James F. Meyers

Development of Doppler global velocimetry as a flow diagnostics tool

The development of Doppler global velocimetry is described from its inception to its use as a flow diagnostics tool. Its evolution is traced from an elementary one-component laboratory prototype, to a full three-component configuration operating in a wind tunnel at focal distances exceeding 15 m. As part of the developmental process, several wind tunnel flow field investigations were conducted. These included supersonic flow measurements about an oblique shock, subsonic and supersonic measurements of the vortex flow above a delta wing and three-component measurements of a high-speed jet.

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>>

LDV Surveys Over a Fighter Model at Moderate to High Angles of Attack

The vortex flowfield over an advanced twin-tailed fighter configuration was measured in a low-speed wind tunnel at two angles of attack. The primary test data consisted of 3-component velocity surveys obtained using a Laser Doppler Velocimeter. Laser light sheet and surface flow visualization were also obtained to provide insight into the flowfield structure. Time-averaged velocities and the root mean square of the velocity fluctuations were obtained at two cross-sections above the model. At 15 degrees angle of attack, the vortices generated by the wing leading edge extension (LEX) were unburst over the model and passed outboard of the vertical tail. At 25 degrees angle of attack, the vortices burst in the vicinity of the wing-LEX intersection and impact directly on the vertical tails. The RMS levels of the velocity fluctuations reach values of approximately 30% in the region of the vertical tails.

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.

Measurement of Leading Edge Vortices from a Delta Wing Using a Three Component Laser Velocimeter

A demonstration of the capabilities of a three component laser velocimeter to provide a detailed experimental database of a complex flow field i s presented. The orthogonal three component laser velocimeter was used to measure the leading edge vortex flow field above a 75 degrees delta wing at angles-of-attack of 20.5 degrees and 40.0 degrees. The resulting mean velocity and turbulence intensity measurements are presented. The laser velocimeter is described in detail including a description of the data processing algorithm. A full error analysis was conducted and the results presented.

Shock-Strength Determination With Seeded and Seedless Laser Methods

Two noninvasive laser diagnostics were independently used to measure time-averaged and spatially resolved pressure change across a two-dimensional (2D) shock wave. The first method is Doppler global velocimetry (DGV) which uses water seeding and generates 2D maps of three-orthogonal components of velocity. A DGV-measured change in flow direction behind an oblique shock provides an indirect determination of pressure change across the shock, when used with the known incoming Mach number and ideal shock relations (or Prandtl–Meyer equations for an expansion fan). This approach was demonstrated at Mach 2 on 2D shock and expansion waves generated from a flat plate. This technique also works for temperature change (as well as pressure) and for normal shocks (as well as oblique). The second method, laser-induced thermal acoustics (LITA), is a seedless approach that was used to generate 1D spatial profiles of streamwise Mach number, sound speed, pressure and temperature over the same oblique waves. Excellent agreement was obtained between DGV and LITA, suggesting that either technique is viable for shock-strength measurement.

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.

Investigation of Measurement Errors in Doppler Global Velocimetry

While the initial development phase of Doppler Global Velocimetry (DGV) has been successfully completed, there remains a critical next phase to be conducted, namely the determination of an error budget to provide quantitative bounds for measurements obtained by this technology. This paper describes a laboratory investigation that consisted of a detailed interrogation of potential error sources to determine their contribution to the overall DGV error budget. A few sources of error were obvious; e.g., iodine vapor adsorption lines, optical systems, and camera characteristics. However, additional non-obvious sources were also discovered; e.g., laser frequency and single-frequency stability, media scattering characteristics, and interference fringes. This paper describes each identified error source, its effect on the overall error budget, and where possible, corrective procedures to reduce or eliminate its effect.