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ILP Institute Insider

January 23, 2017

Taking flight with numerical models

Jaime Peraire builds simulations for hypersonic flows, lightning strikes, bats, and other physical systems in aerospace.

Eric Bender

As private firms join governments in the global race to space, aerospace engineers grapple with practical problems such as understanding how air flows around hypersonic vehicles re-entering the atmosphere. “We rely heavily on the ability to simulate those flows on the computer, because there are still many unknowns and getting experimental data is very difficult,” says Jaime Peraire, MIT’s H.N. Slater Professor of Aeronautics and Astronautics.

Peraire’s research on hypersonic flows is one example of his work in simulating physical systems in aerospace. His efforts also encompass finding novel ways to protect planes built of new materials from lightning, analyzing the flight characteristics of bats, and solving a diverse set of other puzzles.

“We try to predict the behavior of a system before it is actually built, and come up with designs that avoid problems,” he says. “In addition to building numerical models, and simulating them with efficient algorithms that exploit modern computer architectures, it’s very important to quantify the uncertainty in our predictions. There are many problems of practical interest in aerospace that are still beyond the capabilities of computational simulations given the problems’ complexity and their large scales.”

Peraire, who heads MIT’s Department of Aeronautics and Astronautics, emphasizes that partnerships with government agencies and industry are key for taking on these tough projects. “One of the things we do very well at MIT is engage with industry or government agencies to address long-term challenges,” he says. “We work together to define the problems, analyze them, extract the important features, and come up with the strategies that allow us to develop effective solutions.”

Jaime Peraire
H.N. Slater Professor & Department Head
Aeronautics and Astronautics

When lightning strikes out

One challenging case in point is protecting planes from lightning strikes, a tricky issue raised by Boeing, a longstanding Institute and department partner.

Guarding against lightning is an evolving challenge in aircraft manufacturing “given that it is very expensive to protect aircraft from lightning, and given the increasing use of composites to replace metals,” Peraire explains. “Over the years this problem has been largely overlooked by the scientific community. If it had not been brought up by industry, it’s very unlikely that we would have approached it. We’ve been studying that problem with a number of MIT faculty, and we will soon release our first important results.”

Historically, lightning has been addressed reactively—putting new rules and guidelines in place after incidents occur. But Peraire sees an opportunity to move proactively toward more effective solutions.

“The problem can be approached from two perspectives,” he says. “One is to mitigate the consequences of lightning striking an aircraft. Another is to come up with strategies for avoidance—are there ways you could actually control the electrical charge in an aircraft so that lightning would not strike?”

Going to bat

Other simulation research focuses on new vehicles that push the envelope, such as the micro air vehicles that are typically called drones. “The challenge there is trying to design vehicles that can efficiently reach long range, and are highly maneuverable and able to hover, yet are as small as possible,” Peraire says. “These are conflicting requirements.”

In a long-running U.S. Air Force program on biologically inspired flight, “our focus was to understand the mechanics of bat flight and extract potential mechanisms and physical strategies applicable to micro air vehicles,” he says.

Bats are efficient and highly maneuverable fliers, with widely varying size, and, as such, offer valuable lessons about flapping flight. “We came up with estimates for the power required to fly an object the size of a bat at a certain speed, and compared those with our estimates of the energy consumed by bats,” Peraire says.

Doubling air traffic

Research in the Department of Aeronautics and Astronautics focuses on a wide range of technical challenges, with the department identifying three main strategic areas for significant growth in the next ten to fifteen years: air transportation, autonomous systems and small satellites.

With global air traffic forecast to double or triple by the middle of the century, “the infrastructure that we’ve developed in the last 50 or 60 years will have to be doubled in the next 10 or 20 years,” Peraire notes. “Most of this increased demand will come from the developing world. We’ll need many new aircraft and a lot of infrastructure.”

As this global expansion accelerates, the environmental impact of air travel represent an enormous concern, and the MIT department is one of the first in academia to begin addressing this.

“While aviation only contributes five percent to greenhouse emissions, it is the fastest-growing contributor to greenhouse gas emissions, and finding replacements for hydrocarbon fuel is very difficult,” Peraire says. “Additionally, most of the emissions coming from aviation is at 30,000 feet, and there are unique pathways for these chemicals to dissolve and to disperse within the atmosphere that are not fully understood.”

“We have the challenge of environmental impact and we also have the challenge of increased cost of energy,” he says. “The only way we can resolve these challenges is with better aircraft, better technologies, and better operations.”

Among better aircraft designs, Peraire hopes that the National Air and Space Administration will soon select the MIT D-8 “double bubble” commercial aircraft concept as one of the next X-plane programs. The D-8 features numerous design innovations, such as a lift-inducing double-width fuselage and efficient engine placement on the rear top of the fuselage. “We estimate that the D8’s savings in fuel consumption could be as high as 70%,” he says.

“Even in the very conservative domain of air transportation,” Peraire adds, “if there is a good time for making a leap into new designs and technologies, it’s right now.”

Adding autonomy and thinking small

As in aircraft design, MIT offers unparalleled resources for research into autonomous aerospace systems—not just within Aerospace and Aeronautics, but also in centers such as the Computer Science and Artificial Intelligence Laboratory and the Laboratory for Information and Decision Systems.

“In addition to growth in autonomous systems like drones, we see an increase of autonomy in other areas, such as manufacturing,” Peraire says. “Manufacturing cars is already highly automated, but we still have a long way to go to automate the manufacturing of aircraft. These two processes are very different. The most productive aircraft lines might produce one vehicle a day, as opposed to car lines, which may produce 1,000 cars a day. We need to use automation in a different, more intelligent way.”

Autonomous capabilities also are essential to space exploration. “It makes all the sense in this day and age to use autonomous exploration in space,” he says. “Even when you want to send humans, you can send robots ahead of time to create infrastructure and to facilitate human presence at a later stage.”

Small satellites present dramatic new opportunities as well. “Satellites are important for our everyday life, and the ability to miniaturize systems and sensors is allowing us to use much smaller satellites to do the same functions,” Peraire notes. Smaller satellites can be much cheaper and more cost-effective than their traditional counterparts. Additionally, in some roles, a single large satellite can be replaced by a constellation of smaller ones that provide numerous benefits over their larger counterparts, such as cheaper launch costs and the ability to repair or replace single small elements in the event of failure.

“MIT has unique capabilities in two technologies that are important in this domain, space propulsion and optical communications,” he comments.

As with so many other MIT aerospace projects, work on small satellites is moving forward in industry partnerships.

Corporate collaborations are also crucial to strengthening education and training programs, a critical need in aerospace. “As aerospace research, development, and production are on the upswing and as the aerospace workforce is aging and retiring, industry and government foresee a shortage in the qualified workforce,” he says. “Consequently, we see a tremendous opportunity for continuing education. Technology dictates the pace of development in aerospace, and skills need to be constantly updated and honed. It’s very important to have the infrastructure for continued retraining and education.”

“Our mission in AeroAstro is to educate tomorrow’s leaders through innovative programs, generate novel technologies for aerospace challenges and provide leadership to aerospace and engineering communities,” Peraire says. “With the growing need for new aerospace technologies and the highly skilled people to develop them, our mission becomes all that more important.”