Research

The characteristics of electromagnetic waves are considerably affected when they propagate through a turbulent medium. For example, small variations in temperature or density lead to fluctuations in refractive index that in turn perturbs the phase and amplitude of the propagating wave. This optical turbulence can significantly distort the final wave front and result in degrading effects such as beam spreading, scintillation and jitter. Some of these effects can be characterized by optical path differences which has, thus, been studied extensively in different flows such as shear layers, wakes and turbulent boundary layers. These effects
are also consequential in long-range laser communication systems. Understanding these effects is important for fundamental and practical reasons. For example, if the characteristics of the turbulence are known, then one can predict, and thus compensate for, the distortion of the wavefront. Conversely one can also use the information from the aberrated wave front to characterize both the medium and the inhomogeneities encountered along the path.

At TACL we use theory and highly resolved numerical simulations of both turbulence and optical phenomena to gain better fundamental understanding of laser distortion as well as develop models for better prediction and correction. Here are a few recent results in paper and paper.

Numerical simulations are an important tool in understanding complex problems in physics and engineering systems. Many of these phenomena are multi-scale in nature, and are governed by non-linear partial differential equations (PDEs). With a wide range of scales at realistic conditions, like turbulence phenomena in fluid flows, the numerical solution of these equations becomes computationally very expensive. It is known, at extreme scale, that data communication as well as synchronization between PEs pose a major challenge in the scalability of scientific applications. So, there has recently been a major interest in developing numerical methods that minimize communications and relax data synchronization at the mathematical level. This is the main thrust in this NSF-funded project. We have developed asynchrony-tolerant (AT) schemes that will be able to efficiently use future Exascale system with millions or billions of processing elements. The schemes we developed can accurately capture the rich dynamics of turbulence even at the smallest scales (!), while mitigating (or even eliminating) the overheads associated with communication and synchronizations on very large processor counts.

In this project we collaborate with computer scientists to develop the software abstraction layers to implement AT schemes efficiently and in a portable manner through (see here, and here). It is also possible to devise modified equations — proxy equations — that when solved asynchronously, one recovers the solution of the original equation.

Collaborators at Texas A&M: L. Rauchwerger, (Computer Science), R. Bhattacharya (Aerospace Eng.), S. Girimaji (Aerospace Eng.).
Students: Komal Kumari, Bryan Mahoney.

Read more: Paper, paper, paper, paper.

A distinguishing feature of compressible turbulence is the appearance of fluctuations in thermodynamic variables. In many situations of practical and fundamental interest, it can be assumed that the flow is in thermodynamic equilibrium, in which case thermodynamic states can be determined by two thermodynamic quantities alone. When the flow is turbulent, both hydrodynamic as well as thermodynamic variables exhibit nonlinear fluctuations in time and space over a range of scales which increases with the Reynolds number. And, these fluctuations are expected to depend on the level of compressibility in the flow, which is typically quantified by the turbulent Mach number, Mt. We have created a massive database of DNS of isotropic compressible turbulence with resolutions up to 2048^3, Taylor Reynolds numbers up to 400 and turbulence Mach numbers up to 0.6. We have been studied a number of issues and have been able to identify for example a transition at Mt~0.3 where the flow changes qualitatively. We have studied the behavior of temperature, density and pressure statistics  as well as the solenoidal and dilatational components of the velocity field. A number of further results can be found in here and here.

Recent exciting results on universality scaling for compressible turbulence can be found here.

Interested in studying full velocity, density, pressure and temperature fields or any other derived quantity from these DNS? Contact Dr. Donzis to get access.

The interaction of shocks and turbulence is a fundamental phenomenon of fluid dynamics, and this interaction is critical in many areas such as aerodynamics, combustion, and astrophysics. Because of the wide range of spatial and temporal scales that exist in these flows and the difficulty in capturing the shock in simulations and measuring it in experiments, the knowledge of the interaction remains limited. The research of shock-turbulence interactions (STI) in TACL is aimed at fundamental understanding of the two-way coupling of shock and turbulence using massively parallel shock-resolving direct numerical simulations. The main focus is on STI when turbulence is strong, where mean Ranking-Hugoniot relations are no longer applicable.  We have recently shown that the shock cannot, in general, be treated as a discontinuity as has been the case since the 50’s. Instead, we showed that effects of Reynolds and turbulent Mach numbers the shock structure can be characterized by a new non-dimensional parameter that correspond to a case of incomplete similarity. This collapses our new data as well as that in the literature. The understanding of STI is crucial for supersonic flight and is expected to improve the performance of flying vehicles in the future.

 

Some papers on shock-turbulence interactions include theoretical efforts towards finding universal features of the effect of the shock on the turbulence, the structure of the shock, highly-resolved detailed simulations, and some modeling efforts.

 

 

 

The importance of properly accounting for internal molecular structure of gaseous flows is well established across a number of disciplines including high-temperature aerodynamics and combustion.

Using massive Direct Numerical Simulations we have investigated the complex two-way coupling between turbulent fluctuations and molecular energy modes in and out of equilibrium. We found that turbulence can indeed alter the distribution of mean energy across molecular modes (translational-rotational-vibrational), a phenomenon that becomes stronger as turbulent fluctuations become stronger (see figure) and can have important practical consequences in, for example,  high-speed flight.

Collaborators: R. Bowersox (TAMU), S. North (TAMU), W. Hase (Texas Tech).

Read more Paper, paper.

Interested in studying full velocity, density, pressure and temperature fields or any other derived quantity from these DNS? Contact Dr. Donzis to get access.

Check also our collaborative work as part of the National Aerothermochemistry Lab.