The Development and Characterization of Femtosecond Laser Velocimetry Methods

Abstract

Measurements in high speed wind tunnels and reactive testing facilities require remote, non-perturbing, high-fidelity diagnostic tools. Femtosecond Laser Electronic Excitation Tagging (FLEET) is a recent addition to ultrafast laser diagnostics for fluid dynamic measurements. FLEET is uniquely suited for making difficult measurements: in its most basic form, it requires only a nitrogen-containing flow, a laser system, focusing optics and a gateable camera. The further development of FLEET as a velocimetry tool is presented in this work.

Three aspects of FLEET velocimetry are examined in this work. The first extends FLEET from primarily nitrogen and air environments to flows with argon, oxygen, helium, carbon dioxide, methane, water vapor and freon mixtures. Time-resolved and time-integrated emission are captured to study the temporal behavior of the signal, and spectra are gathered to study molecular and atomic species present in the dynamics associated with FLEET. A zero-dimensional kinetics model is developed to study excited species decay following femtosecond laser excitation in argon-nitrogen and oxygen-nitrogen mixtures. Argon gas, which is frequently used in arcjet facilities, shock tubes, and plasma experiments, enhances the signal and lifetime of FLEET emission when combined with nitrogen. Oxygen has a non-monotonic quenching effect on the signal when it is combined with nitrogen gas.

Secondly, experiments are performed in well-developed subsonic turbulent flow and acoustic measurements are made of the laser pulse to determine the effects of laser heating on the gas. Turbulence statistics are computed for different sets of optical parameters and compared to expected values. Strong laser focusing and high pulse energies appear to perturb the flow enough to mask small features, and weak magnification also introduced artificial correlations into the flow.

Lastly, outcomes from the previous two efforts are synergized to develop FLEET as a tool for near wall measurements, specifically to resolve the viscous sublayer in supersonic flows for skin friction characterization. To minimize flow perturbation, tagging is performed using less than a millijoule of energy per pulse at 400 nm, and magnification and gain are chosen to avoid intensity saturation or cutting off any of the measured region.