Fluid Mechanics and Homeland Security
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AR266-FL38-04 ARI 11 November 2005 16:24
Fluid Mechanics and
Homeland Security
Annu. Rev. Fluid. Mech. 2006.38:87-110. Downloaded from arjournals.annualreviews.org
Gary S. Settles
by UNIVERSITY OF NOTRE DAME on 05/10/06. For personal use only.
Mechanical and Nuclear Engineering Department, Pennsylvania State University,
University Park, Pennsylvania 16802; email: gss2@psu.edu
Annu. Rev. Fluid Mech. Key Words
2006. 38:87–110
counterterrorism, microfluidics, plumes, explosions, ventilation
The Annual Review of
Fluid Mechanics is online at
fluid.annualreviews.org
Abstract
doi: 10.1146/annurev.fluid. Homeland security involves many applications of fluid mechanics and offers many
38.050304.092111 opportunities for research and development. This review explores a wide selection of
Copyright
c 2006 by fluids topics in counterterrorism and suggests future directions. Broad topics range
Annual Reviews. All rights from preparedness and deterrence of impending terrorist attacks to detection, re-
reserved sponse, and recovery. Specific topics include aircraft hardening, blast mitigation,
0066-4189/06/0115- sensors and sampling, explosive detection, microfluidics and labs-on-a-chip, chem-
0087$20.00 ical plume dispersal in urban settings, and building ventilation. Also discussed are
vapor plumes and standoff detection, nonlethal weapons, airborne disease spread,
personal protective equipment, and decontamination. Involvement in these applica-
tions requires fluid dynamicists to think across the traditional boundaries of the field
and to work with related disciplines, especially chemistry, biology, aerosol science,
and atmospheric science.
87
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INTRODUCTION
Homeland security, or counterterrorism, offers many opportunities in fluid mechanics
at a time when interest remains high but some traditional fluids applications are
waning. Homeland security combines established fluids topics like plume dispersion
with others that are new, such as microfluidics. This “new” field encourages us to work
with chemists and biologists, rewards us for innovation, and engages our theoretical,
experimental, and computational talents.
Homeland security is a new name for the ancient need to protect our families
and cities from incursion by barbarians. Fluid mechanics has played a role in this
since prehistoric times by limiting access (e.g., the castle moat) and by providing
defensive weapons (via ballistics). The rise of terrorism poses new challenges for fluid
Annu. Rev. Fluid. Mech. 2006.38:87-110. Downloaded from arjournals.annualreviews.org
dynamicists, who can now serve the public good in some ways more direct than those
offered by the earlier cold-war and space projects.
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There is a current notion, elaborated by the U.S. National Research Council (NRC
2002), that the asymmetrical threat of terrorism can be met by advanced technology.
Sensors forever on guard for the environmentally unusual, for example, can maintain
vigilance. That view is adopted here within limits, but without the expectation for
technology to compensate for everything up to and including global politics gone
awry.
Although at first glance homeland security technology may seem to be all comput-
ers and lasers, in fact there is fluid mechanics in it almost everywhere. Moreover, this is
largely the fluid mechanics that we already know. For example, bioterrorism is a form
of disease spreading by aerosol, and plume dispersion from chemical, biological, and
radiological (CBR) weapons is similar to that from environmental pollution sources.
Scope and Goals
The subject matter is vast and the length of this review is strictly constrained. There-
fore, a mere outline of selected fluids topics within homeland security must suffice,
including limited citations chosen for accessibility and to provide more depth to the
reader. Note especially several cited NRC reports that form the backbone of current
scientific thinking on homeland security topics.
The choice of coverage is admittedly subjective. For example, oceanographic issues
in homeland security must await a different venue and a qualified author. Likewise,
meteorology is treated only to the extent of plume dispersion, and the effects of
nuclear weapons are well covered elsewhere (Glasstone & Dolan 1977). What remains
is an eclectic group of topics that generally proceed from preparedness and deterrence
of impending terrorist attacks to detection, response, and recovery. Throughout,
special attention is given to opportunities for fluids research and development.
PREPAREDNESS AND DETERRENCE
Terrorism favors certain targets over others in order to best further its political goals.
These targets are well known and, within limits, can be made resistant to attack. So
doing forces the terrorist to consider falling back on less attractive alternative targets.
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Aircraft Hardening
Commercial aviation is a traditional terrorist target. Thus, measures are needed to
harden aircraft against catastrophic in-flight failure due to the detonation of concealed
explosives. The Pan Am 103 tragedy and other incidents prove that modest amounts
of explosives are capable of destroying an aircraft by holing and splitting its fuselage,
leading to structural failure and aerodynamic breakup. Still, not all terrorist blasts have
succeeded in bringing down the targeted aircraft. Despite several costly full-scale blast
experiments, the gas-dynamic phenomenology of these onboard explosions remains
poorly understood.
Settles et al. (2003) gives a detailed review of this topic. Briefly, interior explo-
sions in aircraft (and also in buildings) are complicated by shock wave reverberation
Annu. Rev. Fluid. Mech. 2006.38:87-110. Downloaded from arjournals.annualreviews.org
and overpressure amplification when shock waves reflect from surfaces. The relative
importance of shock overpressure, shock attenuation by luggage, fragment puncture,
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quasistatic overpressure, and the fuselage service pressure difference is not known.
Edwards & Owen (1995) conducted small-scale blast experiments that revealed the
explosive tearing of petalled holes in aircraft aluminum skin of the sort also discovered
in the wreckage of Pan Am 103 (U.K. Dep. Transp. 1989).
