Joseph W. Lee

Characterization of measurement error sources in Doppler global velocimetry

Doppler global velocimetry uses the absorption characteristics of iodine vapour to provide instantaneous three-component measurements of flow velocity within a plane defined by a laser light sheet. Although the technology is straightforward, its utilization as a flow diagnostics tool requires hardening of the optical system and careful attention to detail during data acquisition and processing if routine use in wind tunnel applications is to be achieved. A development programme that reaches these goals is presented. Theoretical and experimental investigations were conducted on each technology element to determine methods that increase measurement accuracy and repeatability. Enhancements resulting from these investigations included methods to ensure iodine vapour calibration stability, single frequency operation of the laser and image alignment to sub-pixel accuracies. Methods were also developed to improve system calibration, and eliminate spatial variations of optical frequency in the laser output, spatial variations in optical transmissivity and perspective and optical distortions in the data images. Each of these enhancements is described and experimental examples given to illustrate the improved measurement performance obtained by the enhancement. The culmination of this investigation was the measured velocity profile of a rotating wheel resulting in a 1.75% error in the mean with a standard deviation of 0.5 m s-1. Comparing measurements of a jet flow with corresponding Pitot measurements validated the use of these methods for flow field applications.

Pulsed-laser vibrometer using photoelectromotive-force sensors

We demonstrate experimentally significant improvement in the sensitivity of photoelectromotive-force (photo-EMF) laser vibrometers using pulsed-light sources. The vibrating surface is discretely sampled by individual laser pulses and recorded by the photo-EMF sensor via the generation of photocurrent pulses whose magnitudes are proportional to the instantaneous surface displacements. With a sufficiently high sampling rate, reconstruction of the vibration wave form can be achieved by conducting envelope (or peak) detection of the resultant series of photocurrent pulses. Significantly higher peak optical power levels of the probe laser pulses, which can be orders of magnitude greater than those of continuous-wave interrogation lasers with the same average power, lead to proportional enhancement in the photo-EMF response and remarkable improvement in detection sensitivity when the photodetection process is initially amplifier noise current limited.

Signal processing schemes for Doppler global velocimetry

Two schemes for processing signals obtained from the Doppler global velocimeter are described. The analog approach is a simple, real-time method for obtaining an RS-170 video signal containing the normalized intensity image. Pseudocolors are added using a monochromatic frame grabber producing a standard NTSC video signal that can be monitored and/or recorded. The digital approach is more complicated, but maintains the full resolution of the acquisition cameras with the ability to correct the signal image for pixel sensitivity variations band to remove background light. Prototype circuits for each scheme are described, and example results from the investigation of the vortical flow field above a 75 degrees delta wing are presented.<<ETX>>

Boron nitride nanotube: synthesis and applications

Scientists have predicted that carbon’s immediate neighbors on the periodic chart, boron and nitrogen, may also form perfect nanotubes, since the advent of carbon nanotubes (CNTs) in 1991. First proposed then synthesized by researchers at UC Berkeley in the mid 1990’s, the boron nitride nanotube (BNNT) has proven very difficult to make until now. Herein we provide an update on a catalyst-free method for synthesizing highly crystalline, small diameter BNNTs with a high aspect ratio using a high power laser under a high pressure and high temperature environment first discovered jointly by NASA/NIA/JSA. Progress in purification methods, dispersion studies, BNNT mat and composite formation, and modeling and diagnostics will also be presented. The white BNNTs offer extraordinary properties including neutron radiation shielding, piezoelectricity, thermal oxidative stability (> 800°C in air), mechanical strength, and toughness. The characteristics of the novel BNNTs and BNNT polymer composites and their potential applications are discussed.

Single-pulse Multi-point Multi-component Interferometric Rayleigh Scattering Velocimeter

A simultaneous multi-point, multi-component velocimeter using interferometric detection of the Doppler shift of Rayleigh, Mie, and Rayleigh-Brillouin scattered light in supersonic flow is described. The system uses up to three sets of collection optics and one beam combiner for the reference laser light to form a single collimated beam. The planar Fabry-Perot interferometer used in the imaging mode for frequency detection preserves the spatial distribution of the signal reasonably well. Single-pulse multi-points measurements of up to two orthogonal and one non-orthogonal components of velocity in a Mach 2 free jet were performed to demonstrate the technique. The average velocity measurements show a close agreement with the CFD calculations using the VULCAN code.

