Fluctuating hydrodynamics of multi-species, non-reactive mixtures

In this paper we discuss the formulation of the fuctuating Navier-Stokes (FNS) equations for multi-species, non-reactive fluids. In particular, we establish a form suitable for numerical solution of the resulting stochastic partial differential equations. An accurate and efficient numerical scheme, based on our previous methods for single species and binary mixtures, is presented and tested at equilibrium as well as for a variety of non-equilibrium problems. These include the study of giant nonequilibrium concentration fluctuations in a ternary mixture in the presence of a diffusion barrier, the triggering of a Rayleigh-Taylor instability by diffusion in a four-species mixture, as well as reverse diffusion in a ternary mixture. Good agreement with theory and experiment demonstrates that the formulation is robust and can serve as a useful tool in the study of thermal fluctuations for multi-species fluids. The extension to include chemical reactions will be treated in a sequel paper

A Numerical Study of Methods for Moist Atmospheric Flows: Compressible Equations

We investigate two common numerical techniques for integrating reversible moist processes in atmospheric flows in the context of solving the fully compressible Euler equations. The first is a one-step, coupled technique based on using appropriate invariant variables such that terms resulting from phase change are eliminated in the governing equations. In the second approach, which is a two-step scheme, separate transport equations for liquid water and vapor water are used, and no conversion between water vapor and liquid water is allowed in the first step, while in the second step a saturation adjustment procedure is performed that correctly allocates the water into its two phases based on the Clausius-Clapeyron formula. The numerical techniques we describe are first validated by comparing to a well-established benchmark problem. Particular attention is then paid to the effect of changing the time scale at which the moist variables are adjusted to the saturation requirements in two different variations of the two-step scheme. This study is motivated by the fact that when acoustic modes are integrated separately in time (neglecting phase change related phenomena), or when sound-proof equations are integrated, the time scale for imposing saturation adjustment is typically much larger than the numerical one related to the acoustics

Field dependent competing magnetic ordering in multiferroic Ni3V2O8

The geometrically frustrated magnet Ni3V2O8 undergoes a series of competing magnetic ordering at low temperatures. Most importantly, one of the incommensurate phases has been reported to develop a ferroelectric correlation caused by spin frustration. Here we report an extensive thermodynamic, dielectric and magnetic study on clean polycrystalline samples of this novel multiferroic compound. Our low temperature specific heat data at high fields up to 14 Tesla clearly identify the development of a new magnetic field induced phase transition below 2 K that shows signatures of simultaneous electric ordering. We also report temperature and field dependent dielectric constant that enables us to quantitatively estimate the strength of magneto-electric coupling in this improper ferroelectric material.Comment: 18 pages, 4 figures. Accepted for publication in Euro. Phys. Let

On the high fidelity simulation of chemical explosions and their interaction with solid particle clouds

High explosive charges when detonated ensue in a flow field characterized by several physical phenomena that include blast wave propagation, hydrodynamic instabilities, real gas effects, fluid mixing and afterburn effects. Solid metal particles are often added to explosives to augment the total impulsive loading, either through direct bombardment if inert, or through afterburn energy release if reactive. These multiphase explosive charges, termed as heterogeneous explosives, are of interest from a scientific perspective as they involve the confluence and interplay of various additional physical phenomena such as shock-particle interaction, particle dispersion, ignition, and inter-phase mass, momentum and energy transfer. In the current research effort, chemical explosions in multiphase environments are investigated using a robust, state-of-the-art Eulerian-gas, Lagrangian-solid methodology that can handle both the dense and dilute particle regimes. Explosions into ambient air as well as into aluminum particle clouds are investigated, and hydrodynamic instabilities such as Rayleigh- Taylor and Richtmyer-Meshkov result in a mixing layer where the detonation products mix with the air and afterburn. The particles in the ambient cloud, when present, are observed to pick up significant amounts of momentum and heat from the gas, and thereafter disperse, ignite and burn. The amount of mixing and afterburn are observed to be independent of particle size, but dependent on the particle mass loading and cloud dimensions. Due to fast response times, small particles are observed to cluster as they interact with the vortex rings in the mixing layer, which leads to their preferential ignition/ combustion. The total deliverable impulsive loading from heterogeneous explosive charges containing inert steel particles is estimated for a suite of operating parameters and compared, and it is demonstrated that heterogeneous explosive charges deliver a higher near-field impulse than homogeneous explosive charges containing the same mass of the high explosive. Furthermore, particles are observed to introduce significant amounts of hydrodynamic instabilities in the mixing layer, resulting in augmented fluctuation intensities and fireball size, and different growth rates for heterogeneous explosions compared to homogeneous explosions. For aluminized explosions, the particles are observed to burn in two regimes, and the average particle velocities at late times are observed to be independent of the initial solid volume fraction in the explosive charge. Overall, this thesis provides useful insights on the role played by solid particles in chemical explosions.Ph.D.Committee Chair: Menon, Suresh; Committee Member: Jagoda, Jeff; Committee Member: Ruffin, Stephen; Committee Member: Thadhani, Naresh; Committee Member: Walker, Mitchel

