SAFL welcomes Mechanical Engineering faculty
Assistant Professor Jiarong Hong arrived in the fall of 2012 with a joint appointment in Mechanical Engineering and the St. Anthony Falls Laboratory. Hong grew up in southwestern China, in a city called Chongqing, one of the most populous cities in the world at 32.8 million people that stretches across the Yangtze and Jialing Rivers among the mountains upstream from the Three Gorges Dam. On his first visit to Minnesota, Hong noted the parallel—that Minneapolis is also situated on the upper reaches of a major river, the Mississippi. He liked that connection.
Hong developed an early interest in engineering from his father. Although his father worked as a Chinese language teacher, he was a hands-on engineer: “He could fix anything, a refrigerator or the TV,” said Hong. “When I was ten years old and interested in astronomy he helped me build my own telescope.” Other interests included nature, science and mathematics.
His interest in scientific discovery with an engineering perspective led him to study at the University of Science and Technology in Hefei, Anhui, China, where he pursued studies in thermal physics. With this sound foundation he was accepted as a graduate student at Johns Hopkins University where he worked with Professor Joseph Katz, a renowned experimentalist in fluid mechanics. This was challenging at first, as Hong had approached science from a theoretical perspective. Yet, he discovered, “lots of things have uncertainties, and in experiments you see exactly what happens, and you have the privilege to demolish an existing theory. That is the exciting part of doing experiments – you discover what is truly there.”
While at Johns Hopkins, Hong worked on an experimental technique for measuring complex flows called particle image velocimetry (PIV). To apply this technique, the flow field is seeded with particle tracers and a light sheet is generated to illuminate the particles in the field.
A camera, oriented perpendicular to the light, is used to capture the motions of lightened tracers, which are used to calculate fluid motions in this region. With high-speed cameras, this technique allows for quantifying the dynamics of fluids in the entire imaging region at high spatial and time resolutions, providing opportunities to characterize coherent structures in the flow field, such as vortices of different scales. “What we have done in the lab is to push this technique to the cutting edge,” he said.
With a background in experimental fluid mechanics, the joint position with the St. Anthony Falls Laboratory and Mechanical Engineering offers unique opportunities for Hong’s research. At the Eolos Wind Energy Research Station at UMore Park in Rosemount, Minnesota, Hong uses PIV to study flows around a 2.5 MW full-scale wind turbine. Most of the previous wind energy research using PIV were performed in laboratory wind tunnels using miniature wind turbines with a maximum field of view of less than 3 m × 3 m. But with the size of modern wind turbines approaching heights of 100 meters and their interaction with wind significantly different from that of the small-scale turbine models, there is a pressing need for characterizing wind turbine performance at full-scale flow conditions. The UMore Park facility has provided a laboratory at this scale. Hong and his team—two graduate students, two undergraduate students and another faculty member—have been conducting experiments onsite.
“There are many challenges to implementing PIV at this large scale,” said Hong. First, a strong light source is needed to provide uniform illumination in a flow field at scales larger than 50 m × 50 m. Second, the particles served as flow tracers must be environmentally friendly – you don’t want to introduce any hazard or pollutant. You also need to supply an abundance of seeding particles persistently, and using any artificial particle injection device would disturb the original flow field. So they realized they could use the power of nature itself to provide the particles—snow. Luckily, Minnesota has had no shortage of that substance. It is entirely environmentally friendly with no additional cost. However, conducting research at night in a Minnesota snowstorm is no one’s idea of an easy task.
Over the winter they kept improving their instruments, learning how to generate a uniform light sheet, and improving the alignment of the light sheet with wind direction and imaging devices (not easy at this large scale). Though he admits their techniques were primitive—it worked! “We were lucky, every time we went out there all systems worked fine, and we have acquired some very exciting data,” said Hong.
In 2011, the state of Minnesota ranked 4th in the nation in terms of wind energy percentage of total energy generation (wind energy was 12.7% of total electricity generated). However, significant renewable energy growth is required if Minnesota is going to achieve its stated goal of cutting greenhouse gas emissions by 30% by 2025. Professor Hong’s research has the potential to improve the efficiency and longevity of wind power generation through optimization of the structural design and layout of wind turbines.
