- Dual-Cavity Lasers Keep Up with the Flow
Dr. Callum Gray
Fluid-flow analysis is critical in the aeronautical, automotive, biomedical, chemical, meteorological and environmental industries. Turbulence in pipes or valves can change the flow rate, which can have dire consequences. For example, an instrument that could reveal regions of strong shear in the blood flow through and around artificial heart valves could help prevent the damage to platelets and, ultimately, the thrombosis that such shear can cause.
Finding the right tool, however, can be a challenge. Understanding how a steady flow can change to turbulence has become a multidisciplinary study involving engineers, physicists and mathematicians. Recently, researchers from Delft University of Technology in the Netherlands used a laser to study laminar transitions in a turbulent section, or "puff," along a pipe flow.
They used the FlowMaster HS, a particle image velocimetry system from LaVision GmbH of Göttingen, Germany, to create two- and three-dimensional velocity maps based on imaging the light scattered by small particles in the pipe flow illuminated by a laser light sheet. The system includes two digital cameras used in a stereoscopic configuration, a pulsed laser system from New Wave Research of Fremont, Calif., and a computer that runs control and processing software.
It measures whole velocity fields by taking two consecutive images of the flow and calculating the distance that individual particles have traveled. All three components of the velocity vector field are obtained from a stereoscopic reconstruction of the measured displacements from the two cameras. Velocity is calculated from the known time difference and the measured displacement.
Pulsed lasers are well-suited for these measurements because the flow is very fast and they can pulse at speeds of up to 1 kHz. Using other methods would result in blurred images. For example, employing continuous-wave lasers and flashlamp-based systems, the light-sheet quality is inferior and the flow velocities are too fast.
A cylindrical lens shapes the laser beam into a sheet of light that bisects the flow under study. Microparticles introduced into the flow scatter the laser light and, when captured by the cameras on consecutive frames, reveal their velocity and trajectory. Images analyzed on a point-wise basis produce a grid of velocity values that represent a "snapshot" of the flow in a particular instant.
The CMOS cameras used for the experiment delivered 2000 fps at full resolution and even higher frame rates at partial resolution -- well beyond the capability of conventional CCD cameras. CMOS technology, however, was the easy part. The challenge for high-acquisition particle imaging velocimetry is finding a laser that can provide correspondingly high repetition rates, without compromising beam quality.
In the past, lower acquisition systems used single-cavity Nd:YAG and Nd:YLF sources. These are sufficient for studies of spatial flow and, if necessary, can accelerate their repetition rates by emitting double pulses, releasing part of the cavity's energy in the first pulse and the remainder in the second. This approach, however, introduces restrictions in the available pulse separations, pulse energy and beam quality.
The researchers used a dual-head, diode-pumped Nd:YLF source that generates collinear infrared beams that are intracavity-doubled to deliver 527-nm output. Besides having a much smaller laser head, this dual-head configuration allows the laser to generate pulses with better energy distribution and to accommodate a broader range of experimental conditions than is possible with double-pulsed single-cavity arrangements.
The high sampling rate made it possible for the researchers to fully resolve the temporal evolution of the flow and to map complicated dynamics, such as the time evolution of the kinetic energy in a pipe flow. Specifically, it enabled them to measure a large spike in the kinetic energy that, at conventional sampling rates, would not have been evident. Previously, such measurement of 3-D flow fields and their structure had been possible only through computer modeling (see figure).
Particle image velocimetry, a technique used to characterize flow fields, produced this double-vortex vector map, capturing the motion of vortex rings across the measurement area.
One area that is ripe for development is the extension of time-resolved particle imaging velocimetry to measuring velocities through a volume. Two or more CMOS cameras viewing at oblique angles can map particle trajectories in a volume space. Another method is to scan the rapidly pulsing laser sheet through a volume to make a detailed map of the velocity field. Thus, higher camera and laser repetition rates may lead not only to further temporal information, but also to increased volume velocity information.
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