Engineers at the Arnold Engineering Development Complex and the American company MetroLaser are using a new type of laser measurement system to test the speed of engine exhaust and help find ways to reduce aircraft noise.
In November 2021, a team from MetroLaser used the J85 engine testbed maintained by Arnold Engineering Development Complex (AEDC) at the University of Tennessee Space Institute’s Propulsion Research Facility (PRF) near Arnold Air Force Base, Tennessee, to study its three-component planar Doppler. velocimetry (PDV), a system that allows non-intrusive optical measurements of the velocity of gases at the exhaust of an engine.
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Members of MetroLaser and the US Navy have partnered with several other programs with AEDC, which helped them decide to use PRF for this particular research.
“The Propulsion Research Facility’s engine test bed facilities have been critical to the success of the MetroLaser SBIR program,” said David Mayo, US Navy mechanical engineer and technical point of contact for MetroLaser. “PRF engineers and staff provided critical support for validation testing of MetroLaser’s POS technology.”
Instrumentation and diagnostics expert Robert Howard noted that MetroLaser and the Navy knew the PRF would be “an excellent low-cost opportunity to demonstrate this measurement technique on a full-scale turbine engine.”
He added that the Navy’s interest in the PDV stems from the need to understand velocity distribution in jet engine exhaust as a key to developing aircraft noise reduction strategies.
“Measurements are needed for comparison with engine exhaust gas flow field models,” Howard said. “Current measurement methods involve intrusive probes or optical methods that require seeding the flow with particles. The optical method developed by MetroLaser relies on the soot particles already present in the exhaust gases and thus makes this measurement technique more versatile for performing velocity measurements on engine mounts and winged aircraft.
With an extended laser beam illuminating a band of the exhaust flow field, the MetroLaser approach relies on measuring scattered laser light with three strategically placed camera systems mounted outside the exhaust.
“Specialized iodine filters are then used to help discern the Doppler frequency shift of the scattered laser light illuminating the camera systems,” Howard said. “The change in laser frequency results from the scattering of laser light by high-velocity soot particles in the exhaust.”
AEDC J85 test operations engineer Brad Besheres said the test conducted in November 2021 was actually the second of a two-phase effort in the process.
“In the fall of 2020, AEDC assisted MetroLaser in installing and evaluating the performance of a basic laser and camera system comprising the POS system components near the relatively harsh vibrating acoustic environment of the jet engine. operating across its power range, from idle to afterburner operations,” Besheres said.
“The information and lessons learned from this test were used to design an assembly structure for the final demonstration, which led to a very successful test campaign that provided MetroLaser with ample data to declare a successful demonstration of the system. POV to their Navy sponsor.”
Tom Jenkins, president of MetroLaser, said the PDV technique was also previously used at AEDC’s National Large-Scale Aerodynamics Complex at NASA’s Ames Research Center in Mountain View, California. In the 80 by 120 foot wind tunnel. The technique has been shown to be able to resolve velocities as low as 2 m per second from a single laser pulse and to be applicable at viewing distances greater than 40 m.
These capabilities, in addition to PDV’s minimum requirements on the optical characteristics of aerosols in flow, make it a means to measure three-component velocity vector fields in time-dependent flows in large-scale wind tunnel installations. . A high-quality three-camera PDV system was assembled and installed in the wind tunnel to enable flow field measurements between the moving blades of a full-scale rotor.
PDV can also be applied to measurements of the motion of solid objects whenever there are unique advantages to the use of localized laser targeting and the spatial discrimination provided by velocity field images. Examples include individual flight paths and the spinning or tumbling speeds of multiple targets, the local speeds of moving pieces of machinery, and the movement of biological targets.
In the case of measurements on solid objects where the trajectory of motion is known, only one camera system is needed to obtain the full velocity along the trajectory and the laser illumination can be volumetric rather than a light sheet.
