needed to solve any real world solution with Direct Navier-Stokes (DNS).

That leaves us with generalization. Turbulence models designed to reproduce the results of physical simulations are applied, rather than direct solutions, easing the computational load for those small scales. More complex models can better model more complex flows, but in turn are more computationally intensive, giving us a sliding scale between solution quality, and solution speed.

Computer simulation was never intended to replace physical simulation, merely supplement it. Physical simulations will always be limited in the environments they can produce, the duration of testing, and the data that can be measured. Computer simulation works on inference. If the simulation matches the physical test's data points at certain test conditions, the rest of the flow field not physically measured and be considered reasonably accurate. The same simulation can be performed at different test conditions, and as long as they are bracketed by the testing, can also be considered reasonably accurate. The simulation can be performed at test conditions outside the parameters of the physical testing, extrapolating behavior with reduced accuracy.

Re:Been there (1)

hitmark (640295) | more than 2 years ago | (#38288550)

"So typically problems with fluid flow models come from making too many simplifications, having incorrect initial conditions, or making bad assumptions"

Sounds like what is plaguing economic thinking as well...

Re:Been there (0)

Anonymous Coward | more than 2 years ago | (#38286968)

Although Chaotic systems cannot be modelled accurately in the sense of accurate predictions, you can get valuable information as to the shape and properties of the attractor in phase space. A chaotic system has a fractal attractor with endless detail at all levels of scale. Just because we cannot see "all" of the attractor does not mean that we cannot see enough to be useful. After all, the weather is also chaotic, yet the models that forecast 1 to 2 days ahead are pretty good. In special circumstances they even work 5-7 days ahead. Knowing where the "shaggy bits" of the attractor are as compared to the "smooth bits" may be enough for some valuable insights. Just because it aint perfect doesn't mean it has no value.

Re:Been there (1)

arkenian (1560563) | more than 2 years ago | (#38287896)

Any model that is inherently chaotic (read: almost all of them) cannot be simulated on a computer accurately. There will be a cumulative error which will grow extremely rapidly (this is known as the "Butterfly Effect" after a well-meaning but ultimately damaging title of a research paper on the subject). Fluid dynamics is a major headache when it comes to chaotic systems, since the equations are violently unstable at most points most of the time. The only reason anyone can get any work done at all with CFD is that most research groups simplify the equations to one very narrow, very specific range of conditions. Even then, CFD is treated as a first approximation. A physical model of some sort is almost always built, somewhere along the line.

So I'd like to take a few exceptions to your statements. First to say "cannot be simulated on a computer accurately" is both entirely true and entirely deceptive... For an example I take weather, which, while it cannot be accurately simulated in the general case, is, though a better understanding of measurements of initial conditions and better computational models, more accurately able to be simulated for short term results . . . weather predictions out to even the 5 day forecast are getting remarkably accurate these days. Also, when you say "cannot be simulated on a computer" you imply that computer models are inherently more prone to error than other models. ALL models make simplifying assumptions of various sorts, and at best attempt to determine boundary conditions for solution. Computer models tend to make a particular SET of simplifying assumptions, but scale models ALSO make various simplifying assumptions. I would agree that, in the case of fluid dynamics, most likely the computer model still makes MORE simplifying assumptions than a masterwork scale model, and totally agree that a scale model is a good thing, but I think we need to be careful about how we define our terms. (I would also argue that multiple types/forms of models to test varying assumptions and boundary conditions is likely necessary for major projects on exceptionally sensitive non-linear dynamic systems)

Re:Been there (1)

MaskedSlacker (911878) | more than 2 years ago | (#38289138)

There's nothing wrong with the Navier-Stokes equations, except that they're unsolvable. From a theoretic standpoint however, they're perfectly accurate. Unless you want to claim that mass, momentum, and/or energy are not conserved--because that's all the Navier-Stokes Equations are. Each one of the Navier-Stokes equations is a particular representation of the corresponding conservation law. Except for relativistic systems, or systems where quantum effects become relevant, the equations are perfectly accurate, and perfectly unsolvable (analytically, that is). Even in those systems, it's not Navier-Stokes that breaks down, but the Equation of State for the fluid that is necessary to form a complete system of equations. ALL of the problems we have solving Navier-Stokes computationally come down to grid size (which is just a computational resource problem), and EOS.

