The Air Force Office of Scientific Research (AFOSR) recently awarded Mitul Luhar, assistant professor for the Department of Aerospace and Mechanical Engineering, a $360,000 research grant through its Young Investigator Research Program (YIP). The three-year grant, announced in mid-October, will fund Luhar’s work investigating the effects of porous and patterned surfaces on airflow to control turbulence.
This year, 58 awardees were selected from over 230 proposals, all of which designed to enhance the future capabilities of the Air Force. The YIP supports scientists and engineers across the nation who have received a Ph.D. or equivalent in the last five-years and show great research ability and promise.
By controlling turbulence, which affects aircraft speed and vibrations, Luhar’s research will impact fuel-efficiency, flight time and aircraft maintenance. He plans on developing multifunctional surfaces that, in addition to providing structural integrity, will also influence the surrounding airflow. To do so, he will design and test various rough porous and patterned surfaces, veering away from the typical smooth surface designs.
“That was the original idea—you make your surface as smooth as possible,” Luhar said. “In many ways that’s still our best bet, but there’s research that suggests in many cases it’s actually beneficial to have specific surface patterns.”
The key challenge Luhar faces is the nature of turbulent flow. In general, turbulence is the motion of fluid particles that creates swirling vortices, known as eddies. This disruptive motion leads to an increase in drag, or air resistance, that slows the aircraft down. Additionally, turbulence influences the noise produced by the aircraft and can cause vibrations that damage the plane.
The ultimate goal is to, “suppress the turbulence, or push it away from damaging frequencies to frequencies that are maybe less damaging from a vibration or noise perspective” Luhar said.
His research will further explore the influence of complex surfaces on turbulence, building upon decades of previous studies that have attempted to find optimal surface structures.
“The way people have been doing it so far is, you either do this in an experiment and then it’s a bit of a trial and error process, or you do it in numerical simulations where you have to resolve everything from the largest eddies in the flow to the smallest eddies,” Luhar said. “And that essentially, even with current computational capabilities, is impossible for something at the scale of an aircraft.”
During his post-doctorate years at the California Institute for Technology, Luhar developed simplified computer models that were able to reproduce the results of complicated simulations, but in a fraction of the time. Using this method, he will be able to rapidly test the effects of a variety of surface patterns, narrowing in on an optimal design before manufacturing and testing it in the lab.
“I don’t expect perfect results right off the bat, but it’s a much better way than trial and error,” Luhar said. “Because the parameter space is huge—you can design any sort of porous or patterned surface—how do you know it’s going to be helpful? So this is one way of doing it intelligently.”
Once optimal surface designs have been identified using the computer model, they will be fabricated for lab testing. For this, Luhar plans to take advantage of the continuing advances in materials science and additive manufacturing, commonly known as 3-D printing, which will give him freedom over surface design choices. In addition, with the creation of the Center for Advance Manufacturing, directed by Professor Satyandra K. Gupta, his designs can get even more creative.
“The possibilities are really endless,” Luhar said. “You can imagine having surface treatments that vary depending on where on the aircraft you are, depending on what sort of turbulence you are expecting. Skin friction reduction is maybe the most obvious example, but there might be other things that we can do that people haven’t even thought about yet.”