When water flows over an acoustic sensor, non-acoustic pressure fluctuations caused by turbulence can decrease the signal-to-noise ratio and make it difficult to sense incoming acoustic waves. The Quiet Water Tunnel Facility at the Naval Undersea Warfare Center in Newport, RI is a unique test facility capable of investigating these pressure fluctuations and evaluating new and existing technologies aimed at reducing flow noise and drag due to skin friction. These technologies include modifications to the surface itself, such as riblets or compliant coatings, or modifications to the flow, such as suction or injection of water into the boundary layer.

Background

Water tunnel components and flow schematic.
The Quiet Water Tunnel Facility was built at the Naval Underwater Sound Laboratory in New London, Connecticut in 1965. When constructed, the facility included only a circular test section for studying fully developed turbulent pipe flow. A rectangular test section was added in 1974 to allow for flat plate wall pressure measurements and the investigation of compliant coatings beneath turbulent boundary layers. The facility was subsequently moved to the Naval Undersea Warfare Center in Newport, Rhode Island in 1995.

The Quiet Water Tunnel depicted in the accompanying diagram is a recirculating flow facility that contains approximately 2000 gallons (7600 liters) of fresh water. Mass flow is controlled by a 745.7 watt induction motor that is coupled with a centrifugal pump. Maximum mass flow is approximately 3,300 gal/min (210 L/s). Static pressure is kept between 20-40 psi (140-210 kPa) during testing to prevent pump cavitation, and water temperature can be maintained from 60-90 °F (15.5-32 °C) with a counterflow heat exchanger.

As seen in the diagram, circular and rectangular test sections are installed in parallel and can be run independently or concurrently with one another. The circular test section consists of an acrylic pipe with an inner diameter of 3.5 inches (89mm) and a wall thickness of 0.5 inch (13mm). Flow enters the pipe from a transition section which is connected to the upper plenum chamber via a rubber hose. Centerline velocities up to 80 ft/s (24.5 m/s) are possible in the circular test section, resulting in Reynolds numbers based on pipe diameter up to 2.4×106. The generated boundary layer is half the pipe diameter, or approximately 1.75 inches (45mm) thick.

The rectangular test section is 83 inches (2.11 meters) long, with a constant interior width of 12 inches (305mm). In order to compensate for the growth of the boundary layers on the walls and to maintain a zero pressure gradient flow, the interior height increases from 4 inches (102mm) at the inlet to 4.41 inches (112mm) at the exit. If needed, the bottom plate of the test section can be reconfigured to establish an adverse or favorable pressure gradient. Free stream velocities up to 20 ft/s (6.2 m/s) are possible in the rectangular test section. A rectangular contraction nozzle upstream of the test section in the middle plenum chamber is used to accelerate the flow into the test section while minimizing free stream vorticity, resulting in a turbulence intensity of approximately 1% in the free stream. Also, the test section has minimal spanwise variation in the boundary layers on the top and bottom walls. The side wall boundary layers have minimal effect on measurements that are taken from the center of the channel on the top or bottom walls.

Custom instrumentation can be easily installed in each test section. Both test sections have a modular design with easily removable and replaceable fixtures. In the circular test section, sections of the acrylic pipe can be removed and replaced with instrumented sections. In the rectangular section, six ports in the top of the test section can be removed and machined in order to accommodate a variety of sensors and test fixtures, including piezoelectric wall pressure sensors, flush mounted hot film wall shear stress sensors, pitot tubes, and static pressure taps. For example, one current port has a pressure sensor array consisting of 48 tightly-spaced piezoelectric sensors flush mounted at the fluid/solid interface, allowing direct wave-number-frequency measurements of turbulent boundary layer wall pressure fluctuations to be made.

Strengths of the Quiet Water Tunnel

Side view of the rectangular test section. Water flow direction would be from right to left.
There are both advantages and complications to working in water rather than air. For undersea applications at moderate to high Reynolds numbers, working in water is often required. In particular, for studies regarding turbulence control and drag reduction, accurate scaling of results from air to water can be exceedingly difficult. The boundary layer thickness and Reynolds numbers achieved in the Quiet Water Tunnel are directly applicable to undersea applications. Use of the water tunnel can be particularly beneficial as a lower cost option for testing in early stages of research, as an alternative to utilizing a tow tank or conducting lake tests or sea trials. Each of these options involves progressively higher cost and gives less control over the testing parameters. However, full scale testing at sea is typically required to establish the performance of an entire system and eliminate uncertainties related to scaling factors.

Among similar water tunnel and tow tank facilities, the acoustic isolation of the Quiet Water Tunnel makes it truly unique. Acoustic noise in the test sections is minimized in several ways, the foremost being the use of rubber hoses to provide vibration isolation between the major components. Also, the plenum chambers, rectangular test section, and circular test section are sufficiently structurally rigid to minimize flow induced vibration. Finally, the pump was specifically designed to minimize radiated acoustic energy. The resulting acoustic isolation allows the non-acoustic pressure fluctuations at the fluid-solid interface of a turbulent boundary layer to be studied without background noise.

Applications

Rectangular test section, as seen from above. Water flow direction would be from top to bottom.
Studies conducted in the water tunnel have relevance to a variety of undersea applications. Knowledge from water tunnel investigations is particularly important in optimizing the structural design of submarines and surface ships through drag reduction, as well as maximizing the performance of sensors and SONAR arrays through flow noise control. Computational Fluid Dynamics (CFD) projects can also benefit from implementing the experimental data gathered at the water tunnel.

Drag and flow noise reduction can be accomplished through control of the physics of the turbulent boundary layers. Various techniques for turbulence control have been tested at the Quiet Water Tunnel, including isotropic and nonisotropic compliant walls, hot and cold water injection into the boundary layer, riblet coatings, large eddy break-up devices, and thin urethane coatings. These and other efforts have also led to the development of unique instrumentation and experimental techniques.

Summary

The Quiet Water Tunnel Facility at the Naval Undersea Warfare Center in Newport, RI is specifically designed to measure the mean and fluctuating wall pressure and shear forces exerted by a turbulent boundary layer. Both the circular and rectangular test sections can be used to achieve a wide range of moderate to high Reynolds number flows. The test sections are well acoustically isolated at frequencies above those of the structural noise generated by the centrifugal pump. The Quiet Water Tunnel is a unique facility ideally suited for studying boundary layer control for drag and flow noise reduction, and is a valuable asset for basic and applied research.

This article was written by Jillian Kiser, William Keith, and Alia Foley; Devices, Sensors, and Materials R&D Branch, Naval Undersea Warfare Center (Newport, RI). For more information, Click Here .


Defense Tech Briefs Magazine

This article first appeared in the December, 2011 issue of Defense Tech Briefs Magazine.

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