Settles et al. (2003) simulated the Pan Am 103 explosion and hull holing in a
3/5-scale model of a luggage container and lower widebody aircraft fuselage cross-
section, as illustrated in Figure 1. Forensic evidence from Pan Am 103 showed that
the terrorist bomb was located in a hard-shell suitcase near the outer wall of a luggage
container and within half a meter of the aircraft skin. The accident report conjectured
shock wave motion and reverberation inside the fuselage as well, but this had never
been observed or studied. Thus, the purpose of the simulation was to provide such
observations, and to illustrate the type of optical results possible in full-scale aircraft
tests. Figure 1 shows the fireball and shock wave motion about 2 ms after blast
initiation, including shock reflections inside the partially filled luggage container.
Blast Mitigation for Buildings
Another favorite terrorist tactic is the truck-bomb attack on an important and vulner-
able building (Murrah Federal Building, Khobar Towers, U.S. Embassy attacks, first
World Trade Center attack, etc.). There are now several guides available on building
design to deter and mitigate such explosions, e.g., FEMA (2003) and NRC (1995).
The anticipated bomb mass in TNT equivalent and the standoff distance define
the threat to a building. Classical air-blast similarity theory (e.g., Glasstone & Dolan
1977) reveals that a given shock wave overpressure occurs at a distance proportional to
the cube root of the explosion energy. Where possible, keeping vehicles at a distance
is an important design strategy. Also, building design shapes that trap the shock wave
amplify the damage, whereas rounded shapes are better at reflecting the shock away.
Casualties in these attacks result, in large part, from wounds caused by debris
fragments hurled at supersonic speed by the expanding gas following the shock wave.
Glass shards and fragmented building cladding become a hail of deadly bullets. Un-
derstanding this phenomenon promotes safer building design, especially including
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Figure 1
Schlieren image of a scale-model simulation of the terrorist blast in a luggage container that
destroyed Pan Am flight 103. A cross-sectional view of the left forward hold of a Boeing 747
aircraft is simulated. The fireball, jagged hull hole, and shock wave motion about 2 ms after
blast initiation are shown in this 5-µs exposure. From Settles et al. (2003); photo by author.
laminated safety glass windows. Also, buildings must be more massive at their bases
to prevent the type of blast-induced structural failure and collapse that occurred in
the Murrah Federal Building attack.
Blast-effects codes such as CONWEP, BLASTX, AIRBLAST, and EBLAST are
available to aid the design process (NRC 1995). These predict not only shock over-
pressure, but also fragmentation and even estimated injuries. These are approxima-
tions, though, not full 3D computational fluid dynamics (CFD) solutions. An example
of the latter is Löhner et al. (2004), which includes fluid-structure interactions as well.
Research Opportunities in Blast Mitigation
Better computer simulations and more experimental input are needed in blast miti-
gation, aircraft hardening, and blast containment. There is also a special opportunity
here for cheap, safe, quick simulations of blast effects using scale models and optical
shock wave imaging. Such scaled experiments compare well with costly, dangerous,
time-consuming full-scale tests (Edwards & Owen 1995, Reichenbach 1992, Smith
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et al. 1992). High-speed digital video cameras are now available to record shock
position versus time by schlieren or shadowgraphy (Settles 2001), from which all
postshock fluid properties can be determined. The TNT equivalent of gram-sized
explosive charges is well established (Kleine et al. 2003), making them available for
use with scale models to simulate shock diffraction and overpressures about planned
buildings. Upon being dusted off, university-scale shock tubes can also see gainful
employment in studies of building and vehicle materials fragmentation under shock
loading. One optical method of shock wave imaging, retroreflective shadowgraphy,
is even robust enough to leave the lab and venture outdoors (Settles et al. 2005).
More fundamental study of interior blasts is also needed. At least in the open
literature there are piecemeal investigations but no general synthesis of the problem.
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Applied research on aircraft explosions is needed to inform the design of future
blast-resistant aircraft. Faced with increased aviation security, some terrorists have
turned their attention from aircraft to high-density ground transportation (buses
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and trains). Blast resistance, antifragmentation, shock wave absorption, and other
potential counterterrorism design features in these vehicles are not as weight sensitive
as they are in aircraft design. The vehicle ventilation systems could also be used to
advantage for explosives detection.
DETECTION
Because the terrorist chooses the time and place of an attack, a general elevated level
of vigilance is needed if these attacks are to be anticipated and thwarted. This has
prompted a flowering of all sorts of detection technology, wherein fluid mechanics is
seldom the star of the show but is often a key supporting player.
Sampling
Many studies cry out for better sampling of the environment to monitor airborne
threats, whether they are from chemical vapors, particulates, or bioaerosols. Some
current CBR detectors for field use have only rudimentary air samplers or none at
all. Bioaerosol samplers (Griffiths & Decosemo 1994, Macher & Burge 2001) must
collect live undamaged airborne bacteria and viruses in order for the subsequent iden-
tification step to succeed. Airborne particle sampling is highly developed following
years of monitoring environmental pollution (Chow 1995, Hering 2001, Lippmann
2001, Marple et al. 2001), although new impactors, cyclones, and filters are still being
devised.
