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.
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.
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 [email protected].
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 [email protected].
(For more information, contact Fluent Inc., 10 Cavendish Court, Centerra Resource Park, Lebanon, NH 03766; Phone: 603-643-2600, Fax: 603-643-3967; www.fluent.com, or the Boeing Co., P.O. Box 3707, Seattle, WA 98124, mail code OT-29; www.boeing.com.