Neither of these explosions 
are actually real.
This one, known as a physical, or special effect,
happened on set,
 and is much more colorful than in reality.
And this one, a computer animation, is
created using an algorithm.
So what are we technically seeing onscreen?
We have to create these effects that
people have come to think of as explosions
that really aren't explosions at all;
they're fire reactions.
The real explosion takes place very quickly,
because you have energy moving
over 20,000 feet per second, so one minute the car is there, then the next minute there's nothing.
The movie version of the explosion
happens thousands of times slower,
always involves fire, always involves smoke.
Steve Wolf has developed and executed stunts like these and hundreds more for over 30 years
on film and television sets.
There are two ways explosions release energy:
by deflagration and detonation.
The explosions Steve usually creates are deflagrations.
They produce pressure waves that travel slower than the speed of sound, which makes them subsonic.
They are easier to control, like these chemical
reactions with powder.
Supersonic explosions release by detonation.
These pressure waves, which travel faster than the speed of sound, are created by high explosives like C4.
So, sometimes I want a lot of flame, sometimes I want something to physically move,
so we have to choose the materials that we're going to use to create that effect.
Most of the time, high explosives are
not going to be the answer.
To control the fire and the effect, Steve removes everything from the site of the explosion,
in this case, the interior of a test car.
The only thing that's burning 
are the fuels that we put in there
so we can control exactly the
quantity of the fuel, the nature of the fuel,
what reactions are going to take
place inside the vehicle,
and make sure that there's nothing else involved.
We want to use the least amount 
of kinetic energy that's necessary
to create the effect,
so we pre-cut the car.
We cut the hinges on the doors and just hot meld
them back on, and the same thing with the hood,
so that just a small amount of
energy will make the car fall apart.
This method of special effects is still
popular with filmmakers
because it is relatively inexpensive
and easily repeatable.
Now compare it to this: 
a computer animation.
Traditionally, effects like these 
have been complex and costly,
taking days or weeks to render over multiple machines.
By researching complex natural phenomena,
Dr. Theodore Kim has been able to drastically improve this process over the last few decades.
Complex natural phenomena is nonlinear phenomena.
Basically, almost all of what you see
in the natural world is nonlinear
because most things don't just 
travel in a straight line.
And if it doesn't travel in a straight line then the math that you learned in high school,
it doesn't quite get the job done.
So you're gonna have to
start using something a little bit more advanced
and that's where we come in.
A big breakthrough in fluid simulation was by a Canadian researcher named Jos Stam back in 1999.
It enabled artists to manipulate 
a fluid simulation in real time,
so something like smoke rising from a cigarette,
 or steam from a coffee cup
something like that.
One of the limitations of this method, though, was
that the fluid that it produced looked very viscous
so you could get very large
very broad swirls out of it,
but the thing that makes, for example, large
explosions look like large explosions,
is that there's lots of tiny little curls
that then spin off from the main explosion.
It didn't quite excel at
generating those specific details.
Theodore answers this in his paper called
'Wavelet Turbulence'.
He combines Stam's algorithm with a
modified version of a pre-existing formula
that quickly generated those
tiny swirls.
But the details weren't quite moving
with the laws of physics.
So, any fluid simulation, you have to use velocity and you have to use pressure, in some form.
What we did is we added small-scale noise 
or high-frequency noise.
Theodore used a tool called wavelet transform, 
which could detect
when and where tiny details appeared off.
It uses something called a Texture Advection.
First, we detect where would some small swirl have actually appeared?
So we go ahead and inject the swirl, and have it move along with existing flow.
Advection just means the velocity is pulling something along, so a leaf for example is advected in a stream—
it's just being pulled along by the stream. We then have to analyze the Jacobian of the
texture coordinate, because what can
happen, as with any flow, it can try to
pull something apart or it can try to
push it back together. So we detect when
that happens and then we delete that
vortex so it's not allowed to exist anymore.
By building and perfecting this algorithm, 
Theodore could synthesize all these details quickly
so that artists could get the look they wanted for explosions in fast enough time.
The tool was so effective and featured in so many movies that it earned Theodore an Academy Award.
My favorite one was probably 'Super 8.'
there's a big train sequence at the beginning and
it was used for a lot of the fire effects
for the train crash.
Mechanical engineering is usually not concerned with artistic purposes
so while we use much of the similar underlying math,
we do tweak it in our own way to make it 
so that it's artistically controllable.
There's really just a handful of laws of physics that control how everything works.
Whether we do something amazing with it or not
is just by how we take those simple
concepts and how we put them together.
This episode was presented by the U.S. Air Force.
Learn more at Air Force.com.
For more episodes of Science in the Extremes
check out this one right here.
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Thanks for watching.
