COLUMBUS, Ohio -- Ohio State University researchers have developed a new technique for measuring airflow into jet engines.
The technique, which uses lasers and optics to measure airflow, will help aircraft manufacturers boost fuel economy and engine performance.
For this study, which appeared in a recent issue of AIAA Journal, the researchers took advantage of Rayleigh scattering -- the ability of gas molecules to capture laser light waves and then emit, or scatter, light of a different frequency.
As the number of molecules in a gas increases, so does the scattering effect. For this reason, scientists often judge the density of a gas by casting a beam of laser light through it and measuring the amount that is scattered.
“But the light signal that we get from Rayleigh scattering carries much more information than just the density,” said Mohammad Samimy, professor of mechanical engineering and principal investigator of the study. “The question is how to extract that information.”
Samimy and his colleagues improved upon traditional Rayleigh scattering by designing a new system of cameras and optics to pull more information from scattered laser light. The accompanying technique, which the researchers dubbed filtered angularly resolved Rayleigh scattering (FARRS), measures the velocity, density, and temperature of air as it enters an engine, and uses that information to calculate the mass flow, the actual amount of air flowing through the engine.
The only other way to measure mass flow involves inserting an invasive probe into the airflow, which causes a disturbance, and alters the measurements. The beam of laser light in the FARRS system doesn’t interfere with airflow at all.
To put these measurements to good use, however, the researchers had to find a way to simplify the data. Mass flow depends on the velocity and density of tiny groups of individual molecules over the entire region of airflow, in this case, a full-sized jet engine.
“The equations that we normally use to describe airflow over a small area become more difficult when we’re dealing with a large area, like an automobile or aircraft. Today’s computers simply cannot handle the calculations -- not even supercomputers,” said Samimy.
Once Samimy and his colleagues used FARRS to find out how velocity, density, and temperature relate to each other, they were able to write software that simulates the behavior of molecules over a large area. This enables today’s computers to calculate the airflow equations in many practical applications.
“This technique combines all the small-scale measurements into a larger-scale model that the computer can handle,” said Samimy.
FARRS provides an optical means for aircraft computers to measure the amount of air flowing through a jet engine. Armed with that information, the computer can calculate the right amount of fuel to maximize engine performance.
The Ohio State researchers built FARRS out of off-the-shelf laboratory components. With further work, FARRS will operate as a single unit that will measure airflow for both commercial and military aircraft. For now, the researchers are working to make the scattering measurements more accurate. For example, commercially available lasers, like the one in FARRS, don’t always generate light at an exact frequency. As the frequency fluctuates -- even minutely -- so do the scattering measurements. Recently the researchers adapted the FARRS system to measure the amount of fluctuation in the laser beam and compensate for it in the calculations.
Researchers tested FARRS in a wind tunnel at Ohio State’s Aeronautical and Astronautical Research Laboratory. While initial errors ranged from 7 percent to 10 percent, monitoring the laser frequency fluctuation cut error down to 3 percent.
Contact: Mohammad Samimy, (614) 292-6988; Samimy.1@osu.edu
Written by Pam Frost, (614) 292-9475; Frost.18@osu.edu