However, sniffing for chemical traces—as in canine detection (Syrotuck 1972)—is
considerably less developed. There is much to learn from nature (Settles 2005): For
example, the slit nostril of the dog is both an inlet and a variable-geometry outlet
flow diverter. The olfactory apparatus of fish is especially interesting in its simplicity,
not having to share a flow path with respiration. Sampling by flight vehicles may be
a current topic in homeland security, but of course birds pioneered it. Seabirds with
Pitot-tube-like nostrils can follow the trace odor of a food source for many kilometers
over the ocean, providing an example for micro-air-vehicles such as those described
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by Mueller & DeLaurier (2003). Fluid dynamicists have several opportunities here
in the development of new bio-inspired sampling systems.
Explosive Trace Detection
Given terrorists’ fondness of improvised explosive devices (homemade bombs), the
detection of abnormal traces of nitrates in the environment now has a high homeland
security priority (Fainberg 1992, Steinfeld & Wormhoudt 1998). Dogs are the tradi-
tional gold standard for detection (Furton & Myers 2001, Syrotuck 1972), but recent
incursions have been made by specialized detectors and electronic noses (reviewed by
Settles 2005). Volatile explosives like nitroglycerin and triacetone triperoxide (TATP)
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are directly detectable by their vapors, whereas nonvolatiles like RDX (the active com-
ponent of C-4 plastic explosive) must be detected by way of trace particles that happen
to have an affinity for surfaces.
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“People portals” (Hallowell 2001) combine a fast, sensitive detector—usually an
ion mobility spectrometer—with an aerodynamic means of sampling vapors and parti-
cles (usually air-jet impingement followed by collection of the resulting trace-bearing
air volume and preconcentration of the trace signal). Two such portals are currently
commercial products fielded by Smiths Detection and General Electric (Linker et al.
1999, Settles 2000). A few seconds are required to screen each person, the screening
is essentially nonintrusive, and false alarms are rare. This technology has its primary
application in passenger screening for aviation security. It is, however, still in its
infancy.
The aerodynamics of the human body plays a key role in such trace sampling of
passengers. In still air, a buoyant flow of some tens of liters per second (L/s) rises
from the body: the “human thermal plume” (e.g., B.A. Craven & G.S. Settles, paper
in preparation). At a walking speed beyond about 0.2 m/s the plume gives way to the
“human aerodynamic wake” (Edge et al. 2005), as illustrated in Figure 2. Chemical
traces originating as passive scalars on the body are found in these flows and can be
sampled. Patterns of secreted proteins in the human plume or wake can be indicators
of the early stages of a CBR attack (NRC 2002). It may even be possible someday to
test mitochondrial DNA from skin flakes in the human plume as a biometric means
of access control (NRC 2003c).
Cargo screening for trace explosives is complicated by the impracticability of
manually opening and unloading millions of containers for inspection. Instead, a
way to sample a container interior through its air vents is badly needed. Previous
approaches are reviewed by Settles (2005), but here is another opportunity for fluid-
dynamic innovation.
Sensors
A renaissance in sensor technology is currently underway. Real-time change detec-
tion and intrusion detection are central to the homeland security mission of prevent-
ing a terrorist attack. Special emphasis is placed on widely distributed cheap-sensor
networks (NRC 2003c), and biosensors for pathogens like anthrax, smallpox, and
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Figure 2
The instantaneous trace contaminant concentration in the wake of a walking person, from
Edge et al. (2005). Frames a and c are RANS solutions using a blended k-ω/k-ε 2-equation
turbulence model and a simplified representation of the human body. Frames b and d are
drawn from flow visualization experiments of an actual walking person. A side view is depicted
in frames a and b; frames c and d show the top view. The walking speed is 1.34 m/s and the
Reynolds number, based on a body width of 0.58 m, is 53,000.
plague, as well as for microbial contamination of food and water (Hobson et al.
1996). Such environmental biosensing is complicated by the presence of a large back-
ground bioaerosol level (Jones & Harrison 2004). “Motes” (Culler & Mulder 2004)
are tiny computers linked in networks and fitted with sensors of all sorts. In addition
to trace explosive, chemical, and biological sensing, such sensors might also perform
anemometry and monitor the ambient pressure for signs of shock waves. At the other
end of the size spectrum is the truck-scale military Joint Biological Point Detection
System (JBPDS). Once again, according to NRC (2002), understanding the olfactory
acuity of certain animals can provide an important input to new sensor design.
Microfluidics and Lab-on-a-Chip
Fluidics had an abortive first life in the 1970s, but now it appears that microflu-
idics is permanent, having already spawned two archival journals. It embodies a new
paradigm in analytical chemistry and biochemistry: Ponderous lab-bench instruments
are giving way to cheap, rapid microlabs-on-a-chip, also known as µTASs (micro
total analysis systems). These run faster than traditional lab processes and require
less reagent because of their smaller size. Applications include cell cultures, DNA
testing, drug development, biomaterials analysis, and a variety of physicochemical
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measurements (Beebe & Folch 2005, Schulte et al. 2002, Sharp et al. 2002,
Srinivasan et al. 2004).
Actually, microfluidics is much broader than just homeland security, but its porta-
bility and rapidity are the keys to homeland security applications. These include
field-deployable rapid immunoassay, field DNA analysis, and detecting and identify-
ing chemical, biological, and environmental threat molecules on the battlefield or in
an urban terrorist attack. None of these counterterrorism applications allow one to
sit around for days awaiting a traditional lab analysis.
The fluid mechanics of microlabs is as fascinating as it is unusual (Stone et al.