Chaos means that the system is deterministic but not predictable. Which is no different from saying that you have a superposition of probablistic outcomes where the actual outcome is only knowable by observing the system.

This is not even wrong. You've confused quantum systems and chaotic systems.

Moreover, there are plenty of alternative ways to model chaotic systems. Most of them are less useful than brute-force computation or models for engineering purposes (except as a way of identifying the general types of solutions you should expect). For mathematical theory (which may, or may not yield future engineering benefits) they're useful.

Re:Been there (1)

jd (1658) | more than 2 years ago | (#38293686)

First, Quantum Mechanics is chaotic. Second, probability waves are a product of Information Theory.

Re:Been there (1)

socrplayr813 (1372733) | more than 2 years ago | (#38283864)

This.

Anyone who uses computer simulations to model physical things knows that computers just plain can't do everything for you. ie. In the case of a physical part, you still need to do hands-on testing and at least verify your simulation results.

I want to say I'm a bit surprised that physical models aren't used more, but I'm really not. I just pray it's because people are trying to trim costs and are rolling the dice that the simulations are good enough and not because they actually believe the simulations are perfect... At least if it's a cost-cutting measure, they probably know they could run into trouble. Maybe.

Re:Been there (1)

TheTurtlesMoves (1442727) | more than 2 years ago | (#38289032)

Real models are still used extensively, for example wind tunnels. Especially for hard fluid problems. However GP is a not really correct, there are plenty of flow regimens where the NS equations are not chaotic. That is small perturbations do not grow, but are damped out.

Note that if they are always generally chaotic there would be no such thing as climate predictions for one.

Re:Been there (1)

socrplayr813 (1372733) | more than 2 years ago | (#38289560)

Absolutely, they are still used. I'm certainly not saying they aren't. I wish they were used MORE. I'm just pointing out the fact that way too many manager types think computer simulations are good enough. Whether it's because of TV or some kind of strange trust in salesmen, I don't know, but it's becoming a bigger problem in some industries.

Both. Are. Models. (0)

Anonymous Coward | more than 2 years ago | (#38294576)

n/t

We are the slashdot's trolls (-1, Troll)

asshole36535 (2525948) | more than 2 years ago | (#38283082)

We are the slashdot's trolls, my friend
And we keep on trolling till the end.
We post goatse [goatse.ru] links.
, We post goatse [goatse.ru] links.
, No time for lusers (Linux users),
'cause we troll slashdot...

Re:We are the slashdot's trolls (0)

mug funky (910186) | more than 2 years ago | (#38285976)

at least fit that shit to the song. i was singing along then fell on my face because you got no rhythm. ...or is that part of the troll?

Re:We are the slashdot's trolls (1)

tehcyder (746570) | more than 2 years ago | (#38290824)

at least fit that shit to the song. i was singing along then fell on my face because you got no rhythm. ...or is that part of the troll?

Of course it's part of the troll, it manages to annoy both people who love Queen and people who loathe Queen.

Don't miss the Mississippi (5, Interesting)

at10u8 (179705) | more than 2 years ago | (#38283096)

There was also the Army Corps of Engineers model of the entire Mississippi/Missouri/Ohio/Arkansas/Red river basins. It was built by POW labor near Clinton, MS. See what's left of it here. http://maps.google.com/maps?ll=32.30606,-90.316173&spn=0.003922,0.006335&sll=36.977452,-121.987122&sspn=0.118622,0.202732 [google.com]

Re:Don't miss the Mississippi (4, Interesting)

Anonymous Coward | more than 2 years ago | (#38283172)

Or check this out: Mississippi River Bason Model in decay [blogspot.com]

Re:Don't miss the Mississippi (5, Informative)

kybred (795293) | more than 2 years ago | (#38283236)

Yes, it's 200 acres in size!

More (much more) info [designobserver.com]

Re:Don't miss the Mississippi (4, Interesting)

Anonymous Coward | more than 2 years ago | (#38284916)

I worked for Brown and Butler a consulting firm in Baton Rouge that did beta testing of the first PC port of the Corps HEC-1 mainframe hydraulic simulation software. John Haestad of Haestad Methods developed the software which is currently available from Bentley Systems at bentley.com. In the course of my duties I visited the COE Mississippi River model in the mid 1980's.

