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Resolving Nonequilibrium Gas Kinetics in Supersonic Neutral Flows with Coherent Rayleigh Brillouin Scattering

Atulya Kumar, Gabriel Alfaro, Marios Kounalakis, Stefan Karatodorov, Alexandros Gerakis

Abstract

We present a characterization of high-speed flows in nonequilibrium thermodynamic conditions using single-shot coherent Rayleigh-Brillouin scattering(CRBS). The technique is applied on a highly underexpanded jet using 200ns laser pulses, enabling simultaneous probing of multiple spatial locations. We map the jets average axial velocity and density distributions and resolve local velocity gradients, providing access to parameters relevant to turbulence characterization. The measurements are validated against numerical simulations, showing good agreement overall. We find that in most cases individual single-shot spectra exhibit substantial deviation from the bulk averaged lineshapes, reflecting the non-Maxwellian velocity distributions generated by shock-induced regimes. The results presented here establish single-shot CRBS as an important tool for direct measurements of flow velocity components, velocity gradients, and density in complex, unsteady supersonic environments.

Resolving Nonequilibrium Gas Kinetics in Supersonic Neutral Flows with Coherent Rayleigh Brillouin Scattering

Abstract

We present a characterization of high-speed flows in nonequilibrium thermodynamic conditions using single-shot coherent Rayleigh-Brillouin scattering(CRBS). The technique is applied on a highly underexpanded jet using 200ns laser pulses, enabling simultaneous probing of multiple spatial locations. We map the jets average axial velocity and density distributions and resolve local velocity gradients, providing access to parameters relevant to turbulence characterization. The measurements are validated against numerical simulations, showing good agreement overall. We find that in most cases individual single-shot spectra exhibit substantial deviation from the bulk averaged lineshapes, reflecting the non-Maxwellian velocity distributions generated by shock-induced regimes. The results presented here establish single-shot CRBS as an important tool for direct measurements of flow velocity components, velocity gradients, and density in complex, unsteady supersonic environments.
Paper Structure (8 sections, 6 equations, 11 figures, 1 table)

This paper contains 8 sections, 6 equations, 11 figures, 1 table.

Table of Contents

  1. Introduction
  2. Experimental methods
  3. Single-shot Coherent Rayleigh Brillouin Scattering
  4. Experimental Apparatus
  5. Data processing and Results
  6. Discussions
  7. Conclusion
  8. Acknowledgments

Figures (11)

  • Figure 1: Schematic representation of the structure of a highly under-expanded jet based on Ref. snedeker1964experiments.
  • Figure 2: The four wave mixing process ($\chi^{(3)}$), illustrating the interaction of the pump $I_{pp_1}$, chirped pump $I_{pp_2}$, and probe beams $I_{p}$ that generates the CRBS signal $I_{signal}$.
  • Figure 3: (a) Optical setup of the experiment. (b) Orientation of the jet with respect to the laser beams.
  • Figure 4: (a) A schlieren image of the under-expanded jet with $6.5$ NPR. The box indicates the region of interest of the jet and the green circle highlights the tracer. (b) All probe locations are marked with respect to the schlieren image. Blue and red lines indicate locations corresponding to $PD_A$ and $PD_B$ respectively. Location B6 and E6 have been highlighted for further discussions. The yellow scale bar indicates $1$ mm of length.
  • Figure 5: Single-shot CRBS spectra from photodiodes $PD_{A}$ and $PD_{B}$ in quiescent conditions. The dotted gray lines represent the local speed of sound in air. The dotted green curve shows the mean heterodyne used for velocity mapping.
  • ...and 6 more figures