2004). Even though Stokes flow reigns and turbulence is left far behind, multiple
simultaneous effects preclude any simple back-of-the-envelope analysis. The no-slip
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boundary condition usually still holds, but thermal convection is damped by viscosity
in flows where the wall dominates and there is no freestream. Fluid is electrokinetically
moved through on-chip microchannels without pressure drop. Because distances
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are small, diffusion can separate molecules and small particles according to their
various diffusion coefficients (Weigl & Yager 1999). Laser-induced fluorescence and
µm-range particle image velocimetry (PIV) (Devasenathipathy et al. 2003) quantify
chemical species and make velocity measurements.
Stone et al. (2004) list a wealth of opportunities for future work, including mi-
croflow visualization (Sinton 2004), flow path optimization, 3D microflows, viscoelas-
tic microflows, and the eventual scale-down to the nanoflow regime. Drops are often
the units of micromass flux, and their formation, coalescence, translation, internal
mixing, and breakup need better understanding, as do contact angles, wetting, and
surfactant effects at the channel walls. Finally, there are design issues associated with
jets, sprays, microvalves, pumps (Laser & Santiago 2004), and processes. Stone et al.
(2004) expect these topics to engage fluid dynamicists for generations to come.
Some have already made the jump successfully, but this alternate universe of Re ∼ 1
is not for everyone. Still, for those willing to change, it is a young field with plenty of
opportunity: “Lego-block fluid mechanics,” it might be called, or perhaps an attempt
to realize the sci-fi vision of a Star-Trek Tricorder.
Vapor Plumes and Standoff Detectors
Turbulent plumes figure prominently in homeland security. Background on plume
motion is given in chapter 6 of Turner (1973) and List (1982a,b). First, however, we
consider the relatively small-scale vapor-trace-bearing plumes that might arise from
vents in cargo containers, open car windows, building ventilators, truck trailers, and
even individual suicide bombers. These plumes are generally turbulent and are almost
always buoyancy- or momentum-driven, the trace signal of interest being merely a
passive scalar (Warhaft 2000).
For example, take the most volatile terrorist explosive, TATP, which has a vapor
pressure of about 7 Pa at saturation. Even at this comparatively high concentration
the explosive cannot play a significant role in the plume dynamics, which is governed
instead by the plume momentum, the ambient wind speed, and the temperature dif-
ference. The chemical trace signature of the plume from the ammonium nitrate-fuel
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oil (ANFO) truck bomb that destroyed the Murrah Federal Building in Oklahoma
City was still weaker than this by several orders of magnitude. Therefore, we must dis-
tinguish these vapor-trace-bearing plumes from the dense-gas plumes (Britter 1989)
that might result from a terrorist attack on a chemical plant, for example, in which
case the chemical plays a significant role in the plume dynamics.
The structure and behavior of turbulent trace-bearing plumes was studied as part
of a DARPA project on chemical plume tracing in nature, e.g., Atema (1996), Murlis
et al. (2000), Webster & Weissburg (2001). More recently, a NRC report (NRC
2004), which, despite its standoffish title, is replete with mostly correct fluid me-
chanics, sparked interest in small-scale explosive-vapor-trace-bearing plumes. In it,
understanding the dynamic behavior of an explosive vapor plume is expected to as-
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sist the success of standoff optical spectroscopy detectors, particularly in the case
of detecting volatile explosives like nitroglycerin, EGDN, and TATP. The detector
standoff distance is 10 m or more, so this process is distinct from the point-detection
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sniffing discussed earlier.
Standoff detectors are usually outside the danger zone of a terrorist bomb and they
can survey a wide area. The role of fluid mechanics is to point the way for them. The
plume shape and position, time-dependent behavior, particle content, response to
ambient breeze, and adjacent-surface effects are all important to detector guidance.
These standoff detectors are optical. They include infrared thermography, UV
absorption and fluorescence, Coherent Anti-Stokes Raman Spectroscopy (CARS),
and Light Detection and Ranging (LIDAR), which produces actual images of threat
plumes (NRC 2002, 2004). Particles on the surfaces of plume-generating objects
can also be interrogated. In addition to explosive-vapor plumes, the aerosols as-
sociated with CBR agents are likewise potentially detectable by optical standoff
sensors.
As an example of the role of fluid mechanics in this process, consider Figure 3.
A uniform wind at 0.45 m/s approaches a generic “cargo container” sitting on a
ground plane. The color of the vectors represents the local airspeed on the ground
plane and on the outside of the container, ranging from 0.45 m.s (red) to 0.02 m.s
(blue). A chemical vapor plume is emitted from a vent centered on the facing side
of the container. This plume (shown in solid blue) initially travels forward to the
leading edge before separating (not shown) and eventually becoming entrained in the
container’s wake.
Figure 3 is for illustration purposes only and typifies the 3D separated flows
about cubical protuberances described by Castro & Robins (1977), Higson et al.
(1994), Hunt et al. (1978), and other studies. The computation of this class of flows is
nontrivial (Lakehal & Rodi 1997, Rodi et al. 1997). More-sophisticated simulations
are possible using large-eddy simulation (LES), e.g., Rodi et al. (1997) and Moin
(2002). Nevertheless, the point is that standoff detectors of chemical plumes need
input from fluid dynamicists in order to know where to point the detector, and why.