We were using the HEC software on a Leading Edge 8088 PC, 640K ram and 30mb hard drive, researching the effects of a proposed reservoir on the Amite River near Darlington, Mississipi to prevent annual flooding downstream in increasingly populated areas. The COE eventually decided against the B and B proposed reservoir and instead build a diversion canal to the Mississippi River, a project which is still in progress. The project decided upon still has not been completed 40 years after the initial studies began and is still facing opposition to its completion.

Here are the coordinates of one of the control structures:

3037'14.26"N 9114'24.11"W

Editors aren't editing (-1)

Anonymous Coward | more than 2 years ago | (#38283102)

Man is the editing on this article awful. Just terrible.

Oh, wait, timothy article that should be in idle, carry on.

Re:Editors aren't editing (0, Funny)

Anonymous Coward | more than 2 years ago | (#38283188)

AAA+++. Enjoyed random; Punctuation. Felt like - Shatner. Would read "again". Good stuff.

The Need Will Never Be Replaced (0)

Anonymous Coward | more than 2 years ago | (#38283114)

Computer modeling is grand but it will never replace the need for physical models or prototypes. You simply can't display on a small screen every aspect of something physical. At the least there are intangibles, feel, smell, whatever.

In the future, as is now, computer design and modeling will be the first step. Then there will be physical modeling or prototyping. Then comes implementation.

Damming San Francisco Bay, well, that's just silly.

Re:The Need Will Never Be Replaced (1)

wagnerrp (1305589) | more than 2 years ago | (#38285810)

In the future, as it is now, physical modeling will be the first step. We're nowhere near the computational power needed for direct simulation of complex shapes, and were not likely to be any time soon. That means physical tests to verify simplified computational models which can then be used to interpolate and extrapolate beyond the scope of those physical tests.

Railroad scale (1)

vlm (69642) | more than 2 years ago | (#38283124)

What model railroad scale is the closest? I have no interest in CA, so I don't know if 1.5 acres makes that bigger than G scale or smaller than Z scale or something in between. The live steamers might want to turn it into a live steam park, if allowed. Around here, the live steam parks are not quite as elaborate as this sounds.

You have to see this (4, Interesting)

wsanders (114993) | more than 2 years ago | (#38283316)

The model cover about a 3000 sq mile area. The Golden Gate Bridge is about 6 inches long IIRC.

You have to see the Bay Model if you visit the Bay Area. There is a fully restored Nike missile silo nearby that is also a must-see of Bay Area nerdy sights.

Re:You have to see this (1)

Anonymous Coward | more than 2 years ago | (#38283452)

Why a shoe company thought it was a good idea to buy the naming rights to an AA missle project is beyond me.

Re:You have to see this (2)

gknoy (899301) | more than 2 years ago | (#38285368)

I realize you're likely joking, but I bet Nike (the shoe company) is named after Nike, the goddess of victory. ( http://en.wikipedia.org/wiki/Nike_(mythology) [wikipedia.org] )

Re:You have to see this (1)

morethanapapercert (749527) | more than 2 years ago | (#38286498)

but so is the missile...

Nike Missile project : circa 1945

Nike Athletics : First named Blue Ribbon Sports, renamed Nike in 1978

According to Wikipedia, BOTH were named after the goddess of victory....

Re:You have to see this (1)

jd (1658) | more than 2 years ago | (#38283744)

I hope they remembered to pay the estate of Emperor Norton royalties (imperialties?) for the bridge.

Mythbusters (2, Informative)

d3ac0n (715594) | more than 2 years ago | (#38284198)

Isn't this the same model that was featured on the Mythbusters episode [wikipedia.org] about escaping from Alcatraz?

Re:You have to see this (0)

Anonymous Coward | more than 2 years ago | (#38284318)

wsanders is right both of these things are very cool and very close to one another.

The Nike site is only open one day a month. Don't go there without checking ahead of time.

