Research

Rayleigh-Taylor Instability

The Rayleigh-Taylor instability is a fundamental problem in fluid mechanics that occurs when a heavy fluid is placed on top of a light fluid. As everyone knows from experience, that is an unstable situation resulting in the inversion of the density stratification. The resulting flow can become very complicated as the two fluids mix through turbulent motions. The turbulence is marked by a wide range of scales of motion, and is thus challenging to describe using numerical simulations. Our approach is to use Implicit Large Eddy Simulations to parse the hierarchy of motions expected in such flows. We use the three-dimensional multispecies code RTI-3D (developed by Dr. Malcolm Andrews, Texas A&M University) to describe various aspects of this complex flow.

 

Such flows are important because they have been observed in nuclear fusion, particularly in the Inertial Confinement Fusion (ICF) processes. In ICF, fusion is initiated by laser impingement of a target fuel capsule that leads to implosion of the fuel layers. Here, Rayleigh-Taylor instability can result from irregularities on the fuel-pusher interface. Rayleigh-Taylor instability is also of significance to supernovae explosions. In a supernova detonation (more here), a radially outward blastwave can accelerate the dense stellar material in to the emptiness of space through the Rayleigh-Taylor mechanism.

 

This work is performed in collaboration with Dr. Guy Dimonte (Los Alamos National Laboratory).

 

Richtmyer-Meshkov Instability

When a shockwave crosses a density jump, it creates conditions conducive to what is known as the Richtmyer-Meshkov instability. If the density contact is seeded with imperfections, they can grow under the influence of the vorticity deposited by the incident shock. Thus the interface will continue to grow long after the shock has left the scene. The details of the flow depend on a wide selection of parameters such as the Mach number of the original shock, the compressibilities of the gases, the density difference between the gases and the nature of the initial perturbations at the interface. Similar to the Rayleigh-Taylor Instability, the Richtmyer-Meshkov flow plays a prominent role in nuclear fusion and in supernovae explosions.

 

This work is performed in collaboration with Dr. Guy Dimonte (Los Alamos National Laboratory).

What is a Supernova?

When a dense star known as a white dwarf gains mass through an accretion process from a companion star, its core can become so heavy that it collapses on itself triggering a runaway thermonuclear reaction. This process plays out over several billions of years, but the subsequent shockwave that blasts radially outward lasts only a few seconds, and is capable of ejecting the stellar matter far in to the emptiness of space. It is believed that the material thus ejected in to space is critical to the formation of earth-like planets and ultimately life. The luminosity of a supernova explosion can be as bright as that of several million suns. The turbulent mixing initiated by the advancing flamefront from the nuclear reaction can lead to detonation and a shockwave that travels radially outward. The outward bound shockwave can in turn accelerate matter contained in the heavier inner core past the less dense outer regions of the star this is similar to the instability caused by placing a heavy fluid over a light fluid under the influence of gravitational acceleration.

Implicit Large Eddy Simulations of Turbulent Combustion

Turbulent Combustion is a challenging problem in engineering because it is marked by a proliferation of scales, and the presence of several simultaneous physical phenomena. The details of the species mixing in a combustor are critical in determining the reaction rate, and must be described accurately. Direct Numerical Simulations of such flows, where all the scales of motion are faithfully resolved, are all but computationally intractable. The computational demands are further exacerbated by the need to include additional physics due to reacting flows, spray modeling where applicable etc. Our approach is to use so-called Implicit Large Eddy Simulations, where the large scales are explicitly resolved while a numerical viscosity enforces small-scale dissipation. The results will be used to evaluate the accuracy and computational efficiency of turbulence models. This work is performed in collaboration with Dr. Mesbah Uddin (UNC Charlotte).