Burst-mode femtosecond laser electronic excitation tagging (FLEET) of nitrogen is introduced for tracking the velocity field in high-speed flows at kilohertz–megahertz (kHz–MHz) repetition rates without the use of added tracers. A custom-built Nd:glass femtosecond laser is used to produce 500 pulses per burst with pulses having a temporal separation as short as 1 µs, an energy of 120 µJ, and a duration of 274 fs. This enables 2 orders of magnitude higher measurement bandwidth over conventional kHz-rate FLEET velocimetry. Characteristics of the optical system are described, along with a demonstration of time-resolved velocity measurements with ∼ 0.5 % precision in a supersonic slot jet.

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A novel, to the best of our knowledge, optical arrangement is evaluated for performing single-shot femtosecond laser electronic excitation tagging in a 16-point grid (Grid-FLEET) with single-ended optical access. The optical arrangement includes a diffractive optical element beam splitter to produce a grid of laser beams in a simplified, flexible, and efficient manner for tracer-free multi-component molecular tagging velocimetry in a two-dimensional field. Analysis of the optical element with respect to beam forming is described, and Grid-FLEET measurements are evaluated relative to the precision of previously described single-point FLEET measurements using Lagrangian tracking for flow in a laminar jet and around a sharp corner. Utilizing a conventional 1-kHz laser source coupled to a high-speed intensified camera, it is also feasible to achieve measurement rates of 100 kHz or higher by mapping the Lagrangian grid to one or more Eulerian measurement points. The data further indicate that enhancement of the instantaneous vector fields and spatial velocity gradients can be analyzed to enhance the understanding of multi-dimensional flow physics in applications in which the use of tracers may be difficult and where multi-directional optical access may be limited.

Shock waves appear in numerous high-speed propulsion applications, including intakes, nozzles, and transonic and supersonic turbomachinery. The aerodynamic performance in bladeless turbines, which is designed for work extraction under such conditions, is dominated by flow separation induced by shock-wave pressure gradients. The large velocity gradients pose limitations on flowfield characterization using particle-based optical diagnostics, such as particle image velocimetry and laser Doppler anemometry. These limitations, along with challenges in seeding the flow, can be overcome by tracking the molecules already present in the flow. This paper presents kHz-rate femtosecond laser electronic excitation tagging (FLEET) to excite long-lived fluorescence of nitrogen molecules, acting as in-situ flow tracers. A multi-point variation of this approach was demonstrated in an optically accessible linear turbine test section, developed to investigate bladeless turbines. The femtosecond laser is coupled with an intensified CMOS camera with a frame rate of 200 kHz. High-speed measurements were made of the steady and unsteady performance in the bladeless turbine, with particular attention to capturing flow structures, spatial velocity gradients, and transient events such as unstarting of the supersonic passages.