Re:You have to see this (2)

vlm (69642) | more than 2 years ago | (#38285018)

The Golden Gate Bridge is about 6 inches long IIRC

Thats what I needed. I looked it up and the GGB is about nine thousand feet long, and a typical railcar is "about half a hundred feet" (there is no standard) so the GGB if it were a railcar bridge (maybe it has a rail deck, for all I know or care) then it would hold about 180 railcars, if I divided that in my head correctly (10K/50 = 200 minus 1K/50 = 20 means about 180, right?). I don't think there is any model railroad gauge where 180 railcars fits in 6 inches. Although that would look pretty cool if it ever existed.

Re:You have to see this (2)

morethanapapercert (749527) | more than 2 years ago | (#38286588)

Google up the world's smallest train set sometime. If I remember correctly, a guy basically took a tiny plastic tube, carved a train consist along the circumference of one end and inserted it up into a 1/4" block which was decorated for scenery. IIRC, the scale was something on the order of 1:3200. 180 cars at ~50' each (neglecting couplers) is 4000 feet. at 1:3200 the Golden Gate would be about 2 1/2 feet So even the smallest scale model train I have ever heard of (aside from lab demos) is still roughly twice the size needed to fit the model in question.

*Yes, I know my numbers are very rough, I couldn't be bothered to be more precise. But if anyone else feels like giving us more reliable numbers, please feel free

Re:Railroad scale (4, Informative)

hawguy (1600213) | more than 2 years ago | (#38283366)

What model railroad scale is the closest? I have no interest in CA, so I don't know if 1.5 acres makes that bigger than G scale or smaller than Z scale or something in between. The live steamers might want to turn it into a live steam park, if allowed. Around here, the live steam parks are not quite as elaborate as this sounds.

You don't have to have an interest in CA to read the first few paragraphs of the article:

its 1.5 acres replicate a 1,600-square-mile area that runs from the Pacific Ocean to the Sacramento Delta

1600 mi^2 is 1024000 acres, so it's a 1.5:1024000 (or 1:682666) scale if you believe the article.

However, the bay model's webpage tells a different story:

http://www.spn.usace.army.mil/bmvc/bmjourney/the_model/facts.html [army.mil]

Model Scales (Model to the Bay)

Horizontal: 1 foot = 1000 feet
Vertical: 1 foot = 100 feet
Velocity: 1 foot/ second = 10 feet/second

So using their numbers, it's a 1:1000 scale.

I have no interest in model trains, but Wikipedia tells me that Z-scale is the smallest commercially available scale, and is 1:220, so this is a much smaller scale than any model train system.

Re:Railroad scale (3, Informative)

Anonymous Coward | more than 2 years ago | (#38283538)

The 1:682666 and 1:1000 aren't really very far apart...you based the 1:682666 on area, but scale is normally based on linear dimension. The square root of 682666 is about 825, so the two aren't really that different. Since the model isn't square, and actually twists part of it to make it smaller, the two are pretty close...

Re:Railroad scale (1)

hawguy (1600213) | more than 2 years ago | (#38283650)

The 1:682666 and 1:1000 aren't really very far apart...you based the 1:682666 on area, but scale is normally based on linear dimension. The square root of 682666 is about 825, so the two aren't really that different. Since the model isn't square, and actually twists part of it to make it smaller, the two are pretty close...

Ahh yes, good point! I forgot to convert from area to linear scale!

Re:Railroad scale (1)

tehcyder (746570) | more than 2 years ago | (#38290874)

The 1:682666 and 1:1000 aren't really very far apart...you based the 1:682666 on area, but scale is normally based on linear dimension. The square root of 682666 is about 825, so the two aren't really that different. Since the model isn't square, and actually twists part of it to make it smaller, the two are pretty close...

Ahh yes, good point! I forgot to convert from area to linear scale!

Now you know how those NASA engineers felt...

Re:Railroad scale (2)

bjorniac (836863) | more than 2 years ago | (#38283620)

Linear and quadratic scales aren't the same thing - if you double a length scale, you quadruple the area. Thus 1:1000 in length -> 1:1000000 in area, which is closer to the number you got from the article, modulo rounding (1.5 acre could be anything from 1.25-1.75 if they just rounded to nearest half integer).

