Turbulence for future hypersonic aircraft that travel at speeds of Mach 5 and above could be catastrophic. A researcher working at the Energy Department's Sandia National Laboratory has characterized the vibrational effect of the pressure field beneath one of these 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.”

Aerospace engineer Katya Casper. Source: Sandia National Laboratories by Randy MontoyaThe pressure field is key to understanding how intermittent turbulent spots shake an aircraft flying at Mach 5 or greater. Hypersonic vehicles are subjected to high levels of fluctuating pressures and must be engineered to withstand the resulting vibrations.

Over the past several years, Casper’s experiments have moved from 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 optically measure the pressure fluctuations.

Earlier this year Casper won the American Institute of Aeronautics and Astronautics Lawrence Sperry Award, given for notable contributions in the field by a person age 35 or younger. The organization cited Casper’s breakthrough in characterizing hypersonic turbulent spots and her work with novel fluctuating pressure instrumentation when announcing the award.

How turbulent spots vibrate hypersonic vehicles

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.

(Click to enlarge.) The pressure footprint of one hypersonic turbulent spot at Mach 6. As turbulent air flows over an object, thousands of such spots occur every second causing severe vibration. Credit: Sandia National LaboratoriesUsing 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. “This would be a worst-case scenario for the flight," she said. Her work means that engineers now have an improved means of predicting such a scenario and adapting to it.

Pressure-sensitive paint

In 2018, Casper migrated similar pressure diagnostics to Sandia’s blast tube to demonstrate in larger field tests the pressure-sensitive paint technique first used in the wind tunnels. She combined lighting, high-speed cameras and pressure-sensitive paint to capture the effect of a shock wave rolling across a vehicle.

Like the turbulent spots in the wind tunnel, the shock wave created unsteady pressure loading that can vibrate a flight vehicle.

With an explosive charge detonated at one end of the six-foot diameter blast tube, a shock wave traveled through the tube before hitting a model at the other end. Traditionally, hundreds of small pressure sensors would be placed on the model to measure the force. Instead, Casper proposed using pressure-sensitive paint, allowing for a much broader collection of data.

In August, the paint was airbrushed on a model nose cone. Four high-powered, water-cooled ultraviolet lights were shone on the pressure-sensitive paint, causing it to fluoresce. The more oxygen the paint was exposed to, the less it fluoresces. The greater the pressure, the greater the oxygen. So as the shock wave from the blast passed over the model, increasing pressure on its surface, the intensity of the paint’s glow decreased.

Caught on a high-speed camera shooting at 25 kilohertz (or 25,000 cycles per second) with a filter used to block the ultraviolet lighting, the result was a dark shadow growing over the model from the tip to the base. As a reflected shock passes by, the shadow encroaches from base to tip.

Casper and team conducted eight blast tube runs over two days and learned a few valuable lessons from the tests. For example, the tests collect better data when the environment is dark, or at least cloudy, as sunlight interferes with the paint’s fluorescence.