Professor Hong’s research also involves flow diagnostics at the opposite end of the scale - the sub millimeter scale. By using a 3-D holographic and optical refractive index matching technique, he is studying the behavior of flows at close proximity of solid boundaries, such as those on a shark’s skin. When viewed under a micro¬scope (see photo on right) the shark’s skin has these very complex, geometrical, denticle structures. Because of their tiny size, it is extremely difficult to characterize the flow around them.
A well-known hypothesis of the physiological function of these structures is that they can reduce drag. But recent studies showed that adding similar structures onto rigid surfaces does not favor this hypothesis. In fact, as Hong points out, shark skin is not really rigid – as the body continually undulates as it cruises in the ocean.
Another hypothesis is that the denticles inhibit bacteria colonization (anti-fouling) on the shark’s skin. In the ocean, micro-algae can quickly form a film on solid surfaces, allowing further aggregation of barnacles. This fouling process greatly increases the hull roughness and hydrodynamic drag on vessels, which can result in powering penalties up to 86% at cruising speed for ocean ships.
“By being able to measure the flows around a piece of shark skin, we can analyze whether and how these structures contribute to the hyperdynamic performance on surfaces,” said Hong. “So this research at this tiny scale is also related to energy,” he added.
Aside from his research and teaching responsibilities, Professor Hong enjoys sports, especially soccer, badminton and tennis, although his professorial duties leave little time to for him to pursue them. He finds the Twin Cities a great combination of natural beauty with all the amenities of city life. He has found very good Sichuan food (“probably the best Sichuan restaurants in the country”) conveniently located near his home in St. Paul. He did mention one drawback – “it is a little bit cold,” but for a researcher who relies on snow, he is in the right place.
Associate Professor Lian Shen arrived fall 2012 from Johns Hopkins University. Born in Zhejian Province, south of Shanghai, Shen received his B.S. degree from the University of Science and Technology of China in 1993. He attended graduate school at the Massachusetts Institute of Technology, attaining his Sc.D. in Fluid Mechanics in 2001. In 2004, he joined the faculty of the Johns Hopkins University as assistant professor in the Department of Civil Engineering and Center for Environmental and Applied Fluid Mechanics.
As a young boy, Shen was not overly studious, preferring games and sports. His father was a teacher of Chinese and Western literature, and his mother was a physician. No one in his family had an engineering background. But as he grew older, a strong curiosity to understand how things work mechanically, and especially with regard to how forces make things move, led him to study mechanical engineering and mechanics.
Currently, Shen studies fluid mechanics using computer simulations. He studies flows in the context of the natural environment, in the ocean or the atmosphere. Even with large supercomputers, with thousands of CPUs, a typical simulation can take several weeks to run. So Shen uses lots of computer power, supplied by the Department of Defense at their supercomputer centers nationwide.
One of his projects is researching offshore wind energy harvesting. Typical land based wind farms have limitations, like the availability of land, the noise created, and their visual impact on the landscape. Moving wind turbines to offshore environments (into the deep ocean) have several advantages—they would not be visible from shore, the wind out on the ocean is even stronger, and the areas to place the turbines are vast. Shen admits this idea is years from being practical, with many engineering and structural challenges, but he is performing some leading edge research sponsored by the National Science Foundation.
Another topic of Shen’s research is studying the fundamental mechanisms of multiphase flows at the boundary layer. This work involves applications with heat and mass transfer, “so I am very glad I came to this department, because it is very strong in this area,” said Shen. He is looking at the transfer of greenhouse gasses between the atmosphere and the ocean. “The ocean can absorb CO2, but how much and how fast is controlled by a very thin boundary layer at the air and water interface,” he explained.
He is also looking at another renewable energy resource – ocean waves. Trying to find out how the energy of waves could drive some mechanical system to produce energy. Shen is currently recruiting students to work in his group, “I want to have a big research group,” he said. Though much of his work is with simulations, he is also interested in doing experiments. With his joint appointment with the St. Anthony Falls Laboratory, he is looking forward to doing multidisciplinary research and experiments at this unique research facility.
Shen’s interest in fluids goes beyond engineering science – he loves water – “seeing water makes me feel good, and there are so many lakes here, so many places to catch fish!” he exclaimed. “I have come to the right place.” Having lived in New England for several years, Shen is not perturbed by our Minnesota winters, “I know there are lots of activities available and I am looking forward to them, and actually, I am quite experienced in digging my car out of snow,” he added.
--Reprinted from the ME News with permission from the Department of Mechanical Engineering.