Re:Railroad scale (0)

Anonymous Coward | more than 2 years ago | (#38283428)

It's 1:1000 scale, so it's about 1/5 the size of Z scale. There are probably practical limitations in the universe that would prevent you from making a working model steam locomotive engine less than a half inch long.

Education (5, Insightful)

Zrako (1306145) | more than 2 years ago | (#38283130)

Computer models may be far more efficient for research but physical models still have a very important role. Children, while far more efficient then ever at using technology, do not have the ability/opportunity to learn from these computer models like a researcher would. I think physical models are still very important for teaching the next generation about different science and engineering principles, like the tide cycle from the article. The ability for kids to physically see and experience the science and engineering topics is what gets them interested and engaged in learning about those topics in the future. Models like this are essential for future education for kids. I mean what kid doesn’t start to wonder about the world around them after going to the local science center?

Re:Education (1)

blair1q (305137) | more than 2 years ago | (#38284384)

You can make a computer model fit a physical model.

Your physical model will never fit your computer model. At least, not if your physical model is supposed to be natural.

Re:Education (1)

VortexCortex (1117377) | more than 2 years ago | (#38289784)

Children, be sure to take home these permission slips and get your parents to sign them. We'll spend an ENTIRE day the next FOUR WEEKS touring various fluid simulation sites.

vs

Children, Please download the Fluid Simulation Software to your workstations or laptops. Please load, and view each of the 20 simulation demos before our next class either in the Lab, or AT HOME. Each situation faces a water problem. RE-ARRANGE THE LANDSCAPES in the simulation and see if you can solve the problems.

Since when do Children need Five Nines of accuracy to learn about fluid dynamics? You can run the simplified sims on your home computer... You: Look, but don't touch, and try to learn. Me: Here, have a hands-on experience.

If you're talking about much smaller scale sims, well then, good luck setting them all up each time the banks wash away. It'll devolve into a splashing mud-slinging free-for all. Great fun, but less learning. Trust me, I've tried.

The page (-1, Troll)

Capt James McCarthy (860294) | more than 2 years ago | (#38283138)

I didn't know their web page was written in the 50's as well.

Re:The page (0)

smooth wombat (796938) | more than 2 years ago | (#38284578)

You mean it's not a horrendous conglomeration of Flash ads, moving type, slideouts/slideins and inconsistent use of font sizes?

I'm glad they went with a simple, easy to use to design that allows one to get to what they want without having to wade through piles of cruft.

Quite obviously they didn't let a web designer design their site.

Re:The page (1)

tehcyder (746570) | more than 2 years ago | (#38290996)

Quite obviously they didn't let a web designer design their site.

What's the difference between a web designer and a telephone sanitiser?

About thirty thousand quid a year.

This is nothing new... (2)

icebrain (944107) | more than 2 years ago | (#38283140)

Wind tunnels and subscale models are still used in aerospace even with the availability of CFD and other computerized tools.

Re:This is nothing new... (2)

rwa2 (4391) | more than 2 years ago | (#38283530)

I work in an old wind tunnel today! They use it as a server room now.

Unfortunately they don't use it to cool the servers, because that would be just too awesome.

Re:This is nothing new... (1)

CompMD (522020) | more than 2 years ago | (#38286350)

Skin drag of the air along the interior walls of the wind tunnel will generate heat and will probably destroy your servers.

I used to work in a closed circuit wind tunnel. It was about 30 ft tall, with the upper test section measuring about 6'x5'x4'. The top speed in the upper test section was about 200mph. After running tests for 4 hours at speeds no greater than 60mph, the temperature inside the test section was 105F. And no, it wasn't from the motor, that was actually outside the tunnel and a shaft protruded in with a prop at the end.

Re:This is nothing new... (1)

hankwang (413283) | more than 2 years ago | (#38288782)

No, it's the work done by the prop that heats up the air. Any heating up near the walls is balanced by cooling down elsewhere in the tunnel. The interesting thing about friction heat in a gas pipe is that a temperature increase causes the gas to expand, which costs exactly the same amount of energy as you put in. The net effect is that the gas expands along the pipe without a net increase in temperature.

Re:This is nothing new... (0)

Anonymous Coward | more than 2 years ago | (#38283546)

A simple thing like a flag flapping in the wind can't be simulated accurately. People haven't yet come to a complete and perfect understanding of every field of physics, so physical models are still useful.

