Browsing by Subject "Compressible Turbulence"
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Item Reynolds and Mach Number Scaling in Stationary Compressible Turbulence Using Massively Parallel High Resolution Direct Numerical Simulations(2014-07-24) Jagannathan, ShriramTurbulence is the most common state of fluid motion in both natural and engineering systems. Many real world applications depend on our ability to predict and control turbulent processes. Due to the presence of both hydrodynamic and thermodynamic fluctuations, simulations of compressible flows are more expensive than incompressible flows. A highly scalable code is presented which is used to perform direct numerical simulations (DNS) aimed at understanding fundamental turbulent processes. The code is parallelized using both distributed and shared memory paradigms and is shown to scale well up to 264144 cores. The code is used to generate a large database of stationary compressible turbulence at world-record resolutions and a range of Reynolds and Mach numbers, and different forcing schemes to investigate the effect of compressibility on classical scaling relations, to study the role of thermodynamic fluctuations and energy exchanges between the internal and kinetic modes of energy, and to investigate the plausibility of a universal behavior in compressible flows. We find that pressure has a qualitatively different behavior at low and high levels of compressibility. The observed change in the likelihood of positive or negative fluctuations of pressure impacts the direction of energy transfer between internal and kinetic energy. We generalize scaling relations to different production mechanisms, and discover a plausible universal behavior for compressible flows, which could provide a path to successful modeling of turbulence in compressible flows. Our results, unprecedented in size, accuracy and range of parameters will be helpful in addressing a number of additional open issues in turbulence research.Item Toward Understanding and Modeling Compressibility Effects on Velocity Gradients in Turbulence(2011-02-22) Suman, SawanDevelopment of improved turbulence closure models for compressible fluid flow simulations requires better understanding of the effects of compressibility on various underlying processes of turbulence. Fundamental studies of turbulent velocity gradients hold the key to understanding several non-linear processes like material element deformation, energy cascading, intermittency and mixing. Experiments, direct numerical simulation (DNS) and simple mathematical models are three approaches to study velocity gradients. With the goal of furthering our understanding of the effects of compressibility on turbulent velocity gradients, this dissertation (i) employs DNS results to characterize some of the effects of compressibility on turbulent velocity gradients, and (ii) develops simple mathematical models for velocity gradient dynamics in compressible turbulence. In the first part of the dissertation, effects of compressibility on velocity gradient invariants and the local topology of compressible turbulence are characterized employing DNS results of compressible decaying isotropic turbulence. Joint statistics of second and third invariants of velocity gradient tensor and the exact probability of occurrence of associated topologies conditioned upon dilatation (degree of compression/expansion of fluid) are computed. These statistics are found to be (i) highly dependent on dilatation and (ii) substantially different from the statistics observed in incompressible turbulence. These dilatation-conditioned statistics of compressible turbulence, however, are found to be fairly independent of Mach number and Reynolds number. In the second part of the dissertation, two mathematical models for compressible velocity gradient dynamics are developed. To take into account the significant aero-thermodynamic coupling that exists in compressible flows, the models are derived explicitly using the continuity, energy and state equations, along with the momentum equation. The modeling challenge involved in the development of these models lies in capturing the inherently non-local nature of pressure and viscous effects as a function of local terms to derive a closed set of ordinary differential equations. The models developed in this dissertation are evaluated in a variety of flow regimes - incompressible limit (low Mach number); pressure-released limit (extremely high Mach number); and intermediate (sub-sonic Mach numbers) - and are shown to recover a range of known compressibility effects.