Non-intrusive measurement technology validated

Nickolas Galyen, an engineer in Aerospace Testing Alliance’s Integrated Test and Evaluation department, aligns a calibration target during a series of tests to validate the use of stereo particle image velocimetry (PIV) in the 4-foot transonic wind tunnel at Arnold Engineering Development Center. Images of the precisely machined target are used to spatially calibrate particle images to measure three-dimensional velocity airflow fields.

Nickolas Galyen, an engineer in Aerospace Testing Alliance’s Integrated Test and Evaluation department, aligns a calibration target during a series of tests to validate the use of stereo particle image velocimetry (PIV) in the 4-foot transonic wind tunnel at Arnold Engineering Development Center. Images of the precisely machined target are used to spatially calibrate particle images to measure three-dimensional velocity airflow fields.

Nickolas Galyen, an engineer in Aerospace Testing Alliance’s Integrated Test and Evaluation department, aligns a calibration target during a series of tests to validate the use of stereo particle image velocimetry (PIV) in the 4-foot transonic wind tunnel at Arnold Engineering Development Center. Images of the precisely machined target are used to spatially calibrate particle images to measure three-dimensional velocity airflow fields.

Nickolas Galyen, an engineer in Aerospace Testing Alliance’s Integrated Test and Evaluation department, aligns a calibration target during a series of tests to validate the use of stereo particle image velocimetry (PIV) in the 4-foot transonic wind tunnel at Arnold Engineering Development Center. Images of the precisely machined target are used to spatially calibrate particle images to measure three-dimensional velocity airflow fields.

ARNOLD AIR FORCE BASE, TENN -- An Aerospace Testing Alliance (ATA) technology and analysis branch team recently broke new ground at Arnold Engineering Development Center (AEDC) by conducting a series of tests to determine the feasibility of using an optical method to accurately measure and assess the airflow quality in the four-foot transonic (4T) wind tunnel in the Propulsion Wind Tunnel facility.
"Ideally, we want the tunnel airflow to be as straight and uniform as possible to realistically simulate those conditions an aircraft or ordnance store in flight would experience," said Dr. Joe Wehmeyer, a project engineer with the technology and analysis branch of ATA's test and evaluation department.
"Due to the physical nature of a wind tunnel, some cross flow, vortices, eddies and those types of anomalies do occur - these are not overly significant factors, but ones we may try to minimize.
"These tests were intended to evaluate the application of this system, which is relatively non-intrusive and provides close to real-time analysis and data at a reasonable cost. These tests proved the system is practical and from all indications, it gave us the results we need."
This method, known as stereo particle image velocimetry (PIV), was used to measure velocities and uniformity of the air flow in a small, pre-selected region of Tunnel 4T. PIV is considered a non-intrusive method, one that does not introduce unwanted changes or disturbances to the airflow. Fully and successfully implemented, PIV data may replace traditional relatively costly surveys of the airflow using a cruciform probe to assess flow quality.
The PIV system consists of two digital cameras, two high-powered lasers, a cylindrical
lens to convert each laser's output to a sheet of light, a cloud of microscopic oil droplets
injected into the airflow being measured and a calibration target.
The laser's output acts like a strobe light to illuminate the airborne droplets. The calibration target is a finely machined metal plate with a precisely spaced grid pattern on each side for calibrating the camera system.
PIV technology is not new and has been used successfully at the Sandia National Laboratory in New Mexico, NASA Laboratories in Cleveland, Ohio, Hampton, Va., and Palo Alto, Calif., and the Air Force Research Laboratory at Wright-Patterson Air Force Base, Ohio.
The decision to test the PIV technology in 4T at this time was based on a combination of relatively recent improvements in high-end digital camera miniaturization, overall cost avoidance and availability of more sophisticated computer-aided analysis software.
"It was just a matter of this being the right time and the appropriate facility to test this technology," Wehmeyer said. "We're pleased with the results."