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NIOSH and Boeing are using computer simulation to investigate
the potential spread of viruses and bacteria through the air in
the cabin of a commercial airliner.
By Chao-Hsin Lin, Ph.D., P.E., and Sutikno
Wirogo
The question of infectious diseases spreading through an
aircraft's ventilation system is a serious concern to air
travelers and flight crews.
Increased interest in this issue by airline employees prompted
the Federal Aviation Administration (FAA) to sponsor a study by
the National Institute for Occupational Safety and Health (NIOSH)
to consider new methods to investigate the potential for
airborne disease transmission on commercial flights.
An aircraft's ventilation system plays a key role in reducing
the airborne spread of pathogens. The cabin air supply system
pulls air in from the compressor stages in the aircraft's jet
engines. This pressurized air is cooled and may be mixed with an
almost equal amount of highly filtered air from the passenger
cabin. The mixture is then blown into the cabin through overhead
supply outlets. In the cabin, air typically flows in a circular
pattern and exits through floor grilles. About half of the air
exiting the cabin is immediately exhausted from the airplane,
while the other half is filtered and remixed. The filtered air,
called recirculated air, normally passes through a
high-efficiency particulate air (HEPA) filter, before it is
mixed with the air from the compressors. The concentration of
particulate matter in the cabin is reduced by dilution by the
entering air mixture.
The subject of transmission of infectious diseases through
aircraft ventilation systems has traditionally been investigated
on a smaller scale or with bulk volume approximations, all of
which showed low risk for airborne disease. This project was
initiated by the FAA in 1997 through an inter-agency agreement
with NIOSH. Since tracking disease transmission on aircraft by
standard epidemiological methods is difficult, the FAA and NIOSH
decided to take a multifaceted approach to address this issue.
The project includes performing Computational Fluid Dynamics (CFD)
simulations, testing airflow in a mock-up of a full-sized
aircraft cabin section, and measuring levels of bioaerosols
during commercial flights.
A team of aircraft ventilation engineers from Boeing Commercial
Airplanes and NIOSH engineers helped ensure accurate simulations
by using a new CFD code with leading-edge turbulence
formulations by first modeling a simplified representation of an
aircraft cabin. The results from this simulation were used to
calibrate a fully detailed cabin model, ensuring results would
match physical testing. The full simulation was configured to
model the movement of particles throughout the cabin and
evaluate the potential exposure, if any, of passengers sitting
in different parts of the plane.
Boeing Commerical Airplanes was the primary developer of the
analytical model based on guidelines from NIOSH. The University
of Illinois ran experiments in a five-row, full-scale cabin
mockup to provide independent quantitative and qualitative data
on cabin airflow and for comparison with the model. NIOSH
performed sampling on commercial aircraft flights for
bioaerosols, including airborne fungi and bacteria. Sandia
National Laboratories reviewed the model development and
experimental methodologies and worked with NIOSH to identify
future areas of research.
Using CFD
Because of the difficulty of testing in an actual aircraft cabin
during flight conditions, the primary focus of this project has
been to perform numerical simulation of the cabin airflow. A
numerical simulation can predict the effect of conditions that
could never be tested, and many different conditions can be
examined in far less time and at a lower cost than would be
required for physical testing. In addition, simulation provides
results at every point in the computational domain while testing
results are limited to the areas and conditions that can be
measured.
"CFD was the obvious tool to perform this simulation because it
can predict airflow within an enclosed space under a variety of
conditions and configurations," said Kevin Dunn, mechanical
engineer for the Centers for Disease Control and Prevention.
Besides predicting airflow, CFD makes it possible to introduce
particles into the cabin model and track them as they are driven
by the airflow in the cabin. If particles pass through the
breathing zone of a passenger, then the potential exists for
that person to become infected.
Overcoming Obstacles to Accuracy
Of course, a critical challenge in every computer simulation is
ensuring that the simulation accurately predicts real world
observations. Engineers began their simulation effort using the
traditional k-_ turbulence model.
Correlating the predictions of these simulations with recent
experimental measurements using a hot wire anemometer showed
that the traditional turbulence model substantially
under-predicted the turbulence in the cabin.
NIOSH and Boeing engineers worked together to increase the
accuracy of the model using state-of-the-art CFD codes. This
work drew on Boeing's experience in modeling a wide range of
airflow problems in their broad range of aerospace products. For
example, Boeing has long used CFD to design cabin sections, but
has more recently begun creating very large models of the entire
cabin. In designing such critical areas of the aircraft,
simulation accuracy is essential, so considerable time has been
spent evaluating the effectiveness of different physical models.
