Clifford Hou Makes Sense out of Whale Flipper's Bumps

Clifford Hou first learned about the Pratt Fellows program when he visited Duke as a prospective student during Blue Devil Days. He remembers thinking then that the program – which provides students an opportunity to do intensive research in their engineering major, including course credit and paid summer research -- was “something to strive for.”

Now in his senior year, the mechanical and biomedical engineering double major and Pratt Fellow from St. Louis has conducted research aimed at unraveling the purpose for once mysterious bumps found on the flippers of humpback whales. As a result of his findings, along with those of others in the lab of associate professor of mechanical engineering Laurens Howle, bumps that mimic those of the whale “tubercles” could wind up on helicopter propellers or underwater autonomous vehicles, according to Hou.

For Hou, the fellowship program represents everything that appealed to him most about Duke from the very beginning.

“Duke is such a great school, it’s hard to ignore,” Hou said. “Pratt is small enough to be intimate, but its programs, particularly that in biomedical engineering, are world-renowned. I felt like I had something to contribute here – and a lot to gain.”

Hou confessed to a few other perks involved in the move to North Carolina. He and his family -- his Taiwanese parents and identical twin brother Kirk -- had lived in Charlotte, N.C. for a few years when he and Kirk were much younger.

“I wanted to come back to North Carolina,” Hou said. “I love the weather, the people – not to mention the sweet tea and fried chicken.” It’s hard to imagine, but Hou said he once spoke with a southern drawl. For him, the decision to enroll at Duke was simple.

Hou started out as a biomedical engineering (BME) major. But he quickly realized that a double major in BME and mechanical engineering and materials sciences (MEMS) would “capture everything” -- offering him an ideal cross-section of chemistry, biology, physics and math.

In the fall of his junior year, Hou applied to be a Pratt fellow in either the BME or MEMS departments, with Howle’s lab a top choice.

“It was a tight knit group of graduate students and Pratt fellows that offered a lot of interaction with the professor,” Hou said of the Howle lab. “On top of that, I would have my own project to work on – a well-defined experimental investigation into the optimization of air flow performance using leading edge tubercles inspired by humpback whale flippers.”

Howle’s group had earlier shown in wind tunnel tests of scale-model humpback flippers that the whale’s scalloped flipper is a more efficient wing design than that currently used by the aeronautics industry on airplanes. The tests show that bump-ridged flippers better withstand air flow interruptions known as stall – a factor that could increase planes’ ability to maneuver at steep angles. The whale-inspired wings also produced more lift and less drag than comparably-sized sleek flippers, they found. (

“They already had data indicating that the bumps provided an aerodynamic advantage,” Hou said. “But, why?”

To answer that question, Hou conducted experiments to visualize the flow of air over the surface of a model flipper. He attached fluorescent tufts very precisely over the surface of the model, blacked out the wind tunnel and took pictures under a black light. Hou said the technique, first applied in the 1940s, remained the simplest and most cost-effective method to see air flow over the whale-like wings.

“When the air flow is smooth and laminar, the threads remain parallel to one another,” Hou explained. “Under turbulent flow, the tufts lift off the surface and flutter. We could look at the onset of stall -- when the tufts first start to lift -- and get an idea of how the stall spreads as a function of the angle of attack.

“We could see, for example, how steeply an airplane with such a wing could climb before stall occurs.”

Hou found that the tubercles act as “span-wise stall mitigation” factors. In other words, they prevent stall from spreading along the length of an airfoil or flipper.

“It’s a revelation that has a lot of application in the real world,” he said.

In the case of the whales, the bumps probably offer an advantage to the mammoth humpbacks as they attempt to catch much smaller prey, Hou said. The same design incorporated into airfoils or rudders might also prove beneficial, he added.

The most obvious possibility is airplanes, but Hou said most planes wouldn’t need to fly at a steep enough angle to make bumpy wings useful.

“More advanced technologies are more likely to encounter the extreme conditions needed to get stall,” he added. For example, the principle might be applied to underwater autonomous vehicles – particularly auto-submarines that need to explore tight areas such as coral reefs.

The design could also be of interest in specialty racing sail boats, where a scalloped rudder might allow for sharper turns, or in helicopter propellers, he said.

“Helicopters experience a wide range of operating conditions that may push the limits,” Hou said. He is currently working on a project to examine the effect of bumps for propeller efficiency.

The Pratt fellow program has offered more than just research opportunities, Hou pointed out. For example, he’s also gained valuable experience through presentations and personal interaction with faculty members and other accomplished people in the field, he said.

Hou recalled that at one such poster presentation for the Pratt Board of Visitors, he talked to someone who had been trained as a pilot and was intrigued by the counterintuitive benefits of the leading edge tubercles on whale flippers.

“It turned out it was Mr. Pratt,” he said, the son of Edmund Pratt Jr. for whom the Pratt School is named. “That’s an example of the kind of interaction that the fellows program makes possible.”

Despite his accomplishments, Hou will be the first to tell you, for him college isn’t all about studies. His intramural football team won the undergraduate championship two years in a row, a fact he takes pride in.

“I made a conscious decision not to push too hard and to enjoy my time at Duke and all it had to offer – in an academic and a social sense.”

Hou thinks he is likely to go on to medical school, but plans to take a year or two after graduation to expand his research experience in aerodynamics into the biomedical arena, where he would like to study cancer or the immune system. He expects that the rising need for sophisticated computational biology -- a result of the Human Genome Project and other advances that have led to an explosion of information about gene pathways and their interactions -- will make his engineering skills an asset.

“The strong shift to computational biology requires a lot of modeling and programming,” Hou said. “The future of medicine lies in the ability to break down specific pathways in the cell and then go beyond the molecular or cellular level to the level of entire organisms. Modeling is going to play a huge part in the ability to cure disease and understand fully how body systems react.”