TensorFlow reinforcement learning quick start guide: get up and running with training and deploying intelligent, self-learning agents using Python

This book is an essential guide for anyone interested in Reinforcement Learning. The book provides an actionable reference for Reinforcement Learning algorithms and their applications using TensorFlow and Python. It will help readers leverage the power of algorithms such as Deep Q-Network (DQN), Deep Deterministic Policy Gradients (DDPG), and ..

A Multi-Species Modeling Framework for Describing Supersonic-Jet Induced Cratering in a Granular Bed: Cratering on Titan Case Study

The dynamics of cratering caused by a multi-component supersonic jet due to its interaction with the granular soil on the surface of Titan is investigated using a two-phase model and numerical simulations. Both fluid and particles are mathematically described in an Eulerian framework. The fluid is modeled using multi-species Large Eddy Simulation accounting for the details of diffusion among species, and the solid phase is described by a Kinetic-Theory-based model. The general framework is that of a recent model that successfully predicted specific details of Mars/Earth cratering observed by the Mars Science Laboratory (Balakrishnan and Bellan, International Journal of Multiphase Flow, 99, 1–29, 2018), however the distinctive differences between the present and previous model allows new physics to be uncovered that is peculiar to cratering in the presence of a dense atmosphere. Unsteady, three-dimensional simulations are performed to elucidate the morphological features of the crater and the dynamics of its evolution with time. Parametric variations of the initial conditions are conducted to understand the effect of the surface evenness, jet Mach number, particle size in the granular bed, jet composition, and the inter-granular stress coefficient. The new observation of crater-in-a-crater formation at early times is discussed and explained; this phenomenon is transient and the subsequent consumption of the inner crater by the surrounding crater walls is explained in detail. The jet exhibits complex morphological features revealed by vorticity and helicity analysis. The primary-crater walls display ripples along their surface that are traced to the interaction with the dense jet fluid which, having reached the crater bottom, changes direction to exit the crater using the spaces unoccupied by the jet and rubs the crater walls. Detailed analyses of the particle volume fraction and the particle momentum revealed the detailed morphology of the craters and the ejections from the craters, and related them to the initial conditions. A larger jet Mach number results in increased erosion due to the higher momentum content of the jet. A smaller increase in the jet momentum obtained by increasing the jet-fluid molar mass jet results in a crater that is more angled at the top, this feature being due to the increased jet expansion rate. The inter-granular stress coefficient does not affect the large-scale features of the crater, but does affect the peak compaction values. Species diffusion is investigated and it is found that both regular and uphill diffusion occur, the latter being primarily concentrated in the vicinity of the jet and the depths of the crater. The presented detailed formulation and numerical methodology for investigating supersonic jet-induced cratering on granular soil are sufficiently robust to accommodate plume-induced cratering on a variety of planetary bodies having an atmosphere