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Carderock's Anechoic Flow Facility Experiences Unprecedented Turbulence

By Dustin Q. Diaz, Naval Surface Warfare Center, Carderock Division Public Affairs | February 14, 2017


Years of work by engineers at Naval Surface Warfare Center, Carderock Division's Anechoic Flow Facility (AFF) to revitalize the facility are paying off today, with unprecedented research now underway thanks to new gear.

The AFF is a quiet, closed-loop, low-speed air-flow facility with low turbulence levels, built in the 1960s to conduct flow-generated noise studies. According to Jonathan Forest, a research engineer in the Hydroacoustics and Propulsor Development Branch (Code 725), the facility's new Active Turbulence Grid (ATG) will now allow it to fulfill a different function, one completely new to the facility.

"When this tunnel was first designed, the intention was to measure things like mean flow and acoustic phenomena under very low turbulence intensities," Forest said. "This upgrade allows us to create a very different flow field with a lot of turbulence and be able to control that. There really aren't any studies out there that have done a comprehensive measurement of unsteady lift due to large-scale turbulence, because there aren't a lot of places that can create turbulence this large."

The ATG, or as the team calls it, the "Meat Grinder," is a mechanically-agitated turbulence generator measuring four square meters and comprising nine intersecting horizontal and vertical bars, each individually controlled by a servo motor. Each shaft has 10 diamond-shaped spoilers designed to create large vortical structures when rotated in an oncoming flow, and it can randomize the behavior of those spoilers, according to Emilia Kawashima, another research engineer in Code 725.

"We can rotate them at different speeds at different times and create various turbulence flow fields for testing," Kawashima said. "The ATG was installed specifically so we can conduct testing to create a large data set for the acoustic community -- not only government facilities, but also for academia and industry."

Kawashima said this testing paradigm, new to the AFF, came about because while the physics of basic foil turbulence are well-known within the community, data to validate prediction models for unsteady lift due to high Reynold's number turbulent flows is scarce. The Navy and Marine Corps commonly work with airfoils like propellers, wind turbines, and aircraft engines, and unsteady loading on these foils operating in turbulent flow fields is known to lead to undesirable structural loading, vibration, and sound. Carderock's engineers have traditionally used the AFF's closed-jet test section or the anechoic chamber to gather data by mounting and testing models which can be up to 20 feet in length. The AFF's 2,140-horsepower, 300 rpm fan feeds air through the system at a maximum of 118 knots for testing.

"We use the wind tunnel to introduce a flow over a structure and use microphones to listen to all the sources of noise radiating from that structure," said Kent Bartlett, AFF tunnel engineer for Code 725. "We can sample the flow-generated noise coming off of a structure to isolate areas that need further design considerations. We've done tests for surface panels of submarines, landing gear associated with NASA aircraft, and motorcycles to see where noise is generated. The testing we do could be done in a water tunnel, as well, but doing it in an air tunnel makes it much easier to manipulate and work on the model affording us the ability to try multiple configurations in quick succession. After testing, we can then scale the results to accurately predict how the structure would react in other fluids such as water."

The AFF was built on a foundation of crushed gravel on top of solid bedrock to reduce background noise and isolate the chambers from structural vibrations. Other features built into the facility to that end include custom acoustic mufflers located upstream and downstream of the flow-generating fan, acoustic treating on prominent surfaces and isolation joints to separate sections of the wind tunnel, among others. Testing flow-generated noise on ships and submarines was done on full-scale ships or in wind or water tunnel test facilities before it was done at the AFF, but these all had drawbacks. Full-scale ship testing was too expensive and hurt operational readiness, while water tunnel testing was less flexible and more expensive than wind tunnels which produced too much noise to collect valuable data, according to Bartlett.

While the AFF's function until now was to test under low turbulence, Naval Sea Systems Command (NAVSEA) with the help of the Office of Naval Research (ONR) is now sponsoring a multi-year project to expand the acoustic community's database of comprehensive validation data for foil-turbulence unsteady force models which account for chordwise thickness and camber. The ATG was installed in August and Forest, Kawashima, and their colleagues then got to work doing initial tests in the anechoic chamber to characterize the flow under these new conditions. Forest said the triple-component hotwire data they gathered was a vindication for the team after years of persistent work to reach that point.

"It's been almost three years since we put in our first purchase request for this project," Forest said. "Procurement took some time. Now that we have all the equipment and have started tests, we're really excited. We started taking data and the flow was very spatially uniform. We were able to achieve length scales and turbulence intensities above what we were hoping for. The data we've gotten thus far, while limited, has been fantastic."

Forest said the team is set to begin airfoil testing with the ATG under the NAVSEA and ONR project soon, with other testing under discussion. Dr. Jason Anderson, senior scientist and propulsor technical specialist; Devin Stewart, project manager; Zachary Kaler, Ryan Catlett, Jason Joiner and other members of Code 725 have also contributed to the project.

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