Re:This is nothing new... (5, Interesting)

YackoYak (153131) | more than 2 years ago | (#38283588)

I imagine testing of scale (including full scale) models is still used in most industries. Computers help you reduce the number of mistakes and shorten the iterative development cycle so you make fewer elementary errors.

In Oil & Gas, almost all of the major manufacturers have been modeling all their new components in 3D for the last decade. You can have whole departments dedicated to running Finite Element Analysis on these 3D models. It's not as simple as putting in your constraints (loads, fixed and pinned reaction points, etc) and hitting "solve". You still have to make certain assumptions, tailor the matrix (the 3D lattice/approximation made of pins and beams) or make simplifications to the model if you want it to converge toward a solution. The computer solves the cases iteratively. Once you think you're close, you perform a test at scale and verify your assumptions. Then your match it to your computer model and hopefully you're not too far off. For example, things like incorrect friction factors between materials matter a lot when they are amplified by huge forces. The manufacturer of a particular coating may claim it's .07 (if they even tell you). You learn it's more like 0.08 through repeated testing. At this point there is no way of calculating things like that.

Of course, this assumes that you have the time, money, and server time to run such experiments. Most of the time you focus on the one or two critical areas of a design for FEA and use past experience or classic formulas on the rest. Customers still want final tests before product ships, so technically you have one last point to catch a failure/issue before it ships.

Re:This is nothing new... (1)

icebrain (944107) | more than 2 years ago | (#38284298)

That's exactly how it is in the aerospace field, too. Thank you for explaining it in better terms than I could.

I will say it does tick me off when (usually in response to flaws discovered in testing) people say things like "We have been using aluminum for a few decades, and we have computers now. There should be no excuses for any design that isn't perfect."

ahh yes, a nice quick slashdotting of the site... (0)

Anonymous Coward | more than 2 years ago | (#38283204)

great... I didn't want to read about it anyway

both with 3D printing? (1)

peter303 (12292) | more than 2 years ago | (#38283270)

You could realize the model now and then by printing it.

Re:both with 3D printing? (0)

Anonymous Coward | more than 2 years ago | (#38283390)

As soon as we get a 3D printer that can properly print bedrock, dirt, forests, and San Francisco.

Re:both with 3D printing? (4, Funny)

sexconker (1179573) | more than 2 years ago | (#38283518)

As soon as we get a 3D printer that can properly print bedrock, dirt, forests, and San Francisco.

I thought that was what the magenta ink tank was for.

Insomniac (1)

Baby Duck (176251) | more than 2 years ago | (#38283372)

I remember seeing that model on Insomniac with Dave Attell. Exciting to behold!

Ah Hah! (1)

SnarfQuest (469614) | more than 2 years ago | (#38283582)

replicates a 24-hour tidal cycle in just 14 minutes.

So, thus is where all those "faster-than-light" particles are coming from.

Question for experts (2, Insightful)

Anonymous Coward | more than 2 years ago | (#38283596)

Something I've always wondered about physical models is, how can you compare them to real situations at different scales? So many relationships in physics are non-linear. It seems like the model behavior must differ both quantitatively and qualitatively from the real behavior.

Re:Question for experts (4, Interesting)

cellocgw (617879) | more than 2 years ago | (#38283932)

Something I've always wondered about physical models is, how can you compare them to real situations at different scales?
IIRC the folks who design America's Cup sailboats have (had) a rule of thumb that a hull model less than 1/3 scale would not accurately predict the drag of the full-size design. Off-topic: if you think water and fluid dynamics are tough to figure out, take a look at what little we know about how ice skating works -- or to be exact, just how the surface of ice behaves to allow gliding motion.

Re:Question for experts (2)

RoverDaddy (869116) | more than 2 years ago | (#38284526)

That's a good one. I was taught years ago that the pressure of the blade on the ice momentarily melts the water beneath, but since then I've read that there isn't enough pressure to do that (and I always wondered by the skater wouldn't leave thin puddles behind). We really don't know for sure how ice skating works.

