New research shows that airflow over a double cone loses its symmetry at high speeds. (Photo courtesy of University of Illinois at Urbana-Champaign)
A detailed study of airflow around shapes designed for high speeds reveals surprising manifestations of turbulence, according to a new study. The findings, published March 7 in the journal Physical Review Fluids, could have implications for the design of future high-speed vehicles.
As part of the study, the scientists used 3D modeling to identify unexpected disturbances around fast-moving cones.
At hypersonic speeds—above Mach 5, more than five times the speed of sound (3,836 mph or 6,174 kilometers per hour)—the airflow around a vehicle becomes complex and irregular. Most simulations assume that the flow is symmetrical around the entire cone, but until recently, studies of the transition from aerodynamic to turbulent conditions could only be performed in two dimensions, making it difficult to verify that the flow around a three-dimensional structure was not asymmetrical.
The findings could help engineers create stronger, faster aircraft that can withstand the extreme temperatures, pressures and vibrations experienced during hypersonic flight.
“Transient flows are inherently three-dimensional and unstable, regardless of the flow geometry,” study co-author Irmak Taylan Karpuzku, an aerospace engineer at the University of Illinois at Urbana-Champaign, said in a statement. “Experiments were conducted in three dimensions in the early 2000s, [but] they did not provide enough data to detect any three-dimensional effects or instability because there were not enough sensors around the cone-shaped model. This was not a mistake. It was just all that was possible at the time.”
Using the Frontera supercomputer at the Texas Advanced Computing Center, Karpuzku and aerospace engineer Deborah Levin simulated how airflow around a cone-shaped object — often used as a simplified model for hypersonic vehicles — changes in three dimensions at high speed. They studied both a single cone and a double cone, which allows the scientists to analyze the interaction of multiple shock waves.
“You would typically expect the flow around the cone to be in concentric layers, but we noticed flow discontinuities within the shock layers in both single and double cones,” Karpuzku added.
These disruptions were particularly common around the cone's apex. At high speeds, the shock wave is located closer to the cone, squeezing air molecules into unstable layers and increasing instability in the airflow. The team confirmed their findings by running a program that tracks each simulated air molecule and analyzes how collisions between molecules affect the airflow.
The disturbances also appear to develop at high speeds. “As the Mach number increases, the shock wave gets closer to the surface and contributes to these instabilities. It would be too expensive to run the simulation at every speed, but we ran it at Mach 6 and did not observe flow disruption,” Karpuzku said.
Karpuzku noted that the gaps could impact design decisions for hypersonic vehicles used in shipping, weapons and transportation, as engineers will need to account for newly discovered gaps.
Sourse: www.livescience.com
