Multi-Parameter Measurement in Unseeded Flows using Femtosecond Lasers, Year 3

Our approach is to use new turn-key femtosecond laser technology along with new high-speed CMOS camera technology to build a multi-parameter measurement system based on the Princeton-discovered Femtosecond Laser Electronic Excitation and Tagging (FLEET) technique. Our partners at Princeton and others have recently begun using FLEET to study combustion environments and hypersonic flows. We at NASA realized that FLEET was ideally suited for transonic cryogenic facilities like Langley's National Transonic Facility (NTF) and we discovered that the signal intensity scales directly with density near atmospheric conditions, meaning that it works even better under cryo conditions than at room temperature. Another innovative aspect has been to use high-speed camera acquisition technology, allowing velocity, acceleration and pathlines to be measured from each laser excitation pulse. We have recently demonstrated such measurements in attached and separated airfoil flows in the Langley 0.3 meter cryo tunnel which is the pilot facility for NTF. One goal for this year is to analyze and evaluate this airfoil data. Another is to improve our ability to measure temperature with the technique, having demonstrated a rudimentary method this past year. Finally we will investigate how well the technique works in supersonic and hypersonic flows this year. Funding from another program (ARMD AETC) is being used to implement optical access into the NTF for future measurements. SBIR funding is supporting our partner company to investigate alternate versions of FLEET called STARFLEET, which uses less energy thereby perturbing the flow less, and PLEET which uses a picosecond laser which can operate at ~100 kHz (about 100x faster than FLEET). After this year we anticipate bringing the FLEET, STARFLEET and PLEET technologies to NTF to study the freestream and also flowfields around vehicles.

Currently there is limited or no off-body flow measurement capability in many of Langley's large wind tunnels, including the National Transonic Facility (NTF). Quantitative, multi-parameter flowfield measurements will enable improved understanding of flow physics and provide validation data for computations. Such data may inspire new fluid models (e.g. turbulence models) and would also provide needed boundary conditions for flow computation. The current state of the art technology includes physical probes (pitot, static pressures, thermocouples). Also some laser-based techniques use particles or gases seeded into a flow for velocity measurement. However, probes and seed particles/gases cannot always be used. Sometimes they perturb the flow or impact the facility. Thus there is a significant need for molecular-based measurement techniques for air or N2 flows. Measurements of velocity, density, pressure, temperate and concentration are needed.

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