When traveling at five times the speed of sound or faster, the tiniest bit of turbulence is more than a bump in the road, said the Sandia National Laboratories aerospace engineer who for the first time characterized the vibrational effect of the pressure field beneath one of these tiny hypersonic turbulent spots.
“The problem is that these patches of turbulence are really fast and really small,” said researcher Katya Casper. “There are thousands of turbulent spots every second in hypersonic flow, and we need really fast techniques to study their behavior.”
The pressure field is key to understanding how intermittent turbulent spots shake an aircraft flying at Mach 5 or greater, Casper said. Hypersonic vehicles are subjected to high levels of fluctuating pressures and must be engineered to withstand the resulting vibrations.
Simply put, being able to characterize and predict these pressure spots leads to better vehicle design.
“The understanding of unsteady pressure fields is extremely important for modeling of hypersonic flight vehicle applications for a variety of national security programs,” said Basil Hassan, senior manager in Sandia’s Advanced Science and Technology Program office.
“This advanced diagnostic development work forms unique datasets for fundamental discovery and model validation at Sandia and has been used to improve flight predictions for several national hypersonic flight programs,” Hassan said.
Over the past several years, Casper’s experiments have progressed from the use of miniature electronic sensors to advanced imaging techniques with pressure-sensitive paint, which is applied to a model tested in a wind tunnel and viewed by specialized cameras to measure the pressure fluctuations optically.
The American Institute of Aeronautics and Astronautics recently cited Casper’s breakthrough in characterizing hypersonic turbulent spots and her work with novel fluctuating pressure instrumentation when announcing earlier this year she had won the organization’s Lawrence Sperry Award, given for notable contributions in the field by a person age 35 or younger.
Casper’s experiments characterizing hypersonic turbulent spots used innovative diagnostic techniques to provide insight into the interaction between pressure fluctuations and vehicle structural response.
With advanced imaging techniques and high-speed sensors, the work showed that transitional pressure fluctuations are generated by intermittent turbulent spots that pass by in a millisecond. As the spots grow, they merge into a fully turbulent layer. The data Casper captured was instrumental in improving predictive computer simulations developed by her colleagues at Sandia.
Using a cone-shaped model with an integrated thin panel embedded with pressure sensors and accelerometers at Sandia’s hypersonic wind tunnel, Casper studied the response, or vibration, to turbulent spots.
When the frequency of the passing turbulent spots matched the natural structural frequency of the panel, strong resonance was generated with vibration levels more than 200 times larger than when the spots were mismatched to the panel, she said. “This would be a worst-case scenario for the flight.” Now engineers have an improved means of predicting such a scenario and adapting to it.