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Light Sheets and Cameras Track Fluid Flow

Photonics Spectra
Apr 2004
Hank Hogan

At Dantec Dynamics A/S in Skovlunde, Denmark, they like to go with the flow -- and to help others to do likewise. The company and its subsidiary, Dantec Dynamics Inc. of Mahwah, N.J., specialize in fluid flow measurement systems that enable airplane manufacturers, drugmakers and others to investigate and optimize everything from the flow of air over a wing to the proper way to uniformly mix cough syrup.

Light Sheets and Cameras Track Fluid Flow
Time-resolved particle image velocimetry systems are used in fundamental research, such as the investigation of flow fields around a 20-mm cylinder. Courtesy of Universität Rostock.

The Dantec systems and the particle image velocimetry they provide depend upon two key photonic components: a laser capable of producing double pulses and a high-performance CCD or CMOS camera. "Once you've got a laser and a camera, you can measure a lot of other things than just fluid velocity," said Carsten Westergaard, division manager of the parent company. Additional parameters that can be extracted with these related methods include temperature and concentration. Other techniques, such as laser-induced fluorescence, also can shed light, literally and figuratively, on what's happening in a given process.

In particle image velocimetry, flow is measured by comparing two snapshots of the target area. The target area is seeded with small particles, such as 1- to 5-µm oil droplets in air and 0.2- to 100-µm polystyrene, polyamide or hollow glass spheres in water and other fluids. The choice of size and material is determined based on the need to have the particles flow within the fluid under study and on their ability to detectably scatter light.

Light Sheets and Cameras Track Fluid Flow
The particle image velocimetry systems from Dantec Dynamics A/S incorporate CCD- and CMOS-based cameras from various OEMs.

The latter is important because the technique works by sending a laser pulse into the target area and capturing an image using a camera. The pulse travels through cylindrical optics, transforming it into a light sheet that strikes the seed particles, causing them to sparkle. The light from those shimmering points is imaged by optics onto a CCD- or CMOS-based camera. A second pulse results in a subsequent snapshot of the particle field. The two snapshots are broken down into small interrogation areas.

Because the light travels in a sheet, comparing the two frames results in a flow measurement for the entire target area. This is only in two dimensions, but a third dimension can be added by mounting a second camera at an angle to the first.

The laser can be of any type that can produce two pulses with a specific delay. "The laser that we use should preferably be able to fire two pulses that can be separated in time down to some hundred nanoseconds and up to several seconds," said Kim Jensen, president of Dantec's US subsidiary. The fluid flow velocity and the system's spatial resolution determine the necessary pulse delay.

There must be enough movement of the seed particles to make a measurable difference but not so much that correlation within an interrogation area is lost. As a rule, the particles should travel less than 25 percent of the distance across the area between each laser shot. There also must be enough particles -- 10 or more -- within each interrogation area to make the correlation work and to make the measurements relatively immune to noise.

The laser choice is driven by both the quantum efficiency of the camera and the power required, but the beam color is typically green, such as the 532-nm light emitted by a frequency-doubled Nd:YAG. Green often is used because the sensors efficiently capture light at this wavelength and because it suitably matches the blue-green transmission window of water. In terms of power, even if each shot requires only 10 mJ, repeating this thousands of times a second demands a laser that can produce tens of watts, perhaps even 100.

Dantec uses CCD- and CMOS-based instruments from five OEMs, including Redlake of San Diego, which supplies both types. For the CCD systems, Westergaard said, it is important that the cameras be able to frame-straddle, where one exposure is taken, followed by another within about 200 ns.

A camera also must have both a high framing rate and a high pixel count. The two fundamentally affect the system and its capabilities, and they play a role, along with the optics, in determining the size of the interrogation area. They also determine the spatial and temporal resolution possible with the system. Today's cameras, with framing rates of approximately 5 kHz and resolutions of a few megapixels, are adequate for the current Dantec systems, but the company's designers would prefer hardware with more muscle.

"We'd like to have a camera that would ideally hold something like four megapixels with a pixel pitch of 5 µm, and it should be able to run at a framing rate of 10 kHz," Jensen said. "Those specifications would be enough to put a lot of gray hairs in any camera manufacturer's life."


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