The ability to capture the unsteady dynamics of high-speed compressible flows requires the use of ultrafast imaging diagnostics. Application of particulate-based scattering techniques to these types of flows also requires the use of high-power,pulsed illumination sources such as Nd:YAG solid-state lasers. High-accuracy flow-field measurements having both high spatial and temporal bandwidth have not been previously available because of the combined constraints imposed by the limited frame rate of high-sensitivity digital image sensors and the limited pulse repetition rates of commercially available solid-state lasers. To address this shortcoming and the need for a flow diagnostic system that satisfies the requirements mentioned, researchers at the NASA Glenn Research Center developed a high-resolution, high-framing-rate quantitative flow-measurement imaging system.
Left: MHz-repetition-rate pulse-burst-mode laser. Right: MHz-frame-rate camera.
The imaging system is composed of a megahertz- (MHz-) repetition-rate pulse-burst-mode laser (built in-house) and a commercial MHz-framing-rate charge-coupled device (CCD) camera. The laser and camera are shown in the left and right photographs, respectively. The laser contains six flashlamp-pumped Nd:YAG amplifier stages and can produce a burst train of high-energy pulses (on the order of 10) during each flashlamp discharge period, with individual pulse energies exceeding 75 mJ at 532 nm. Novel features of the laser include the use of fast-acting Pockels cells for pulse-train generation and a phase-conjugate mirror for removing parasitic background light in the pulse train. The camera, having a single optical input, comprises an internal image splitter unit and four individual intensified, frame-straddling, high-resolution 1280- by 1024-pixel CCD camera modules. The intensifiers allow for a sequence of eight full-resolution frames to be acquired in rapid succession. The combined laser and camera system provides imaging frame rates ranging from 50 kHz to 2 MHz, suitable for use in high-speed, unsteady, light-scattering-based velocimetry measurement techniques.
The new imaging system was used to obtain, for the first time, MHz-repetition-rate digital particle image velocimetry (DPIV) data from a supersonic nozzle flow with pulsed microjet actuation. The pulsed train from the laser was formed into a sheet, which was used to illuminate the particulate seeded nozzle flow. The high-speed imaging camera was used to record the positions of the articles at each pulse in the illumination train. The acquired image data were cross-correlation processed to obtain the velocity fields. The resulting sequence of velocity vector maps for the pulsed nozzle flow is shown in the following figure, where a high-speed fluid packet is ejected from the nozzle core flow.
Sequence of velocity vector maps of supersonic nozzle flow with pulsed microjet excitation obtained via MHz-frame-rate DPIV.
Ongoing work to extend the capabilities of the imaging system includes making planar Doppler velocimetry measurements by replacing the existing camera optics with nonpolarizing optical components, reducing the laser head form factor to improve portability, and enhancing the temperature stabilization of thermally sensitive laser components to improve ruggedness in harsh testing environments.
Wernet, M.; and Opalski, A.: Development and Application of a MHz Frame Rate Digital Particle Image Velocimetry System. AIAA-2004-2184, 2004.
Find out more about this research.
QSS Group, Inc., contact: Dr. Anthony B. Opalski, 216-433-3908, Anthony.B.Opalski@grc.nasa.gov
Glenn contact: Dr. Mark P. Wernet, 216-433-3752, Mark.P.Wernet@nasa.gov
Authors: Dr. Anthony B. Opalski and Dr. Mark P. Wernet
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