The paper provides an overview of the use of coherent anti-Stokes Raman scattering (CARS) and spontaneous Raman scattering for diagnostics of low-temperature nonequilibrium plasmas and nonequilibrium high-enthalpy flows. A brief review of the theoretical background of CARS, four-wave mixing and Raman scattering, as well as a discussion of experimental techniques and data reduction, are included. The experimental results reviewed include measurements of vibrational level populations, rotational/translational temperature, electric fields in a quasi-steady-state and transient molecular plasmas and afterglow, in nonequilibrium expansion flows, and behind strong shock waves. Insight into the kinetics of vibrational energy transfer, energy thermalization mechanisms and dynamics of the pulse discharge development, provided by these experiments, is discussed. Availability of short pulse duration, high peak power lasers, as well as broadband dye lasers, makes possible the use of these diagnostics at relatively low pressures, potentially with a sub-nanosecond time resolution, as well as obtaining single laser shot, high signal-to-noise spectra at higher pressures. Possibilities for the development of single-shot 2D CARS imaging and spectroscopy, using picosecond and femtosecond lasers, as well as novel phase matching and detection techniques, are discussed.

https://iopscience.iop.org/article/10.1088/0022-3727/47/43/433001/pdf

Coherent anti-Stokes Raman scattering and spontaneous Raman scattering diagnostics of nonequilibrium plasmas and flows

The hypersonic Mach number independence principle of Oswatitsch is important for hypersonic vehicle design. It explains why, above a certain flight Mach number (M∞ ≈ 4−6, depending on the body shape), some aerodynamic properties become independent of the flight Mach number. For ground test facilities this means that it is sufficient for the Mach number in the test section to be high enough, that Mach number independence exists. However, the principle was derived for calorically perfect gas and inviscid flow only. In this paper a theoretical study for blunt bodies in the case of viscous flow is presented. We provide numerical results which give insight into how attached viscous flow behaves at high Mach numbers. The flow past an axisymmetric configuration is analysed by applying a coupled Euler/second-order boundary-layer method. Wall boundaries are treated by assuming an adiabatic or radiation-adiabatic wall for laminar flow. Calorically perfect or equilibrium air is accounted for. Lift, drag, and moment coefficients, and lift-to-drag ratios are given for several combinations of flight Mach number and altitude, i.e. Reynolds number. For blunt bodies considered here, which are pressure dominated, Mach number independence occurs for the adiabatic wall, but not for the radiation-adiabatic wall assumption.

The hypersonic Mach number independence principle in the case of viscous flow

Broadband single pulse coherent anti-Stokes Raman scattering (CARS) experiments employing a folded-box phase-matching geometry in a pulsed hypervelocity blunt body flow are presented. Rovibrational spectra of molecular nitrogen, produced in the freestream and within the shock layer at moderately high enthalpy (8.4 MJ/kg), are examined. Difficulties peculiar to the application of a single pulse optical technique to a high enthalpy pulsed flow facility are discussed and measurements of flow temperatures are presented. Theoretically calculated values for temperatures based upon algorithms used to determine freestream and shock layer conditions agree well with experimental measurements using the CARS technique. The measurements indicate that thermal non-equilibrium conditions exist within the freestream, and that near thermal equilibrium exists at the point of measurement within the shock layer. The comparison between the experiment and theory in the shock layer is improved by using the measured freestream temperatures as input to the shock layer computations.

Rotational and vibrational temperature measurements using CARS in a hypervelocity shock layer flow and comparisons with CFD calculations

A portable fluorescence imaging system has been developed for use in NASA Langley s hypersonic wind tunnels. The system has been applied to a small-scale free jet flow. Two-dimensional images were taken of the flow out of a nozzle into a low-pressure test section using the portable planar laser-induced fluorescence system. Images were taken from the center of the jet at various test section pressures, showing the formation of a barrel shock at low pressures, transitioning to a turbulent jet at high pressures. A spanwise scan through the jet at constant pressure reveals the three-dimensional structure of the flow. Future capabilities of the system for making measurements in large-scale hypersonic wind tunnel facilities are discussed.

