Molecular Tagging Velocimetry in Hypersonic Flows
Summary
Molecular tagging velocimetry (MTV) has emerged as a cornerstone non-intrusive diagnostic for measuring velocity fields in hypersonic wind tunnels and impulse facilities. By introducing trace amounts of a taggable molecular species into a high-speed flow and using short-pulse lasers to excite or dissociate that species along a line or grid, MTV techniques create visible markers that deform and advect with the gas. Time-resolved imaging of these markers then yields quantitative velocity profiles with sub-millimetre spatial resolution and microsecond temporal resolution. In hypersonic regimes, where extreme temperatures, low densities and shock-induced chemistry challenge traditional probes, MTV provides unique access to freestream velocities, boundary-layer development and shock–boundary-layer interactions. Advances in femtosecond and picosecond laser systems, together with optimised tagging chemistries such as acetone and nitric oxide, have extended MTV measurements to Mach 6–8 flows and beyond. These developments support validation of computational fluid dynamics, inform the design of thermal protection systems and guide the development of next-generation hypersonic vehicles.
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Molecular Tagging Velocimetry in Hypersonic Flows publication trend
The graph below shows the total number of articles in molecular tagging velocimetry in hypersonic flows across all publications each year (not limited to Nature Index journals).
Technical terms
Molecular tagging velocimetry: A laser-based method that marks and tracks fluid regions via tagged molecules to measure velocity fields non-intrusively.
Femtosecond laser electronic excitation tagging (FLEET): An MTV variant using ultrashort (∼10^–15 s) laser pulses to excite gas molecules, creating high-contrast fluorescent lines for velocimetry.
Picosecond laser electronic excitation tagging (PLEET): A related approach employing slightly longer (∼10^–12 s) pulses to generate tagged structures, often at higher repetition rates and beam energies.
Ludwieg tube: A pulse-tube wind-tunnel facility that produces stable, short-duration supersonic or hypersonic flows for high-fidelity testing.
Boundary layer: The thin region of velocity gradient adjacent to a surface, whose structure is critical in hypersonic heat-transfer and drag predictions.
References
- 100 kHz PLEET velocimetry in a Mach-6 Ludwieg tube.. Optics Express (2020).
- Velocity measurements in a hypersonic flow using acetone molecular tagging velocimetry.. Optics Express (2022).
- Hypersonic FLEET velocimetry and uncertainty characterization in a tripped boundary layer. Measurement Science and Technology (2023).
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