Thus, there is a research opportunity here for fluid dynamicists to study and
eventually develop a taxonomy of explosive vapor plume scenarios, e.g., the plume
exiting a vent on a cargo container sitting in the sun, or tracer behavior in the wake of
a moving vehicle. Simplified models of these scenarios can be tested in wind tunnels to
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Figure 3
RANS computation of the flow over a generic “cargo container” sitting on a ground plane
with a uniform wind of 0.45 m/s from the left. A warm (T = 20 K) chemical vapor plume is
emitted from a vent centered on the facing side of the container. The Reynolds number based
on container height is 11,520. The incoming boundary layer on the ground plane is 0.19 h in
thickness. FLUENT simulation using a k-ω turbulence model by W. J. Smith, Jr.,
Pennsylvania State University Gas Dynamics Laboratory.
explore the flow physics, establish a database, and validate computational modeling.
Of course, full use should be made of what we already know about such flow types,
including comparable environmental pollution scenarios. The payoff of this effort to
homeland security is a general understanding of small-scale vapor plume behavior
and guidance for developers and users of standoff plume detectors.
Finally, NRC (2004) recommends imaging explosive-vapor plumes, perhaps by
UV absorption or the schlieren technique, as a way of studying and understanding
them. Although this is possible and useful in the laboratory, it is unlikely that the vapor
concentration of most vapor plumes in the field is independently detectable this way.
Schlieren optics images refractive patterns in the air (Settles 2001), but these predom-
inantly arise from the abundant temperature differences that occur outdoors. Leaks
of heavy or light gases can be distinguished from the background fluctuations, but
explosive vapors usually cannot (except possibly for the highest volatiles like TATP).
Instead, we should concentrate on understanding the thermal plumes that are the
likely carriers of the trace vapors of interest. UV, schlieren, smoke flow visualization,
LIDAR, etc. are complementary ways of imaging plumes in an overall approach to
build a better knowledge base on this topic.
96 SettlesAR266-FL38-04 ARI 11 November 2005 16:24
RESPONSE
Despite the best preparedness, deterrence, and eternal vigilance, terrorism will oc-
casionally succeed in attacking homeland targets. When this happens, effectively
equipped first responders and hospitals must be ready to minimize casualties and to
evacuate the population where necessary.
Nonlethal Weapons
Urban conflict is expected when confronting terrorism, and a range of nonlethal hu-
manitarian combat alternatives is needed to minimize civilian casualties. One example
of how fluid mechanics plays a role is Sandia’s sticky thermoplastic foam gun (Valenti
1994), designed to incapacitate a combatant at up to 10-m distance without harm.
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Similar effects are obtained from high-pressure water guns that not only project mo-
mentum, but also possibly pepper spray. A vortex-ring gun, still under development,
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aims to upset a combatant by the impact of a formidable combustion-driven ring
vortex, similar to but stronger than that produced by the commercially successful
AirZooka toy. Another nonlethal weapon focuses disturbing low-frequency acoustic
energy up to 1 km away (SARA Inc.). “Flashbangs” are essentially grenades without
the lethal fragmentation, producing a temporarily blinding flash, a deafening blast,
and even a nauseating smell. The multispectral smoke bomb creates an aerosol cloud
that is opaque to the eye but transparent to special vision systems. Fluid dynamicists
have not been very involved in nonlethal weapons developments to date, but there is
still plenty of opportunity for bright ideas.
CBR Plumes and Atmospheric Dispersion
Short of a nuclear blast, perhaps the most fearsome prospect of terrorist attacks is
the large-scale airborne release of a CBR agent in a city, causing massive casualties
and overwhelming emergency facilities. The CBR plume would likely be invisible,
odorless, and silent. Dense plumes from chemical-plant sabotage are also included in
this scenario, although the plume dynamics is somewhat different.
Of many aerosolizable CBR agents available to terrorists, the most dangerous
are (a) Bacillus Anthracis (anthrax) in the respirable 1–10-µm size range, because of
its robustness and extreme lethality; (b) smallpox virus, less lethal than anthrax but
exceptionally contagious; and (c) radioactive fallout from a “dirty bomb.” In each case
there have been recent precedents, mostly nonterrorist related, from which painful
lessons were learned.
Precedents. Weaponized anthrax was accidentally released from a military labora-
tory in Sverdlovsk, U.S.S.R., in 1979 (Dixon et al. 1999, Meselson et al. 1994). The
plume spread into the countryside under the prevailing wind. Sixty-six people died
in the city and many cattle succumbed to anthrax further downwind. More recently,
anthrax in the U.S. postal system in 2001 caused relatively few casualties but greatly
heightened the public awareness of this terrorist threat (Cole 2003, Fennelly et al.
2004).
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In Yugoslavia in 1972, a natural smallpox outbreak, stemming from a single index
case, infected 175 people of whom 35 died, even though 60% of those infected had
been previously vaccinated (Henderson 1999). The infection is transmitted by aerosol.
The severe acute respiratory syndrome (SARS) viral outbreak in Hong Kong in
2003 was apparently spread through the air shaft of an apartment building (Yu et al.
2004). A total of 187 people were infected.
The Chernobyl nuclear accident in 1986 spread a radiological plume across several
countries, resulting in an estimated 32,000 deaths within 10 years (Shcherbak 1996).
Atmospheric dispersion modeling done at the time was inaccurate.
In Bhopal, India, in 1984, 40 tons of methyl isocyanate gas leaked from the Union
Carbide pesticide plant and spread a dense-gas plume across the sleeping city. Some
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16,000 people died soon thereafter, and perhaps 520,000 were exposed to the poison
(List 1996, Sharan & Gopalakrishnan 1997).
