Velocity-Sensitive Visualization of CO2 Clusters in Underexpanded Jet via Mercury SLIS of UV Rayleigh Scattering

A method of velocity-sensitive visualization using CO2 clusters as a seed for Doppler-shifted Rayleigh scattering is developed based on Slow Light Imaging Spectroscopy (SLIS). In regions of low pressure and static temperature, a CO2 fog with a large scattering cross section forms as the gas nucleates. The Rayleigh scattering signal from these clusters is spectrally resolved using slow light imaging spectroscopy through a mercury vapor cell, where Rayleigh scattering near the mercury resonance is temporally delayed due to the mercury vapor refractive index dispersion. This technique is used to selectively slow down a narrow spectral region corresponding to a specific Doppler-shifted Rayleigh scattering component and to visualize a region of the flowfield with a chosen velocity magnitude and direction. Utilization of mercury-based SLIS has a few key advantages: higher scattering cross-section of the UV light near 254 nm and effective suppression of unwanted spectral components, such as broadband laser scattering, via delaying the camera gate by a few tens of nanoseconds.