An official website of the United States government
In this work, a detailed calibration study is performed to establish non-intrusive one-dimensional (1D) rovibrational temperature measurements in unseeded air, based on air resonance enhanced multiphoton ionization thermometry (ART). ART is generated by REMPI (resonance enhanced multi-photon ionization) of molecular oxygen and subsequent avalanche ionization of molecular nitrogen in a single laser pulse. ART signal, the fluorescence from the first negative band of molecular nitrogen, is directly proportional to the 2-photon transition of molecular oxygen C3Π (v = 2) ← X3Σ (v’=0), which is used to determine temperature. Experimentally, hyperfine structures of the O2rotational branches with high temperature sensitivity are selectively excited through a frequency-doubled dye laser. Electron-avalanche ionization of N2results in the fluorescence emissions from the first negative bands of N2+near 390, 425, and 430nm, which are captured as a 1D line by a gated intensified camera. Post processing of the N2+fluorescence yields a 1D thermometry line that is representative of the air temperature. It is demonstrated that the technique provides ART fluorescence of ∼5cm in length in the unseeded air, presenting an attractive thermometry solution for high-speed wind tunnels and other ground test facilities.
more »
« less
Air resonance enhanced multiphoton ionization tagging velocimetry
Air resonance enhanced multiphoton ionization (REMPI) tagging velocimetry (ART) was demonstrated in quiescent and supersonic flows. The ART velocimetry method utilizes a wavelength tunable laser beam to resonantly ionize molecular oxygen in air and generate additional avalanche-type ionization of molecular nitrogen. The fluorescence emissions from the first negative and first positive bands of molecular nitrogen are, thus, produced and used for flow tagging. Detailed characterization of ART was conducted, including the effects of oxygen resonance to fluoresce nitrogen, nitrogen fluorescence spectrum, laser energy deposition into quiescent flow showing minimal perturbations in flow, fluorescence lifetime study at various pressures, and line tagging without breakdown. Pointwise velocity measurements within a supersonic flow from a nominal Mach 1.5 nozzle have been conducted and characterized.
more »
« less
- Award ID(s):
- 2026242
- PAR ID:
- 10531269
- Publisher / Repository:
- Optical Society of America
- Date Published:
- Journal Name:
- Applied Optics
- Volume:
- 61
- Issue:
- 13
- ISSN:
- 1559-128X; APOPAI
- Format(s):
- Medium: X Size: Article No. 3748
- Size(s):
- Article No. 3748
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Detailed characterizations of picosecond laser electronic excitation tagging (PLEET) in pure nitrogen ( ) and air with a 24 ps burst-mode laser system have been conducted. The burst-mode laser system is seeded with a 200 fs broadband seeding laser to achieve short pulse duration. As a non-intrusive molecular tagging velocimetry (MTV) technique, PLEET achieves “writing” via photo-dissociating nitrogen molecules and “tracking” by imaging the molecular nitrogen emissions. Key characteristics and performance of utilization of a 24 ps pulse-burst laser for MTV were obtained, including lifetime of the nitrogen emissions, power dependence, pressure dependence, and local flow heating by the laser pulses. Based on the experimental results and physical mechanisms of PLEET, 24 ps PLEET can produce similar 100 kHz molecular nitrogen emissions by photodissociation, while generating less flow disturbance by reducing laser joule heating than 100 ps PLEET.more » « less
-
Holger, Babinsky (Ed.)Abstract This paper presents experimental studies on a novel active high-frequency coaxial injector system designed for enhanced flow mixing and control at extreme flow velocity conditions. The flow dynamics and mixing characteristics of the system operating at 15kHz were captured using planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) techniques and compared against its steady and baseline modes. In pulsed mode, this active injection system delivers a pulsed supersonic actuation air jet at the inner core of the coaxial nozzle that provides large mean and fluctuating velocity profiles in the shear layers of an acetone-seeded fluid stream injected surrounding the core through an annular nozzle. The instantaneous velocity, vorticity and acetone concentration fields of the injector are discussed. The high-frequency streamwise vortices and shockwaves tailored to the mean flow significantly enhanced supersonic flow mixing between the fluids compared to a classical steady coaxial configuration operating at the same input pressure. The paper analyses the dynamic and diffusion characteristics of this active coaxial injection system, which may have potential for supersonic mixing applications.more » « less
-
AIAA (Ed.)With a focus on improving mixing at extreme flow velocity conditions, this paper presents planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) studies on the flowfield of a high-speed, pulsed co-flow system integrated with a high-frequency actuator operating at 15 kHz. This active injection system delivers a supersonic pulsed actuation air jet at the inner core of the co-axial nozzle that provides large mean and fluctuating velocity profiles in the shear layers of a fluid stream injected surrounding the core through an annular nozzle. The instantaneous velocity, vorticity, and acetone concentration fields of the injector in three distinct modes of operation – pulsed actuation, steady actuation, and without actuation -are presented. The high-frequency streamwise vortices and shockwaves tailored to the mean flow significantly enhanced supersonic flow mixing between the fluids compared to the steady co-axial configuration operating at the same input pressure. The study analyzes the mixing and dynamic characteristics of this active co-axial injection system, which has the potential for supersonic mixing applications.more » « less
-
As the field of fluid dynamics progresses, the demand for sophisticated diagnostic methods to accurately assess flow conditions rises. In this work, resonantly ionized photoemission thermometry (RIPT) has been used to directly target and ionize diatomic nitrogen (N2) to measure one-dimensional (1D) temperature profiles in a supersonic jet flow. This technique can be considered non-intrusive as the premise uses resonantly enhanced multiphoton ionization (REMPI) to target molecular nitrogen. This resonance excites N2into absorption bands of the P, Q, and R rotational branches of N2(b1Πu). The ideal (3 + 1) REMPI scheme excites from the ground state and ionizes N2(b1Πu←X1Σg+) where de-excitation results in photoemission from the first negative band of ionizedN2+(B2Σu+→X2Σg+) as nitrogen returns to the ground state. The resulting emission can be observed using an intensified camera, thus permitting inference of the rotational temperature of ground-state molecular nitrogen. A linearly regressive Boltzmann distribution is applied based on previous calibration data for this technique to quantify the temperature along the ionized line. This work applies this technique to a pure N2supersonic jet in cross-flow and counter-flow orientations to demonstrate N2RIPT’s applications in a supersonic flow. Temperature variations are observed at different locations downstream of the exit in cross-flow, and axisymmetric in counter-flow, to generate profiles characterizing the flow dynamics. Due to the collisional effects resulting from the number density of N2at higher pressures, a (3 + 2) REMPI scheme is observed throughout this text.more » « less