The Kuwait oil-well-fire plumes in 1991 merged into a single huge plume that
by UNIVERSITY OF NOTRE DAME on 05/10/06. For personal use only.
grew faster than predicted by available dispersion models (Cooper 2005).
Collectively, these precedents emphasize the key role of atmospheric dispersion.
They also show that an accurate predictive capability is crucial, even beyond homeland
security considerations.
CBR agent dispersal. A CBR agent intended to harm the population must be
broadly distributed, primarily through atmospheric dispersion as a gas, an aerosolized
liquid, or a powder of solid particles. Chemical warfare agents, for example, are not
difficult to manufacture in liquid form, but dispersal requires a ground-based aerosol
generator such as a pesticide sprayer, or perhaps an airborne crop duster.
So, although spray-atomizing liquids is an enabling fluid-dynamic step for the ter-
rorist, it is also a critical step that cannot be done haphazardly. For example, depend-
ing on the pressure, atomizing a fluid through a spray orifice can create longitudinal
strains sufficient to damage bacteria and viruses in the fluid.
Other means of aerosolization available to the terrorist include dry mixer-nozzle
dispensers, ultrasonic atomizers, and fluidized beds, to name just a few. These methods
are described in the literature, e.g., Fuchs (1989), Horvath (2000). Bioaerosols are
covered by Henderson (1999) and Griffiths & Decosemo (1994). A combustion-
generated aerosol (Lighty et al. 2000) was the dispersal mechanism at Chernobyl.
Homeland security concerns are likely to generate further aerosol science research,
including means of agglomeration and removal (NRC 2003a).
There is also the issue of aerosol redispersion. Pathogenic particles that have
settled out may become airborne once again due to the wind ( Jones & Harrison
2004), the wake of an automobile, and even the wake of a walking person (Edge
et al. 2005; Figure 2). Recent studies have improved the understanding of particle
redispersion from surfaces (Phares et al. 2000, Smedley et al. 1999), but more work
is needed.
“Dirty bomb” is a compelling popular term for a radiological dispersal weapon.
Its key threat lies in the release of respirable particles (NRC 2002). Fallout can be
washed from the clothing and skin, but if you inhale it, you own it. Available lethal
low-grade radiological materials with long half-lives include cesium-137, cobalt-60,
98 SettlesAR266-FL38-04 ARI 11 November 2005 16:24
americium, plutonium, and the various components of spent reactor fuel. The con-
ventional explosive in a dirty bomb serves mainly to aerosolize these materials. Public
intimidation and the rendering of symbolic targets unapproachable forever are likely
concomitant effects of a dirty-bomb attack.
Atmospheric plume dispersion modeling. Plume dispersion modeling is central
to homeland security. Expert opinion on this topic is summarized in NRC (2003d),
which all interested readers should consult. Some micrometeorological background,
e.g., Arya (2001), is also helpful.
Briefly, successfully predicting and dealing with CBR plumes requires (a) a dis-
persion model to yield the plume path and spread rate, (b) local topography and
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meteorology data, and (c) communication between the atmospheric science and emer-
gency response communities. Emergency responders need a quick, simple, hands-on
prediction capability for plume direction, coverage, and lethality. At the other end
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of the spectrum, supercomputer solutions of the Navier-Stokes equations with LES
turbulence modeling have magnificent resolution (e.g., Boris 2002) but are presently
considered too slow to serve the emergency responder’s needs. In between lies a be-
wildering plethora of dispersion models, some Lagrangian, some Eulerian, and many
based on a simple time-averaged Gaussian plume assumption (see NRC 2003d and
Lee et al. 1997).
These models can reveal some fascinating fluid dynamics. In modeling plume
dispersion in Washington D.C., Boris (2002) resolved the Kármán vortex shed-
ding from the Washington monument. Figure 4 shows a simulation of downtown
Portland, Oregon with a west wind and contaminant dispersal at the small yellow
“+.” Unexpected initial northerly plume propagation illustrates the “urban street
canyon” effect. Without the buildings the plume grows linearly with distance east of
the source, as expected from traditional plume theory. This and other examples of
the peculiarities of urban street vortices and building effects are reviewed by Brown
(2004).
Gaussian models average out all turbulent motion. As illustrated in Figure 5, this
can yield an unrealistic picture of the safe lateral boundaries of a CBR plume. In fact,
peak instantaneous concentrations in an atmospheric dispersion plume may exceed
the local average values by 6:1.
Model validation comes from two sources: field measurements, e.g., Allwine et al.
(2002), and meteorological wind tunnel experiments, e.g., Cermak (1975, 1976),
Meroney & Melbourne (1992). Many earlier wind tunnel, CFD, and field studies
of environmental pollution plume dispersion are equally applicable to CBR-weapon
plumes, e.g., Castro & Robins (1977), Fedorovich (2004), Robins (2003).
NRC (2003d) suggests the further study of urban surface effects on local mete-
orology, more operational urban model development, the inclusion of error bars in
dispersion forecasts, and the assimilation of meteorological and sensor data into the
models. Additional well-designed urban field and wind-tunnel experiments are called
for to test and improve the models.
Recent advances in LES also hold the promise of supplanting the simple dispersion
models. Nondissipative numerical methods for mid-scale eddies, coupled with vast
www.annualreviews.org • Homeland Security 99AR266-FL38-04 ARI 11 November 2005 16:24
Figure 4
Simulation of atmospheric
dispersion of a contaminant
released in downtown
Portland, Oregon. The
wind is from the west and
the contaminant is released
at the small yellow “+.” The
UDM dispersion model,
developed by the U.K.