The ideal approach is to model the entire cabin in an
environment that would have made it possible to utilize the
latest and most accurate turbulence models. But that wasn't an
option when the team started so they used another approach that
they thought would be nearly as effective. They began with a
very simplified version of the cabin geometry that was similar
in Reynolds number to an actual cabin. They used this simplified
model to explore the physics of flow within the cabin using
Fluent CFD software. Fluent offers several turbulence models
that could be used in airflow studies. For instance, one model
is the large eddy simulation (LES) model, in which time
averaging is applied to only the smallest turbulent eddies,
those that are smaller than a typical cell size. Larger eddies
are computed directly in this time-dependent model. Aircraft
ventilation analysts discovered that the traditional model could
be adjusted to duplicate the turbulence levels found in the LES
with the User Defined Functions feature in Fluent. They continue
to use the modified traditional model for further studies
because it is less computationally intensive and therefore
provides results in less time.
Tracking Viral Movement
Jennifer Topmiller, mechanical engineer, and James Bennett,
senior service fellow with NIOSH, are continuing to work with a
team of researchers from Boeing, the University of Illinois and
Sandia National Laboratories to validate the model with
additional experimental data. With the flow fields validated,
NIOSH engineers hope to use the CFD model to continue the study
of the potential for airborne disease transmission within
aircraft cabins.
With a validated model, the analysts can investigate a wide
range of cabin conditions to determine the effect on the
particle movement. For example, they can change the air supply
rates, move the diffusers used to introduce air into the cabin,
and try different cabin seating configurations. Analysts even
have the opportunity to evaluate the spread of different
diseases by changing the particle properties and emission rates.
Results
The results of the simulation were encouraging. The Fluent
solver was robust and consistent for the velocity field and the
particulate fields in the eight-processor ring used to solve the
model. The contour and surface plots quickly conveyed the
necessary information to the user. This technique can be an
extremely valuable tool in the future to improve the design of
aircraft ventilation systems.
"This project is a first step in gaining a better understanding
of how airborne diseases might be spread on commercial
aircraft," said Topmiller.
"As we learn more about cabin airflows and how particles are
transported, we should be able to say more about the risk of
disease transmission on aircraft."
According to the U.S. Centers for Disease Control and
Prevention, the main way that SARS seems to spread is by close
person-to-person contact.
Although this project was started before SARS was even
discovered, there could be some application. Bennett noted that,
"The extent to which SARS is transmitted through airborne
droplets is yet unknown. To this extent, these simulations may
eventually help public health authorities and aircraft designers
to reduce the risk of exposure to the SARS virus during airline
travel."
It is too early to state how effectively the model can be used
to predict the spread of diseases like SARS. As the experiments
are finished, there will be a better indication of model
accuracy. Following the completion of the validation process,
the model will be utilized to look at various cabin ventilation
conditions and configurations and evaluate the impact on the
risk of disease transmission.
(NOTE: The National Institute for Occupational
Safety and Health, a federal agency, does not endorse products
or services.)
Chao-Hsin Lin, Ph.D., P.E., is a member of the Environmental
Control Systems
(ECS) of the Boeing Commercial Airplanes Group. After earning
his doctorate in mechanical engineering from University of
Illinois at Urbana-Champaign, he worked in GM during 1989-1997.
He is currently an associate technical fellow of CFD analysis in
aircraft cabin environment. He has published refereed papers in
the areas of aerosol mechanics, control of diesel soot,
atmospheric dispersion of air pollutants, and CFD
applications/benchmark in automobiles, aero- and spacecraft. He
can be contacted via e-mail at chao-hsin.lin@boeing.com.
Dr. Sutikno Wirogo received his Ph.D. in 1997 from Iowa State
University with specialization in the development of a
high-order conservative numerical discretization scheme for
pressure-based solver. Wirogo joined Fluent Inc. in August 1999,
and has since been supporting major clients in the Aerospace
industry. His specializations include compressible external
aerodynamics flows, aircraft icing, and customizations of Fluent
for user-defined models. He is currently the Customer Service
Team Leader for the Space and Defense Team. Prior to joining
Fluent Inc., he was employed at Sukra Helitek Inc., in Ames,
Iowa, where he was involved in the development and support of
Sukra's CFD code. Wirogo can be contacted at (800) 445-4454,
ext. 247, or by email at
sw@fluent.com.
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