Interview With Christopher Roy, Professor at Virginia Tech
1. Can you tell us about who you are?
Chris Roy, a professor in the Crofton Department of Aerospace and Ocean Engineering at Virginia Tech in Blacksburg, Virginia. I'm also a husband, father of 2, and a triathlete.
2. Why did you want to become an aerospace engineer?
I was studying mechanical engineering as an undergrad at Duke University in the early 1990s and took a graduate course in aerodynamics. I really enjoyed the class and that led me to pursue graduate degrees in aerospace engineering.
3. What do you think is going to be the next biggest breakthrough in aircraft and spacecraft simulation technology?
I think machine learning methods will help us to build better sub-models for complex phenomena like turbulent flow in fluids and damping in jointed structures, but only if we can find a way to constrain those sub-models to obey the appropriate physical laws.
4. What are Euler and Navier-Stokes equations?
These are the equations governing the conservation of mass, momentum, and energy in a fluid (gas or liquid) with friction (Navier-Stokes) or without friction (Euler). The Euler equations, which neglect friction, apply in a surprisingly large number of cases as long as you are not interested in predicting drag or heating on an object. The Navier-Stokes equations contain all of the physics describing the vast majority of fluid flows that scientists and engineers are interested in including turbulent flow.
5. How hard is it to get a simulation as accurate as real life where there are tons of different variables involved?
For complex flows, it is very difficult. There are just so many ways in which you can introduce errors and uncertainties. The grid you solve the problem on may not be fine enough to get a good numerical solution. The sub-models for things like turbulence, multiphase flow, chemical reactions, fluid-structure interactions, etc. are often not accurate. Finally, we are just now learning how to allow our simulations to account for the uncertainties in the environment, geometry, and material parameters that can come from both inherent randomness and from a lack of knowledge of those parameters.
6. What is fluid turbulence?
The nonlinearity of the Navier-Stokes equations can give rise to flow instabilities which ultimately result in a breakdown to turbulent flow. Turbulence is surprisingly hard to define, but some general characteristics include: three-dimensionality, unsteadiness, a (very) wide range of length and time scales, an (almost) chaotic behavior, a cascade of rotational structured (called turbulent eddies) from large scales down to very, very small scales, and enhanced transport of mass, momentum, and energy in the fluid.
7. Can you describe how fuel moves through an aircraft engine?
Aircraft generally store fuel in the wings. Jet engines (i.e., gas turbines) suck in air through a series of compressor blades to increase its pressure, then the fuel is sprayed into the high pressure combustor, mixes with the air, and is ignited. This heat release from burning the fuel expands the combustion products (primarily air, water vapor, and carbon dioxide) at high velocity, which turns the turbine blades (attached to the compressor) and exits the back of the engine at very high speeds producing thrust.
8. What are compressible flows?
We usually consider the flow of a gas to be compressible when the speed gets above Mach 0.3 (when the speed is at least one third the speed of sound, or about 750 mph at sea level). Compressible flows have changes in the gas density produced by the aerodynamic changes in pressure. When the local flow speed gets higher than the speed of sound, then shock waves can form which can have very large increases in pressure over a very short distance. Supersonic vehicles generally create strong shocks at the front and back of the vehicle, giving rise to a sonic boom, a sound wave that can be strong enough to break windows on the ground.
9. What has been your toughest accomplishment so far in your career?
My toughest accomplishment would be writing a book called "Verification and Validation in Scientific Computing" which came out in 2010. With all my research, teaching, and administrative duties, finding time to get this written was a tough challenge. However, it has also been one of the most rewarding experiences of my career.
10. To high schoolers like me and to younger generations that want to become aerospace engineers, what would be your advice to be successful in this career?
Find what inspires and motivates you, pursue it any way you can, don't be afraid to take risks and fail, and when you do fail, use those experiences as learning opportunities to grow as a person. If you constantly and truly challenge yourself, you will fail often, but you will also grow immensely.