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Craig Dolder with the water tunnel he used to test his boundary layer suction device (see below).

Mechanical Engineering graduate student Craig Dolder has recently received the Acoustical Society of America's (ASA) Best Student Paper Award for his research in underwater acoustics, which was presented at a meeting last fall. The ASA and its major publication, the Journal of the Acoustical Society of America, are some of the best known and most prestigious academic resources in the field. Dolder's lab, The University of Texas at Austin's Applied Research Laboratories, heavily focuses on underwater acoustics and sonar, particularly for military applications. Dolder's research was conducted under Dr. Charles E Tinney and Dr. Michael R. Haberman. The Particle Image Velocimetry measurements were performed by Meagan Villanueva (undergraduate in Aerospace Engineering) under Craig's supervision. Craig Dolder, Dr. Michael Haberman, Meagan A. Villanueva, and Dr. Charles E. Tinney are the authors on the paper.

Sonar

Diagram depicting how a sonar system works.

Sonar is a sensory technology in which precise measurements of incoming sound waves and echos are used to detect objects on or under the surface of water. Although these sound waves can originate from the object being detected, many ships and submarines emit their own sound waves to be reflected back by foreign objects, similar to how a radar works or how a bat navigates the air using echolocation.

One significant problem with sonar systems today is the turbulence created along a ship's hull anytime it is moving due to friction and viscosity in the surrounding water. Although negligible while a ship is moving slowly or stationary, this layer of turbulent water called the boundary layer can, at higher speeds, easily create enough ambient noise to drown out the incoming sound waves and prevent the sonar system from detecting objects.

The boundary layer is a layer of water between the side of the ship and the more stable, flowing water outside of it. Inside the boundary layer, the water starts out traveling in a smooth, calm manner and then transitions into an increasingly-strong, chaotic fluctuating flow. Dolder's research centers around a way to use sonar on a faster moving ship by being able to quiet the turbulence inside the boundary layer using suction.

Dolder's Research

Dolder's boundary layer suction device (slit on left) next to a pressure array microphone (circle on right).

In an attempt to tackle the boundary layer problem, Dolder created a "boundary layer suction device" that literally removes a portion of the boundary layer, leaving a vacuum to be filled by more stable water that produces less noise. An elegantly simple solution to a complex problem, the boundary layer suction device consists of no more than a slit in the side of a hull, a pump connected to the slit, and a porous aluminum strip to ensure uniform suction. Although the boundary layer never disappears entirely and builds back up to its full size rather quickly, placing the boundary layer suction device upsteam of a sonar sensor can reduce the turbulence enough to significantly improve the clarity of incoming sound and accuracy of the sensor.

A graph showing different amounts of pressure (equivalent to volume of sound) picked up by pressure array. The solid line represents the amount of pressure without any suction through the boundary layer suction device, while the dotted lines represent pressure with two different amounts of suction.

Dolder tested his boundary layer suction device in a laboratory using a water tunnel, which recreates the flow dynamics of a flat wall flowing through water. The tests involved measuring both the pressure at the wall and the velocity field of the flow above the suction device and sonar array. The pressure reveals the amount of noise the turbulent flow is producing and the velocity field provides clues as to how this noise is created. Information on this array was recently published in a journal article.

The first test involved a row of small pressure sensors called a pressure array, which allow for the measurement of the instantaneous pressure at several locations on the wall and act as underwater microphones. While these small sensors are usually used to detect the echoes from distant objects, they are also sensitive to the chaotic fluctuations caused by the turbulence. Dolder used the pressure array to measure the sound of rushing water which would usually be considered interference.

Two graphs showing the velocity standard deviation of particles in the water tunnel under no suction (top) and high suction (bottom). The dark blue area represents a lower standard deviation, meaning that the water there is more stable and less turbulent. The red bar along the bottom marks the location of the boundary layer suction device.

Dolder's second test involved a technology called particle image velocimetry. Using a sophisticated system of lasers, cameras, and special tracer particles mixed into the water, Dolder was able to compare two rapidly taken images to track individual particles in the water as they were traveling, and track their speeds (see sidebar images). These measurements were performed by Meagan Villanueva (ASE student) under direct supervision of Dolder. By measuring the differences in speed of separate regions of the stream of water, he was able to tell how much turbulence, and consequently sound, was being generated.

Having had the concept validated by both tests, Dolder now hopes to see his boundary layer suction device picked up and used in sonar systems for fast-moving water vehicles. One application of the technology he envisions is use in Coast Guard vessels which are constantly patrolling the coast and scanning for divers, but currently must stop or slow down to take a sonar reading. Reducing the boundary layer of a vehicle also reduces drag, meaning that for certain vehicles that do a lot of traveling (such as unmanned submarines that constantly survey the ocean) Dolder's device could actually improve their energy efficiency and speed. Dolder is also considering developing a passive version of the device similar to a hood scoop that would work by capturing and redirecting the boundary layer without the use of a pump.