Aerospace Sciences

Innovative imaging methods bring new diagnostic capabilities

The Aerodynamic Measurement Technology Technical Committee advances measurement technology for ground facilities and aircraft in flight.

In January, researchers at the University of Michigan developed a new diagnostic method for the detection of mid-infrared emission from species of interest in rotating detonation combustors. The technique, upconversion imaging, is based on a pulsed mid-IR light source. UCI is an alternative to direct mid-IR detection that uses nonlinear optical frequency mixing to shift mid-IR wavelengths carrying a target image to wavelengths of visible light that can be imaged at higher efficiency with high-performance silicon-based charged coupled device/complementary metal-oxide semiconductor, or CCD/CMOS, cameras. UCI has several favorable properties, including high-spectral selectivity, high-temporal resolution and superior low-light detectivity. Researchers applied this technique to the imaging of carbon dioxide emission from an RDC operated with a hydrogen-carbon dioxide-air mixture. They performed imaging in high-pressure and high-temperature regions associated with a detonation front. The resulting measurements demonstrated a high spatiotemporal resolution capable of imaging small structures near the supersonically propagating detonation wave front. The results show how this technique can be used to observe sharp gradients and millimeter-scale structures in the high-temperature, high-pressure zones of RDC flow fields with a temporal resolution of approximately 200 nanoseconds.

In March, California-based MetroLaser Inc. received a Phase II Small Business Innovation Research award from the U.S. Air Force to develop diagnostic tools and facilities to characterize plasma-material interactions in high-enthalpy flows. The program supports hypersonic flight applications and takes advantage of MetroLaser’s expertise in laser-based diagnostics of combustion and aerodynamic environments.

Also in March, MetroLaser developed a fieldable megahertz-rate digital holography system that enables the study of ultrahigh-speed phenomena in three dimensions. Researchers used the system to study the collision of a supersonic projectile and its bow shock with water droplets for applications involving vehicle survivability in adverse weather conditions. They reconstructed and de-twinned (removed the conjugate twin image that always appears due to symmetry reasons) the holograms to probe complex shock wave dynamics and droplet breakup. In October, MetroLaser was awarded a related Air Force program to develop evaluation tools to measure surface quality and chemistry of carbon-carbon composites and coatings. The company expects these two programs to contribute to the development of hypersonic flight vehicles.

Also in October, MetroLaser demonstrated a three-component velocity diagnostic being developed for supersonic aircraft exhaust flows called planar Doppler velocimetry. PDV enables spatially resolved distributions of the total velocity vector in particle-containing flows, such as combustion gases, and is useful for studying three-dimensional flow phenomena related to jet noise. The PDV method was demonstrated on a laboratory scale jet flow in preparation for a full-scale fighter jet engine test.

Contributors: Mirko Gamba and Jacob George

Innovative imaging methods bring new diagnostic capabilities