Defence Science and
Technology Laboratory, was
used in this simulation by
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the Los Alamos National
Laboratory, courtesy
G.E. Streit.
by UNIVERSITY OF NOTRE DAME on 05/10/06. For personal use only.
gains in computing power, now allow the LES simulation of complex turbulent flows
beyond what was once thought possible (Moin 2002). Although this is supercomputer,
not laptop, CFD, reduced-order models based on these high-fidelity computations
could be the next step in field prediction of urban atmospheric dispersion (P. Moin,
personal communication).
Building Interior Airflows
Although terrorists have shown their predilection to destroy prominent buildings,
it may also serve their purposes to harm the occupants by dispersing CBR agents
into building heating, ventilation, and air-conditioning (HVAC) systems. Very few
systems were originally designed with this threat in mind, but ASHRAE, the society
of HVAC engineers, is taking the problem seriously (ASHRAE 2003, Henderson
2004, Persily 2004).
HVAC engineering has traditionally been outside the realm of basic and applied
fluid mechanics, with only occasional crossovers, notably by Linden (e.g., Lin &
Linden 2002, Linden 1999). Fluid dynamicists should not shy away from building
ventilation, though, for it is merely the airflow through an intricate 3D enclosure
100 SettlesAR266-FL38-04 ARI 11 November 2005 16:24
Annu. Rev. Fluid. Mech. 2006.38:87-110. Downloaded from arjournals.annualreviews.org
by UNIVERSITY OF NOTRE DAME on 05/10/06. For personal use only.
Figure 5
Flow visualization images of the (a) instantaneous and (b) time-averaged turbulent plume
downstream of a continuous source, courtesy of the U.S. EPA/NOAA Fluid Modeling
Facility.
with pressure drops, temperature differences, plumes, jets, separated flows, ducts,
and outlets just about everywhere. The situation is further complicated by the many
different classes of buildings that have never been subjected to a proper taxonomic
study (NRC 2003b): office buildings, apartment buildings, airports, train and subway
stations, schools, stadiums, theaters, and manufacturing facilities, to name just a few.
In short, coming up with properly defined “basic” fluid-dynamic problems for future
study in building ventilation is a challenge.
However, at least some essential principles of new-building HVAC design have
arisen in the era of homeland security (ASHRAE 2003, NIOSH 2002). Briefly, one
should filter the intake air [High-Efficiency Particle Air (HEPA) filters trap biological
and radiological particles but not chemical agents], avoid the intake of outdoor threat
agents, and isolate indoor contamination when it occurs. Infiltration of outdoor air is
bad because it is a potential contamination source. Ventilation-isolated “safe areas”
can be designed into new buildings, and distributed CBR sensors can be used to
trigger the isolation of affected areas (NRC 2002, Persily 2004).
Containment and response to an attack requires controllable HVAC to halt con-
taminant spread and a means to capture or neutralize CBR agents in the building.
The HVAC system may need to be shut down, whereupon occupants must either
evacuate the building or shelter-in-place.
The prediction of building HVAC flows is currently done with rapid multi-
zone airflow and transport models, especially NIST CONTAM and LBNL COMIS
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(Haghighat & Megri 1996). Once again, supercomputer CFD is currently regarded
as too slow, but LES methods hold promise for the future. Such CFD solutions,
when they are done, are challenged to provide both a realistic grid for the building
geometry and also enough nodes for realism in the flow. Reshetin & Regens (2003)
present a different modeling approach, well-founded in fluid dynamics, that predicts
the rapid spread of anthrax spores throughout a building.
Meaningful data are badly needed for code validation. For example, fog and tracer-
gas measurements were made to map the airflow in the new San Francisco airport
terminal as part of the Sandia PROACT program.
Laboratory experiments in building ventilation favor water models, e.g., Lin &
Linden (2002) and Thatcher et al. (2004). Dynamic similarity is achievable at a typ-
Annu. Rev. Fluid. Mech. 2006.38:87-110. Downloaded from arjournals.annualreviews.org
ical scale of 1/6, and the water flow is easy to visualize (Settles 1989). These experi-
ments can yield understanding of the flow phenomena and data for code validation
(Finlayson et al. 2004). More of such experiments are needed to better understand the
by UNIVERSITY OF NOTRE DAME on 05/10/06. For personal use only.
spread of contaminants through a building, to guide contamination-control design
and sensor placement, and even to inform decontamination strategies.
Finally, mass transit facilities deserve special attention due to their large open
spaces and their concentration of people (NRC 2002). A CBR release in a subway
system can be transported underground by the piston effect of train movement. At
least one CFD simulation has predicted the threat cloud motion in a generic subway
station: Camelli & Löhner (2004). Distributed CBR sensors are needed, and are
already in place, for example, in the Washington Metro System.
Airborne Disease Spread
The fluid dynamics of airborne contagion is closely related to the building venti-
lation issues just discussed. This is not a new topic, e.g., Sattar & Ijaz (1987), but
recent ominous natural viral outbreaks, such as SARS, and the potential for terrorist
introduction of smallpox give it new emphasis.
The traditional Wells-Riley model of airborne infection (Nardell et al. 1991, Nicas
& Hubbard 2003, Fennelly & Nardell 1998) paints a frightening picture of smallpox.