https://ntrs.nasa.gov/api/citations/20090026074/downloads/20090026074.pdf

Portable Fluorescence Imaging System for Hypersonic Flow Facilities

The validity of the computed three-dimensional perfect-gas inviscid density field of the shock layer about a blunt body in a hypersonic argon freestream has been investigated. Mach-Zehnder interferometry was used to generate interferograms of such a flowfield produced in the T3 free-piston shock tunnel. Two-dimensional phase maps (representing line-of-sight integrated density) were produced from the interferograms using a two-dimensional Fourier transform fringe analysis method. Theoretical maps, computed from the computational fluid dynamics solution, compare extremely well with experimental maps for each of seven different viewing angles used to generate interferograms. The multiple angles remove the ambiguity associated with comparing theoretical and experimental integrated quantities. Thus, confidence can be placed in the validity of the three-dimensional density computations.

Computational fluid dynamics validation using multiple interferometric views of a hypersonic flowfield

Free-piston shock tunnels are ground-based test facilities allowing the simulation of reentry flow conditions in a simple and cost-efficient way. For a better understanding of the processes occurring in a shock tunnel as well as for an optimal comparability of experimental data gained in shock tunnels to numerical simulations, it is highly desirable to have the best possible characterization of the generated test gas flows. This paper describes the final step of the development of a laser-induced grating spectroscopy (LIGS) system capable of measuring the temperature in the nozzle reservoir of a free-piston shock tunnel during tests: the successful adaptation of the measurement system to the shock tunnel. Preliminary measurements were taken with a high-speed camera and a LED lamp in order to investigate the optical transmissibility of the measurement volume during tests. The results helped to successfully measure LIGS signals in shock tube mode and shock tunnel mode in dry air seeded with NO. For the shock tube mode, six successful measurements for a shock Mach number of about 2.35 were taken in total, two of them behind the incoming shock (p
 $$\approx $$
 1 MPa, T
 $$\approx $$
 600 K) and four after the passing of the reflected shock (p
 $$\approx $$
 4 MPa, T
 $$\approx $$
 1000 K). For five of the six measurements, the derived temperatures were within a deviation range of $$6\%$$
 to a reference value calculated from measured shock speed. The uncertainty estimated was less than or equal to $$3.5\%$$
 for all six measurements. Two LIGS signals from measurements behind the reflected shock in shock tunnel mode were analyzed in detail. One of the signals allowed an unambiguous derivation of the temperature under the conditions of a shock with Mach 2.7 (p
 $$\approx $$
 5 MPa, T
 $$\approx $$
 1200 K, deviation $$0.5\%$$
 , uncertainty $$4.9\%$$
 ).

LIGS measurements in the nozzle reservoir of a free-piston shock tunnel

Rotational and vibrational temperatures determined from broadband single pulse Coherent Anti-Stokes Raman Scattering (CARS) experiments 7.2~7.5 MJ/kg) pulsed hypervelocity blunt body flow are presented. Comparing these temperatures with nonequilibrium chemistry nozzle and blunt body CFD codes shows good agreement, despite indicating certain inadequacies in the methods used.

https://link.springer.com/content/pdf/10.1007%2F978-3-642-78832-1_39.pdf

Comparisons of CFD with CARS Measurements in Hypervelocity Nitrogen Flows

CFD validation of real gas air flows over a blunt body using multiple interferometric views

Surface heat transfer rates have been experimentally determined from measurements on a blunt body in hypersonic flows produced by a free piston shock tunnel. These rates have been used to check the validity of a coupled Euler / 2nd order boundary layer Computational Fluid Dynamics code over a range of stagnation enthalpies that includes perfect gas and nonequilibrium chemical effects, and for both monatomic (argon) and diatomic (nitrogen) test gases. Three dimensional perfect gas and equilibrium chemistry, and axisymmetric nonequilibrium chemistry, computations have been performed. Scatter in the experimental results has prevented distinguishing between the chemical models used, but the comparison of the codes with the measurements provides encouragement that the codes correctly predict the surface heat flux over a wide range of conditions.

Ch. Mundt

Papers

Rotational and vibrational temperature measurements using CARS in a hypervelocity shock layer flow and comparisons with CFD calculations

LIGS measurements in the nozzle reservoir of a free-piston shock tunnel

Comparisons of CFD with CARS Measurements in Hypervelocity Nitrogen Flows

CFD validation of real gas air flows over a blunt body using multiple interferometric views

Computational fluid dynamics validation using surface heat transfer measurements of hypersonic flowfields