Traumatic brain injury (TBI) is often called the signature injury of the war in Iraq. Medical experts have yet to determine exactly what causes the condition, but the violent waves of air pressure emitted by an improvised explosive device (IED) or a rocket-propelled grenade are most likely to blame. These pressure waves travel close to the speed of sound and can rattle the brain's soft tissue, causing permanent, yet invisible, damage. In an effort to better understand how the waves shake soldiers' brains, researchers at the Naval Research Laboratory (NRL), in Washington, DC, developed a computer simulation that models the motion of a propagating blast wave using data gathered from laboratory experiments with sensor-studded mannequins.

"The simulation gives us the full 3-D flow field, velocities, and pressure distributions surrounding the head and the helmet," says David Mott, a scientist researching computational physics and fluid dynamics at NRL, and the project leader for the model. "We can actually watch the waves traveling and interacting."

Initial testing has already revealed some compelling results. "We are seeing the focusing of the pressure waves up under the helmet, so that the highest pressure is between the head and helmet, and not on the side facing the origin of the blast," says Mott. The researchers will present their work today at the 61st Annual Meeting of the American Physical Society's Division of Fluid Dynamics, in San Antonio, TX.

"The physics of a blast are quite complex," says Raul Radovitzky, an associate professor of aeronautics and astronautics at MIT who was not involved in the NRL work. Radovitzky has developed a software model incorporating both the physics of pressure waves and the variable properties of the brain's tissues in collaboration with Walter Reed Army Medical Center. "A blast has many different physical components: the pressure wave, kinetic energy of the fragments, chemical products, thermal components, and electromagnetic emissions," says Radovitzky, "and the least understood or studied is the effect of the pressure wave on [a soldier's] head."

In the new work, the NRL researchers are collaborating with a team of researchers at Allen-Vanguard Technologies, in Canada. The group placed Marine Corps ballistic helmets on mannequins equipped with pressure sensors and accelerometers, and these modified mannequins were placed at various orientations and distances from controlled explosions. The researchers collected data from more than 40 different blast scenarios and integrated the data into their computer simulation.

The simulation uses a set of well-established flow-modeling algorithms for simulating reacting and compressible flow to create a 3-D simulation of the pressure wave that would be experienced by a real soldier. "These [algorithms] have been used in the past, but we are combining them in a new way to make software for this particular problem," says Mott. The calculations are done in two steps. First, the algorithms are used to model the initial blast to get a realistic blast profile from the explosion. "This includes the chemistry, so we can get the strength of the pressure waves and the velocity field," says Mott. Second, as the wave approaches the mannequin, this information is fed into a compressible flow simulation that produces a more complex 3-D simulation of the head-helmet geometry. This combined approach makes the calculations more realistic and efficient, says Mott.