CFD Tackles Aerosols

By Erin Hatfield

The average person breathes 3,400 gallons of air each day, according to the American Lung Association. In addition to life-giving oxygen, the air contains numerous toxic solid particles, droplets, and vapors. Not all of our body’s natural defenses,  such as the cilia that line respiratory tracts, can rid the body of these foreign materials. As a result, the United States and other industrialized countries face staggering increases in chronic diseases such as asthma and lung cancer, especially among children and the elderly.

Responding to these concerns, the Environmental Protection Agency (EPA)  and other federal agencies are funding research on particle deposition in the human airways. Using real-world data sets, mechanical engineers at North Carolina State University are creating computer simulations of the trajectories and deposition patterns of particles when inhaled or exhaled.

In Search of Easy Breathing

Previous studies of the human airways focused mainly on micro-particle deposition, but bypassed ultra-fine particle and volatile droplet depositions in the oral airways and deeper lung regions. Since both types of particles can be highly toxic, medical professionals, pharmaceutical researchers, and federal regulation agencies want to know what kind of particles end up where—and at what concentrations.

> > This velocity profile of the oral airway model was produced using CEI’s EnSight to visualize an ANSYS CFX CFD analysis of air particulate trajectories when they enter the oral and nasal airways.

 

The EPA has data on the density, size, and volatility of numerous pollutants—from outdoor pollutants such as soot, fuels, and chemicals, to indoor pollutants such as asbestos fibers from old insulation and aluminum particles from manufacturing processes. People who are exposed to such particles on a daily basis often wear protective masks, but particles on the nanometer scale are not caught and can penetrate deep into the lungs where oxygen transfer occurs, allowing the particles to migrate to other organs of the body.

Data is also available on clouds of evaporating jet fuel droplets that are emitted during aircraft take-off and refueling procedures. Military personnel who conduct such exercises and civilians who live extremely close to military bases or busy airports could be exposed to these emissions.

Recreating the Human Respiratory System

While tracing inhaled particles in the respiratory system might seem like a biomedical application, its roots are in fluid dynamics analysis. The inhaled particles behave much as any particle does when moving through a curved, branching pipe. “Mechanical engineering is embedded in the biological sciences,” says professor Clement Kleinstreuer, who is heading up North Carolina State’s computational fluid-particle dynamics (CFPD) research group in the University’s Mechanical and Aerospace Engineering Department (click here).

“Instead of applying CFPD theories to traditional mechanical engineering projects, such as internal combustion engines, heat exchangers or slurry transport, we are applying CFPD to part of the human physiology to investigate lung aerosol transport and deposition, and other biofluid dynamics problems.”

Kleinstreuer and his group started by creating a 3D model based on a cast of the oral airway and lung bifurcation geometries from previous research. The engineers used the ambient pollutant and fuel droplet data sets as input and created computational representations of the particle transport and deposition, considering resting, light activity, and exercise breathing modes.

Inhalation in Virtual Reality

North Carolina State researchers use CFX, a finite-volume CFD code from ANSYS, in conjunction with their own pre- and post-processing programs and their particle trajectory code, f90, to compute numerical values of the airflow and particle deposition. After iterations for each of the 30,000 to 100,000 particles, the engineers obtain information about particle positions, velocities and deposition sites. These values are imported into CEI’s EnSight software to create visual representations of the airflow and particle dynamics.

EnSight enables Kleinstreuer and his group to see the distinct trajectories that particles take when they enter the oral and nasal airways and lung bifurcations. After revising some of EnSight’s code with help from CEI, the engineers can also see where certain particles linger near the airway walls, recirculate or deposit.

< < An alternate velocity profile of the oral airway model also produced using CEI’s EnSight to visualize an ANSYS CFX CFD analysis.

 

Based on the visualizations, Kleinstreuer now knows exactly how the particle characteristics, airway geometric features, and inlet conditions, as well as changes in airflow structures and secondary airflows,  affect the particle behavior. In general, micro-particles are more likely to deposit in the upper airways and those in the nano-size range typically migrate deeper into the lung.

The results from the computational analyses have been validated by comparing them to several airflow and particle deposition measurements taken in laboratory settings or from humans. While the computational results are based on a relatively ideal lung airway model, they exhibit real flow features as well as representative particle transport behavior and deposition values.

Lower Cost, Non-Invasive Procedures

Such validations of the results are major milestones for Kleinstreuer and the rest of the North Carolina State team, which includes specialists in computational analysis, laboratory measurements, pulmonary medicine,  and flow visualization.

“When you are faced with highly toxic particles, including biochemical agents, it is impossible to conduct tests on humans,”  Kleinstreuer says. “Testing on animals doesn’t scale up well to the human physiology and species pathways and metabolisms. Data from a mouse’s respiratory system is exactly that—potential impact on a mouse’s system.

“With CFPD, you can model true human airways. You can accurately tell what type of particles deposit where and at what concentrations under any given inhalation condition, all without having to test a live specimen.”

Developing “Smarter” Inhalers

Results of this type can help medical professionals and regulatory agencies determine the location of particles to test therapeutic measures. Specifically, North Carolina State’s approach could be used for chemotherapy purposes to target a tumor located in an identified branch of the lung. Medical device manufacturers could use it to develop a “smart inhaler” that mechanically and aerodynamically delivers the optimized drug aerosol stream as predicted in computer visualizations.

> > Based on such visualizations, researchers can see where certain toxic particles linger near the airway walls. The approach could be used for chemotherapy purposes to target a tumor located in a specific branch of the lung and to develop a “smart inhaler” that aerodynamically delivers an optimized drug aerosol stream as predicted in such analyses.

 

Pharmaceutical companies could use findings from Kleinstreuer’s group to determine the optimal characteristics of drug aerosols. His group could also simulate the absorption of the drugs or toxins based on deposition location and known principles of the compound’s local pathways and metabolisms in the human body.
“Our most startling breakthrough is the methodology to optimize drug aerosol delivery, not just to combat lung diseases but also diabetes,  cancer, AIDS and other diseases,” Kleinstreuer says.

“Once we have determined the optimal characteristics of the therapeutic particles—density, diameter, phase, shape—we can develop ideal inhaler specifics for medical situations where it is prudent to use the lung as an entry way.”

Erin Hatfield is a freelance writer covering the computer graphics, IT, and electronics industries. Send your comments about this article through e-mail by clicking here. Please reference “CFD Tackles, October 2006” in your message.

Product Information

CFX
ANSYS Corp.
Canonsburg, PA

EnSight
CEI
Apex, NC

North Carolina State University
Raleigh, NC