Sandia’s Katya Casper Eyes Hypersonic
May 6, 2019
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.
Aerospace engineer Katya Casper
has become known for her innovative techniques measuring the effects of
pressure on hypersonic vehicles at Sandia National Laboratories wind
Simply put, being able to
characterize and predict these pressure spots leads to better vehicle
“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
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
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. 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.
Blasting paint to measure pressure
A lot of Casper’s work occurs at Sandia’s wind tunnels, but it doesn’t
stop there. Last year, 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 intricate lighting, high-speed cameras and the carefully
formulated chemistry of 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 creates
unsteady pressure loading that can vibrate a flight vehicle.
With an explosive charge detonated at one end of the 6-foot diameter
blast tube, a shock wave travels 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.
“With sensors, you can only get pressure readings at the discrete
locations of where they’re placed,” Casper said. “With the paint you can
get data everywhere.”
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 is 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.
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
is a dark shadow growing over the model from the tip to the base; and
then as a reflected shock passes by, the shadow encroaches from base to
The change in the paint’s florescence can be calibrated to the amount of
pressure exerted on the model.
Casper and team conducted eight blast tube runs over two days and
learned a few valuable lessons from the first-of-their kind tests. For
example, the tests collect better data when it’s dark, or at least
cloudy, as sunlight interferes with the paint’s florescence.
“It’s a new approach for measuring pressure taken to the blast tube,”
she said. “Overall, the tests were successful, and with a few
adjustments should ultimately be useful in determining how to protect
objects from shock waves.”