Research

Our interests lay at the coupling of high fidelity models of reactive flows with the state-of-the-art computational methods. Toward this end, we have developed a comprehensive numerical package capable of describing a hypersonic flowfield in the vicinity of a re-entry capsule including the modules of thermodynamics, finite-rate chemistry, multi-dimensional radiation transfer and turbulence.

 

We are interested in designing highly-parallel, scalable and fault-tolerant algorithms for state-resolved simulations of kinetics and thermodynamics of reacting flows.

Molecular dynamics (MD) is a powerful tool for describing individual interactions between particles (atoms and molecules in gases, plasma, and solids).  The centerpiece of any MD-based approach is the potential energy surface (PES) – a function of several variables that gives the forces acting between a pair of atoms. Design of PESs is a multidisciplinary problem itself, making the MD approach a cutting-edge discipline. Having in hands a PES of interest, one can obtain an unprecedented detalization of processes taking place in nature.

In the HTGD, we apply MD methods to study molecular processes in hot air – i.e. in the flow heated by shock waves around flying objects. This research is of interest to DOD, AFOSR, and NASA.

This movie shows the complexity of an actual underlying collisional dynamics in atmospheric species (such as O2 and N2). These mutual “vibrations” and internal energy “scrambling” are due to specifics in the electronic shells of molecular species. The collisional dynamics reflects on the micro- and macroscopic properties of the flow.

For example, we know that ozone (O3) is an important species in the Earth atmosphere which shields us from the harmful UV radiation. Turns out, the reactivity of O2-O collisions produces an unusual behavior of the O2 vibrational mode when it is driven out of equilibrium. It is conventionally assumed that vibrational relaxation becomes more efficient with increasing temperature, however, in the case of O2-O, the trend is opposite. Due to the absence of potential barrier at the O2-O approach, vibrational relaxation in such collision is most efficient at room temperatures while at hypersonic conditions, in the abundance of atomic oxygen, such collisions are less likely to induce an event of vibrational energy transfer.

On the macroscopic level, the state-of-the-art calculations show discrepancies between experimental measurements and theoretical predictions produced by conventional models. The differences are observed in the heat flux and pressure, two critical parameters for the design of a spacecraft. In HTGD lab we develop high fidelity models of chemically reacting flows from first principles, i.e. via propagating trajectories of individual particles and gathering statistical data and coupling it with the state-of-the-art CFD methods.

Interaction of particles with each other often does not occur in a classical manner. As an example, one can think of collisions of heavy particles (atoms and molecules) with electrons and interaction with the light (photons). A quantum approach that is not based on the classical equations of motion is required to adequately capture the macroscopic properties of such matter. However, we encounter situations when the classical approach fails also for heavy particles.

Recent investigations of O2-Ar collisions reveal that vibrational deactivation is not properly captured by solving only classical equations of motions (think of upgraded second Newton Law). We have solved Schrodinger equations to discover that the discrepancies are observed at temperatures as low as 3000K – in a hypersonic regime!

 

 

The process of radiation transfer (emitting, absorbing, scattering and reflecting photons) stands alone among all other processes of energy transfer. This is because the photons propagate at the speed of light, faster than any other processes. On top of this, radiation strongly depends on the wavenumber and the direction of the propagation.

Extra-terrestrial missions, as well as the re-entry process back into the Earth atmosphere, are all subject to a severe heating, partially due to the emission of light from a shock-heated gas toward the surface of a vehicle. The fastest man-made object, capsule Stardust, has collected the star dust from the comet Wild 2 and some cosmic dust and returned to Earth at speed of 12 km/s. The radiation was responsible for 90% of the total heating upon re-entry!

Simulation of the radiation transfer is not a trivial problem due to the spatial and angular dependence of the radiative vector. On top of that, radiation intensity of heated air must be spectrally resolved. In the HTGD laboratory, we advance numerical methods of radiation transfer and couple them with state-of-the-art optical models of various gases.

A propagating shock front will often drive an undisturbed gas out of chemical and thermal nonequilibrium. This means that the internal degrees of freedom (such as vibrational and rotational levels of molecules) and the composition of the flow do not adjust itself instantaneously to altered ambient conditions. In other words, nonequilibrium occurs due to different equilibration times of translational and internal modes of atomic and molecular species. Chemical and thermodynamic nonequilibrium conditions are often coupled to each other.

Under nonequilibrium, the internal temperature, if exists, does not correspond to the kinetic energy of translational motion of particles. Moreover, the population of internal states (vibrational, rotational and electronic) departs from equilibrium and from Boltzmann distribution due to onset dissociation/recombination.

In HTGD Laboratory, we implement conservation equations (master equations) for each individual internal state of species in order to describe the abundance of nonequilibrium processes. The master equation approach is one of the most accurate and sophisticated ways to assess chemical and thermodynamic nonequilibrium.