Arindam Banerjee and his students built the Rotating Wheel Rayleigh-Taylor Instability Experiment, which studies two-fluid mixing to mimic inertial confinement fusion. The lab, known as the Turbulent Mixing Laboratory, took Lehigh students five years to build from scratch. (Video by Stephanie Veto)
Mayonnaise: You either love it, hate it, or use it to study the fundamental hydrodynamics of nuclear fusion.
Arindam Banerjee, an associate professor of mechanical engineering and mechanics who studies fluid dynamics in extreme environments, has taken the third approach—generating new understanding of the “instability threshold” of elastic-plastic material in the process.
Banerjee leads Lehigh’s Turbulent Mixing Laboratory and, with his team, has built several one-of-a-kind, highly specialized devices to effectively investigate the dynamics of fluids and other materials under the influence of high acceleration and centrifugal force. The experiments replicate the conditions of inertial confinement, one of the most promising paths to generating energy through nuclear fusion.
So where does the kitchen staple come into play?
First, you need a basic understanding of how inertial confinement experiments work: Gas (hydrogen isotopes) is frozen inside pea-sized metal pellets. The pellets are placed in a chamber and then hit with high-powered lasers that compress the gas and heat it up to a few million Kelvin—about 400 million degrees Fahrenheit—creating the conditions for fusion.
The massive transfer of heat, which happens in nanoseconds, melts the metal. Under extreme compression, the gas inside wants to burst out, causing an unwelcome outcome: The capsule explodes before the hydrogen ions can fuse.
To study the process, Banerjee and researchers in his field mimic the molten metal by using—yep, you guessed it—mayonnaise. The material properties and dynamics of the metal at a high temperature are much like those of mayo at low temperature, he explains.
One of the lab’s unique devices is the Rotating Wheel Rayleigh-Taylor Instability Experiment. It looks like a high-speed train track in a figure-eight shape and took the team five years and continuous funding from the Department of Energy (National Nuclear Security Administration) through their Stewardship Science Academic Alliance Program, along with subcontracts from Los Alamos National Laboratory and the National Science Foundation (CAREER and regular awards) to build.
Rayleigh-Taylor instability occurs between materials of different densities when the density and pressure gradients are in opposite directions creating an unstable stratification.
"In the presence of gravity—or any accelerating field—the two materials penetrate one another like 'fingers,'" he says.
For the experiment, Hellmann’s Real Mayonnaise was poured into a Plexiglass container. Different wave-like perturbations were formed on the mayonnaise and the sample was then accelerated on the rotating wheel device. The growth of the material was tracked using a high-speed camera. An image-processing algorithm was then applied to compute various parameters associated with the instability. Experimental growth rates for various wavelength and amplitude combinations were then compared to existing analytical models for such flows.
Banerjee’s team discovered that the onset of the instability—or instability threshold—was related to the size of the amplitude (perturbation) and wavelength (distance between crests of a wave) applied. Their results published in a recent article in the journal Physical Review E showed that for both two-dimensional and three-dimensional perturbations (or motions) a decrease in initial amplitude and wavelength produced a more stable interface, thereby increasing the acceleration required for instability.
"There has been an ongoing debate in the scientific community about whether instability growth is a function of the initial conditions or a more local catastrophic process," says Banerjee, who works with Lawrence Livermore National Laboratory and Los Alamos, the two major U.S. labs studying inertial confinement. "Our experiments confirm the former conclusion: that interface growth is strongly dependent on the choice of initial conditions, such as amplitude and wavelength."
This work allows researchers to visualize both the elastic-plastic and instability evolution of the material while providing a useful database for development, validation, and verification of models of such flows, says Banerjee.
He adds that the new understanding of the instability threshold of elastic-plastic material under acceleration could be of value in helping to solve challenges in geophysics, astrophysics, and industrial processes such as explosive welding, as well as high-energy density physics problems related to inertial confinement fusion.