Intracavity Rayleigh-Mie scattering for multipoint, two-component velocity measurement

A simultaneous multipoint, two-component Doppler velocimeter is described. The system uses two optical cavities: a Fabry-Perot etalon and an optical cavity for collecting and recirculating the Rayleigh-Mie-scattered light that is collected from the measurement volume in two parallel but opposite directions. Single-pulse measurements of two orthogonal components of the velocity vector in a supersonic free jet were performed to demonstrate the technique. The recirculation of the light rejected by the interferometer input mirror also increased the signal intensity by a factor of 3.5.

Doppler global velocimetry measurements of the vortical flow above an F/A-18

A Doppler global velocimeter was used to investigate the vortical flow above an F/A-18 model at 25-degrees angle of attack. The measurements indicate that the flow had the same characteristics as the vortical flow above a standard delta wing. The flow pattern indicating transition from stable to burst conditions found above the delta wing was also found at the 440 station above the F/A-18. Measurements downstream at the 524 station found that the flow velocity varied considerably, with standard deviations reaching 30 percent of free stream. However, individual data images indicated that the flow was spatially coherent, and not chaotic as expected.

Investigation of the vortical flow above an F/A-18 using Doppler global velocimetry

The flow above an F/A-18 model, set to 25-degrees angle of attack, was measured using a Doppler global velocimeter, (DGV). The investigation indicated that the complex flow contained many similarities to the vortical flow above a simple delta wing set to a high-angle of attack, including flow standard deviations greater than 30-percent of free-stream. These standard deviation levels were also comparable to results found during a previous investigation of the vortical flow above a YF-17 using fringe-type laser velocimetry. The global measurement capability of the DGV provided the first evidence that the burst vortices above the model were structured. These structures were found to maintain their spatial coherence while the flow varied in an overall sense.

Mobile CARS - IRS Instrument for Simultaneous Spectroscopic Measurement of Multiple Properties in Gaseous Flows

This paper describes a measurement system based on the dual-pump coherent anti-Stokes Raman spectroscopy (CARS) and interferometric Rayleigh scattering (IRS) methods. The IRS measurement is performed simultaneously with the CARS measurement using a common green laser beam as a narrow-band light source. The mobile CARS-IRS instrument is designed for the use both in laboratories as well as in ground-based combustion test facilities. Furthermore, it is designed to be easily transported between laboratory and test facility. It performs single-point spatially and temporally resolved simultaneous measurements of temperature, species mole fraction of N2, O2, and H2, and two-components of velocity. A mobile laser system can be placed inside or outside the test facility, while a beam receiving and monitoring system is placed near the measurement location. Measurements in a laboratory small-scale Mach 1.6 H2-air combustion-heated supersonic jet were performed to test the capability of the system. Final setup and pretests of a larger scale reacting jet are ongoing at NASA Langley Research Centers Direct Connect Supersonic Combustor Test Facility (DCSCTF).

Increased Accuracy in Molecular Filter Based Flow Field Diagnostics Through Direct Frequency Calibration Using Optical Modulators

*† A description of technique and appar atus for determining the transfer function of molecular filters used in laser -based flow diagnostics is presented. The technique employs the use of a frequency stabilized argon ion laser and an Acousto -Optic Frequency Shifter (AOFS) to accurately determine the frequency -to -optical density transfer function of molecular iodine. An argon ion laser that is locked in frequency to a selected location on a molecular filter absorption line provides a stable, low -drift optical frequency input to the AOFS. Advances in crystal technology have resulted in the development of AOFS systems that can be varied in frequency without significant loss of power (<3 dB) in the shifted laser beam. Driving the frequency shifter with a computer -controlled synthesizer provides a very accurate and precise means of controlling the Bragg -shifted laser output from the AOFS. By directing the frequency -shifted laser beam through a molecular filter and recording the laser intensity throughput as a function of the shifted optical frequency, a profile of the filter transfer function has been achieved with rms uncertainties less than 0.1 -percent of the absorption linewidth.