Re:Question for experts (3, Interesting)

BetterSense (1398915) | more than 2 years ago | (#38284848)

I learned the same thing in school. I also learned that airplanes work because air travels a longer distance going over the top of the wing than the bottom, that movies work because of persistence of vision, and that the primary colors were Red, Yellow and Blue.

I learned some things in school but sometimes I wonder if it was worth stuffing my head with all the wrong things I learned.

Re:Question for experts (2)

drcheap (1897540) | more than 2 years ago | (#38285416)

Funny...all these years I thought it had something to do with the drastically reduced coefficient of friction of ice (especially at just-below-freezing temps where there is a thin layer of liquid water present).

Re:Question for experts (1)

idontgno (624372) | more than 2 years ago | (#38285524)

I also learned that airplanes work because air travels a longer distance going over the top of the wing than the bottom

I learned that too. I guess it was sincerely believed for a long time.

that movies work because of persistence of vision

Yeah. The brain-centric nature of motion perception was discovered in the early 20th Century, but I was still being taught that the eye is where image persistence became perceived motion. Kind of disappointing.

and that the primary colors were Red, Yellow and Blue.

They are, if you're in elementary school art class. They don't make many magenta or cyan tempra paints, after all. (And more to the point, those pseudo-primary colors work in a subtractive color system pretty well. The main thing they can't reliably do is mix to a full color gamut.) Now, in my elementary school SCIENCE class, they taught us about TWO different sets of primary colors: additive (Red, Green, Blue) and subtractive (Magenta, Cyan, Yellow). And they made a point of saying not to pay any attention to what the art teacher was saying about primary colors.

Re:Question for experts (1)

Neil Boekend (1854906) | more than 2 years ago | (#38288966)

Some things (like how ice skates work and the way movies are percieved) are found out to be wrong (if you stand still on ice skates you won't melt your way through the ice so the pressure isn't melting the ice. It's a complicated system and I don't know for sure how it works)
Somethings are different from different points of view (the primary colors are different in art and science.)
Some things are explained in only one way, while they can be done in different ways: a plane usually flies by Bernoulli's principle (the difference in speed causes a difference in pressure) but a plane can fly with another effect (deflection of air downward causes a reaction force upward) and even a third effect (creating a high pressure pocket under the plane, if you consider a wing in ground effect vehicle a plane).
Using Bernoulli's principle is the most efficient one at an altitude of above a couple of meters, as in it causes the least drag for the lift it creates. That's why most planes use Bernoulli's principle. But a stunt flier can't just change the form of the wings if he flies upside down, so he must use the second method of staying in the air. The third method has been experimented with by the Russians for a couple of years but when the USSR crashed nobody continued the research.
To conclude: what you learned in school wasn't wrong as such. The teachers told you something was true because it was at that time and in the most cases.
Exept for the movie stuff: the books that told you "lies" about how they worked weren't updated yet.

Re:Question for experts (3, Insightful)

Anonymous Coward | more than 2 years ago | (#38284400)

It depends on what effects are interesting to you. For example, in testing scale models of aircraft, reducing the model size generally means increasing the wind speed, to keep the Reynold's number (viscosity and kinematic effects) constant. However, that can jack up the Mach number (shock wave effects). Or, if you're doing dynamics, then the weight will be very important, which doesn't scale linearly with size. So it's usually a tradeoff.

Re:Question for experts (0)

Anonymous Coward | more than 2 years ago | (#38295250)

http://en.wikipedia.org/wiki/Similitude_(model)

Someone above mentioned the scales on the Bay Area model: 1:1000 horizontal, 1:100 vertical, 1:10 velocity; different parameters are scaled differently to meet similitude requirements. Sometimes in hydraulic models, the working fluid will also be changed from full- to model-scale to also provide scaled densities and viscosities.

Mythbusters (5, Interesting)

Tofof (199751) | more than 2 years ago | (#38283642)

For what it's worth, this is the same Bay Model the Mythbusters used in their season 1 episode Alcatraz Escape.

The accurate reconstruction of the tidal effects allowed them to convincingly show that a raft would be unable to reach Angel Island, and that a more plausible route would have been toward the Marin Headlands - before confirming the model's result experimentally.

