a ballistic missile carrying a chemical or biological agent is traveling
fast toward its targetmilitary or otherwise. What are the
implications of intercepting or destroying that missile in the upper
Part of the answer to that
question depends on knowing what conditions would allow lethal amounts
of the liquid agent to reach the ground.
For instance, consider the
chemical nerve agent VX, an organophosphorous compound that disrupts
the bodys nervous system. Lethal dosesingested, inhaled,
or absorbed through the skincause rapid death. It is estimated
that a lethal dose is contained in a 2- to 3-millimeter-size drop.
A warhead holding 400 kilograms of VX contains about 62 million
lethal doses. If the warhead were to reach its targetsay,
a port or air baseit would saturate the target and cause an
area denial, that is, make the target site unusable
until cleaned up. But what if it were to be intercepted tens of
kilometers above the ground? What would happen to the VX?
The extreme conditions experienced
by a single liquid drop during its reentry into the atmosphere lie
in a regime for which no experimental data exist. To better understand
the physics of what happens at these altitudes, physicist Glen Nakafuji,
analyst Roxana Greenman, professor Theo Theafanous of the University
of California (UC) at Santa Barbara, and research colleagues are
studying how liquid breaks up and evolves in rarefied (thin) atmospheres.
do so, they are using unique hydrodynamic and shock-physics experiments
coupled with advanced chemicalkinetic and hydrodynamics computer
codes. The experiments and codes simulate the supersonic, rarefied
flow environments that reentering droplets of a chemical agent would
experience. Nakafuji is the principal investigator for the project,
which is funded by the Laboratory Directed Research and Development
of photos showing bag breakup of a liquid drop,
in which the round drop deforms into a shape resembling a bowler
Atmospheres, High Velocities, Surface Tension
A number of complicated
factors determine how a body of liquid breaks up and how the individual
drops or streamers break apart and shape and reshape themselves.
The factors include the pressure of the surrounding atmosphere,
the velocity at which the liquid is traveling, and the physical
properties of the liquid. At altitudes of tens of kilometers,
explains Nakafuji, the agent disperses and expands in an atmospheric
pressure that can be ten thousand times less than that at sea level.
Pieces of liquid float out, stretch, and tear in milliseconds, then
fall in an expanding cloud into the atmosphere. From there,
the mass of drops falls through the air, moving at supersonic velocities
through increasing atmospheric pressure. Originally,
notes Nakafuji, people in the field theorized that the liquid
would aerosolize into droplets on the order of 10 micrometers in
diameter and disperse. Initial experiments indicate that this may
not be true. So the question remains open: Would a given liquid
break up into these small-size droplets or not?
a huge gap in experimental data for the behavior of liquids in this
sort of environment, notes Nakafuji. We know how various
liquids break up at sea level, where the atmosphere is dense, and
the air moleculeswhich can be represented as individual particlesare
constantly bouncing off each other, pressing together, and acting
more like a fluid than individual particles. However, higher
up in the atmosphere, the molecules are fewer and more widely dispersed,
acting more like individual particles at altitudes above 30 kilometers.
You add to this the fact that the liquid agent is not in free
fall but is experiencing atmospheric drag, and the problem becomes
very complex, notes Nakafuji. Yet this is the situation
were faced with in examining the physics of droplet breakup.
Weber Numbers and Bag Breakups
physics of a liquid drop breaking up has much to do with the nature
of the fluid (its density and viscosity, for instance) and the forces
acting upon it. The ratio of external aerodynamic forcewhich
tends to pull the drop apartto the liquids surface tensionwhich
tends to hold the drop togetheris a dimensionless quantity
called the Weber number. Drops with different Weber numbers break
up in different ways. Drops with higher Weber numbers (above 100)
tend to have more catastrophic breakup and result in smaller drops.
At very high altitudes, where external aerodynamic forces are small,
the Weber number remains relatively low, below 100. When the team
conducted experiments on drops with a range of Weber numbers characteristic
of high altitudes, interesting findings emerged. For instance, drops
3 to 4 millimeters in diameter tended to oscillate before breakup.
