Abstract
Modern “non-intrusive” optical methods are providing revolutionary capabilities for diagnostics of hypersonic flow fields. They generate accurate information on the performance of ground test facilities and provide local time accurate measurements of near-wall and off-body flow fields surrounding hypersonic test articles. They can follow the true molecular motion of the flow and detect nonequilibrium states and gas mixtures. They can be used to capture a wide range of turbulent scales and can produce highly accurate velocity, temperature and density measurements as well as time-frozen images that provide intuitive understanding of flow phenomena. Recent review articles address many of these methods and their applications. The methods highlighted in this review are those that have been enabled or greatly improved by new, versatile laser systems, particularly including kHz rate femtosecond lasers and MHz rate pulse burst lasers. Although these methods can be applied to combusting environments, the focus of this review is on external high Mach number flows surrounding test articles and wind tunnel core flow properties. The high repetition rates enable rapid time evolving flows to be analyzed and enable the collection of large data sets necessary for statistical analysis. Future capabilities based on the use of atomic vapor filters and on frequency tunable, injection locked MHz rate lasers are promising.
Highlights
As maneuverable air platforms move up in speed from subsonic through supersonic to hypersonic, satisfactory safety, reliability and performance become increasingly difficult to achieve
These models must incorporate the true physics of the air flow and must be able to predict performance, including laminar to turbulent transition, nonequilibrium phenomena, dissociation, ionization, unsteadiness, shock interactions and separation as well as perturbations caused by ablation, surface degradation and structural coupling
Recent work on operating with much shorter pulse lengths, on the order of 100 picoseconds, has enabled high repetition rate nonlinear processes including the development of picosecond laser electronic excitation tagging (PLEET). (Jiang et al 2017a, b)
Summary
As maneuverable air platforms move up in speed from subsonic through supersonic to hypersonic, satisfactory safety, reliability and performance become increasingly difficult to achieve. Of particular relevance for the application of these diagnostics to hypersonic flows is a recent NATO review article that addresses absorption spectroscopy, planar laserinduced fluorescence (PLIF), molecular tagging velocimetry (MTV), focused laser differential interferometry (FLDI), light scattering (Rayleigh and Raman), coherent anti-Stokes Raman scattering (CARS), particle imaging velocimetry (PIV) and optical emission spectroscopy (OES) (Danehy et al 2019) These advances have been enabled by advances in laser technology, including the invention of the tunable dye laser, advancements in high pulse energy solid-state laser technologies, efficient harmonic conversion, injection and narrow linewidth control, rapid multiple pulse capabilities, and femtosecond and picosecond laser technologies. We will focus on methods that are enabled or enhanced by high repetition femtosecond lasers and by narrow linewidth MHz rate pulse burst lasers and produce time accurate measurements of local nonequilibrium and unsteady flow properties, where “time accurate” refers to sampling that is not time or multiple pulse averaged. Recent work on operating with much shorter pulse lengths, on the order of 100 picoseconds, has enabled high repetition rate nonlinear processes including the development of picosecond laser electronic excitation tagging (PLEET). (Jiang et al 2017a, b)
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