The Purdue "simulation depicts how a plane tore through several stories of the World Trade Center north tower within a half-second and found that the weight of the fuel acted like a flash flood of flaming liquid, knocking out essential structural columns within the building and removing fireproofing insulation from other support structures."
Mete Sozen, Purdue's Kettlehut Distinguished Professor of Structural Engineering, says: "To estimate the serious damage to the World Trade Center core columns, we assembled a detailed numerical model of the impacting aircraft as well as a detailed numerical model of the top 20 stories of the building".
Since the simulation is of the "top 20 stories" for "3/4 seconds real-time", don't look for this model (even if correctly defined) to explain the collapse—about 102 minutes later in roughly 10 seconds—of the 110-story North tower of the World Trade Center.
And don't look for an improved model to explain the collapse any time soon.
Since the Purdue simulation of the "3/4 seconds real-time . . . took about 80 hours using a high-performance computer", a simulation of the 102 real-time minutes from impact to collapse, using Purdue's computers, could take 652,800 hours—((102 x 60 / 0.750) x 80) hours, or about 75 years.
The number of assumptions necessary for such a simulation, and cumulative, computational errors, would render the final result worthless.
Saturday, June 16, 2007
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The assumptions are not obvious from the animation, but there is something grossly un-physical about the way the plane interacts with the outer wall of the building. This is the previously noted fact that the plane simply melts into the building with no visible deformation.
We see this especially in the side view, where the plane's fuselage is seen traversing the space between the outer wall and the core with no signs of shortening, crumpling, etc. This makes no sense in terms of the actual mechanics of the impact, in which energy can only be transferred to the outer columns by rapidly decelerating the part of the plane that first contacts the building.
This deceleration creates a wave of crumpling and deformation that travels back through the structure of the fuselage, and the amount of energy transmitted will be directly related to the amount of plastic deformation. In the case of punching a hole through the "intense grid" of 14" box columns, this would have to be a very large amount of energy, and would leave the fuselage compressed to a small fraction of its original length.
For an example of the way a real plane behaves in a crash, see for example:
http://www.youtube.com/watch?v=VMjIVCgRIcg
Some other thoughts on the simulation and the assumptions behind it.
The astonishing description of "a flash flood of flaming liquid, knocking out essential structural columns" indeed fits the claims made by Mete Sozen in the article (http://news.uns.purdue.edu/html4ever/2006/060911.Sozen.WTC.html)
on the simulation in the Purdue University News:
"As a result of the Pentagon research, we have a better understanding of what happens when a tremendous mass of fluid such as fuel hits a solid object at high velocity," Sozen said. "We believe most of the structural damage from such aircraft collisions is caused by the mass of the fluid on the craft, which includes the fuel.
"Damage resulting solely from the metal fuselage, engines and other aircraft parts is not as great as that resulting from the mass of fluids on board. You could think of the aircraft as a sausage skin. Its mass is tiny compared to the plane's fluid contents."
Unfortunately it does not fit anything that fluid dynamics (at least outside the DoD) would predict about the behavior of real fluids.
We are told that "As a result of the Pentagon research, we have a better understanding of what happens when a tremendous mass of fluid such as fuel hits a solid object at high velocity," yet the fuel is certainly not "a tremendous mass" in this context. It's like expecting a water balloon to punch through 1/4" steel plate just because it is tossed at a few hundred mph.
Jet fuel, essentially high grade kerosene, is not a dense fluid, and does not turn into a rigid body at 500 mph. Even if it could somehow do become a solid, because of its low density it would still not carry enough kinetic energy per unit volume to penetrate. Perhaps this is why depleted uranium is used rather than small aluminum tanks of kerosene for armor-piercing rounds.
There are fluids that behave in this way, so called non-Newtonian fluids (search YouTube for some neat examples) that are rigid to rapid movement, but it takes some effort to create such things. Simple hydrocarbon fluids like jet fuel will behave like classical fluids over a very wide range of velocities.
One other comment on the animation: it does not actually show this behavior by the fuel. Instead it disperses as we would normally expect. We also see the animation depict pieces of aluminum from the fuselage slicing edgewise through core columns (no comment). Note also that the connections between the core columns seem to have been left out of the simulation. There were a great many of these, spandrel plates and box columns on each floor connecting every column to its neighbors, as well as diagonal vertical bracing connecting the four columns in each corner of the tower.
The behavior of columns tied together into a grid is very different from unconnected columns waving in the breeze.
According to Kevin Ryan: Purdue University's "short video clip is far from a scientifically-based production, as it actually contradicts several of the government's own, much more intensive studies, and shamefully fails to capture some of the most basic aspects of the related events." He expressed his views in a letter to Purdue's president.
Don't skyscrapers have to sway in the wind? If an airliner hits near the top of a skyscraper isn't it going to deflect and oscillate?
Why don't the CORE COLUMNS move in unison in the Purdue simulation as the plane comes in and pushes against the floor slab, cracking it up? Didn't the survivors just below the impact point report the whole building shaking? Someone said it "moved like a wave".
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