研究目的
To characterize selective two-photon absorptive resonance femtosecond laser electronic excitation tagging (STARFLEET) velocimetry for the first time at high-pressure, low-temperature conditions in a cryogenic wind tunnel.
研究成果
STARFLEET velocimetry was successfully characterized for high-pressure, low-temperature conditions in a cryogenic wind tunnel. The technique demonstrated a standard error of 1.6% in velocity measurements, with precision ranging from 1.5% to 10% of the freestream velocity. Despite limitations such as energy losses and the need for expensive optics, STARFLEET is a viable diagnostic technique for velocimetry measurements in large-scale, high-pressure, cryogenic wind tunnels, especially where surface damage or thermometry must be considered.
研究不足
The technique's precision is worse than measurements made using FLEET or PLEET, partially due to reduced signal-to-noise ratio from energy losses at mirrors. The requirement for expensive MgF2 optics and the significant reduction of energy reaching the measurement volume due to oxygen absorption and mirror reflectivity are also limitations.
1：Experimental Design and Method Selection:
The study utilized STARFLEET velocimetry in the NASA Langley Research Center’s 0.3 meter transonic, cryogenic wind tunnel. The technique involves using a femtosecond laser to excite nitrogen molecules for velocimetry measurements.
2：3 meter transonic, cryogenic wind tunnel. The technique involves using a femtosecond laser to excite nitrogen molecules for velocimetry measurements.
Sample Selection and Data Sources:
2. Sample Selection and Data Sources: The experiments were conducted in pure nitrogen to allow for the highest Reynolds numbers and optimal performance of the STARFLEET technique. Flow conditions spanned total pressures from 100 kPa to 517 kPa, total temperatures from 80 K to 327 K, and Mach numbers from 0.2 to 0.
3：2 to List of Experimental Equipment and Materials:
85.
3. List of Experimental Equipment and Materials: A regeneratively amplified Ti:sapphire laser system (Spectra-Physics Solstice) was used as the laser source. The setup included a fourth-harmonic generator (FHG) to create the 201.75 nm light for writing the STARFLEET line, MgF2 windows and focusing lenses, and a UV high-speed image intensifier (LaVision HS-IRO) coupled to a high-speed CMOS camera (Photron SA-Z) for imaging.
4：75 nm light for writing the STARFLEET line, MgF2 windows and focusing lenses, and a UV high-speed image intensifier (LaVision HS-IRO) coupled to a high-speed CMOS camera (Photron SA-Z) for imaging.
Experimental Procedures and Operational Workflow:
4. Experimental Procedures and Operational Workflow: The laser beam was directed to the wind tunnel, focused within the test section, and the resultant STARFLEET signal was imaged. The imaging system captured 12 frames of data for each run condition, with the first frame used for cleaning, the second for background, and the last 10 for STARFLEET signal at various time delays.
5：Data Analysis Methods:
Data processing included background subtraction, image dewarping, peak-signal determination using Gaussian fitting, and velocity and signal lifetime calculations through linear regression and bi-exponential fitting, respectively.
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