Given the virulence and long lifetime of the virus, even a single virus per m3 of air
is enough to present a serious risk of airborne infection. Ventilation lowers the risk
somewhat by diluting the virus concentration, while a respirator lowers it dramatically.
Still, an infected patient is a massive source of airborne virus-containing parti-
cles, as first shown by Jennison (1942) (see also Papineni & Rosenthal 1996) and
illustrated here in Figure 6. Breathing and talking project some airborne parti-
cles, but a cough and especially a sneeze turn the human mouth and nose into
atomizers.
Fluid mechanics can contribute to this important problem in several ways. The
velocity fields and particle concentrations of breathing, speech, cough, and sneeze
can be quantified using modern optical methods such as PIV. Airflow interactions
and cross-infection between adjacent individuals can be quantified (Bjørn & Nielsen
1996). Both model and full-scale airflow studies can be done to sort out the safest
ventilation scheme for a hospital room, for example. The hospital is central to
102 SettlesAR266-FL38-04 ARI 11 November 2005 16:24
Figure 6
High-speed flash
illumination of a human
sneeze, revealing tens of
thousands of aerosol
particles, courtesy of
A. Davidhazy, Rochester
Institute of Technology.
Annu. Rev. Fluid. Mech. 2006.38:87-110. Downloaded from arjournals.annualreviews.org
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response and recovery from a biological attack. According to Franz & Zaitchuk
(2002), “A sound public health infrastructure, which includes all of us and our re-
sources, will serve this nation well for the control of the disease, no matter what the
cause.”
RECOVERY
The final step in counterterrorism is to clean up after an attack and move on. The rel-
evant fluids issues of personal protective equipment and decontamination are briefly
discussed here.
Personal Protective Equipment
Current gas masks are WWI vintage. They need HEPA filters, for example, to stop
bacteria and viruses (Hawley & Eitzen 2001). Better still, hoods with self-contained
breathing apparatus provide a factor of 10 improvement in protection over masks and
avoid fit problems, though they have other drawbacks. A lightweight, comfortable
design is needed, compatible with breathing and body convection patterns.
Current protective suits are bulky and heavy, and are prone to cause heat stress
(NRC 1999). The Level A suit can only be worn for less than an hour, and wearer
performance is degraded (Goldman 2005). Comfortable CBR protective garments
do not presently exist.
A simpler, more efficient systems solution to these problems will require input
from fluid dynamicists and physiologists. There is a long-term need for effective,
comfortable personal protective equipment.
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Decontamination
Extensively studied for years, decontamination is mostly a chemistry-microbiology-
radiology problem with some fluid mechanics connotations. For background, see
Fitch et al. (2003), Hawley & Eitzen (2001), Raber et al. (2001), NRC (2003b),
and chapter 7 of NRC (1999). Liquids typically decontaminate surfaces, whereas
gaseous decontaminants fumigate spaces. Water sprays can scrub the air and remove
aerosols. Redispersion of spores or fallout by the air disturbance due to workers
is a concern. People portals, described earlier, can screen victims and workers for
trace contamination. The prediction of airborne contaminant dispersion patterns,
also discussed, is a guide to decontamination efforts. Finally, as an example of putting
thermal science to work, the University of Buffalo Bioblower decontaminates air by
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heating it rapidly in a positive-displacement compressor.
Open questions in decontamination include how to deal with porous materials,
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how to decontaminate sensitive equipment, valuable items, and hard-to-reach places,
and how to measure residual contamination levels. Also, recent developments in sur-
face cleaning and coating removal need to be assessed for decontamination potential,
including high-pressure water jets, CO2 - and ice-blasting, flashlamp, laser, and ul-
trasonic surface treatments, and foams.
CONCLUSIONS AND OUTLOOK
Many fluid mechanics R&D opportunities in homeland security were pointed out
along the way. Some of the key ones include airborne sampling—especially bio-
inspired samplers, fluids sensor development for distributed sensing systems, scale-
model simulations of blast effects, microfluidics and lab-on-a-chip, modeling urban
plume dispersion, aerosol generation, agglomeration, and removal, and water-model
experiments in building ventilation. CFD has an important role to play, and will
shortly be available in real time as an input to emergency management. Plumes of
all sorts figure prominently and need further study. Existing fluids facilities, espe-
cially meteorological wind tunnels and shock tubes, will doubtless see more activity.
Opportunities will arise for the design of new facilities as well.
Homeland security needs to be better informed about fluid mechanics. Some of
the cited references show a misunderstanding of plumes, the nature of turbulence, the
pressure-velocity relationship, and the difference between a sound wave and a shock
wave. In every field that is “new” to fluid mechanics, the first step is always to educate.
Here, the defining NRC report (NRC 2002) calls for training and simulation as part
of the overall homeland security effort. Apart from just doing research, we could
profitably develop multimedia materials, for example, explaining the role of fluid
mechanics in homeland security and presenting fluids concepts in layman’s terms
(made more palatable, of course, by a little flow visualization).
As fluid mechanics is inevitably pushed in this new direction, we will educate and
also learn many new things ourselves. Along the way, we can contribute not only to
counterterrorism, but also to better indoor air quality, less environmental pollution,
and an invigorated public health system, possibly saving more lives this way than the
104 SettlesAR266-FL38-04 ARI 11 November 2005 16:24
terrorists can take at their worst. Compared to some of the other options for fluid
mechanics R&D (say, manned space travel or ballistic missile defense), this one looks
pretty good.
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