Dimensionless quantities (2)

bpowah (1119491) | more than 2 years ago | (#38283664)

The primary drawback to physical models is that it typically very difficult or expensive to get the same behavior of the phenomena you are studying at different scales. Depending on the study, you need to try your best to match the dimensionless quantity that is involved in the phenomenon you are studying. Fluids have many properties (surface tension, viscocity, expansion coeffs, etc..) that typically don't scale well: http://en.wikipedia.org/wiki/Dimensionless_quantity [wikipedia.org] Often you use a fluid other than water and a different timescale and sometimes you can adjust the properties of the fluid by changing it's temperature and try to dial in the dimensionless quantity of interest. But I bet it is difficult to swap out all that water in the Bay Model or drastically change the temperature..

Re:Dimensionless quantities (1)

Hentes (2461350) | more than 2 years ago | (#38284604)

They don't necessarily have to be of different scale, wind tunnels are often used to study 1:1 scale models.

Re:Dimensionless quantities (1)

systemeng (998953) | more than 2 years ago | (#38285006)

I saw this problem in school. One of the clinic project teams was given a 1/10 scale or so model of the piping in a copper smelter. They did a semester of research before somebody figured out that Reynolds number did not scale properly for the model. Different Reynolds number meant different flow characteristics and thus a model providing useless results. The project had to be cancelled.

Utility of physical modeling (4, Informative)

Anonymous Coward | more than 2 years ago | (#38283950)

The days of large physical models of tidal hydraulics in large estuarine systems are past because properly calibrated/validated numerical models provide good results at a fraction of the cost.

However, it is paramount the the numerical models are capable of simulating the correct physical processes without over-simplication. For example, the flow hydrodynamics near the Port of Anchorage in the Knik Arm of Cook Inlet are dominated by large gyres that are shed off prominent headlands. A large physical model of the Knik Arm constructed by the Corps of Engineers at their Vicksburg, MS, research facility reproduced the large gyres with a good match to measured field data, and local tug pilots agreed that the flows resembled what they experience daily. Initial attempts at numerical modeling the flow fields produced no gyres, and it was not until a very sophisticated adaptive turbulence closure scheme was added that gyres formed in the numerical model. Both the physical and numerical models required good boundary and initial conditions for success.

Physical models are still useful for simulating processes that are beyond our ability to describe mathematically (required for numerical modeling). Examples in the field of hydraulic engineering include some sediment transport processes, stability of rubble-mound structures such as jetties and breakwaters, erosion of cohesive sediments, wave forces on structures, and resiliency of levee grasses subjected to wave overtopping, just to name a few.

Numerical modeling in hydraulic engineering is making rapid advances, and often any unknown processes can be adequately represented in the model by empirical formulations that have been developed based on physical model tests. Whatever the skill of the numerical model, it is imperative that engineers who apply the numerical model to a problem have a good understanding of what physics are being simulated and what compromises have been made during model development. Failure to understand what the model does will assuredly lead to disaster.

Finally, physical models can be successful provided: (1) The dominant forcing in the real world is correctly represented in the scaled model, (2) any forcing not correctly represented has minor influence, (3) laboratory and scale effects can be minimized or some compensation can be applied, and (4) model results have been validated to the extent possible. A similar set of criteria applies to numerical models.

In the future, physical modeling will continue to be used to validate numerical models, they will provide physical understanding and empirical formulas for use in numerical models, and physical models will continue to address those engineering problems that cannot be formulated mathematically.

Re:Utility of physical modeling (1)

blair1q (305137) | more than 2 years ago | (#38284444)

1. If all you want is "good results", then computer models are fine.

2. it's still expensive to build a model of anything natural, and still can't be perfectly accurate, because you can always make it an even more detailed copy. shorelines are fractal, and so are all the textures and texture mixes. at a certain point, you might as well just run experiments on the actual thing.

Physical models are returning (2)

hweimer (709734) | more than 2 years ago | (#38285764)

There is currently a huge research effort underway funded by tons of millions to make physical models of more complicated stuff: quantum simulators. With classical computers, there's basically no way to calculate properties of things like quantum magnets, quark-gluon plasmas, or high-temperature superconductors. But by using a different quantum system that we can precisely control (for example, ultracold atoms [quantenblog.net] ), we can tune it to simulate the behavior of the material we are interested in.

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