New measurement capabilities demonstrated for 3D flow properties, high-speed facilities and combustion research

By Thomas P. Jenkins|December 2023

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

In January, researchers at the Advanced Flow Diagnostics Laboratory at Auburn University in Alabama demonstrated a novel gridless tomographic reconstruction technique, referred to as Fluid Neural Radiance Field, or FluidNeRF, that provides 3D measurements of flow properties or particles in, for example, engine development research or fluid dynamics studies. In May, the researchers compared FluidNeRF to a common reconstruction technique for flow measurements. They found that FluidNeRF increases reconstruction quality, is more robust to noise and shows flexibility to be applied to larger-scale problems.

FluidNeRF uses a machine learning concept in which a 3D volume can be represented as a continuous function of a 3D location. A feed-forward neural network approximates the emission density of the 3D scene. The FluidNeRF tomography algorithm operates similarly to traditional methods in which the volume is updated by using the difference between measured and rendered perspectives. The FluidNeRF neural network is updated using the mean squared error between the measured and rendered perspectives. The current implementation of FluidNeRF assumes emission-based, optically thin line-of-sight measurements. The continuous approximation of the volume does not cause an inherent limitation on topology like traditional volume discretization methods. The machine learning structure of the method provides modularity for incorporating various inputs, outputs, accurate ray models and underlying physics. FluidNeRF also provides greater data compression. This technique can be instrumental in improving the modeling and understanding of combustion and high-speed flows.

In June, scientists at Spectral Energies, a small business in Ohio, collaborated with the U.S. Air Force Research Laboratory in Ohio and the Arnold Engineering Development Complex in Tennessee to measure flow velocity via krypton tagging velocimetry, KTV, in a Mach 18 hypersonic wind tunnel in White Oak, Maryland, and picosecond laser electronic excitation tagging, or PLEET, in a Mach 6 Ludwieg tube flow facility at Wright-Patterson Air Force Base in Ohio. The scientists used nano- and picosecond high-repetition-rate burst-mode laser systems. With 0.5% krypton as the seeding gas for KTV and nitrogen as the tagging molecule for PLEET, the scientists measured hypersonic flow velocities in the freestream and boundary layers at a rate of 100 kilohertz. Such measurement capability is needed for characterizing high-speed flows while developing hypersonic vehicles.

Collaborators at Purdue University in Indiana and Spectral Energies continued to push the state-of-the-art in high-speed laser-based imaging and spectroscopy for highly dynamic flow fields. This year’s milestones include the first megahertz-rate in situ diesel planar laser-induced fluorescence, which they applied to study the fuel refill process in a rotating detonation engine, 5-megahertz particle image velocimetry in post-detonation blast environments and 20-kHz tomographic 3D volume laser-induced fluorescence of liquid sprays. This work helps the development of advanced propulsion systems for defense applications that may offer increased performance and efficiency.

In September, Spectral Energies, in collaboration with the University of Notre Dame in Indiana, developed and demonstrated a plasma-based pressure sensor on a hypersonic sounding rocket in a wind tunnel at the university. The plasma-based sensor for pressure measurement has a similar form factor to that of a conventional membrane-based sensor and is used similarly. However, the plasma-based sensor has a few advantages. For one, the sensor can withstand adverse environments without needing advanced cooling or other protections. This is due to the fact that the plasma-based sensor uses a controlled plasma contained between two electrodes and not a physical wire or membrane that could rupture. Furthermore, the sensor can automatically reignite if the plasma discharge is lost. Another advantage of the plasma sensor is its megahertz (or higher) sampling speed, which allows it to capture the small time scales that may occur in hypersonic boundary layers or combustion applications.

Contributors: Justin Jiang and Dustin Kelly