Application of femtosecond laser electronic excitation tagging (FLEET) velocimetry in a bladeless turbine

Abstract

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.

Localized time accurate sampling of nonequilibrium and unsteady hypersonic flows: methods and horizons

“…FLEET velocimetry has been also demonstrated in other complex flow environments, such as in combustion (DeLuca et al 2017), in the NASA Langley high-pressure cryogenic tunnel (Burns et al 2018a(Burns et al , 2018b, in bladeless wind turbines (Fisher et al 2020a), in the Quiet Mach 6 tunnel at Purdue (Fisher et al 2021), in a high-enthalpy arc-jet tunnel (UTA 2020) and in the wake of a cone at Mach 8 at Sandia (Zhang et al 2019). Multiple line FLEET has been applied at Sandia (Zhang et al.…”

Multi-point FLEET velocimetry in a Mach 4 Ludwieg tube using a diffractive optical element

“…An excellent review of the theory and previous uses of FLEET is provided by Danehy et al [15]. The FLEET technique has had great success in high-speed wind tunnel measurements, including in the Texas A&M ACE tunnel [16], the Purdue Mach 6 Quiet Ludwieg Tube [17], the AFRL Mach 6 Ludwieg Tube [18], and AEDC Tunnel 9 at Mach 18 [19]. FLEET utilizes a focused femtosecond laser pulse to dissociate nitrogen molecules in air which then recombine and slowly decays back down to the ground state, causing a microseconds-long relatively broadband emission in the visible spectrum [10].…”

“…There has been recent interest in extending the capabilities of FLEET, in particular by writing FLEET lines at multiple locations [17], which provides more information about the local flow field. Zhang et al [20] were able to use periodic masks in front of focusing lenses to write multiple FLEET lines.…”