Gaining New Insights Into Multiphase Flows: Bubbly Shock Waves & Superhydrophobic Surfaces

An ultra water-repellent coating developed at the University of Michigan could help tomorrow’s ships slide through the water more easily, reducing transportation costs for the 90 percent of consumer goods that travel by ship.    Photo by Marcin Szczepanski/Multimedia Director and Senior Producer, University of Michigan, College of Engineering
An ultra water-repellent coating developed at the University of Michigan could help tomorrow’s ships slide through the water more easily, reducing transportation costs for the 90 percent of consumer goods that travel by ship. Photo: Marcin Szczepanski

In his internationally recognized laboratory, Steve Ceccio, the Vincent T. and Gloria M. Gorguze Professor of Engineering, conducts novel experiments to better understand the complex dynamics of cavitation and other multiphase bubbly flows at the macro and micro scales. 

Cavitation refers to the formation of bubbles when liquid changes to vapor in response to a sudden drop in pressure. The phenomenon occurs in many situations, including within narrow flow passages and around ship propeller blades as they move through the water. As the water flow speeds up, the local pressure drops and, if it drops enough, tiny bubbles can grow explosively and collapse, causing performance degradation, noise and vibration and erosion near the cavitating surfaces.

“Cavitation isn't all bad. It's been employed for beneficial medical purposes, sometimes deliberately produced through ultrasound, but in hydraulic systems it's something we usually want to prevent,” Ceccio said. "To understand its effects, we need to see exactly what's going on in the cavitating flow itself."

A towed axisymmetric model is tested in the U-M Marine Hydrodynamics Laboratory to determine the amount of skin friction reduction achieved through application of Super Hydrophobic Coatings on its surface.
A towed axisymmetric model is tested in the U-M Marine Hydrodynamics Laboratory to determine the amount of skin friction reduction achieved through application of Super Hydrophobic Coatings on its surface.

Imaging through a bubbly cloud

One key challenge to visualizing cavitation is the high gas-volume fraction. Ceccio likens the effect to examining a glass of champagne or beer: “You have a lot of nice bubbles, but the bubbly clouds can easily make the liquid opaque.” 

Even a volume-fraction of a few percent can defeat optical probes of these bubbly liquids, but in many cavitation flows, the volume-fraction can reach over 30 percent. So Ceccio and his team turned to X-ray technology.  The group has developed a specialized cinemagraphic X-ray densitometry and imaging system to penetrate the bubbly liquid and directly measure cavitation dynamics. His is one of the first research teams in the country to observe and capture the dynamics of cavitating clouds at rates in excess of 1000 frames per second. 

Ceccio's team used the system to examine the growth and shedding of vapor-filled pockets, analogous to the sheet cavities that form on pump and propeller blades. When the first images came in, they were surprised to see what looked like shock waves within the shedding cavity cloud. 

“The classical explanation for cavity shedding is the presence of a liquid jet forming where the cavity closes on the surface. But we observed bubbly shock wave propagation within the cavity that led to shedding," he explained. 

Since the bubbly cloud is compressible, the presence of shock waves had been predicted theoretically. But the X-ray images revealed them directly.

Ceccio’s group subsequently showed that cavity dynamics are strongly related to the Mach number within the bubbly cavity, with the strongest shedding associated with hypersonic conditions.  

"Now that we know more about the importance of compressibility in these flow dynamics, we can begin to reinterpret a variety of curious cavitating phenomena,” said Ceccio, whose group now is at work on a scanning X-ray tomography system. 

Small Bubbles Lead to Friction Drag Reduction

Many passive and active technologies have been studied in recent decades to reduce skin friction produced by turbulent liquid flows, another area of interest to Ceccio. Reducing skin friction can lead to increased system performance and lower energy consumption. 

James W. Gose, Ph.D. Candidate at the Naval Architecture and Marine Engineering Department of the University of Michigan tests the coating on a specially prepared submarine at the U-M Marine Hydrodynamics lab.One promising possibility is the use of superhydrophobic surfaces (SHSs). Much like lotus leaves whose nanoscale surface features keep the leaves dry by causing water to bead up and roll off, engineered SHSs can have a similar effect. The surfaces have micro- and nanoscale textures that trap small pockets of gas, which can reduce the local skin friction produced by liquids flowing over them. What has remained unclear, however, is whether these surfaces could reduce the friction of turbulent flows. 

In a large, five-year effort to explore this question, Ceccio serves as principal investigator of a Multidisciplinary University Research Initiatives program supported by the U.S. Office of Naval Research.  A team of experts from U-M as well as Massachusetts Institute of Technology, Stanford, University of Minnesota, Johns Hopkins and The University of Texas at Dallas have developed a wide range of SHSs and characterized their interactions with the turbulent flow. 

The team has presented some significant findings. The SHSs can, indeed, lead to meaningful reductions – up to 50 percent -- in the skin friction drag of turbulent flows, so long as the SHSs maintain the small gas pockets within their microscale surface features. When the bubbles remain stable or replenish, friction reductions persist. But when the gas pockets are lost, friction readily returns.

Ceccio’s team is now working to show that SHS coatings can reduce friction on a towed axisymmetric body in U-M’s Marine Hydrodynamics Laboratory.  

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