"Explosions are not comfortable." -- Yevgeny Zamyatin
These days, digital explosions in Hollywood movies are a dime a dozen -- unless Michael Bay is directing of course, in which case they are an Iranian rial a dozen -- but do you ever wonder how filmmakers are actually able to pull them off (you know, beyond "with computer animated wizardry")?
Well luckily for you, Theodore Kim, the professor of Media Arts, Technology and Computer Science at the University of California, is here to explain exactly how Hollywood is able to create realistic explosions in movies with this video, "The Science Behind Hollywood Explosions."
The simple answer to your question is "algorithms." The slightly more complex answer is an algorithm called "Wavelet Turbulence," which was published as part of a study done by the University of California back in 2008.
As Kim explains, "Wavelet Turbulence" is an algorithm that deals with "swirls," the vortices often used to draw smoke, steam, and wind. Using this algorithm, Kim was able to build on previous research and determine how to add hundreds of smaller swirls to one larger, singular vortex. The tool, known as a "Wavelet Transform," combined these two sets of vortices into one cohesive image, and when combined with a method for moving the smaller swirls along with the overall "flow" of the explosion (known as "texture advection"), the end result was a faster resolving, much more realistic animation.
At least, that's what I *think* Kim was getting at. This was all a little over my head.
In any case, Kim's Wavelet Turbulence algorithm has become quite popular amongst Hollywood filmmakers in recent years, being used to create mind-blowing effects in such films as Monsters vs. Aliens, Avatar, Iron Man 3, and Super 8.
Check out Kim's wonderful breakdown of Wavelet Turbulence above, then learn all you need to know about the algorithm over at Cornell.edu