Going with the flow

Rhode Island EPSCoR core facility supports ground-breaking research


Most of us don’t give a second thought to suspension bridges spanning across bodies of water, offshore oil platforms straddling ocean waves, or power transmission cables strung across the sea floor.

As long as the structures perform as designed, what happens beneath the surface remains out of sight and mind. Unless they fail, underwater cables and pipes — typically circular cylinders — withstand the dynamic loading of the ocean without much attention.

A cable bends and vibrates in the laminar flow tank. Below, E. Deniz Gedikli prepares to demonstrate how the tank works and talks with Ed Baker, seawater facility manager, about cables.

However, in the engineering world, the wiggle is a big deal.

In more technical terms, long cylinders in flowing water are susceptible to vibrations caused by the fluid flowing past the structure. As the fluid moves around the cylinder, it forms into eddies or vortices, making the structure vibrate similar to a guitar string. When flow conditions are just right, the vibrations can be destructive, leading to catastrophic failure through fatigue.

The same effects can be observed in air, also considered a fluid in the physics world. Have you ever heard power lines hum in the wind? The same phenomenon of flow-induced vibrations causes these cables to vibrate. The potential for engineering concern can be seen in the 1940 collapse of the Tacoma bridge in a gale wind.

Inside the Ark building on the University of Rhode Island Narragansett Bay Campus, E. Deniz Gedikli, a URI Ph.D. candidate in ocean engineering working with Assistant Professor Jason Dahl, is trying to explain the fundamental physics related to this phenomenon to be able to predict and prevent these vibrations in engineering design.

Forces of nature

One recent morning, Gedikli, in his third year of graduate school, took a break from his research and demonstrated the phenomena of fluid flow and vortex-induced vibration on structures like pipes and cables. He called attention to a small, thin, flexible cylinder mounted in a specialized recirculating test tank that generates a uniform streamlined flow.

The speed of the flow can be precisely controlled, allowing investigators to study the vibration through a range of flow speeds as would happen with changing tidal currents, ocean currents, or variations in wind speed.

Part of the Marine Science Research Facility (MSRF), a Rhode Island NSF EPSCoR core facility, the Ark houses the laminar flow tank, which has a pump that circulates water through large diameter piping, a pre-tank, a test chamber of clear glass and back to the pipes, allowing the water to flow round and round.

An experimental cylinder, about 10 inches long and a quarter inch in diameter, stretched across the test chamber horizontally and perpendicular to the flow.  The tank’s controls allowed Gedikli to precisely dial the rate of flow. As the flow increased, a static, also known as a standing, wave formed on the surface. Soon, the cylinder in the flow began to vibrate.

Ed Baker, seawater facility manager, stood by and observed, noting there were multiple variables to consider in real life applications of the engineering, from phone or power lines suspended in the wind to safety factors with off-shore oil drilling rigs, or a mooring cable for off-shore wind farms.

Consider, he said, an offshore oil rig sinking a well pipe 20 inches or greater in diameter, 1,000 feet or more through the depth of the ocean and then miles into the ocean floor:

“The currents move at different rates and different levels, with different moon tides and sea-state wave action, and different tensions and compression forces on the well pipe. There are many variables to consider. Deniz is in the process of better understanding the phenomenon of vortex-induced vibration in cylinders for better design and for safety factor calculations to prevent oil spills and mooring system failure.”

Without the facility and the equipment, Gedikli said, his research simply would not be possible.

The laminar flow tank was purchased for use at the Bay Campus by a grant to URI Professor Cheryl Wilga, chair of the Department of Biological Sciences, whose research area is evolutionary and functional morphology of sharks. She works on describing the physics of shark locomotion, among other research topics, and uses the laminar flow tank to study near surface fluid dynamics of water passing over the shark’s body and in the wake of the tail and other fins.

Laminar flow tank
The laminar flow tank Gedikli uses for his ocean engineering research originally was purchased by a grant to URI Professor Cheryl Wilga, chair, Department of Biological Sciences.

An international education

Gedikli received his undergraduate degree in naval architecture and marine engineering and a master’s in each of the disciplines from Yildiz Technical University in Istanbul, Turkey, the cost of his education covered by scholarships Turkey provided.

Gedikli’s home country also provides a scholarship for his Ph.D. work here in the US. Upon completion of his degree, Gedikli will return to teach in Turkey, where he already has a faculty position secured for him, with no debt incurred from any of his degrees.

He said he was attracted to the ocean engineering program here at the URI Bay Campus and the lab of Assistant Professor Jason Dahl, in the Department of Ocean Engineering.

“In Europe, people just know about Boston and Michigan and New York,” Gedikli said, smiling. “But, I knew about ocean engineering at the GSO.”

And, now that he has seen Rhode Island for himself, Gedikli said of the Ocean State: “It’s a small, beautiful place.”

Story and photos by Amy Dunkle