For drops with Weber numbers between 12 and 100, the experimenters
observed a phenomenon called bag breakup, in which a
round drop deforms into a shape resembling a bowler hat, with a
flat rim and curved crown. As the drop falls, the bag portion, which
corresponds to the crown of the hat, oscillates in and out. When
the original drop disintegrates, large drops form from the rim,
and smaller ones form from the bag. This happens in tens of
millisecondsmuch slower than anyone expected, says Nakafuji.
Previously, it was observed that such bag breakup would occur
in hundreds of microseconds to 1 millisecond, tops.
A diagram of the vertical wind tunnel used to re-create a drop
falling through the upper layers of the atmosphere. (b) The
ALPHA facility is a one-
of-a-kind experimental system to examine liquid fragmentation.
Goes with the Flow
These experiments were conducted
in the ALPHA facility, a one-of-a-kind experimental system designed
and built by the LivermoreUC Santa Barbara collaboration to
examine liquid fragmentation. The facility is essentially a large,
vertical wind tunnel, consisting of a cylinder about 3 meters long
and 10 centimeters in diameter, that can be pumped down to pressures
of 10 to 30,000 pascals. The methodology for re-creating a drop
falling through the upper layers of the atmosphere is as follows.
An injector releases liquid through a laser beam. The drop breaks
the beam, which makes it act like an optical trigger and causes
a diaphragm to burst. Air rushes up the cylinder past the drop,
in effect simulating the fall of the drop through the atmosphere,
and a high-speed camera records the behavior of the drop. We
have the capability to get air moving at velocities of Mach 5about
1.5 kilometers per second, says Nakafuji. The air flows past
the drop at a nearly constant velocity for about 200 milliseconds
before its speed begins to ebb, long enough to watch a drop fall,
reverse direction, rise, and then burst. This past spring, the group
tested a drop 1.5 centimeters in diameterthe largest drop
yet tested anywhere. We dont test actual agents,
Nakafuji emphasized. We use glycerin and other kinds of fluid,
and extrapolate to agents from there.
examining whether assumptions made at sea level about the breakup
of liquid hold true in rarefied environments, the team is also exploring
the different break-up modes and whether the dynamics of these modes
differ from the dynamics seen for bag breakup. The researchers
efforts have been rewarded. They have documented dynamics that have
never before been seen or predicted. For instance, before
the bag breaks, it oscillates at some frequency, explains
Nakafuji. What we saw for the first timeand which no
one had expectedis that after the drop turns and begins to
move upward, the oscillation frequency doubles. We are now trying
to understand this.
with Livermores ALE3D code, which can predict the drag
on rigid spheres in subsonic and supersonic rarefied flows,
validate a surface-tension model, and test a deformable drop
Details, Drop by Drop
Ultimately, the team would
like to understand and be able to predict the dynamics of specific
liquid drops in any rarefied environment. Wed like to
be able to calculate the onset of breakupwhen a drop will
break up, the configuration the liquid will take, which drops are
stable, and which are not, says Nakafuji, adding, Weve
definitely made strides in that direction, to the point where we
can now accurately predict whether a drop will break up under certain
present goal is to obtain critical hydrodynamics and chemical data
to validate computer models of these simulations. Working toward
this end, the researchers have successfully used the Laboratorys
ALE3D code to predict the drag on rigid spheres in subsonic and
supersonic rarefied flows, validate a surface-tension model, and
test a deformable drop simulation.
experiments and simulations, we are pinpointing the ranges of drop
stability and getting a better handle on the physics of liquid breakup,
explains Nakafuji. In the final analysis, we want to be able
to predict the rarefied atmospheric conditions under which a given
chemical agent will break up into lethal-sized stable droplets.
This is a critical question, one whose answer could affect us all.
ALE3D, ALPH facility, biological agent, chemical agent, lethality,
liquid breakup, nerve agent, rarefied atmosphere.
information contact Glen Nakafuji (925) 424-9787 (firstname.lastname@example.org).