Application of FLEET Velocimetry in the NASA Langley 0.3-Meter Transonic Cryogenic Tunnel

Abstract

Femtosecond laser electronic excitation and tagging (FLEET) velocimetry is demonstrated in a large-scale transonic cryogenic wind tunnel. Test conditions include total pressures, total temperatures, and Mach numbers ranging from 15 to 58 psia, 200 to 295 K, and 0.2 to 0.75, respectively. Freestream velocity measurements exhibit accuracies within 1 percent and precisions better than 1 m/s. The measured velocities adhere closely to isentropic flow theory over the domain of temperatures and pressures that were tested. Additional velocity measurements are made within the tunnel boundary layer; virtual trajectories traced out by the FLEET signal are indicative of the characteristic turbulent behavior in this region of the flow, where the unsteadiness increases demonstrably as the wall is approached. Mean velocities taken within the boundary layer are in agreement with theoretical velocity profiles, though the fluctuating velocities exhibit a greater deviation from theoretical predictions.

FLEET Velocimetry Measurements on a Transonic Airfoil

“…The initial studies done in the freestream of the 0.3-m TCT facility seem verify these behaviors. 17 However, with FLEET there are other practical issues that affect the measurement dynamic range.…”

“…Since the FLEET technique requires no seeding beyond nitrogen (which is present in abundance in most facilities) and is experimentally simple to implement, it is being investigated for its use as a velocity measurement technique that could be applied in TCT facilities such as the NTF. FLEET velocimetry has been previously demonstrated in the NASA Langley 0.3-m TCT facility as an excellent marker for velocity measurements in the tunnel freestream 17 and has the potential to measure thermodynamic properties as well. 18 The present study seeks to examine and document the utility of FLEET for making measurements in much more complex flow geometries, specifically those around models.…”

A Review of Femtosecond Laser-Induced Emission Techniques for Combustion and Flow Field Diagnostics

“…The incident 100 fs laser dissociates the nitrogen molecules into atoms, which subsequently recombine into the excited B state of molecular nitrogen, which emits in the red and near infrared through the first positive transition to the A state. FLEET is used in a wide range of applications, including subsonic [134,[139][140][141], transonic [136,[142][143][144], and hypersonic [145][146][147][148] flow fields. At present, FLEET has been successfully applied in nitrogen [134,140,141,[149][150][151][152], argon [139,[153][154][155], air [141,147,156,157], 1,1,1,2-Tetrafluoroethane [158], helium [159], carbon dioxide [159], oxygen [159], and combustion [141,148,160] flow fields.…”

Selective Two-Photon Absorptive Resonance Femtosecond-Laser Electronic-Excitation Tagging (STARFLEET) Velocimetry in Flow and Combustion Diagnostics

“…Figure 4 shows a comparison of STARFLEET and FLEET spectra associated with the second positive (0-1) band of N2 near 358 nm, obtained in a ~300 K nitrogen flow. With normalized signal intensities, the standard FLEET signal exhibits more pronounced rotational band envelope, a broader band head, and more-pronounced presence of a N2 + peak around 358.3 nm [19] than STARFLEET. The N2 + peak for STARFLEET is below the detection limit, but the presence of the FLEET spectrum implies that there is significantly more ionization of nitrogen molecules with FLEET than that of STARFLEET.…”

“…In some cases, such as in hypersonic flows and combustion, it is critical that the high-intensity probe-laser pulses do not perturb the flow fields or chemistry. For velocimetry applications of FLEET in large ground-test facilities [19], high-energy fs laser pulses could also potentially damage the expensive wind tunnel model. Edwards et al [18] measured the effects of FLEET laser heating by observing spectrally resolved fluorescence from the second positive band (C 3 u-B 3 g) of nitrogen.…”