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Carderock Division Researchers Enhance Data-Gathering Capabilities with New Stereo Particle Image Velocimetry System

By By Dustin Q. Diaz, NSWC Carderock Division Public Affairs | Aug. 17, 2016

WEST BETHESDA, Md. —

Researchers from Naval Surface Warfare Center, Carderock Division were recognized at an all-hands meeting July 21 for creating a self-contained submersible stereo particle image velocimetry.

The scientists and engineers from Carderock's Naval Architecture and Engineering Department not only created, but successfully tested the system in the David Taylor Model Basin at Carderock in the spring.

Dr. Emily Harrison, a naval scientist with the Hydrodynamics and Maneuvering Testing Branch, said the completion of this multi-year project produced the best particle image velocimetry results ever seen at Carderock and will save the government money in testing time moving forward.

"This test produced the largest data set of the highest quality data we've seen to date," Harrison said.

Particle image velocimetry is a technique to measure fluid velocity, done at Carderock by taking multiple images of water and comparing the distance and direction particles in that water move from one image to the next. Harrison said this is done using a laser camera-based craft at Carderock to validate computational fluid dynamics models used in ship design. Conventional particle image velocimetry is done in a two-dimensional grid, providing a velocity field and two components of velocity. Stereo particle image velocimetry offers multiple advantages, including a second camera that introduces a third dimension of data, and this system facilitates the newest iteration of stereo particle image velocimetry in use at Carderock.

"Stereo particle image velocimetry gives you two cameras that look at the same physical field, but it allows you to resolve the third component," Harrison said. "In our case, the two cameras are separated by a 30 degree-looking angle, but both are contained in a single pipe-shaped system we call the 'torpedo.' The advantage of using the torpedo to do PIV is that we are dragging it in one dimension, so we are not creating a giant wake by the measurement system itself. This minimizes the effect of the instrument on the flow field you're measuring."

The new system was designed, assembled and instrumented in house by about 25 employees in five divisions within the Naval Architecture and Engineering Department, led by the Submarine Maneuvering and Control Division. Harrison was the test lead for the system, which has a 2 feet by 2 feet measurement plane with more than 20,000 velocity points that can be traversed beneath the system's carriage.

"This system was built as an enclosed unit, so when you want to go use it, you can just take it, put it in the water, attach to the carriage and you're good to go," Harrison said. "You don't have to take it apart and reassemble it between uses like our previous system."

Harrison said the system takes velocity measurements at eight feet under water to avoid surface effects and also brings a significant increase in camera and laser technology that improves the quality and quantity of data the system generates, which is important in larger facilities like those at Carderock.

"The amount of laser intensity you're imaging gets weaker over a larger field," Harrison said. "Originally, these were used for small water tanks, but as you expand, everything becomes harder. We're now using a more powerful laser, so this was a significant upgrade in laser quality and quantity in the towing basin. The same is true of the camera technology."

The use of this laser during the test, which ran from late February through late May in the J-Basin of the David Taylor Model Basin, was designed to withstand being in the water for so long, since cameras and lasers normally aren't designed to be submerged for so long, Harrison said.

"This one is instrumented with a full environmental monitoring system, so that we can keep an eye on it," Harrison said. "We can make sure the instrumentation is behaving as it should, that it's not in any sort of trouble with temperature and relative humidity, and we can keep it at its peak condition during testing. It paid off in the end; we had no issues.

"We were all very excited to see it test and the quality of the data we got out of the system," Harrison added. "The system itself is a very unique tool that we now have."